Population dynamics of the horseshoe crab, Limulus polyphemus, on Cape Cod: Field data and model simulations of the effects of harvest

The population status of the Atlantic horseshoe crab, Limulus polyphemus, along the East coast of the United States is unclear but it has been suggested that they may be declining, in part because they are harvested as a source of Limulus Amoebocyte Lysate (LAL), a blood compound used to detect endotoxins, for bait for the eel and conch fisheries, and in smaller numbers, for scientific research on vision. To assess the status of horseshoe crabs on Cape Cod I applied a stage-structured matrix population model and collected data for running model simulations for three different estuaries subject to different harvest rates. Field surveys of three Cape Cod estuaries with different harvest pressures (Stage Harbor > Barnstable Harbor > Pleasant Bay) yielded data on growth, survival, and mortality for horseshoe crabs in each of the estuaries. Despite differences in abundance and size distribution among estuaries, similar size cohorts could be identified in populations from all three estuaries, and the mean size of individuals in each cohort increased at similar rates. The field data were used in the model to estimate stable stage distribution, reproductive value, and population growth rate (r). The Pleasant Bay horseshoe crab population, where harvest rates are lowest, had positive population growth (r = 0.06 to 0.08). The Barnstable Harbor population, harvested at an intermediate rate, was poised at near stable growth rates (r = -0.05 to 0.01), and the Stage Harbor population, subject to the highest harvest rates, was declining (r = -0.06 to -0.10). Simulations of different harvest scenarios showed that a low to moderate level of harvest is sustainable, particularly if harvests avoid the smallest adults and occur later in the spawning season. The model simulations also suggested that harvesting crabs prior to sexual maturity has the greatest impact on r; older juveniles are more sensitive than other stages to changes in survival. Modifying harvest strategies in the horseshoe crab fishery could prevent declines in population growth in estuaries on Cape Cod and elsewhere.