A Comparison of Diploid and Triploid Eastern Oysters for Aquaculture Production Under Extreme Temperatures and Salinities

Off-bottom aquaculture of Eastern oysters (Crassostrea virginica) is a nascent industry that is increasingly supported by the use of triploid oysters, which grow faster than diploids. Despite their growth advantage, elevated triploid mortality compared to diploids under high temperature, low salinity, or a combination of these conditions challenge consistent triploid production. Identifying the environmental thresholds at which differential triploid mortality occurs and predicting its economic impact are important to informing decision-making in oyster aquaculture. The goal of this thesis was to compare the biological and economic performance of diploid and triploid oysters under high temperature and low salinity conditions. To accomplish this goal, I first conducted a laboratory study to assess the differential biological performance between ploidies. Forty diploid and forty triploid oysters were placed in each of 12 replicate tanks, which were adjusted to three different temperatures (24, 29, 34oC), held at a salinity of 18, and mortality checked daily. Salinity was then reduced to 2 in half the tanks at each temperature. Mortality was tracked daily, and samples were taken to assess physiological responses. Under low salinity (2), we found that triploids died faster than diploids at every temperature treatment, reflecting their reduced ability to osmoregulate. In contrast, we found that under medium salinity (18), diploids died faster than triploids at 34°C but at around the same rate at 24 and 29°C. To project the impact on farm profitability, we assessed the economic implications of differential ploidy performance under various salinity and temperature combinations using a bioeconomic model. The model combined outputs from the dynamic energy budget model with a modified enterprise budget to estimate profit for aquaculture operations under different salinity and temperature scenarios. Both salinity and temperature affected profits, and outcomes in all environmental scenarios varied by ploidy. These results can help inform seed and site selection, although future investigation using our bioeconomic approach can be applied to inform other best management practices including optimal husbandry techniques and stocking densities. Combined, these studies help reduce risk to triploid growers by assessing the potential biological and economic differences in ploidy performance when exposed to extreme environmental conditions.