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Nonnative Oysters in the Chesapeake Bay (2004)

Chapter: 9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay

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Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
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9
Elements of Risk Assessment for the Introduction of Crassostrea ariakensis in the Chesapeake Bay

BACKGROUND ON RISK ASSESSMENT

The committee was asked to assess whether the breadth and quality of existing information on oysters and other introduced species are sufficient to support risk assessment of three management options: no use of nonnative oysters, open-water aquaculture of triploid nonnative oysters, and introduction of diploid nonnative oysters. We must begin by identifying an appropriate scope for the risk assessment. Structuring a conceptual or numerical risk assessment highlights what is known and what is not known about the modeled system, breaks complex issues into more understandable problems, and helps to identify critical assumptions. Some previous National Research Council (NRC) studies (e.g., NRC, 1983, 2002b) define risk assessment as the identification and characterization of hazards and the determination of the likelihood that hazards will result in harms. In these studies, risk management is defined as a decision-making process that takes into account the probability distribution of harms given exposure to the hazard and the associated conditional costs and benefits. That is, risk assessment and risk management are reduced to characterizing the branches and conditional probabilities of a decision tree and selecting a strategy based on preferences regarding the conditional net benefits. While this may be workable for well-understood systems with well-defined objectives, complex systems require a more general framework for risk assessment and a closer integration of risk assessment with risk management (NRC, 1993, 1996a, 1996b, 2002b). This closer integration is necessary because the risks that we choose to manage determine the risks that need

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

to be assessed. Following Pratt et al. (1995), we define risk assessment as a decision-making technique that incorporates objective and subjective estimates of the probability of uncertain factors and identifies preferred actions with respect to multiple objectives (see Box 9.1).

BOX 9.1
Glossary of Risk Analysis Terms

Bayes theorem: a statistical rule for combining relative frequencies and subjective probabilities or prior information about the likelihood of certain future conditions.

Conditional costs and benefits: outcomes that can be expected given the occurrence of specific management actions or particular stochastic events.

Decision: an active or passive choice.

Harm: costs incurred as a consequence of specific hazards; a conditional payoff. For example, some Chesapeake Bay stakeholders would construe the establishment of a self-sustaining population of nonnative oysters as a harm related to exposure to the hazard of reversion.

Hazard: an action or event that has the potential to result in an undesired outcome. In the context of this study, a hazard occasioned by open-water aquaculture of triploid nonnative oysters is that reproductively competent oysters are released and the undesired outcome is that the nonnative oysters become invasive.

Likelihood: the probability that an outcome will occur given exposure to a hazard.

Multiple criteria decision analysis: procedures for evaluating alternative outcomes with respect to multiple objectives.

Objectives: goals of stakeholders.

Payoffs: conditional outcomes evaluated in terms of management objectives.

Probability and probability distributions: statistical descriptions of relative frequencies, often expressed in terms of expected value, variance, skewness, and kurtosis.

Relative frequency: the frequency of particular outcomes relative to the frequency of observed outcomes

Risk: the possibility of undesired outcomes being realized as a result of management action or natural variability. In the context of this study, a risk associated with openwater aquaculture of triploid nonnative oysters could be defined as the joint probability that some oysters might revert and that their progeny might become established in the Chesapeake Bay.

Risk analysis: synonymous with risk assessment.

Risk assessment: a decision-making technique that incorporates relative frequencies and subjective probabilities of uncertain factors and identifies preferred actions with respect to one or more objectives.

Risk management: the avoidance, mitigation, reduction, shifting, pooling, or buffering of risk.

Risk preferences: faced with a choice between an action that leads to a guaranteed payoff and an action that leads to an equal but uncertain payoff, the selection of a particular payoff by a decision maker. Most decision makers in most circumstances will select the certain payoff; they are said to be risk averse.

Stakeholder: any person, group, or organization interested in the management decision.

Subjective probabilities: estimates of the likelihood of events and their distribution based on expert judgment with limited historical or experimental observations.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

The structure and dynamics of the Chesapeake Bay ecological and socioeconomic systems are complex, not well understood, and subject to environmental, social, and political influences beyond the scope of management control. Consequently, decision makers are faced with uncertainty—uncertainty about the structure and dynamics of integrated physical, biological, economic, and sociocultural systems, uncertainty about how the systems will respond to the actions taken, and uncertainty about the merits of alternative outcomes. Decision making under these conditions entails risk to ecological systems, risk to socioeconomic systems and institutions, and risk that implementation of management actions will lead to unanticipated or undesired consequences. Actions taken to minimize one aspect of risk often increase the level of risk in other dimensions. When the consequences of management actions are uncertain, good decision making involves balancing risks and benefits.

In order to balance risks and payoffs, it is important to understand the characteristics of risk preferences and approaches to solving multiple criteria decision problems. Although individual risk preferences differ, most decision makers are averse to increased levels of risk unless the risk is offset by increased conditional gains. That is, given a choice between a risk-free payoff and an equal but uncertain payoff, most decision makers will select the risk-free payoff. Risks may be symmetric (equal likelihood of being above or below an expected value) or asymmetric (unequal likelihood of being above or below an expected value). Risk preferences are typically asymmetric (we prefer favorable outcomes) and discordant (we disagree on what is favorable). For example, stakeholders who favor the establishment of nonnative oysters might prefer management actions that increase the likelihood of reproduction, while those who oppose establishment of nonnative oysters might prefer management actions that decrease the likelihood of reproduction, decrease the likelihood of reproduction by reverted oysters, or decrease the likelihood of successful establishment of nonnative populations.

Multiple objectives may be balanced through political processes or formally examined using multiple criteria decision analysis methods (e.g., Keeney and Raiffa, 1976; Saaty, 1990). These methods have been used to address a variety of fishery management issues (e.g., Hilborn and Walters, 1977; Bain, 1987; Walker et al., 1983; Healey, 1984; Mackett, 1985; Merritt and Criddle, 1993). Solutions that emerge from the application of multiple criteria decision analysis often favor compromises that minimize maximum losses or maximize minimum benefits. Multiple criteria decision analyses that incorporate multiple stakeholders with overlapping objectives often select management options that enjoy broad support and limited objection. Stakeholders with conflicting objectives may prefer similar options for dissimilar reasons. For example, stakeholders opposed to the

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

establishment of nonnative oysters might strongly prefer that triploid aquaculture field trials be based on genetic triploids (mated tetraploid-diploid crosses) because of the reduced likelihood of reproduction. Stakeholders interested in commercial-scale aquaculture of triploid nonnatives might favor field trials based on genetic triploids to reduce liability of diploid escape.

Risks that cannot be avoided can often be mitigated, reduced, shifted, pooled, or buffered. A policy mandating the early harvest of triploid nonnatives might mitigate the risk of open-water triploid aquaculture by minimizing the number of oysters that would revert from a triploid to diploid condition and reducing the likelihood that reverted oysters would spawn before they are harvested. Risk reduction might entail actions that partially reduce the level of risk. For example, triploids created from tetraploid-diploid crosses have a lower reversion rate than chemically induced triploids. Risk shifting entails a transfer of risk from one individual or stakeholder class to another. For example, oyster farmers may be able to shift the risk and liability associated with broodstock maintenance and larval production to state hatchery facilities. Risk pooling distributes individual risk across a class of stakeholders. Traditional insurance programs are a mechanism for pooling low-frequency risks with severe negative consequences. Buffering consists of actions that increase the resilience of systems to adverse events. For example, oyster farmers may be able to buffer production risk by distributing their oyster beds across a broad geographic region or across salinity gradients.

The three management options entail differing arrays of risks and are subject to the diverse objectives of multiple stakeholders. Development of a risk assessment framework for the decision would involve characterizing and developing probability distribution functions for the various risks, evaluating those risks according to the diverse objectives, and balancing those objectives in a multiple criteria decision analysis. At this stage there is insufficient information for a formal risk assessment of management options concerning the introduction of a nonnative oyster into the bay. Moreover, it is not possible to complete a risk assessment because the objectives and goals for the Chesapeake Bay and its dependent communities are not well defined or fully agreed upon. The objectives of state, federal, and local governing agencies are unclear, conflicting, or both. In addition, the objectives of the watermen, environmental groups, aquaculturalists, and other users are also unclear, conflicting, or both. Until these diverse objectives are sorted out, meaningful risk assessment cannot be undertaken. Nevertheless this chapter reviews areas of risk and identifies specific risk factors, indicates relative degrees of risk in the ecological and socioeconomic realms, and evaluates the adequacy of the information available.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

Ecological risks associated with the options include the environmental and ecological consequences of continued low native oyster population levels, the risk that nonnative oysters would become established and pervasive or fail to become established or pervasive, the risk of further collapse or impaired recovery of native oyster populations, and the risk of adverse consequences for other stocks of fish, shellfish, and vascular plants. Ecological risks could arise through competitive interactions for food and space through technological interactions (e.g., joint harvesting of scarce stocks of native oysters admixed with abundant stocks of nonnative oysters) or through other interactions (e.g., enhancement of predator, parasite, or disease organisms, or fertilization competition between native and nonnative oyster gametes). An example of a qualitative risk assessment for shellfish farming in Tasmania considered the spread of predators or pests, habitat disturbance, and effects on food resources for other filter feeders (Crawford, et al., 2003). The risk assessment model described by Dew et al. (2003) provides a useful example of a simple model of the likelihood of self-sustaining populations resulting from commercial production of supposed triploid nonnative oysters. Examples of more comprehensive approaches to ecological risk assessment would include models coupling hydrodynamics and larval transport, models assessing age- or size-dependent predator-prey and competitive interactions, and models that incorporate variability in environmental conditions (e.g., salinity, temperature, substrate) to oyster growth, survivorship, and reproduction.

The decision to introduce or not introduce C. ariakensis can be expected to generate differing arrays of risk to economic and sociocultural institutions and systems. Economic and sociocultural risks would differ under each of the three management options and would impact, for example, the availability of oysters for public- and leased-bottom fisheries, the opportunity and incentive for consolidating and vertical integrating of harvesting and processing operations, the sustainability of public-and leased-bottom fisheries, household production and the structure of fishery-dependent communities, the net use and option benefits of recreational and amenity services, and non-use benefits. The economic and socioeconomic risks associated with the management options and the adequacy of available data to assess the magnitude and significance of risk to social, cultural, and economic systems and institutions are also examined below.

Implementation risk includes the risk of political objection, the consequences of management actions that may differ from the intent of those actions, the possibility that actions taken by one regulatory entity might adversely affect the efficacy of actions taken by another regulatory entity, and the likelihood that the selected management option might spur unau-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

thorized introductions. Endangered Species Act Section 7 consultations are an example of an assessment of the risk that implementation of a proposed action could jeopardize the recovery of an endangered species or adversely modify its critical habitat. The adequacy of information available for assessing implementation risks is the final consideration discussed below.

