Linking temperature and salinity tolerance to winter mortality of Chesapeake Bay blue crabs (Callinectes sapidus)

Blue crabs (Callinectes sapidus) form one of the most important and largest commercial fisheries in Chesapeake Bay. Blue crabs have evolutionary origins in the tropics, although they currently inhabit temperature estuaries that exhibit major fluctuations in diurnal, monthly, and seasonal environmental conditions. Therefore, harsh winter conditions in Chesapeake Bay are a potentially important source of blue crab stock loss. However, this variable has been largely unexamined. To assess the effects of variation in winter environmental conditions on blue crab survival, we measured winter morality of crabs in the field and conducted laboratory experiments to test the interactive effects of low temperature, salinity, and blue crab life stage. Field studies indicated that blue crabs suffered relatively low winter morality rates (≤3%) during five out of eight winters, when bottom water temperature in February was at or above the 8-year average (3.4 °C). However, in years when bottom water temperature fell below the February average, annual mortality rates rose to 6.0-14.5%. Mortality rates were highest in the coldest regions of Chesapeake Bay, and larger crabs and female crabs were most vulnerable to these stressful conditions. Similarly, in the laboratory, mortality was highest in the lowest temperature (1 °C) and salinity (8 ppt) treatments. Mature females were more sensitive to winter conditions than juvenile crabs. Of the juvenile life stages, recruits (<15 mm carapace width) were least tolerant to winter conditions. These results indicate that temperature, salinity, and blue crab life stage are important variables in predicting survivorship over winter months. Because winter mortality may be a significant source of stock loss for blue crabs, especially during severe winters and in low salinity areas of Chesapeake Bay, these predictions can be used to improve management estimates of stock size prior to each summer fishing season.