Winter Mortality of Chesapeake Bay Blue Crabs

Michelle S Rome, Alicia C Young-Williams, Anson H Hines, Mike R Goodison, Robert Aguilar

A relatively new effort in fisheries management is to establish a minimum stock size threshold which is used to determine a maximum removal rate. Thus, stock size must be accurately estimated to adopt a target removal rate to maintain a sustainable fishery. Blue crabs (Callinectes sapidus), an important commercial fishery, are a fundamentally tropical species and at the northern range of their distribution in Chesapeake Bay. Winter induced mortality is a potentially important, yet unexamined, source of stock loss due to the major fluctuations of winter conditions. To assess the relative importance of winter environmental conditions on blue crab mortality, we conducted laboratory experiments to test the interactive effects of low temperature (1oC, 3oC, 5oC), low salinity (8, 12, 16 ppt), and life stage (recruits <15 mm carapace width, small juveniles 20-65 mm, medium juveniles 80-115 mm, and mature females). Mortality was highest within the lowest temperature and salinity treatments. Mature females were more sensitive to low temperatures and salinities than juvenile crabs, whereby 50%
mortality occurred by the fifth day, and none survived till the end of the experiment (60 d). Recruits (<15mm carapace width) were the most vulnerable size class of juvenile crabs, whereby 50% mortality occurred by the fifteenth day. In a field study throughout the Maryland portion of Chesapeake Bay, blue crabs were placed in field enclosures during a severe winter, mid-December 2002 to late March 2003 (96 d). Mortality was 100%, 97.6% and 95% for mature females, medium juveniles and small juveniles, respectively. These results suggest temperature, salinity and crab size class are important variables in predicting survivorship over winter months.
Furthermore, 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.