RISK FACTORS

Ecological Risk

Disease

The possibility that a new disease-causing organism might be introduced along with C. ariakensis has been one of the major concerns of all agencies and individuals involved in the deliberations about a possible introduction. This fear is not unwarranted since a number of disease outbreaks in oysters have been linked to the introduction and transfer of molluscs for commercial culture (Rosenfield and Kern, 1979; Andrews, 1980). Adherence to the International Council for the Exploration of the Sea protocols, discussed in more detail below, would significantly reduce the risk of bringing in a new disease-causing organism as a consequence of introducing a nonnative oyster. Some of the linkages are more robust than others, and some researchers are more confident of a causal relationship than others. Manifestation of disease following an introduction could result from the concomitant accidental introduction of an exotic pathogen or through the susceptibility of the introduced oyster to an endemic pathogen (Sindermann, 1990). The method mostly commonly envisioned is that the introduced oyster would bring a new pathogen, which would infect the native oyster (or other species). This might occur even though the introduced oyster displayed no disease symptoms. It might be extremely difficult even to detect the pathogen because so few oysters are infected because so few pathogens are present in any individual, or both. Since the native oyster would never have been exposed to the parasite, it would likely be highly susceptible, experience high infection rates, and suffer heavy mortalities. Less frequently considered but equally plausible is the possibility that the introduced oyster would be exposed to an enzootic or “resident” pathogen, which might or might not cause problems in the native oyster (or other) species. In this case it is the introduced oyster that would develop disease. The causative pathogen may never have been recognized simply because it never caused a problem in native species. It must also be stressed that pathogens can be transported by means that are not related to a nonnative introduction for fishery or aquaculture pur-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

poses, including water currents, ballast water, other (vector) animals, or discards from restaurants and processing plants. Case studies illustrating various scenarios are presented here.

Haplosporidium nelsoni (MSX Disease)

When H. nelsoni was first identified in 1958 as the cause of massive oyster mortalities in Delaware Bay, it was a new parasite to the investigators who saw it in tissue sections of the affected oysters. Mortalities of the scale caused by the parasite in Delaware Bay and later in Chesapeake Bay had never before been recorded in those estuaries, and it was assumed that the parasite was new to the region (Ford and Tripp, 1996). Later, two separate investigations (Katkansky and Warner, 1970; Kern, 1976) reported the finding of a parasite that was morphologically identical to H. nelsoni in the tissues of the Pacific oyster, C. gigas, in California and Korea. Friedman and Hedrick (1991) and Friedman (1996) found what appeared to be the same organism in C. gigas from Japan and California. The observed prevalence of H. nelsoni in these samples of C. gigas was low (<2%), and no commercially noticeable mortalities of C. gigas were reported. Burreson et al. (2000) determined that the small subunit ribosomal DNA sequence of H. nelsoni is identical to that of the parasite in C. gigas and concluded that C. gigas was the source of H. nelsoni, which was highly pathogenic in C. virginica even though it caused little damage in the original host. Further, they strongly suggested that the parasite was introduced in shipments of C. gigas for commercial trials. However, they noted a lack of known introductions of C. gigas into the mid-Atlantic in the years immediately preceding the initial outbreaks of H. nelsoni and acknowledged the possibility of other mechanisms of introduction. Subsequent citations of Burreson et al. (2000), including many of the position papers and documents prepared by agencies concerned with the possible introduction of C. ariakensis, state, without qualification, that H. nelsoni was introduced in “unauthorized” oyster shipments. There is little argument that H. nelsoni came from the Pacific where it infects C. gigas, but the pathway of its introduction is simply not known. Introduced C. gigas might well have been the source, but other possibilities must be considered. Particularly noteworthy is the great increase in ship transit between Pacific and Atlantic ports that occurred during and after World War II. Shipping could have introduced H. nelsoni via infected C. gigas attached to ships hulls or via release of H. nelsoni spores in the discharge of ballast water. The spore is a thick-walled stage in the life cycle of H. nelsoni; its role in transmission is not known, but the spore in other species is typically a transmission stage that can remain “dormant” for long periods and is highly tolerant of environmental extremes. MSX must be consid-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

ered a disease in which the causative agent was surely introduced but for which the means of introduction is unknown.

Perkinsus marinus (Dermo Disease)

Although P. marinus was first described and associated with oyster mortalities in the Gulf of Mexico in the late 1940s, it had probably infected oysters in the southern United States for a long time (Mackin and Hopkins, 1962). The parasite was identified in tissue slides of oysters collected in the 1930s, and mortalities similar to those caused by P. marinus were reported in documents dating back to the early part of the 20th century. Further, this parasite was identified in tissues of oysters in the southeastern United States, including the Virginia portion of Chesapeake Bay, as soon as investigators looked in the late 1940s and early 1950s. It was not detected in the northeastern United States, however, and numerous studies documented that the parasite multiplied and killed most readily at elevated water temperatures. However, coincident with large-scale commercial transplanting of infected oysters from Virginia into the New Jersey portion of Delaware Bay for several years in the mid-1950s, the parasite was found in nearby native Delaware Bay oysters, although it caused no mortalities (Ford, 1996). When H. nelsoni began killing oysters in the bay in 1957, an embargo was placed on all imports and exports, and after several years P. marinus was rarely detected in Delaware Bay oysters.

It was argued that without repeated introductions of the parasite, Delaware Bay water temperatures were simply too low for the parasite to maintain a self-sustaining population. Nevertheless, in 1990, a severe outbreak of P. marinus infections and consequent oyster mortalities began in the bay, without the concomitant transfer of oysters from outside the estuary. That outbreak and other outbreaks over a 500-km range north to Cape Cod, Massachusetts, which followed over the next 2 years, occurred during a period of unusually warm water temperatures. Although this range extension was not associated with contemporaneous transplantation of oysters, historical transfers of oysters from the south to overfished regions of the north would probably have introduced P. marinus repeatedly over many earlier decades. In this new, colder region the parasite may have persisted at low and undetectable levels until the temperature became warm enough to stimulate an epizootic, as occurred in Delaware Bay. This account of Dermo disease illustrates what appears to be an example of a pathogen that can be present but suppressed by unfavorable environmental conditions (low temperature), then stimulated to become epizootic when conditions become favorable.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×
Marteilia refringens (Marteiliosis, Aber Disease)

M. refringens infects and causes severe disease in the European oyster, Ostrea edulis. Mortalities ascribed to the pathogen were first noted in Brittany, northern France, in 1967 and 1968 and over the next several years, caused mortalities in most Breton culture areas (Balouet et al., 1979). It is now found south along the French coast and into Spain, Portugal, and the Mediterranean. Like H. nelsoni, the mechanism of transmission and origin of M. refringens are unknown, although recent studies indicate that a copepod may be a second host (Audemard et al., 2002). M. refringens was a “new” parasite when first associated with the O. edulis mortalities; neither it nor any similar parasite had been seen before. Some authors (Andrews, 1980; Farley, 1992) have pointed to the propinquity of the marteiliosis outbreak and the inception of C. gigas introductions into France in the mid-1960s (Grizel and Héral, 1991) as a cautionary example of a disease agent brought with an introduced host. Others are less sure of the connection (Grizel and Héral, 1991). The timing of the introduction of C. gigas, which was found to have very low prevalence of a Marteilia-like parasite (Cahour, 1979) and the outbreak of Marteiliosis in O. edulis, may be simply a coincidence. If it does represent a cause-effect situation, the linkage is less strong than in some other instances. Interestingly, the mussels Mytilus edulis and M. galloprovincialis, which inhabit the same European waters as O. edulis, are infected by very similar parasites (Berthe et al., 2000; Le Roux et al., 2001; Longshaw et al., 2001). An alternative explanation for the appearance of M. refringens is that M. maurini “jumped” to a new host, O. edulis, concomitant with a gene mutation.

Bonamia ostreae (Bonamiosis)

B. ostreae also infects and causes mortality in O. edulis. It came to the attention of molluscan disease specialists when it was implicated as the cause of epizootic mortalities of O. edulis in Brittany, France. The mortalities were first noted at one site in mid-1979 and later during the same year in other Breton oyster farms (Balouet et al., 1983). Within a year or two B. ostreae was found to be causing oyster deaths in Ireland, England, the Netherlands, and Spain. In conjunction with M. refringens, B. ostreae continues to depress European oyster production.

The probable movement of B. ostreae through oyster shipments has an interesting history. Although it was described simply as a “microcell” at the time, the same parasite was found in the mid-1960s in O. edulis reared at the Milford Laboratory in Connecticut (Farley et al., 1988). Some of the Milford progeny were shipped to California where the microcell was later observed in dead and dying oysters (Katkansky et al., 1969). Seed oysters

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

from at least one California hatchery were then transferred to Washington state, Maine, Spain, and France (Katkansky and Warner, 1974; Elston et al., 1987; Figueras, 1991; Friedman and Perkins, 1994). B. ostreae has since been detected in O. edulis grown in California, Washington, and Maine (Elston et al., 1986; Friedman and Perkins, 1994; Zabaleta and Barber, 1996). Since the parasite is directly transmissible between oysters, a case can be made that the source of B. ostreae responsible for the European outbreak was the “large amounts of O. edulis seed transferred to France in the years prior to the detection of the disease there” (Elston et al., 1986). The finding of B. ostreae in O. edulis shipped from the Milford Laboratory to California, where the oysters experienced heavy mortality, and the presence of the same parasite in oysters from the Milford Laboratory held in quarantine at the Oxford, Maryland, National Marine Fisheries Laboratory, suggests that the parasite may have been introduced from the East to the West coasts of North America (Farley et al., 1988). The original shipments of O. edulis to the Milford Laboratory came from the Netherlands, where oyster mortalities caused by B. ostreae were reported only after the French outbreaks. The evidence available thus suggests that the parasite was probably not native to Europe at the time those shipments were made (see Table 3.2). B. ostreae is thus an example of a disease agent that is known to be transmitted directly between oysters, with a relatively welldocumented linkage with the movement of the host oyster for commercial purposes.

Vibrio tapetis (Brown Ring Disease)

Brown Ring Disease is a disease that has caused mortalities of the Manila clam, Ruditapes philippinarum, in Western Europe (Paillard et al., 1994). It is caused by a marine bacterium, Vibrio tapetis, and is characterized by the deposition of a ring of organic material (periostracum) on the inner edge of each valve. Manila clams were introduced to France from the U.S. West Coast for aquaculture because they grow faster than the native clam, R. decussatus. For several years Manila clams did extremely well in culture and became established in the wild, but in 1987 heavy mortalities associated with the brown ring symptom were reported in culture parks in northern Brittany. The disease spread north and south with Manila clam culture in Europe and is now found in England, Spain, and Portugal as well as France, and it affects naturalized populations of R. philippinarum as well as those under culture. It is not found in the northwestern United States or in the native range of the Manila clam along the Asian Pacific coast. The pathogen Vibrio tapetis can be found in environmental samples and the native clam, R. decussatus, is highly resistant to the disease (Maes and Paillard, 1992). Brown Ring Disease appears to be

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

an example of an introduced host being highly susceptible to a resident pathogen, which has little effect on the native clam.

Quahog Parasite X (QPX Disease)

QPX is a newly recognized disease agent in the hard clam, Mercenaria mercenaria (Whyte et al., 1994). Although it has yet to be given a scientific name, QPX is a member of the phylum Labyrinthulomycota, a group of microorganisms that live in marine and estuarine environments on micro and macro algae and detritus. Sometimes they are pathogenic, and they have been associated with mortalities of molluscs in captivity or under culture (Ford, 2001). QPX has been found in clams from Virginia to the Canadian maritime provinces, but it appears to be most serious in the northern culture areas. Recently, it has become evident that serious outbreaks in clams being cultured in New Jersey and Virginia occurred in stocks that had come from South Carolina and Florida and not in those produced locally. This observation is supported by results of an experiment in which brood stock originating in five states from Florida to Massachusetts were spawned in a single hatchery and their offspring grown in side-by-side plots in Virginia and New Jersey (Ragone-Calvo and Burreson, 2002). In both grow-out sites, clams from Florida and South Carolina acquired numerous and heavy QPX infections and suffered high mortalities. Those from Massachusetts and New Jersey had few infections and low mortality. Clams from Virginia exhibited intermediate traits. QPX outbreaks appear to be an example of a disease caused by an enzootic parasite, which may evoke no detectable problems in a resident host but is highly pathogenic to nonlocal stocks of the same species.

