Fisheries Bulletin 101:758-768

A MODEL FOR ASSESSING THE LIKELIHOOD OF SELF-SUSTAINING POPULATIONS RESULTING FROM COMMERCIAL PRODUCTION OF TRIPLOID SUMINOE OYSTERS (CRASSOSTREA ARIAKENSIS) IN CHESAPEAKE BAY

Project Objective:

• To construct a demographic model of both Suminoe and Eastern oyster populations
• To determine the sensitivity of population growth of triploid Suminoe oysters to the reversion rate of triploid to mosaic individuals (having both diploid and triploid cells) under various management scenarios in the Chesapeake Bay through model simulations

One hundred years ago, the Eastern oyster (Crassostrea virginica) population filtered the volume of water in the Chesapeake Bay in four to five days. Now in year 2000, it takes the current oyster population approximately four hundred days to filter the same volume of water. In 1993, the eastern oyster population was at the lowest level ever recorded in the Chesapeake Bay due to years of overharvest, habitat destruction, and diseases. The water quality has been decreased in part because of the overabundant, unconsumed phytoplankton that are consequent to eutrophication. The introduction of a new oyster species, such as the Suminoe oyster (Crassostrea ariakensis) could improve environmental conditions and supplement the current eastern oyster fishery.

A collaborative group at the Virginia Institute of Marine Science (VIMS) and Virginia Tech is currently evaluating the Suminoe oyster under the "Rational Plan for Testing Application of Non-Indigenious Oyster Species". The goal of this plan is to provide a scientific basis for reaching public policy decisions concerning the introduction of non-native species. This is accomplished by screening the Suminoe oyster to determine its suitability to the local environment under quarantine conditions and by assessing the environmental risks associated with the introduction.

The introduction of the Suminoe oyster can impose many environmental risks. The Suminoe oyster is to be introduced as a reproductively sterile triploid; therefore, a lower risk is involved in the introduction. However, there is a newly documented phenomenon of the "reversion" of triploid Suminoe oysters back to diploidy. This creates a reproductive potential for the Suminoe oysters. Other possible risks identified with the introduction of the triploid Suminoe oyster are competition with the native eastern oyster, introduction of parasites and pathogens, and the possibility of the establishment of a self-sustaining population.

My project pertains to the quantitative estimation of risk factors and the development of a model of risks based on the large-scale deployment of triploid Suminoe oysters. The risk assessment model is a demographic and genetic model of Suminoe oyster population growth rate using parameters such as reversion rates, reproduction, recruitment, mortality rates (both natural and harvest), and a range of management options. The model is used to predict the risks of introducing the Suminoe oyster as both triploid and diploid into the Chesapeake Bay. Policy makers will be able to see potential outcomes of policy decisions, and have the ability to make science-based policy decisions about the introduction of the Suminoe oyster into the bay.

Project Abstracts:

Suminoe Oyster Model Abstract:

Culture of a non-native species, such as the Suminoe oyster (Crassostrea ariakensis), could supplement harvest of the declining native eastern oyster (Crassostrea virginica) fishery in Chesapeake Bay. Because of possible ecological impacts from introducing a fertile non-native species, introduction of sterile triploid oysters has been proposed. However, recent data show that a small percentage of triploid individuals progressively revert toward diploidy, introducing the possibility that Suminoe oysters might establish self-sustaining populations. To assess the risk of Suminoe oyster populations becoming established in Chesapeake Bay, a demographic population model was developed. Parameters modeled include salinity, stocking density, reversion rate, reproductive potential, natural and harvest mortality, growth rates, and effects of various management strategies, including harvest criteria. The probability of a Suminoe oyster population becoming self-sustaining decreased when oysters are grown at low salinity sites, certainty of harvest is high, minimum shell length-at-harvest is small, and stocking density is low. Based on model results, I suggest using these management strategies for decreasing the probability of a Suminoe oyster population becoming self-sustaining. Policy makers and fishery managers can use the model to predict potential outcomes of policy decisions, supporting the ability to make science-based policy decisions about the proposed introduction of triploid Suminoe oysters into the Chesapeake Bay.

Jodi Dew
Email: jdew@vt.edu