BOTTON, M. L.*; LOVELAND, R. E.: The Diminishing Abundance of Horseshoe Crabs in Delaware Bay: Potential Impacts on Migrant Shorebirds

Migratory shorebirds on staging areas typically concentrate their feeding on unusually abundant prey species, enabling them to rapidly increase mass prior to the next leg of their migration. Delaware Bay is the most important staging area on the Atlantic coast of North America for shorebirds during their Spring (northward) migration; here, birds primarily feed upon horseshoe crab eggs on sandy intertidal beaches. The commercial fishery for horseshoe crabs, which collects the animals for bait in eel and whelk traps, expanded substantially from about 1990 to 1996. Based on surveys of six Delaware Bay, NJ beaches, horseshoe crab egg abundance has decreased in conjunction with the expansion of this fishery. During the period 1985-1991, relatively few surface sediment samples (0 to 5 cm depth) contained zero horseshoe crab eggs, and at least 30% of these samples contained >10,000 eggs per square meter. In contrast, in the period 1996-1999, the frequency distribution is strongly shifted toward the lower egg density intervals. The likelihood of a shorebird encountering surface sediments with zero eggs has increased from about 10% in 1985 to over 50% in 1999. These trends in egg density are also corroborated by significant decreases in the abundance of stranded horseshoe crabs over the period 1978-1999. Shorebirds in Delaware Bay may be adversely impacted by further decreases in the horseshoe crab resource, by extending foraging time and/or intensifying intra- and interspecific competition for eggs.