Considerations for Disease Risk Assessment
Implications of Detecting Pathogens in Disease Surveys

One of the questions raised about the potential introduction of C. ariakensis is the lack of knowledge of parasites and diseases of the species in its native range. A number of samples have been examined at the Virginia Institute of Marine Science (VIMS) and more are planned (see Chapter 4). So far the only identified pathogen has been a haplosporidian found in one of 155 oysters (0.6%) in a sample from northern China. This very low prevalence is not unusual for parasites that are present in “resistant” hosts but can cause epizootic mortalities when they infect susceptible hosts (e.g., H. nelsoni in C. gigas versus C. virginica). It is worth noting that the prevalence of H. nelsoni in various samples of C. gigas (considered “resistant” to H. nelsoni) from Korea, Japan, and California averaged <1%

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

(Kern, 1976; Kang, 1980; Friedman and Hedrick, 1991; Friedman, 1996; Burreson et al., 2000). After it appeared on the East Coast of the United States, infection prevalence in C. virginica was commonly 50 to 90% (Ford and Tripp, 1996). Thus, low prevalence in one host does not mean that, if introduced, a parasite would be harmless in a new environment.

Effectiveness of Diagnostic Methods

The standard diagnostic method for the detection of infectious agents in molluscs is the examination of fixed and stained tissue sections by light microscopy. The method is good for relatively large parasites such as worms or most protozoans, although the amount of tissue examined in the section is only a very small fraction of the total available and very light infections can easily be missed. Nevertheless, this is the technique that detected the low prevalence of a haplosporidian in C. ariakensis in China, the marteiliodes-like parasites in the same species from Hong Kong, as well as the low prevalences of H. nelsoni in C. gigas. In the last case, sample sizes of hundred to thousands of specimens were examined to document prevalences of <1%.

Tissue section histology is much less effective for the smaller organisms like bacteria, unless they form aggregates, such as is the case for the bacterium that causes “nocardiosis” in C. gigas (Elston et al., 1987; Friedman and Hedrick, 1991). Viruses are particularly difficult to detect using light microscopy unless infections cause obvious cellular damage in the host. Tissues displaying damage not attributable to an obvious cause can be examined by electron microscopy, which can detect viruses but is extremely labor intensive and expensive. Molecular techniques can detect bacteria and viruses not easily found using other methods, but they can also suffer from a need to limit the amount of tissue examined to a relatively small fraction of the total animal. Further, they require that the organism already have been isolated and characterized in order to develop the appropriate assay tools. Thus, such methods are not particularly helpful in screening for “unknown” pathogens. A DNA-based assay is available for the herpes virus that is associated with larval and juvenile bivalve mortalities in hatcheries and nurseries and is being employed at VIMS to examine C. ariakensis from China.

The International Council for the Exploration of the Sea Code of Practice

The greatest risk of C. ariakensis introducing a disease agent would occur if seed or adult oysters were to be placed in Chesapeake Bay directly from another location (e.g., “rogue” introductions from China or

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

the U.S. West Coast). If, on the other hand, the International Council for the Exploration of the Sea (ICES) protocols (see Appendix F and http://www.ices.dk/products/misc.asp) are followed, the chances of introducing a pathogen, such as the parasites that have been cointroduced with oysters in the past, would be greatly minimized. The guidelines direct that the exotic oysters would be used only as brood stock in a quarantined hatchery and would be destroyed after spawning. Only the offspring, produced in the hatchery, would be placed in Chesapeake Bay and strictly monitored for evidence of disease. Hatchery protocols should further ensure that gametes and embryos are well cleansed to remove any parasites potentially included in the gamete mix. Although washing would never be 100% effective, the operation would provide an added measure of safety to the already strict ICES protocol. Neither the ICES protocol nor any washing procedure would, however, prevent the transmission of pathogens that infect the gametes themselves and are passed directly (vertically) to the offspring. Most such pathogens are viruses. Further, ICES protocols cannot prevent disease outbreaks caused by pathogens that are transported by means other than the commercial species under consideration for introduction. Among the case studies presented earlier, only the movement of B. ostreae would almost certainly have been prevented by proper use of ICES protocols. On the other hand, the ICES protocols would not have prevented the introduction of H. nelsoni into the United States or the northward-range expansion of P. marinus if these events resulted from commercial shipping, transport by water currents, or overboard disposal of infected animals by restaurants, processors, or private individuals. Nor would ICES guidelines have prevented disease outbreaks caused by “resident” parasites such as QPX or the bacterial agent of Brown Ring Disease.

A papovavirus is known to infect eggs of C. virginica, causing a condition known as viral gametocytic hypertrophy or VGH (Farley, 1978). The virus particles replicate inside the gametes, eventually resulting in large “inclusion bodies” that are easily visible under the microscope. It is not known whether the virus is transmitted vertically, but the prevalence of VGH is extremely low and causes no mortality. Viral diseases of oyster larvae and juveniles (e.g., herpes virus) have been described in hatcheries and nurseries in a number of countries. Although herpes virus has been found in the gonads of adult oysters (Arzul et al., 2002), its presence inside eggs, which would be evidence of true vertical transmission potential, has not been demonstrated (T. Rheault, Moonstone Oysters, Pt. Judith, RI, personal communication, 2002). It should be noted that herpes virus was found in C. virginica on the East Coast as early as 1972 in oysters that had been subjected to elevated temperatures in a power plant discharge pipe in Maine (Farley et al., 1972).

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

No bacteria have been reported to infect gametes of molluscs. On the other hand, protozoans belonging to microsporidia have been found infecting the eggs of several species of oysters, including C. gigas and O. edulis (Bower et al., 1994), and a Marteilioides has been found in the eggs of C. gigas (Comps et al., 1986). Vertical transmission of any of these parasites has never been reported; however, it should be noted that microsporidia can be vertically transmitted via the eggs of a number of insect and small crustacean (amphipods and copepods) species (Raina et al., 1995; Dunn et al., 2001; Bigliardi and Carapelli, 2002).

Crassostrea ariakensis as a Reservoir for Enzootic Pathogens

One concern raised about the possible introduction of C. ariakensis is that it may act as a reservoir for H. nelsoni and P. marinus, even though it does not succumb to the parasites. Although the possibility cannot be dismissed, the reservoirs for both of these disease agents (C. virginica and perhaps other molluscs in the case of P. marinus and “unknown” in the case of H. nelsoni, whose mode of transmission is unknown) are, and historically have been, plentiful enough to result in extremely high infection levels in oysters wherever environmental conditions are permissive. If C. ariakensis did act as a reservoir, its low parasite burdens would make it insignificant compared to the reservoirs already present.

Crassostrea ariakensis as Host for a “Native” Pathogen

The possibility that C. ariakensis might acquire a resident pathogen should not be ignored in the debate about its possible introduction. For instance, C. ariakensis may be highly susceptible to B. ostreae. While being held in a hatchery in southwestern France, a cohort of C. ariakensis suffered high mortalities associated with infections of a parasite identical morphologically to B. ostreae (Cochennec et al., 1998). The hatchery received raw seawater from a locality where B. ostreae was present in the native O. edulis, and it is probable that the C. ariakensis became infected and died with heavy B. ostreae infections (see Chapter 4). In this connection it should be recalled that B. ostrea is present, albeit at low infection levels, in O. edulis in Maine (Friedman and Perkins, 1994; Zabaleta and Barber, 1996).

The Risk of Disease

C. ariakensis has performed extraordinarily well in field trials conducted in the lower bay (Calvo et al., 1999). It grows rapidly and does not succumb to MSX or Dermo diseases (see Chapter 4). If ICES guidelines

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

are strictly followed in an introduction process, the risk of introducing a new pathogen will be almost completely eliminated. However, the “risk of disease” associated with a possible introduction is not zero. Despite its admirable performance so far, C. ariakensis should not be considered disease resistant. Its susceptibility to the bonamia-like parasite described above is a case in point. Even C. gigas, known for being resistant to the parasites that have devastated both C. virginica and O. edulis, suffers mortalities in Japan, the West Coast of the United States, and more recently in France from causes not well understood (Koganezawa, 1974; Cheney et al., 2000; Lacoste et al., 2001). Finally, large-scale aquaculture in which juveniles are reared at high density with limited water exchange typically favors disease outbreaks in hatcheries and nurseries regardless of the species involved.

Ecological Risks Directly Associated with C. ariakensis

Since the potential ecological risks and benefits of nonnative species are difficult to quantify, it is not surprising that scientists differ on the value of deliberate introductions of species (Ewel et al., 1999). For example, some believe that the need for restoring ecosystem function is so great that concerns about possible harmful effects of deliberate introductions are not warranted. Others, in contrast, place primary emphasis on the biological, economic, and social costs of the introductions. All recognize that once an aquatic species is introduced it is virtually impossible to control its spread. Marine ecosystems have few biogeographic barriers, and the dispersal capabilities of nonnative species do not necessarily coincide with political and economic boundaries. A species that is desirable in one location may be regarded as a nuisance or as undesirable in another. There is also increasing evidence that the human alterations of ecosystems often influence the probability that an introduced species will become invasive and that time lags of several decades or longer often exist between the initial introduction of an organism and when that species becomes a nuisance.

The major ecological concerns centered on the proposed introduction of C. ariakensis deal with illegal or rogue plantings or placing reproductively viable diploid oysters (even with adherence to ICES guidelines) into the bay. If reproductively viable diploid organisms are introduced, the primary issues are fourfold: Where will C. arakensis grow in the bay and how might the oyster affect other resident species, especially the native Eastern oyster? Will C. ariakensis provide similar ecosystem services to the bay as the native oyster? Will C. ariakensis become a “nuisance species,” which would result in negative impacts on the bay’s ecosystem? What are the chances of the nonnative oyster dispersing to regions out-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

side the bay? If an illegal introduction of C. ariakensis occurs, there is increased concern that disease agents or other species (e.g., “hitchhikers”), which may be attached to the oysters, would be introduced into the bay. The primary ecological risk associated with deploying triploid oysters in the bay is the probability that a self-sustaining population of nonnative oysters will be established there, because of the direct introduction of a small percentage of diploid individuals among mated triploids or the reversion of triploids to diploids. If this should happen, the ecological risks would be similar to those of introducing reproductively viable oysters into the ecosystem. Assessing the relative risk of the aforementioned issues is severely constrained by the lack of fundamental ecological information on C. ariakensis. Little is known about the ecology of the oyster in its native range and how it will interact with other species if it is introduced into the Chesapeake Bay.

Is C. ariakensis capable of establishing reproductively viable populations in the bay? If so, will it compete with C. virginica or other resident species in the bay?

C. ariakensis is well adapted to living in estuarine habitats, which characteristically experience a wide range of temperature and salinity variation and contain high levels of suspended material concentrations in the water column (Chapter 4). Field trials using triploid oysters conducted at several sites in lower Chesapeake Bay indicate that C. ariakensis grows well under a relatively wide range of salinity conditions (Calvo et al., 2001). Also, preliminary laboratory studies indicate that C. ariakensis is capable of reproducing over a similar salinity range as C. virginica (M. Luckenbach, Virginia Institute of Marine Science, Gloucester Point, personal communication, 2002). Coupled with its relatively rapid growth and resistance to MSX and Dermo diseases, it is very likely that C. ariakensis is capable of establishing wherever C. virginica was established historically in the Chesapeake Bay, with the exception of areas where sedimentation now prevents or inhibits larval settlement.

