Documenting habitat use and residency of adult Atlantic horseshoe crab (Limulus polyphemus) in Delaware Bay through passive acoustic telemetry

The Atlantic horseshoe crab (Limulus polyphemus) is a critically important species in the mid-Atlantic region providing numerous ecological and economic services being used as bait for commercial fisheries, use of its blood in biomedical tests, and its eggs are the primary food source for several species of migratory shorebirds. Due to increased pressure from multiple user groups, concerns have been raised regarding the status of horseshoe crab populations. Through this study, we monitored the sub-tidal movement patterns of adult horseshoe crabs (n=19) in Delaware Bay via passive acoustic telemetry. Adult horseshoe crabs were outfitted with external acoustic transmitters (VEMCO Ltd. V-164H, 69.0, > 2 year battery life), and released during September 2008. An array of 42 passive acoustic receivers (VEMCO Ltd. VR2) located throughout Delaware Bay and surrounding nearshore coastal waters was utilized in this project. Through August 4, 2009, we have relocated 16/19 (84%) of our telemetered horseshoe crabs a total of 8561 detections on 18 receivers. Preliminary analyses indicate seasonal migrations with telemetered horseshoe crabs exiting Delaware Bay in the fall for coastal waters and then returning again the following spring. Mean temperature at time of departure for telemetered horseshoe crabs was 21.0°C, while the mean temperatures at the time of return were much cooler (14.6°C). Importantly, we feel the method of attachment utilized in this study provides a mechanism for the long term attachment of transmitters to terminal molt horseshoe crabs given the high detection rates in addition to sightings of tagged horseshoe crabs during their spring spawning events. These data provide support for the feasibility of future larger scale studies focused on developing a more robust understanding of horseshoe crab sub-tidal habitat needs. Continued research via biotelemetry can provide heretofore lacking critical information on sub-tidal behavior of horseshoe crabs.