Aquaculture of Triploid

Crassostrea ariakensis
in Chesapeake Bay

A Symposium Report

Summaries of presentations on C. ariakensis

Biology of C. ariakensis in its Native Range, Roger Mann, Virginia Institue of Marine Science (VIMS)

Roger Mann discussed our limited knowledge of C. ariakensis biology in its native range and outlined limitations in the ability of scientists to predict outcomes were diploid (fertile) C. ariakensis introduced into the Chesapeake Bay. One reason that the Crassostrea species is so dominant is its ability to withstand large temperature and salinity ranges. He pointed out the need for biosecurity to guard against possible management mistakes. He described the dimensions of possible benefits and possible risks posed by introduction of triploid C. ariakensis, both of which were large in magnitude.

Disease Certification, Eugene Burreson, VIMS

The importance of MSX and Dermo in the decline of the Eastern oyster underlines the importance of disease certification for imported oysters. There is no information on diseases and pathogens in the native range of C. ariakensis; therefore, only offspring that are bred from non-native broodstock should be used to produce triploids. These protocols have been adhered to in VIMS research to date: C. ariakensis deployed in Virginia were the offspring of imported individuals, which eliminated many possible bacterial or protozoan disease problems. While hatchery spawning eliminates most diseases and pathogens, this is not necessarily the case for viruses. Viruses cannot be diagnosed without molluscan cell culture or electron microscopy. Overall, the risk of viral disease seems low, though it is not absent.

Field Trial Studies of C. ariakensis and C. virginica , Mark Luckenbach, VIMS

Trials of C. ariakensis and C. virginica were conducted at six sites of high, medium, or low salinities (from 36 parts per thousand (ppt) down to 8 ppt). VIMS scientists found that C. ariakensis grew faster than C. virginica at all sites; mortality of C. ariakensis was low at all sites, while mortality of C. virginica was high; prevalence and intensity of Dermo were low in C. ariakensis and very high in C. virginica (Calvo et al., 2001).

In preliminary experiments that compared the growth of post-set juveniles in quarantined conditions, VIMS scientists found that C. virginica was somewhat more competitive than C. ariakensis, gauged by increase in shell height and weight. Under these conditions, C. virginica grew faster and had higher survival; C. ariakensis had poorer survival and grew more slowly when C. virginica were present. It is important to note that these findings were based only on juveniles and did not account for the effect of oyster diseases. Thus, says Mark Luckenbach, extrapolation to field conditions is difficult.

Industry Trials of C. ariakensis , James Wesson, Virginia Marine Resources Commission

Field trials by members of the Virginia Seafood Council were conducted at six sites at three salinity ranges. Trials begun in August 2000 showed low mortality, rapid growth, and excellent yield of shucked meat of C. ariakensis. Some C. ariakensis deployed in trials begun in June 2001 had reached 5 to 6 inches in shell length by October. The oysters grew well in a variety of confinements, including corrals, floating cages, and sunken trays. Consumer acceptance was very high. Hence, there is great interest in the commercial sector for producing C. ariakensis.

Triploid Strategy and Risks, Standish Allen, VIMS

Standish Allen discussed the general approaches to developing a triploid C. ariakensis aquaculture industry and possible risks posed by commercial production in Virginia waters of the Chesapeake Bay. For strain development (domestication), identification of the best-performing C. ariakensis stocks from native populations has begun. Stocks from western Japan, northern China, and southern China are being reared at VIMS. For developing sterile stocks, triploids are produced. Production of triploids through chemical induction is imperfect and infeasible for commercial production. Development of tetraploids for crossing with diploids is more efficient and reliable, and hence can provide the numbers of triploid seed needed for an industry. The availability of tetraploids is now the limiting factor for stepped-up commerical production. Risk factors associated with the triploid aquaculture of C. ariakensis include:

Reliability of triploidy induction (i.e., the possible occurrence of diploids among triploids even using tetraploid crosses).
Sterility of triploids (the possible production of euploid gametes among the many aneuploid gametes with unmatched chromosomes).
Stability of triploids (triploidy can be unstable in a certain proportion of the population and over time, this instability yields individuals that have dipliod and triploid cells simultaneously, called mosaics).
Fertility of mosaic individuals (mosaics may recover reproductive capability given time, i.e., years, although this phenomenon has not been clearly demonstrated in studies so far).

Risk Model, Jim Berkson and Jodi Dew, Virginia Polytechnic Institute and State University

Interactions of scientific uncertainties and management options must be considered to reach an informed decision on whether and how to go forward with commercial production of triploid C. ariakensis. In this context, it is useful to develop and use a simulation model as a tool for approaching a range of "What if?" questions. Jim Berkson and Jodi Dew presented a risk assessment model for considering issues posed by culture of triploid C. ariakensis. The model was developed in Visual Basic because of its user-friendly interface and is available from Virginia Sea Grant (Dew et al. 2001). Simulation of population growth under specified ecological conditions and management strategies was used to evaluate the likelihood of a population becoming self-sustaining at a given site. Mortality, growth, and reproductively effective reversion rates were modeled as stochastic processes. Because knowledge is lacking for C. ariakensis, an existing model for fertility in C. virginica was used. The model predicted that the likelihood of a self-sustaining population becoming established increases as salinity increases, as minimum harvest size increases, as certainty of harvest decreases, and as population density in the culture system increases.

Strategies for Minimizing the Risk of C. ariakensis Aquaculture, Standish Allen, VIMS

In the context of the predictions of the risk model, Standish Allen presented a strategy for commercialization of triploid C. ariakensis production while minimizing potential risks posed to the Chesapeake Bay ecosystem. A seed-stock production enterprise would need a more elaborate structure than that in the shellfish production industry currently. A "technology" group would be involved in broodstock development, especially in development of a stock of tetraploid males and domesticated lines of diploids, used together for triploid production. Aquaculture operations would involve the following:

Oversight and training, diligence with confining larvae, and certification and tracking of batches of seed-stock.
Nursery stage would entail separate rearing and tracking of individual batches.
Grow-out methods would have to be custom designed for site characteristics, notably salinity.
Harvest would have to be complete, with accountability for complete harvest, and harvest would have to occur before spawning.

Dr. Allen also suggested co-stocking with C. virginica to serve as gamete traps, or as "Trojan oysters."

Attitudes about Non-Native Aquaculture, Ratana Chuenpagdee, VIMS

Because the decision of whether and how to go forward with commercial production of a non-native species is value-laden, stakeholder attitudes were considered by Ratana Chuenpagdee, who reported the results of her survey of six key constituencies. Her paired comparison-based survey showed that watermen and members of the seafood industry most highly valued possible impacts of C. ariakensis on human health and on other marine species. Oyster experts, scientists, environmental non-governmental organizations, and the general public most highly valued impacts on other marine species, human health, and wild oyster populations. No group was highly concerned with the costs of government oversight.