Quantitative description of temporal and spatial patterns in spawning activity of the horseshoe crab, Limulus polyphemus

The horseshoe crab supports a fishery of growing value along the mid-Atlantic region. The commercial value of horseshoe crabs is primarily derived from their use as bait and their importance in medical research and diagnostics. Because of increasing harvest over the past ten years, there is concern for the horseshoe crab population. Data from Delaware Bay imply that the number of spawning crabs has decreased in recent years. Because the design of sampling surveys is crucial for detecting changes in the abundance of spawners, there is a need to develop a rigorous sampling protocol that will be reflective of the status of the horseshoe crab population. In 1996 and 1997, I quantified the temporal and spatial patterns of horseshoe crab abundance on spawning beaches to assist in establishing a statistically valid index of spawning stock.; There appeared to be an increasing trend in the average density of spawners on the days preceding full and new moons, and a plateau in spawning activity on the days immediately following full and new moons. Horseshoe crab concentrations were slightly larger two days after full and new moons in 1996 and one day after in 1997. Horseshoe crabs were greater in concentrations on the higher of the two daily high tides and peaked in number one hour after high tide. Daylight did not appear to have an influence on spawning activity. The greatest density of spawners was found at the shallowest water depth (0--33cm).