Eastern oyster production in the Gulf of Mexico is increasingly relying on aquaculture. However, Gulf farms have experienced losses in recent years during summer mortality events. The causes of summer mortalities are not fully understood yet but are hypothesized to be related to excessively high temperature during the late summer months, and also to low salinity events affecting oyster production sites during heavy rain events. In this study, the feasibility of improving tolerance to high temperature and low salinity stresses by selection was evaluated using challenge experiments applied to a mixture of 160 families (10 non-overlapping 4 males x 4 females mini factorial crosses) raised in a common garden. Thermal tolerance was evaluated at the seed (2 months post set) and adult stages (14 months post set) to determine if breeding values for tolerance to high temperature stress in market-size oysters can be predicted during the first year of life. Challenges simulated a heat wave where temperature was maintained between 38-39 °C until all oysters expired. Similarly, tolerance to an acute low salinity stress was tested in a challenge where salinity was reduced from 14.5 psu to 2.5 psu over a period of 4 days. This challenge was applied to juveniles (9 months) and adults (15 months). Survival duration during both challenges was recorded for all individual oysters and tissue samples were taken for genotyping and parentage assignment. The condition index was also recorded for juvenile oysters to characterize the level of stress under challenge.
Oyster seed mortalities peaked after 11 days at 38 °C and complete mortality was recorded after 22 days. Adult oyster mortalities under the same challenge conditions peaked after 5 days at 38 °C and all tested oysters expired after 7 days of challenge. Juvenile oysters tested in the low salinity challenge experienced minimal mortality (2.3% over a period of 40 days). Their level of tolerance will be characterized based on the condition index. In contrast, a peak in mortalities was observed after 14 days at low salinity in the adult challenge followed by complete mortality at 25 days. During both challenges conducted on adults, oysters spawned during the acclimation period, suggesting that their reduced ability to deal with the stressors was related in part to spawning activity.
Challenged oysters will be assigned to parents using a 192 SNP array. Heritability of tolerance to the two challenge stressors and genetic correlations between the two traits will be estimated using general linear model and proportional hazards models