Seasonal movement of the American horseshoe crab \u3cem\u3eLimulus polyphemus\u3c/em\u3e in a semi-enclosed bay on Cape Cod, Massachusetts (USA) as determined by acoustic telemetry

American horseshoe crabs Limulus polyphemus were tracked using acoustic telemetry and traditional tagging in a semi-enclosed bay on Cape Cod (Pleasant Bay), Massachusetts, USA, to determine seasonal movement patterns. Fifty-five actively spawning females were fitted with transmitters in 2008 and 2009 and were tracked using acoustic telemetry from May 2008 through July 2010. Fifteen crabs with transmitters also had archive depth-temperature tags attached. In addition, over 2000 spawning crabs (males and females) were tagged with US Fish and Wildlife (USFWS) button tags over the same period. Ninety-one percent of the crabs with transmitters were detected during this study. In the spring, crabs were primarily located in the northern section of the bay near spawning beaches, whereas in the fall crabs moved towards the deeper portions of the bay, and some may have overwintered in the bay. There was evidence that a majority (58%−71%) of the females with transmitters spawned in two sequential seasons. One archive tag was recovered resulting in a year-long continuous record of depth and temperature data that, when integrated with telemetry data, indicated that the crab overwintered in the bay. The live recapture rate of crabs with USFWS button tags was 11%, with all re-sighted crabs except one observed inside Pleasant Bay. Eighty-three percent of recaptures were found within 2.5km of the tagging location, and 51% were observed at the same beach where they were tagged. This study provides further evidence that horseshoe crabs in Pleasant Bay may be philopatric to this embayment