From the limited information available, it appears that environmental conditions tolerated by C. ariakensis broadly overlap those favored by C. virginica; there is little evidence suggesting the two species would occupy different types of habitats within the bay ecosystem. While there is conflicting information about the reef-building characteristics of C. ariakensis, a number of records indicate the presence of C. ariakensis reefs in certain coastal areas in China (Chapter 4). Given their functional and ecological similarities, it seems likely that both oyster species will utilize similar food and spatial resources. The intensity of competition between the species and whether C. ariakensis will outcompete C. virginica, however, is

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

more difficult to predict. Often when ecologically similar marine species are competing for space, those exhibiting faster growth rates competitively dominate slower-growing species (Branch, 1984); however, there are exceptions to this pattern (e.g., Lang, 1973; Lang and Chornesky, 1990). M. Luckenbach (Virginia Institute of Marine Science, Gloucester Point, personal communication, 2002) has performed a series of interspecific competition experiments in the laboratory with juvenile life stages of both species. He found that C. virginica was the more effective spatial competitor, despite the fact that field trials indicate C. ariakensis grew much faster than C. virginica. Clearly, additional field and laboratory work is needed to more fully understand the nature of interactions between the two species and how variations in environmental factors and the presence of the oyster disease organisms may influence competitive interactions for space. In addition, little is known about the reproductive cycle of C. ariakensis in the bay and whether the species would spawn sooner and set more heavily than C. virginica. For species that are competing for similar settlement substrates, the outcome of competition is often controlled by the order in which they colonize a habitat (e.g., Osman, 1977; Sutherland and Karlson, 1977). However, if C. ariakensis became very abundant, it could act as an important competitor with other fouling species (e.g., barnacles, ascidians, bryozoans) resident to the bay. Also, if C. ariakensis does not form three-dimensional reeflike structures and the population expands by growing horizontally over the seafloor, it could have negative impacts on other resident species. For example, as zebra mussels expanded their populations in the Great Lakes by first colonizing hard substrates (e.g., rocks, pilings, intake pipes), they subsequently began forming dense aggregations across sedimentary habitats (Berkman et al., 2000). If C. ariakensis displayed a similar type of habitat shift following its introduction, it could begin outcompeting and/or smothering infaunal bivalve species (e.g., Macoma balthica, Mya arenaria) that are important food items for blue crabs.

Both oysters are also very likely to compete for food resources. Again, the degree of competition is difficult to discern given the general lack of knowledge about the physiological and feeding ecology of C. ariakensis. R. Newell (Center for Environmental Science, University of Maryland, Cambridge, personal communication, 2002) has conducted some preliminary laboratory experiments comparing the clearance rates of both species, finding that both exhibit similar feeding rates. This result is somewhat surprising given C. ariakensis grows much faster than C. virginica. Bayne (2002), for example, demonstrated that the competitive advantage the nonnative Pacific oyster, C. gigas, has over the native Australian oyster, Saccostrea glomerata was due to faster rates of feeding, particularly at high food concentrations, which resulted in greater metabolic efficiencies for

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

both feeding and growth. Possibly, C. ariakensis has a higher metabolic or assimilation efficiency than C. virginica, which translates into a faster growth rate.

The lack of ecological information on C. ariakensis also greatly limits the ability to predict how it would impact other species in the bay. If C. ariakensis occupies a niche similar to that of the native oyster, it seems unlikely that C. ariakensis would outcompete other resident species. However, if C. ariakensis populations became very abundant, they could compete with other fouling organisms in the bay. Also, if the Suminoe oyster does not form reef structures, it could colonize habitats currently occupied by other sedentary species.

Another serious threat is the impact that a rogue introduction of the oyster would have on the bay ecosystem, since it is highly likely that other species attached to the oyster shell would also be introduced along with the oyster. There are many examples of the impact that “hitchhiking” species have had on coastal ecosystems (see Chapter 3).

Will C. ariakensis provide ecosystem services similar to those provided by the native oyster?

As mentioned in Chapter 4, very dense and spatially extensive populations of suspension-feeding bivalves can provide a number of estuarine ecosystem services. These include bio-filtering of water and removal of suspended materials and improving water clarity. Reef-building suspension-feeding species can provide important habitats for economically important species such as striped bass and blue crabs. The reefs also act to enhance biodiversity relative to surrounding soft sediment habitats. It seems likely that C. ariakensis is capable of providing similar types of ecosystem services as the Eastern oyster if sufficient population densities existed in the bay. While both oyster species are functionally similar, the degree to which C. ariakensis could influence the bay ecosystem in a manner similar to C. virginica would depend on the amount of oyster biomass in the bay and the role of oysters in the ecosystem.

Most of the ecosystem services provided by C. ariakensis will consist of the benefits provided through water filtration activities. The degree and types of ecological services provided by aquaculture of triploid Suminoe oysters would depend on the spatial scale in an individual commercial operation and on how the oysters are grown to marketable size. In situations of intense aquaculture grow-out with high concentrations of oysters, one should see positive benefits on water clarity. However, aquaculture of triploid nonnative oysters is likely to be limited in spatial extent initially, owing to high production and biosecurity costs, and thus unlikely to contribute substantially to total oyster filtration capacity of the

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

bay. Nevertheless, if aquaculture of triploid C. ariakensis proves successful and more grow-out areas in the bay are established, presumably the ecosystem services provided by biofiltration would increase proportionately (e.g., Bayne and Warwick, 1998). Water clarity could be magnified locally if intensive aquaculture operations are located in some of the smaller “trap” or retentive estuaries and tributaries within the bay that have more restricted water movement. Finally, over the longer term, were on-bottom aquaculture of nonnatives permitted on a large scale—entailing a much greater risk of diploid establishment—more profound impacts on filtration capacity and water quality could be expected to follow. Indeed, there is a counterexample; intensive on-bottom aquaculture in France exceeded the carrying capacity of the areas (e.g., Grant et al., 1993; Raillard and Menesguen, 1994), decreasing oyster growth rate and depleting the food available to other filter-feeding organisms in the area. This is also likely if aquaculture operations were located in some of the smaller “trap” or retentive estuaries and tributaries within the bay that have more restricted water movement. If aquaculture of triploid C. ariakensis proves successful and more and more grow-out areas in the bay are used, presumably the ecosystem services provided by biofiltration would increase proportionately (e.g., Bayne and Warwick, 1998). The reef-type ecosystem services (e.g., habitat for other species) provided by C. ariakensis would be based on the length of time the oysters were grown in the water and whether they are grown on the seafloor or suspended in the water column. In both grow-out scenarios these services would be limited both spatially and temporally when compared to naturally occurring oyster reef habitat. Presumably, mobile organisms would be attracted to the oysters or the structures in which the oysters were being reared. There could be negative impacts on the bay ecosystem if culturing operations proliferated in the bay without following best management practices to minimize impacts on other parts of the ecosystem. For example, Everett et al. (1995) concluded that oyster stake culture methods adversely impacted eelgrass through increased sedimentation and physical disturbance associated with planting and harvesting. Limiting stake or other culture structure density in areas with submerged aquatic vegetation (SAV) could mitigate negative effects while still allowing oysters to provide the valuable ecological services noted above and in Chapter 4. Peterson and Heck (2001) and Heck and Orth (1980) demonstrated that benthic mussels (Modilous americanus) in Saint Josephs Bay, Florida, when cultured at appropriate densities, provide a variety of ecological functions that enhanced seagrass (Thalassia testudinum) productivity. Similar effects have been reported by clam farmers on the Eastern Shore of the Chesapeake and by shellfish farmers in the Pacific Northwest. As with bottom culture, aquaculture activities relying on suspended grow-out techniques, sited inappropri-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

ately or cultured at too high a density, may have negative impacts. Excessive accumulation of biodeposits and shell rubble may affect the benthic habitat beneath these operations (ICES, 1988). Enhanced amounts of anoxic sediments have occurred in several shallow bays in Japan as a result of oyster culture where large amounts of pseudofeces are produced (Nose, 1985). Conversely, when cultured at lower densities, suspended shellfish culture was shown to have little impact on the benthic environment in Tasmania (Crawford et al., 2003). On one culture site in Tasmania, dense beds of eelgrass were observed under suspended oyster trays as well as outside the boundary of the farm. Accumulations of shell rubble may alter benthic species composition and provide substrate for oyster settlement.

If a “wild” fishery for C. ariakensis was established in the bay through the introduction of reproductively active diploid oysters, the degree of ecosystem services is again dependent on the amount of oyster biomass and the extent of population distribution in the bay. As mentioned previously, it may have taken the Eastern oyster hundreds of years, with minimal fishing pressure, to form the extensive reefs in the bay’s tributaries (Hargis, 1999). If C. ariakensis was not harvested for a number of decades, sufficient quantities of oysters might develop that would mimic ecosystem services provided by the Eastern oyster prior to its decline through overfishing, disease, and habitat degradation. The extent and time frame of this possible event are also highly dependent on how well C. ariakensis could adapt to the bay’s environmental conditions and how quickly wild populations of oysters would become established and proliferate into sufficiently dense reefs to have an effect like that of the historical native oyster population.

Will C. ariakensis become an invasive or nuisance species that may negatively impact the bay ecosystem?

As mentioned in Chapter 3, it is very difficult to predict which species will become an invasive or nuisance species and which will not. While some attributes are hypothesized for successful aquatic invaders (see Table 9.1), few generalizations have been confirmed, and each has exceptions (e.g., Simberloff, 1989; Lodge, 1993; Ricciardi and Rasmussen, 1998). These attributes can, however, be used to provide a general guideline to facilitate the identification of potential invasive species. For example, species possessing wide environmental tolerance limits and natural mechanisms for rapid dispersal (e.g., zebra mussels, Dreissena polymorpha) are likely to colonize a large geographic range. Other studies have shown than species possessing broad ranges of distribution are often a good predictor of invasive ability (e.g., the Asian clam, Corbicula

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
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TABLE 9.1 Some Hypothesized Attributes of Aquatic Invasive Species

1.

Broad environmental tolerance

2.

Rapid growth

3.

Early age (size) of reproductive maturity

4.

High reproductive capacity

5.

Possessing multiple mechanisms of dispersal

6.

Release from natural predators, parasites, and diseases

7.

Short generation time

8.

Broad diets

9.

Gregariousness

10.

Abundant and broadly distributed in native range

 

SOURCE: Based, in part, on Groves and Burdon (1986), Ehrlich (1986), Morton (1987, 1997), and Lodge (1993).

fluminea [Morton, 1997]; water hyacinth, Eichornia crassipes [Groves and Burdon, 1986]).

Many of the attributes that make C. ariakensis an attractive species for the establishment of a fishery in Chesapeake Bay are the same characteristics that have been attributed to aquatic nuisance species. For example, C. ariakensis has a very broad native distributional range (Chapter 4) and an equally wide range of tolerance to environmental conditions such as variations in salinity and temperature. It lives in estuarine habitats, both intertidally and subtidally, and is tolerant of turbid and eutrophic water conditions. C. ariakensis also grows relatively fast, has a high reproductive capacity, and is able to reproduce within several months following larval settlement. While nothing is known about what species may prey on C. ariakensis, it does appear immune to the oyster diseases that now plague the Eastern oyster in the bay. A number of oyster predators that reside in the bay (e.g., blue crabs, flatworms, cownose rays) might act to control the growth of the oyster population. As mentioned in Chapter 4, the shell of C. ariakensis does not appear to be a strong as C. virginica, which may make it more vulnerable to shellfish predators (particularly crab predators). It should be noted that some mollusc species have the ability to rapidly respond to the effects of crab predators by increasing the thickness of their shells (e.g., Trussell and Smith, 2000).

