Horseshoe Crab Survey


Delaware Bay supports the largest spawning population of horseshoe crabs (Limulus polyphemus) in the world. This species is closely managed because it is used by humans in several ways and it is also of great ecological importance:

  • Horseshoe crabs are collected so that their copper-based blue blood can be extracted for biomedical applications. In particular, proteins in their blood are used to concoct the Limulus amoebocyte lysate (LAL) test, which is capable of detecting bacterial endotoxins in medical resources like injectable medications, tools, vaccines or implantable medical devices.

  • Horseshoe crabs are also harvested by commercial fishermen to be use as bait in conch and American eel fisheries.

  • Horseshoe crab eggs released during spawning provide an essential food source for many migratory shorebirds including the federally threatened red knot. Each spring, this imperiled species travels from South America to spend several weeks at Delaware Bay beaches feasting on horseshoe crab eggs prior to pressing on to their arctic breeding grounds.

The Atlantic States Marine Fisheries Commission’s Interstate Fishery Management Plan for horseshoe crab requires that several states, including Delaware, implement an annual horseshoe crab spawning survey. The Delaware Division of Fish and Wildlife oversees spawning surveys conducted in Delaware and New Jersey. Surveys at individual beaches across the state and region are implemented by private citizens, state and federal agencies, corporations and NGOs. The surveys provide an index of spawning activity and distribution in the region, illuminates the relationship between environmental factors and spawning activity, and enhances public awareness of this species’ important ecological role.

As of 2019, Delaware Sea Grant coordinates and implements the horseshoe crab spawning survey at Big Stone Beach, which is owned by our partner, Delaware Wild Lands, Incorporated. The surveys take place on 12 distinct nights to coincide with the full and new moons in May and June, when peak spawning takes place. For more information about the horseshoe crab spawning survey at Big Stone Beach, contact Coastal Ecology Specialist, Kate Fleming at