Factors Affecting Horseshoe Crab Limulus polyphemus Trawl Survey Design

Abstract

There is currently a lack of abundance information for effectively managing horseshoe crabs Limulus polyphemus. A trawl survey program that specifically targets horseshoe crabs would provide that information. We conducted a study to examine the factors that would influence the trawl survey design. Depth, topography, and time of day were examined as potential survey design influences. Horseshoe crab catches were typically greater inshore (within 5.6 km of shore), in troughs (≥2.4 m deep, ≤1.8 km wide, and >1.8 km long), and at night, but specific results varied by maturity categories. In addition, some interactions existed among factors. Trawl survey protocols incorporating those factors were developed separately for all horseshoe crabs combined and for mature females only because these categories were considered most important for management purposes. Stratified random sampling designs were more efficient for surveys than simple random sampling designs, and optimum effort allocation was more efficient than allocations proportional to stratum area. Nighttime sampling was more precise than daytime sampling. Sample size requirements and effort allocation depended on time of day and survey target group.