Assessing the movements of American horseshoe crabs (Limulus polyphemus) around a marine protected area in Cape Cod, MA, USA

Marine protected areas (MPAs) can be useful for conservation efforts, yet the effectiveness of MPAs relies on their size and location, as well as the spatial ecology of the focal species. We investigated the effectiveness of a small MPA adjacent to Cape Cod, MA (USA) for protecting American horseshoe crabs (Limulus polyphemus). We used a fixed acoustic telemetry array to study the spatial ecology of 24 adult horseshoe crabs off the coast of Chatham, Massachusetts, from June 2010 to November 2011. Two groups of horseshoe crabs were tagged in spawning habitats (separated by ∼2.0 km) with differing commercial harvesting pressure: one group inside Stage Harbor where harvesting is permitted, and the other within Monomoy National Wildlife Refuge and the Cape Cod National Seashore (the MPA) where harvesting is prohibited. In the first year of study, crabs tagged in the MPA did not go to Stage Harbor, but instead remained in the MPA and adjacent high-energy beach, whereas crabs tagged in Stage Harbor moved throughout the entire study area. In the second year of study, crabs from both tagging locations used all areas. All crabs generally moved greater distances in the MPA than in other areas, and the majority of movement we recorded was in late spring and summer. All crabs that we tagged used similar areas around Cape Cod. The MPA only provides protection when crabs are in that area, which may only be for a few months of the year. Given that all crabs used Stage Harbor, they were all exposed to potential harvest, but the MPA did provide some protection for all crabs.