Background
The management of American horseshoe crab (Limulus polyphemus) populations along the Atlantic coast faces significant challenges due to threats such as bait harvest, biomedical bleeding, and commercial fishing bycatch. Habitat loss and degradation have further jeopardized recovery efforts, leading the IUCN to classify the American horseshoe crab as “Vulnerable” to extinction. Historically, horseshoe crabs were harvested for fertilizer and later for bait, with dramatic increases in the 1990s causing significant population declines, reduced egg densities, and cascading impacts on the migratory shorebird and fish species that rely on horseshoe crab eggs to survive. Delaware Bay, hosting the world’s largest spawning population of horseshoe crabs, is a critical habitat for these crabs and an essential stopover for migratory shorebirds traveling between South America and the Arctic.
To monitor and support recovery efforts, New Jersey biologists have collected data on horseshoe crab populations in Delaware Bay since 1983. Methods include tracking spawning crab numbers, egg densities, and population trends. These metrics help assess habitat conditions and inform management strategies for both horseshoe crabs and shorebirds. Delaware Bay serves as a model for similar efforts along the Atlantic Coast, where effective site-specific management is needed.
Each year, teams sample Delaware Bay's beaches to determine yearly horseshoe egg abundances as an indicator of spawning activity and horseshoe crab habitat quality. Surface egg densities are one of the main indices of shorebird foraging conditions and have been correlated with survival and breeding success in red knots. However, collecting and counting surface eggs is labor intensive, expensive and subject to inherent variability related to site factors and egg consumption by breeding and migrant birds. Also, surface egg counts are not likely to be informative in most smaller populations, because at lower densities few buried eggs reach the surface. In 2016, Delaware Bay scientists developed a less intensive method using densities of egg clusters buried in the sand. This method reduced variability and has correlated significantly with surface eggs.