Several studies have noted that one of the most consistent attributes of a invasive species is the use of dispersal mechanisms that involve human activity (e.g., Ehrlich, 1986; Carlton and Geller, 1993; Morton, 1997; Ricciardi and Rasmussen, 1998). C. ariakensis possesses a planktonic larvae that could easily be transported in ship ballast water, and adults can attach to the hulls of ships. In addition to these unintentionally transported vectors, it seems highly likely that adult oysters will be intentionally transported by human activity as part of normal aquaculture or fish-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

ery practices. The relative importance of these vectors is generally dependent on whether the species is capable of establishing reproductively viable populations.

If the nonnative oyster became invasive and the population was not kept in check by harvesting or by native predators, it is conceivable that C. ariakensis could reach sufficient densities to shift the bay ecosystem back toward benthic dominance rather than pelagic dominance. Of course, the same thing could happen if the native oyster rebounded, but this is considered unlikely due to disease and harvest pressure. Reducing standing stocks of phytoplankton might facilitate improvement of water quality and reduce populations of gelatinous zooplankton (Chapter 4). An increase in SAV could have beneficial secondary effects on associated invertebrates and waterfowl. Altering the bay from pelagic to benthic dominance may also result in shifts in species composition and abundance at higher trophic levels. For example, pelagic finfish (e.g., menhaden, striped bass) populations may be reduced, while species that directly or indirectly rely on benthic productivity (e.g., sheepshead, bluefish) may be positively affected (see Baird and Ulanowicz, 1989). Rapid population expansion of the nonnative species may also displace native oysters and other fouling species. Rapid population expansion, however, could enhance rates of denitrification and alter water clarity in the bay. Lastly, the nonnative oyster could become a major fouling species, thereby increasing the economic costs associated with maintenance of water input pipes, boat hulls, and so forth.

What are the chances of the nonnative oyster dispersing to areas outside the bay?

If reproductively viable populations of C. ariakensis are established in the bay, it is highly likely that individuals will eventually spread outside the bay. As mentioned previously, the species is capable of dispersing through a variety of unintentional and intentional mechanisms (e.g., larval transport by water currents, transport of larvae and adults by ship traffic, human movement of adults) that will act to amplify its spread to regions outside the bay.

Rates of dispersal of the species outside the bay are difficult to predict. If dispersal is primarily through transport of larvae in the water column, movements will be dependent on the prevailing water circulation patterns, the degree of water column stratification, and flushing time and larval behavior (e.g. Dekshenieks et al., 1996). Once outside the bay, oyster larvae would be transported by prevailing long-shore current systems. There is limited evidence that larval swimming behavior of C. ariakensis may differ from C. virginica (M. Luckenbach, Virginia Institute of Marine Science, Gloucester Point, personal communication, 2002).

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

While the ecological ramifications of this behavior are unclear, it may result in differences in larval dispersal ability compared to the Eastern oyster. Additional work to couple hydrodynamics with larval behavior and transport, both inside and outside the bay, is needed before any attempts at estimating potential rates of dispersal can be made. Studies of larval settlement patterns, postsettlement mortality rates, and growth in natural conditions are also needed.

Dispersal of C. ariakensis outside Chesapeake Bay by other vectors is highly likely and may occur over much shorter timescales than larval transport via water currents. For example, larvae could be entrained in ship ballast water and/or attach to the hulls of ships. As vessels move from port to port along the eastern seaboard, larvae may be released with ballast water exchange or from adults attached to the bottoms of the vessels. In addition, intentional movement of the species along the eastern seaboard by humans is likely, especially if the species proves to be an economically attractive one relative to the native oyster.

Risk to Social, Economic, and Cultural Systems

Human Health

Assuming that monitoring of water quality and shellfish sanitation practices are followed, there is no known reason to expect the human health risks of consuming triploid or diploid C. ariakensis harvested from the Chesapeake Bay to be any different from those of consuming C. virginica harvested from the bay.

Economic Effects
Price

Oyster harvests from the Chesapeake Bay have declined to less than 3% of the total U.S. live, fresh, and frozen supply. Therefore, a doubling or even tripling of Chesapeake Bay oyster harvests over several years is likely to have only minimal impact on U.S. oyster prices. This is likely to be the case with the introduction of hatchery-based triploid C. ariakensis or with cautious introduction of diploid C. ariakensis. Nevertheless, changes in local harvests may influence price in local markets. The market for oysters is dynamic. Prices and sales volume vary across species, production region, seasonal and intergenerational changes, consumers’ preferences, product-form innovations, and marketing efforts. It is unlikely that triploid aquaculture or even the introduction of diploid C. ariakensis will result in sufficient increases in production volume to contribute to an

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

observable impact on prices in the shortterm. Media attention associated with the introduction of C. ariakensis could lead to increased or decreased consumer demand, depending on the message. However, it must be cautioned that in the event introduction of C. ariakensis or recovery of C. virginica leads to explosive growth in harvest, the price effect is likely to be negative. It is unlikely that price volatility will change significantly in the short run under any of the proposed options.

Oyster Harvests from Public Bottoms

Assuming there is no directed release to public oyster beds in the short run, the segment of the oyster industry dependent on public bottoms is unlikely to experience significant direct benefits or costs in the near term. This is because restoration activities associated with the noaction option are not expected to result in significant near-term increases in the stock of native oysters, and nonnative triploid-based aquaculture is not expected to result in sufficient production to affect regional and national oyster prices. It is possible that introduction of diploid nonnative oysters could result in rapid colonization of the Chesapeake Bay and provide a basis for a public-bottom fishery, but even if nonnative oyster populations expanded rapidly, it would take several years for significant numbers of adult oysters to recruit to the fishery. It is possible that the traditional public-bottom fishery could be adversely affected to a significant degree if the introduction of nonnative oysters led to the accidental introduction of new diseases or parasites.

Oyster Harvests from Private Bottoms/Aquaculture

Assuming disease, parasites, or other uncontrollable effects are managed to eliminate their likelihood, the introduction of C. ariakensis is most likely to have a positive influence on harvests in this sector. Oystermen with private leased bottoms, the majority of which are located in Virginia, are most likely to benefit from any introduction of hatchery-based C. ariakensis.

Assuming that sanctioned introductions adhere to ICES protocols and that rogue introductions do not occur, the inception of triploid-based C. ariakensis aquaculture will probably have a positive influence on harvests by those watermen with leased bottoms who can adapt to somewhat more intensive aquaculture-based management of their sites.

Processing Sector

The processing sector earns net revenue primarily by adding value to live oysters through processing, distributing, and marketing. With only

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

minimal supply from the Chesapeake Bay, regional processors rely on oysters from outside the region. This clearly puts them at a competitive disadvantage relative to processors in regions where the supply is more abundant, such as in the Gulf of Mexico or Washington state. Any increase in the bay’s supply, whether native or nonnative, is likely to have a positive effect on this sector as long as the price of the Chesapeake oysters is competitive with oysters from other regions.

Recreational and Amenity Services

Interdiction of nonnative oyster culture, inception of open-water aquaculture of triploid oysters, or outplanting of nonnative diploids could be expected to generate differing arrays of risk to recreational and amenity services. However, the information available to quantify these risks is very limited. In Chapter 5, it was suggested that the key linkages between the three management options and the magnitude of use and option benefits are probably through their effects on water quality, substrate characteristics, and the composition of benthic vertebrates, invertebrates, vascular plants, and algae. In addition to being affected by the same factors that influence use and option value, non-use benefits are likely to be influenced by individual preferences regarding the benefits or costs of the introduction of an alien species.

Public Institutions

Substantial change in public policy is usually accompanied by some degree of institutional risk. Shifts in public policy related to intensive aquaculture of triploid C. ariakensis or to the managed introduction of diploid C. ariakensis would involve institutional changes associated with implementing new management strategies. Changes associated with both propositions may result in profound differences in management paradigms with concomitant institutional risks. Risks may be associated with encountering divergent public opinions, the need for new and more complex regulatory mechanisms, the implementation of new management policies, and changes in institutional infrastructure.

Current oyster resource management policy is a product of extensive negotiation among stakeholders and representatives of state and federal agencies and is based on specific common objectives, including restoring natural oyster populations, restoring ecological services associated with functioning oyster reefs, and sustaining a traditional commercial oyster fishery. This common management policy is the basis for a multifaceted and multilevel approach to managing natural oyster resources. The scope of the public policy is demonstrated by the numerous partners that share this common management approach. However, commitment to the com-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

mon management policy is largely dependent on the confidence that each party has in the commitment of other parties. In addition, commitment to the common management policy is dependent on real and perceived changes in the probable success of restoration efforts and the likelihood that nonnative oysters will be introduced.

Shifts away from this common management approach become more complicated because of the scope of existing management policy. For example, Maryland, Virginia, and the Chesapeake Bay Program have implemented long-range plans, such as the Oyster Recovery Partnership, the Oyster Heritage Program, and the Oyster 2000 Agreement. These partnership programs are the product of difficult political negotiations filled with compromise and are dependent on coordinated support from multiple funding sources. Changes in any element of these agreements may jeopardize support for current programs.

Institutional change, translating to institutional risk, can come as shifts in policy, politics, agency infrastructure, employment, strategic plans, and funding. Explicit institutional risks include reallocation of funding, loss of influence and/or power, fear of underfunded mandates, and added responsibilities and redirection or undermining of the institution’s long-term goals. The least complex challenge may involve changes in management strategies within the same agency in response to such functions as permitting, compliance monitoring, and law enforcement as an agency shifts from managing a wild fishery to managing aquaculture-based production. A more complex challenge (i.e., in response to the managed introduction of nonnative oysters) results when the activity shift involves multilevel involvement by numerous other stakeholders, typically state and federal agencies. Shifts in activity-based responsibilities among agencies bring about institutional challenges that relate to risks, competition for funding, political support, and public interest. Multitiered institutional structures with overlapping regulatory responsibilities and diverse objectives contribute to the potential that one action may produce various outcomes, each with its own perceived risk and reaction.

There is a risk that institutional structures, such as the Virginia Marine Resources Commission (VMRC) and the Maryland Department of Natural Resources, will react differently to the management alternatives associated with the introduction of nonnative oysters. For example, the VMRC may be better situated to optimize outcomes related to aquaculture on privately held leases. Hypothetically, this scenario may prompt Maryland or another state to take action that is considered to be controversial and as such an institutional and public-interest risk. Leasing submerged public lands to private entrepreneurs may be considered bad public policy, creating a dilemma for resource managers.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

There is also an inherent risk in the practice of relying on interested parties to provide oversight of the design and monitoring of protocols intended to minimize the unintentional release of sexually competent nonnative oysters and to prevent the cointroduction of disease or disease organisms. It seems appropriate to ask: “Sed quis custodiet ipsos custodes?” (Who is to guard the guards themselves?) when those charged with assessing and reducing the risk of introduction are simultaneously charged with devising a strategy for renewed economic opportunity in the fishery.

The institutional risk can be positively or negatively correlated with public-interest risk. For example, agency actions to support a specific management strategy may be strongly discouraged by public opinion when the management strategy is generally held to be inconsistent with prevailing environmental ethics. A potential loss of public support is an institutional risk, because shifts in public opinion influence political support and future funding.

