ed by human modification of habitats), deliberate introductions to "improve nature" or for aquaculture or horticulture, and a wide variety of accidental introductions. CBC seems to have a better safety record than other types of introduction. It is not clear whether this is because the activity is basically benign, because the safety precautions work well, or because CBC involves small organisms that pose smaller risks than larger organisms. The worst failures in all categories have occurred in insular environments such as islands and lakes.
The assessment of risks posed by introductions has been addressed separately by scientists in different disciplines (e.g., agriculture, freshwater and marine ecology, and nature conservation). Communication between the disciplines is poor, and several sets of criteria, procedures, and protocols have been developed independently. Whereas the U.S. Department of Agriculture has adopted flow charts as a way to systematize decision-making, other agencies (e.g., the International Council for the Exploration of the Sea) have concluded that too little is known about ecosystem functioning for flow charts to be useful.
Dr. Policansky commented that risk assessment for species introductions is difficult to fit into the four-step Red Book paradigm. Hazard is taken for granted (because it is the introduction of the species itself); dose-response and exposure are yes-no categories, not continuous variables, because the more important point is whether the species is present or not, not how much of the species is present. A more suitable paradigm might be that presented in the 1986 NRC report Ecological Knowledge and Environmental Problem-Solving: Concepts and Case Studies, which placed more emphasis on problem-scoping and problem-solving than on categorical activities.
CASE STUDY 1:Uncertainty and Risk in an Exploited Ecosystem: A Case Study of Georges Bank
M. J. Fogarty, A. A. Rosenberg, and M. P. Sissenwine, National Marine Fisheries Service
This paper addressed the risks of overexploitation of harvested marine
ecosystems, with specific application to Georges Bank, a highly productive area off the northeastern United States. In this context, risk assessment involves determining the probability that a population will be depleted to an arbitrarily predetermined "small" (e.g., 1% or 5%) size. The "quasi-extinction" level may be defined (Ginzburg et al., 1982) as (1) the population level below which the probability of poor recruitment increases appreciably or (2) the smallest population capable of supporting a viable fishery.
The primary determinant of the long-term dynamics of any population is the relationship between the adult population (stock) and recruitment. The null hypothesis is that the relationship is linear, i.e., that recruitment is independent of density (Sissenwine and Shepherd, 1987). Compensatory changes in survival or in reproductive output result in nonlinear stock-recruitment curves. Nonlinearity permits stable equilibrium under harvesting pressure (i.e., under increased mortality rates), up to a critical exploitation level, beyond which the population will decline to quasi-extinction. Stochastic variation in the stock-recruitment relationship or in multispecies interactions can increase risks of adverse effects at moderate exploitation levels. In practice, because of uncertainties resulting from stochastic variations and measurement errors, it is often impossible to reject the null hypothesis of no compensation. Assuming there is no compensation will, in general, result in a conservative assessment of production capacity and its ability to withstand exploitation.
Haddock populations on Georges Bank fluctuated about relatively stable levels between 1930 and 1960 when the fraction of the total haddock population killed per year by fisherman (annual fishing mortality rate) varied between 0.3-0.6, but collapsed after the fishing mortality rate increased to 0.8 during the 1960s (Grosslein et al., 1980). The empirical relationship between stock and recruitment was extremely variable with little indication of the form of the underlying curve. Analysis of the population dynamics showed that a density-independent null model could not be rejected and gave a neutral equivalent harvest rate of 0.5, which agrees well with the stable period of the fishery. In contrast, the compensatory model is over optimistic with respect to the long-term harvest rate.
The decrease in populations of haddock and other groundfish was accompanied by increases in other species, notably elasmobranchs (rays and sharks). The biomass of predatory species increased dramatically