WOLCOTT, T.G.*; WOLCOTT, D.L.; HINES, A.H.; NC State Univ., Raleigh; NC State Univ., Raleigh; Smithsonian Env. Res. Ctr., Edgewater MD: Exploring migration of female blue crabs to spawning grounds, using biotelemetry and CTD dataloggers.

After mating, female blue crabs (Callinectes sapidus) must migrate seaward (up to 200 km in Chesapeake Bay) to begin brood production. We are exploring several hypotheses about the timing and routes of migration. If selection has acted to maximize “r” and mimimize energetic costs, we would expect females (which mate throughout the summer) to minimize generation time by migrating as soon as they had received sperm; to use ebb tidal currents for transport and seaward orientation; and to travel in the marginal shallows where food is most available. Trawling revealed that females are not in the water column during summer ebbs; tracking and mark-recapture showed that they remain near the mating areas until Sept/Oct. Biotelemetry (depth and foraging) provided a detailed picture of behavior in a few individuals that were followed continuously for several days. They foraged at all times of day/night; moved up to several km/day, generally toward deeper water; and did not swim up off the bottom. To obtain data from many more animals than could be individually followed, we developed 19g (2g in water) modules that log conductivity/depth/temperature and can upload a year’s data upon recovery. In Sept/Oct 2002, 130 loggers were affixed to females and released in upper Chesapeake Bay; 23 were returned by the fishery. The uploaded data show that vertical migration and ebb-tide transport occurred only rarely. Females usually remained on the bottom, suggesting that they walk most of the distance and rely on bottom currents for orientation. Most travel through the deepest parts of the Bay, not the shallow marginal waters, and move seaward at variable speed along the sides and bottom of the main channel. (Support: NOAA/CBSAC)