One issue that has been discussed by numerous resource managers is related to the long-term commitment to restoring native oyster populations. Resource managers have acknowledged that there is an institutional risk involved in moving toward management options based on the purposeful introduction of nonnative oysters. The perceived risk is associated with the shift in funding to implement alternative management options and away from current programs to restore native stocks. Substantially more institutional risk is associated with the managed introduction of reproductively competent C. ariakensis, which may compete directly with current and future programs for funding. Stakeholders have reiterated the institutional risk associated with nonnative introductions, recognizing the potential consequence that support for future efforts to restore native oysters may be lost.

The degree of institutional risk cannot be ascertained without knowledge of how each institutional entity will react to the three management options. However, in an era of governmental budget constraints and mandates for increased efficiency, institutional risks are often magnified as agencies compete for funding. Additionally, the likelihood is that the institutional risk is high as agencies develop management structures and practices to deal with the introduction of nonnative oysters, because the overall challenge of managing the Chesapeake Bay’s natural resources is a high-profile activity.

Management Efficacy

The three alternatives place some common and some unique burdens on management. The status quo involves the costs of monitoring and enforcement of seasons, catch limits, closed areas, and shellfish testing. If

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

diploid nonnative oysters were intentionally introduced and became established with sufficient numbers to support a commercial fishery, it is likely that similar costs would be involved in monitoring and enforcement of seasons, catch limits, closed areas, and shellfish testing. In addition to the costs associated with monitoring and enforcement in an ongoing native oyster fishery, aquaculture of triploid nonnative oysters would involve monitoring and enforcement costs intended to reduce the risk that a diploid population of nonnative oysters could become established in the Chesapeake Bay. In addition, both in the case of intentional diploid introductions and the case of triploid-based aquaculture, monitoring and enforcement costs will increase to cover measures to reduce the risk of accidental cointroductions of exotic disease organisms and nuisance species. Measures that might be taken to reduce these risks include, for example, review and regulation of aquaculture operations plans; mandatory bonding of aquaculture facilities; random sampling of aquaculture oysters during the growing period to determine reversion rates, maturation, and the cause of unusual mortalities; genotyping of aquaculture brood stock; and sampling of adjacent grounds to detect the establishment of escaped nonnative oysters. Impatience with efforts to restore native oyster stocks coupled with the perception that C. ariakensis is a promising replacement increases the risk that unsanctioned introductions will occur under the no-introduction and triploid aquaculture options. The risks occasioned by unsanctioned—rogue—introductions are discussed below.

Community Structures and Social and Cultural Systems

In considering the social and cultural risks associated with the introduction of C. ariakensis into the Chesapeake Bay, it is important to consider both the community level and a broader baywide level. At the community level, the focus of a risk assessment is the possible impacts on watermen’s livelihood, beliefs, and values upon which the identity of watermen communities are based. At the baywide level, the focus shifts to consumers, bay advocates, and the general public.

Community-Level Risk Factors

Continued decline in the long-term productivity and harvests of oysters will almost certainly increase pressure on the social and economic fabric of watermen communities and watermen living outside these communities. As noted in Chapter 5, oyster harvests have declined significantly over the past few decades and, due to recent drought conditions, the oyster harvest for 2002 is projected to be one of the lowest on record.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

Because of differences in salinity levels and the associated prevalence of Dermo and MSX diseases, Virginia-based watermen have suffered greater economic losses than their Maryland counterparts in the past, but now both industries have been severely impacted. Public testimonies to the committee by leadership of watermen associations for both Maryland and Virginia corroborate harvest- and household-level information that the oyster fishery is becoming economically unsustainable.

Continued decline or low levels of productivity and harvest of the bay oysters could increase pressure on both blue crab and oyster fisheries. With continued low availability of native oysters, watermen may decide to continue commercial crabbing longer into the fall, during the period when the oyster and crab seasons overlap, and thus may increase pressure on blue crab stocks. Increased effort in fall crabbing may have a particularly strong adverse biological effect on blue crab reproduction because it is during this period that inseminated mature female crabs migrate south toward the mouth of the bay in order to spawn in warmer, high-salinity waters.

Continued low harvests of native oysters could also result in the opening up of more bottom for power dredging of oysters, as has recently occurred in Maryland. The risks and benefits of opening up more bottom for power dredging are uncertain and controversial. The risk is that dredging, which is one of the most efficient types of gear currently used by watermen to catch oysters, will lead to increased depletion of existing oyster beds and bars. Power dredging has been promoted by watermen who believe this activity cleans the beds of silt and turns over buried shell, with benefits for future spat sets and harvests, but this claim is controversial and has yet to be substantiated.

An additional risk of continued low levels of harvests is the decline in the number of young watermen willing to enter the profession. Without a viable and sustainable fall/winter oyster fishery, it may not be economically viable for watermen to make the investment in gear, repairs, and associated fees required to remain profitable in either the blue crab or oyster fishery. Decline of the oyster fishery is an important factor contributing to the continued decline in the number of commercial watermen and an increase in the average age of those who remain on the water. The prospect of augmentation of current and future oyster harvests is a critical factor in watermen’s support for the common management plan.

The introduction of nonnative oysters, on the other hand, brings a risk of differential economic returns to watermen, depending on their state of residence and on whether the introduction is based on triploids used in aquaculture or reproductively competent diploids for wild harvest. As noted in Chapter 5, existing legislative interest and support, industry configurations (leased bottom, emphasis on aquaculture of oys-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

ters), and ongoing scientific activities make it almost certain that Virginia would have an advantage over Maryland in aquaculture of triploid oysters. Compared to C. virginica, the biological advantages of triploid C. ariakensis (resistance to MSX and Dermo diseases and enhanced growth rate) raise concerns about the ability of Maryland watermen, who are reliant on harvesting native oysters from public bottoms, to compete in local and national markets. If Maryland watermen are not able to compete effectively, many of the same risks and benefits associated with interdiction of nonnative oysters may occur. Moreover, there may be decreasing support for current restoration efforts with the increasing economic success of triploid aquaculture in Virginia. While Maryland policy and support for oyster aquaculture may improve in the immediate future, partly in response to any successful triploid C. ariakensis aquaculture in Virginia, many commercial watermen may not be able to withstand short-term declines in harvests and income in order to participate in longer-term oyster aquaculture development.

Open-water aquaculture of triploid oysters could result in a more vertically integrated oyster industry like the bay region’s poultry-growing industry. Rather than extensive production by small-scale harvesters, the result may be fewer and larger growers and processors. Such a development would bring many changes to watermen’s livelihood and their communities, along with a change in public valuation of watermen’s livelihood and communities as part of Chesapeake Bay cultural and environmental heritage. However, without information about the costs of operation, the relationship between operational costs and scale, or about market opportunities available to niche producers or larger integrated operators, it is difficult at this time to evaluate the likelihood of triploid aquaculture resulting in increased vertical integration.

Inception of triploid nonnative oyster aquaculture may provide a much-needed economic boon for Virginia watermen, though it would be important to evaluate who and how many within the Virginia watermen community might benefit. A key question is the degree of access that traditional small-scale operators would have to the technology, capital, and markets required to grow triploid C. ariakensis in an open-water aquaculture setting. In Virginia many of the watermen may have difficulty switching to aquaculture of triploid C. ariakensis, owing to the higher production costs and related capital and informational needs of this form of aquaculture compared to those of the more traditional practice of rearing and harvesting native oysters on private leased bottoms. Although there could be employment in aquaculture for some watermen, who have primarily participated in the public-bottom fishery, it is unlikely that large numbers of public-bottom watermen would be so employed because this

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

would conflict with summer crabbing and possibly with the limited oystering still available on public-bottom or leased beds.

If biologically and ecologically successful, the introduction of diploid nonnative oysters has the potential of restoring the commercial harvest of not only Maryland but also Virginia watermen. The Maryland Watermen’s Association supports this option, arguing that the oyster fishery has declined to such a low level that something significant and different needs to be done. It is also relevant to consider how the introduction of a diploid nonnative oyster would affect the development of oyster aquaculture in Virginia. Key risk and benefit questions here include what would be the economic interactions between diploid nonnative aquaculture and public-bottom harvest of nonnative diploids. Which sector would be more profitable? Would continued small-scale public- or leased-bottom oystering compete with aquaculture of nonnative diploids? Would this result in an extensive and diverse industry that could support both aquaculture and public- or leased-bottom harvesting by watermen?

Baywide Social and Cultural Risk Factors

On a broader level, it is interesting to consider that interdiction of nonnative oysters could have a positive effect on the willingness of watermen, scientists, and oyster resource managers to form new partnerships to restore and profitably harvest from a smaller oyster fishery. The Oyster Recovery Partnership represents an initial framework and experiment along the lines of forming new relationships and exchanges among oyster scientists, watermen, and resource managers. While the challenges are great, these partners have begun the process of working together to manage reserves and sanctuaries with the goal of promoting the ecological and economic services of the oyster. There may also be employment opportunities for watermen to work alongside scientists and resource managers, undertaking such activities as moving spat, dredging bottoms, self-policing, etc. It might also provide a foundation for small-scale aquaculture by watermen who traditionally have worked public bottoms. However, as Chapter 6 suggests, current restoration efforts face serious challenges. While these challenges must be addressed, restoration of the native oyster fishery fits well within broader environmental ethics and values throughout the Chesapeake Bay area, values that emphasize restoration based on native species. The use of native species in restoration efforts can be an important environmental platform for innovative alliances among stakeholders that redefine traditional social roles and relationships toward resource management, including fisheries. (It may be true that the obverse is also the case: use of a nonnative species reduces

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

the role of environmental values as a motivator for restoration and resource management.)

In the bay region there is a risk that triploid oysters could be perceived or publicly cast as “not natural,” given the chromosomal changes required to induce sterility. Given the strong emphasis in the region on “native” and “pristine,” any significant increase in public perception of triploid ariakensis as unnatural could reinforce existing concerns about the C. ariakensis being nonnative. It should be noted that the public perception of C. ariakensis will be largely influenced by such factors as how the product is marketed and labeled and the reaction of consumer and environmental organizations. Consumer preference for native versus nonnative oysters could reduce the market value of the C. ariakensis half-shell market, reducing the profitability of nonnative aquaculture (Grabowski et al., 2003).

Finally, introduction of a diploid nonnative oyster would likely run the risk of a public cultural-environmental backlash driven by ethics and values or preserving the bay’s natural heritage. The important question would be who would benefit from such a backlash and how they might advance their concern in the public and policy arenas. It could be argued that the Chesapeake Bay is different than many coastal or estuarine areas in the strong cultural-environmental emphasis on restoration of native species and ecosystems.

Implementation Risk

Risk of Political Objection

Inception of triploid nonnative aquaculture or the sanctioned introduction of diploid nonnative oysters could be obstructed through objections raised in the regulatory approval process or through legal challenges brought by concerned parties. Several potential avenues for challenge were explored in Chapter 8. This section briefly explores two broadly constructed stakeholder classes—the fishing and environmental communities—that could challenge implementation of the options.

