Estuarine habitats frequently contain large percentages of nonnative species because these areas are heavily used for shipping, fishing, and recreation. Introductions of nonnatives occur mainly through human activities. Nonnative shellfish have been transported from region to region for hundreds of years, and oysters have been intentionally transported more than any other marine species.
The incentive for introduction of commercial marine species has been to replace depleted natural stocks or to diversify the types of species used in aquaculture. Although molluscan aquaculture typically has relatively small environmental impacts relative to culture of fish and shrimp, diseases and “hitchhikers” have been transported along with the shellfish. A number of these unintentional introductions have caused severe impacts on shellfish production and ecosystem structure and function.
While there are no documented cases that an intentional introduction of a marine bivalve species in U.S. waters has caused pronounced environmental change, there are a number of well-documented cases of how unintentional introductions have resulted in both positive and negative impacts.
It is exceedingly difficult to predict whether a marine species has the potential to become an “invasive” or a “nuisance” species. Species that exhibit fast growth and high reproductive rates and are tolerant of a wide range of environmental conditions generally tend to be viewed as candidate “invasive” species.
Ecosystems with reduced biodiversity or that are environmentally degraded may be more vulnerable to invasion. In addition, the southern and northern boundaries of faunal provinces may be more susceptible to invasion by nonnative species as a consequence of climate change.
A review of a number of case studies of intentional shellfish introduction indicates that there are both benefits and risks dependent on the region. In France, introduction of the Pacific oyster, C. gigas, led to revitalization of the oyster industry, but in New Zealand and Australia, the Pacific oyster outcompeted the commercially viable native oyster in some areas.
The oyster industry on the U.S. West Coast relies almost exclusively on nonnative species. The success of this industry is largely based on culture of several types of molluscs and regulations that allow private ownership of tidelands.