Fishing Community

As documented in Chapter 5 and above, the fishing community is heterogeneous across regions, modes of production, degree of concentration and integration, access to capital, level of financial stress, and ability to respond to the new opportunities represented by the options. Various segments of the fishing community could view themselves as advantaged or disadvantaged under one or more of the options. It is unlikely that

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

members of the fishing community would mount legal or regulatory challenges to the ongoing native restoration plan unless the plan were revised to close public bottoms or otherwise further restrict harvest activities. Elements of the fishing community might object to aquaculture based on triploid nonnative oysters, particularly if they were concerned that triploid nonnative aquaculture might impinge on traditional public-bottom fishing zones or if they were concerned that production from triploid nonnative aquaculture operations might reduce access to product markets for their harvests of the native oyster. The first objection is more likely to affect decision making in Maryland than in Virginia, due to the limited extent of leased-bottom submerged lands in Maryland. The second objection is most likely to be raised if it is perceived that buyers will not differentiate between native and nonnative oysters. Elements of the fishing community are most likely to object to the intentional introduction of the nonnative oyster if there are regional differences in the introduction. Introduction of the nonnative offers the possibility of continuation of the traditional mix of leased- and public-bottom fisheries. If introduction is permitted in Virginia but not in Maryland, the Maryland fishing community may object to introductions in Virginia through opposition to permitting or through legal action. Alternatively, the Maryland fishing community might respond to introductions in Virginia with political and legal actions to allow introductions to occur in Maryland waters as well.

Environmental Community

The environmental community is also heterogeneous with multiple, potentially incompatible objectives. A large segment of the environmental community is primarily concerned with water quality issues associated with recreational activities (e.g,. swimming, boating, waterfowl viewing, recreational fishing) but who may not be politically well organized. These individuals may be more concerned with water quality improvements than with the means used to obtain those improvements. They may be unlikely to object to a continuation of the status quo, with restoration efforts utilizing broodstock selection. They may similarly be unlikely to object to the introduction of diploid nonnative oysters if such introduction is represented as accelerating water quality improvements. They may object to aquaculture of triploid nonnative oysters because they do not anticipate aquaculture being sufficiently extensive to result in large-scale water quality improvements and because they may perceive aquaculture as a competitive claimant on funds presently allocated to restoration efforts.

Other segments of the environmental community, in particular nongovernmental organizations with political clout, may be more concerned

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

with the naturalness of the ecosystem and may object to actions that could result in changes in species composition and abundance. This segment of the environmental community could object to any of the options involving C. ariakensis and could mount regulatory, legal, or political challenges to continuation or implementation of the options. Objections to the introduction of diploid nonnative oysters could arise because of concerns about the ecological effects of C. ariakensis on native species, concerns that the introduction might serve as a vector for introducing exotic disease, and concerns that localized introductions could expand and displace otherwise healthy populations of the native oyster throughout the eastern seaboard and into the Gulf of Mexico. Objections to aquaculture of triploid nonnative oysters could be based on the risk that diploids will inevitably be introduced, owing to the imperfect fidelity, stability, and sterility of mated triploids (see Chapter 4). This organized environmental community could even object to the status quo option. Continued decline of native oyster stocks could stimulate interest in prohibiting commercial and recreational harvests. Objections could be mounted as well to certain restoration activities, such as those employing selectively bred, disease-resistant stocks to artificially supplement natural populations, based on the perceived risks to natural diversity.

Risk of Rogue Introductions

A rogue introduction would be a nonsanctioned direct release of diploid reproductive Suminoe oysters into the Chesapeake Bay, likely executed without benefit of adherence to the ICES protocols. The chief hazards of a rogue introduction are that the nonnative oyster would become established and pervasive; the incidental introduction of other nontarget plant, animal, or microbial species that could become invasive; or the incidental introduction of new pathogens and parasites that could attack native oysters or other bivalves both inside and outside the bay. Often the impact of associated species is as great as or greater than that of the species targeted for introduction. For example, Carlton (1999b) reports finding seven species of algae and invertebrates in a single container of hatchery-reared Pacific oyster seed imported for research purposes to the Woods Hole Oceanographic Institution and cites another study that found an additional 29 species of algae, diatoms, protozoans, and invertebrates in the water of other oyster shipments. Of the 30 nonindigenous molluscs found on the U.S. Pacific Coast, 10 gastropods and 10 bivalves were introduced along with the Eastern and Pacific oysters imported for commercial culture (Carlton, 1992b). Seaweeds and seagrasses, which were used as packing material for imported oysters, have transformed thousands of square kilometers of open mudflat habitat in Pacific Coast bays into stands

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

of intertidal vegetation that harbor completely different invertebrate faunas than existed previously (Posey, 1988; Carlton, 1989).

Another hazard might be the introduction of a nontarget oyster species, caused by lack of care or ability to discriminate among morphologically similar species, subspecies, or physiologically distinct races. The taxonomy of Asian cupped oysters is poorly known and in a state of flux, depending on sporadic studies undertaken for various reasons (e.g., Buroker et al., 1979a, b; Banks et al., 1994; Ó Foighil et al., 1995, 1998; Boudry et al., 1998; Hedgecock et al., 1999; Day et al., 2000). In particular, two distinct geographic races of C. ariakensis have been uncovered through analysis of mitochondrial and nuclear DNA sequences since researchers at VIMS turned their attention to this species as a candidate for nonnative introduction (Francis et al., 2001). A nontarget oyster might not perform as well as the particular strains of C. ariakensis that have been tested to date in Chesapeake Bay field trials.

The likelihood of rogue behavior cannot be quantified but is judged to be substantial and depends on which management strategy is chosen (see below). Rogue introductions of the Pacific oyster have occurred previously in the Chesapeake Bay and elsewhere on the East Coast. This led Maryland to adopt legislation against the introduction of this species (see Table 3.2; Andrews, 1980). Hopes for the recovery of the oyster industry have been fueled by reports about the impressive survival and growth of C. ariakensis in experimental trials. The economic motive for carrying out an illegal rogue introduction is present and is likely to build over time if native oyster populations remain depressed. Although human behavior is unpredictable, shipment of live oysters from Asia to the United States would not present an obstacle to a rogue introduction. All life stages of cupped oysters (D-hinge or later larvae, spat, and adults) are readily shipped live via air courier. Adult Asian oysters could be readily obtained and imported live to the region, probably through normal channels of seafood supply. Hundreds of adults can be shipped live in a box no larger than a typical picnic cooler; if kept moist and refrigerated, adults can survive out of water for more than a week.

Obtaining larval and seed stages of C. ariakensis would require locating a cooperating hatchery in Asia, but these early life stages are attractive targets for rogue introduction because of the much larger numbers of oysters that could be imported. Commercial oyster hatcheries routinely supply farmers with late pediveliger or eyed larvae in very large numbers for remote setting; when screened from their culture water, for example, 2.5 million eyed larvae constitute a golf-ball-sized mass that is easily shipped in a small package. Eyed larvae remain competent for setting for about a week if kept moist and refrigerated. Remote setting requires little technology and minimum infrastructure.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

The likelihood of a rogue introduction resulting in the establishment and spread of a population of nonnative oysters depends on the life stage introduced, the number and density of animals introduced, the spatial scale of introduction, the spawning and recruitment potential at the sites of introduction, and the frequency of introduction. The number of adults that could be shipped and introduced at any one time would likely be limited to a few hundred individuals. A small inoculum of adults could successfully found a population, in principle, owing to the high fecundity of the oyster. Nevertheless, the chances of establishment and spread would be governed by the likelihood that environmental conditions conducive to spawning, larval development, retention in a local area, and recruitment in sufficient density for successful spawning in the next generation were met. The chances of successful spawning and recruitment are classically difficult to predict for most marine animals, including the native oyster. It is noted that successful introduction of the Pacific oyster into France was made possible by massive importation of adults and spat (Chapter 3).

Much larger numbers of seed oysters could be introduced via a shipment of eyed larvae. The percentage of eyed larvae that can be successfully set is variable but probably in the range of 10 to 30%; of these, perhaps 10 to 20% might survive to a suitable size of about 8 mm. This means that from 2.5 million eyed larvae 100,000 seed could be reared and planted, of which thousands or tens of thousands might survive to reproductive maturity. With that size of inoculum, the chances of successful recruitment, establishment, and spread would be greatly increased though not guaranteed.

MANAGEMENT OPTIONS

The biological and social factors likely to be impacted by each of the three management options for introducing the Asian oyster, C. ariakensis into the Chesapeake Bay are listed in Table 9.2. The body of the table contains a qualitative assessment of potential outcomes for each factor under the three management options. Lack of information precludes definitive characterization of every hazard, particularly the ecological ones. Moreover, the very different ecological, economic, and social hazards cannot be weighted with respect to one another or summed to derive an overall relative risk for each management option. Table 9.2 primarily characterizes the various factors likely to be affected by the choice of management options. In the table the likelihood of a particular outcome (listed as positive, negative, or neutral) represents the committee’s assessment for each management option under a short time frame (1 to 5 years). However, there are many uncertainties and potential scenarios that could af-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

TABLE 9.2 Assessment of Potential Outcomes Under Each Management Option

Biological and Social Factors

No Introductiona

Triploid Introductionb

Diploid Introductionc

Ecological

 

 

 

Disease introduction

 

Disease reservoir

 

 

 

Susceptibility to endemic pathogens or parasites

 

Impacts on ecosystem

 

Competition with C. virginica—space, food, habitat

 

Competition with other species (relative to C. virginica) Invasion

 

Dispersal beyond the bay

 

Genetic interactions

 

Water quality

+

+

Reef structures and services

+

+

Economic/social/cultural

 

 

 

Human health/pathogen Price

 

 

Viability of traditional fishery

+

Fishery employment

+

Viability of aquaculture

+

+

+

Aquaculture employment

+

+

+

Tourism, recreational, sports fishery

 

+

+

Public institutions

 

 

 

Management effectiveness

Employment

 

+

+

Watermen communities

+

Watermen culture

+

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

Biological and Social Factors

No Introductiona

Triploid Introductionb

Diploid Introductionc

Cultural perception of restoration, environment

+

Political impact

 

 

 

Fishery

+

+

Environmental

+

Restoration efforts

+

+

Likelihood of rogue introduction

+

+

 

Impact of rogue introduction

Biological and social factors are likely to be affected by selection of one of the three management options with regard to introducing the Suminoe oyster into the Chesapeake Bay. The assessments given here are developed for short-term (1 to 5 years) outcomes and are listed as +, positive; −, negative; and blank, no effect. The rationale for each of these values is explained in detail in the text. There are large uncertainties associated with each outcome; therefore, these values serve as an illustration, but not a prediction, of how the various management options might compare.

aEcological and economic and social outcomes assume no rogue introduction.

bOutcomes assume eventual production of diploids and the establishment of small reproducing populations.

cOutcomes assume large managed introduction of diploids using ICES protocols.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

fect outcomes. The qualitative characterization of risk presented in the table is meant to provide a starting point for reviewing the hazards associated with each of the three management options the committee was charged to consider.

Option 1. Status Quo, No Introduction of Nonnative Oysters

The first management option is simply to maintain the status quo by forbidding the introduction of all nonnative oysters into the Chesapeake Bay, whether diploid or triploid. The chief consequences likely to be associated with this action, were it successful in maintaining the status quo, would be:

  • continued decline of the oyster fishery and erosion of the traditional economies and cultures of Chesapeake Bay watermen;

  • possible increased pressure in the blue crab fishery;

  • possible further declines in bay water quality, owing to loss of oyster filtering capacity, though scientific evidence that water quality is tightly coupled to oyster abundance is weak;

  • possible continuing or accelerating losses of aquatic vegetation and oyster reef habitats, with cascading effects on the structure and stability of the bay’s estuarine communities, though scientific evidence for these assumptions is lacking;

  • possible reduction of bay acreage protected under the Clean Water Act’s shellfish bed water quality preservation mandates; and

  • erosion of confidence in governmental management of the living marine resources of the Chesapeake Bay.

The economic or ecological harm from these hazards can be reasonably extrapolated from recent trends in the fishing sector and in bay water quality and ecology.

The chief benefits of maintaining the status quo would be:

  • avoidance of risks identified with either of the alternative options for introducing a nonnative oyster;

  • increased emphasis on aquaculture of native oysters selectively bred for resistance to MSX and Dermo diseases;

  • increased employment in the native oyster aquaculture sector, especially with new strains of disease-tolerant C virginica; and

  • affirmation of cultural value on conserving native species and natural habitats.

Simply banning the introduction of nonnative oysters into the Chesapeake Bay, however, will not necessarily maintain the status quo. A no-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

introduction policy would increase the likelihood of a rogue introduction, that is, a nonsanctioned direct release of diploid reproductive Asian oysters, executed surreptitiously and without benefit of adherence to ICES protocols. The economic desperation created by the collapse of the traditional oyster fishery of the Chesapeake Bay, coupled with widespread awareness of the performance of triploid C. ariakensis in previous field trials and the ease with which live animals could be acquired through traditional fish markets, makes rogue introduction an easy response to the perception of management inaction. Industry representatives, who addressed the committee, made this hazard explicit. The risks associated with rogue introductions include the risks identified under sanctioned introductions that employ ICES protocols, as well as those incurred by circumventing ICES protocols, and would remain for as long as the population of a native oyster remained depressed. If a self-reproducing population of C. ariakensis were established as the result of a rogue introduction, the resulting harms and benefits would probably increase through time, with an increase in the abundance of the nonnative oyster. Unfortunately, the specific ecological, economic, or cultural harms or benefits of a rogue introduction cannot be specified nor can their magnitudes be predicted. Finally, under this option, management would presumably be burdened with monitoring for rogue introductions and with eradication of diploid nonnative oysters were they detected. Eradication of introduced marine species is extremely difficult or impossible, as recent experiences with the invasive seaweed Caulerpa in the Mediterranean Sea at-test (Thibaut et al., 2001).

Any attempt to maintain the status quo should certainly be coupled with scrutiny of why the restoration of native oysters has failed so far. Such an examination was not part of the charge of this study. Clearly, however, successful restoration of native oysters and the traditional fishery would largely have precluded the present controversy over introduction of a nonnative oyster.

Option 2. Open-Water Aquaculture of Triploid Oysters

Because the fidelity, stability, and sterility of mated triploids are not likely to be 100%, expanding the introduction of mated triploid C. ariakensis in controlled aquaculture settings risks establishing a diploid self-reproducing population in the Chesapeake Bay. This hazard, however, can be broken down into components, most of which can be quantified and modeled, as attempted by Dew et al. (2003), for example, who simulated the population growth and local establishment of nonnative oysters introduced by triploid aquaculture under specified ecological conditions and management strategies. Furthermore, many of the hazards

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

associated with open-water aquaculture of triploid nonnative oysters can be managed to reduce specific elements of risk. For example, increasing the containment of, and accountability for, planted stock could lessen the risk that triploids would remain in the bay long enough to revert to the diploid state. Likewise, the density of planted stocks could be managed to reduce the risk that gametes released by the small percentage of diploids that might be produced along with mated triploid seed would be able to find and fertilize each other. The number of triploids introduced could be constrained to reduce risk, but this would also reduce potential economic or ecological benefits.

Minimizing the duration and scale of the triploid culture effort would minimize the risks of this management option. Indeed, introduction of triploids could be used as a management strategy to buy time for restoration of native oysters, which could result either artificially (from the development of new and more successful approaches to restoration) or naturally (from a return to the more typical conditions of colder winters and wetter summers, which would inhibit parasite proliferation and provide the native oyster with more freshwater refuges from disease). Recovery of native oyster populations would reduce the incentive to introduce a nonnative species or would reduce the scale of any aquaculture sector based on the nonnative relative to the scale of a resuscitated traditional fishery.

Aside from the hazard of establishing a self-reproducing population of a nonnative oyster, some short- and long-term negative impacts of this management option are:

  • continued declines in the traditional oyster fishery or possibly accelerated declines as hope for recovery is lost and extraction is maximized;

  • economic hardships for watermen communities, unless they switch from fishing to aquaculture;

  • no marked improvement in bay water quality in the near term, owing to only a marginal increase in oyster filtration capacity from triploid aquaculture;

  • continued threat of rogue introductions of diploid nonnative oysters;

  • potential introduction of pathogens that may not be excluded by adherence to ICES protocols;

  • potential introduction of other pathogens owing to inadvertent breaches of ICES protocols;

  • susceptibility of nonnative oysters to endemic pathogens or parasites;

  • conflicts with cultural value placed on conservation of native species and habitats;

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×
  • erosion of confidence in resource management; and

  • political resistance or legal challenges by environmentalists or states from outside the Chesapeake Bay.

As under the first option, management could face a considerable burden for monitoring bay waters for the establishment of diploid populations or for subsequent eradication of any diploids detected. Genetic markers could be profiled in all tetraploid and diploid stocks used to make triploids, so that the provenance of any diploids that might subsequently be detected or become established could be determined.

Some short- and long-term benefits of this option, aside from those attending the establishment of a diploid population, are:

  • management control over most aspects of the authorized introduction;

  • viability of aquaculture;

  • aquaculture employment;

  • possible retention of tourism, recreational, and sports fishery benefits associated with Chesapeake Bay oysters, even though nonnative; and

  • increased incentive for restoring the native oyster, if it serves to rally the political constituents of restoration.

One important benefit to the controlled introduction of triploid C. ariakensis could be opportunities for research on the biology of C. ariakensis in the Chesapeake Bay. The likelihood of ecological harm or benefit could be more accurately assessed if basic information were available on the season of reproduction (triploids, though sterile, still go through an annual reproductive cycle), susceptibility to native pathogens and parasites, competition with C. virginica for space, and propensity to sustain old or restored reefs or to build new ones. The risks of expanded industrial trials could be partially offset by the inclusion of parallel ecological experiments designed to generate information critical to evaluating the risk that triploid aquaculture will eventually produce a diploid population.

Option 3. Introduction of Reproductive Diploid Oysters

This management option has strong local support because introduction of an oyster that can survive and grow in the Chesapeake Bay appears, to many, as the only hope of improving water quality and the bay’s ecosystem, recovering the traditional fishing industry and sustaining watermen culture. Behind the hope is the assumption that purpose-

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

ful introduction will quickly yield a large viable population of C. ariakensis, with little or no adverse effects on the remnant native oyster population or other species. However, introductions are not always successful. Initial trials with triploid Pacific oysters in the Chesapeake Bay showed, for example, that this nonnative oyster, though resistant to the diseases that kill native oysters, was susceptible to infestation with the shell-boring polychaete worm Polydora, which made them unacceptable in the market. Still, some introductions of oysters and other bivalves have been successful in establishing industries without untoward ecological harm. The introductions of the Pacific oyster to the west coasts of North America and Europe had positive impacts on fishing and farming industries. The Pacific oyster proved noninvasive on the West Coast of North America; hence, there were no pronounced ecological changes, with the important exception of problems stemming from cointroductions (e.g., Spartina alterniflora to the U.S. West Coast; Naylor et al., 2001). The risks of cointroductions, today, would be greatly reduced by the use of ICES protocols. Finally, opponents of diploid introduction can cite counterexamples of negative ecological impacts from introductions of oysters or other bivalves. The Pacific oyster, C. gigas, in New Zealand and Australia threatens endemic oyster species; the zebra mussel, Dreissena polymorpha, has caused widespread fouling problems in the Great Lakes and other regions in North America; and the Asian clam, Potamocorbula, has greatly modified the soft benthic fauna and primary productivity of the San Francisco Bay and delta. What mix of outcomes—no impact, positive impact, or negative impact—would follow a clean introduction of C. ariakensis into the Chesapeake Bay cannot be predicted.

Short- and long-term negative impacts of introducing diploid C. ariakensis into the Chesapeake Bay could include:

  • disease introduction, though greatly reduced, would still present an unknown hazard from vertically transmitted pathogens even if ICES protocols are followed and perfectly effective;

  • negative ecological impacts, such as competition with C. virginica or fouling of boats, marinas, and other marine structures;

  • spread of nonnative oysters outside the bay, where competitive displacement of healthy native oyster populations might be possible;

  • susceptibility to endemic pathogens or parasites;

  • decreased management effectiveness;

  • abandonment of attempts to restore the native oyster;

  • conflicts with conservation ethic; and

  • political resistance or legal challenges by environmentalists or states from outside the bay.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×

The chief benefits of a diploid introduction would ostensibly be the same as those deriving from recovery of the native oyster population, though hard scientific evidence supporting these presumed effects is limited or lacking:

  • possible improvements in water quality;

  • increases in aquatic vegetation;

  • deposition of new reefs and increases in reef habitat for fish and other invertebrates;

  • resuscitation of the traditional oyster fishery and fishery employment;

  • continued viability of aquaculture and increased aquaculture employment;

  • potential increases in tourism, sports fishery, and a recreational economy;

  • maintenance of watermen communities and culture; and

  • reduced likelihood of a rogue introduction.

All of these benefits assume that an introduction of diploid C. ariakensis would result in a large population of reef-building oysters, an outcome that is uncertain.

FINDINGS

  • The three management options (no introduction of nonnative oysters, introduction of triploids for aquaculture, and introduction of diploids) entail differing arrays of ecological, socioeconomic, institutional, and implementation risks.

  • The risk of a disease outbreak in either the native or nonnative oyster populations following an introduction is not zero, even if ICES protocols are followed. If ICES protocols are applied, the risk of disease outbreak has low probability but potentially high impact if it occurs.

  • Assessing an array of ecological risks is severely constrained by lack of fundamental ecological information on the Suminoe oyster, C. ariakensis, and even by lack of sufficiently detailed ecological information for the native oyster and the Chesapeake Bay. Various ecological risks that can be postulated have unknown probabilities and unknown impacts.

  • No human health risks are apparent. The risk to human health has a very low probability and a low impact.

  • Assessment of the risks to institutions with responsibilities for managing the living resources of the Chesapeake Bay have unknown probabilities and unknown impacts.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×
  • The risks of rogue introductions are likely high under the no-introduction management option; may remain high to moderate under the triploid aquaculture option, particularly among the “have not” stakeholders; and are likely low to moderate under the diploid introduction model. The potential impact of a rogue introduction is high, owing to the substantial ecological impacts that have been documented following the unintended cointroduction of other organisms besides the oyster.

  • The breadth and quality of existing information on oysters and other introduced species are not sufficient to support a comprehensive risk assessment of the three management options.

  • Comprehensive risk assessment is also not practicable, owing to the lack of well-defined and/or conflicting objectives and goals among Chesapeake Bay management agencies and users.

Suggested Citation:"9. Elements of Risk Assessment for the Introduction of Crassostrea Ariakensis in the Chesapeake Bay." National Research Council. 2004. Nonnative Oysters in the Chesapeake Bay. Washington, DC: The National Academies Press. doi: 10.17226/10796.
×
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×
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Nonnative Oysters in the Chesapeake Bay discusses the proposed plan to offset the dramatic decline in the bay’s native oysters by introducing disease-resistant reproductive Suminoe oysters from Asia. It suggests this move should be delayed until more is known about the environmental risks, even though carefully regulated cultivation of sterile Asian oysters in contained areas could help the local industry and researchers. It is also noted that even though these oysters eat the excess algae caused by pollution, it could take decades before there are enough of them to improve water quality.

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