Thirty Years of Horseshoe Crab Surveys Reveal Increase in Species

The Takeaway: Each year since 1990, volunteers from around the Delaware Bay estuary join forces to count and document these important, joint-legged arthropods.

The Delaware Bay is the most critical breeding ground for the iconic horseshoe crab, a keystone species in the estuary’s ecosystem. This crab species also is a valuable resource to the medical community. The Delaware National Estuarine Research Reserve plays an important role in annual surveys that document numbers of spawning horseshoe crabs. Many volunteers from the community, and from federal and state governments and nonprofit organizations, join forces for this event.

Each May and June, hundreds of thousands of horseshoe crabs convene on the shore of the Delaware Bay to spawn on Delaware and New Jersey beaches. This major migratory bird stop along the Atlantic Flyway hosts as many as one million birds, which feast on horseshoe crab eggs to replenish their energy reserves before continuing their journey to Arctic breeding grounds. Their continued abundance is not just important to birds but also to the medical community, which uses the species’ blood to test pharmaceuticals for harmful bacteria. Horseshoe crabs are gathered, sent to the lab for blood collection, and returned to the ocean unharmed.

Volunteers undergo training before gathering at peak spawning tides—the evening high tide during full and new moons and two days before and after—to collect data at 24 locations around Delaware Bay.

The 2019 seasonal spawning activity for the Delaware Bay was 3,397,246 (1,291,799 New Jersey, 2,105,447 Delaware), a record since the beginning of the data series in 1990. In total, this effort has recorded over 42 million spawning horseshoe crab sightings.

This highly coordinated effort, now led by the Atlantic States Marine Fisheries Commission, provides detailed procedures for how volunteers collect and record information. The Delaware National Estuarine Research Reserve is an organizational point of contact for several survey locations along the Delaware shore. (2020)

More Information: Horseshoe Crab Survey

Partners: Adventure Aquarium, Cape Henlopen State Park, Delaware Division of Fish and Wildlife, Delaware National Estuarine Research Reserve, Delaware Nature Society, Ecological Research and Development Group, Jenkinson’s Aquarium, Lucky Dog Custom Apparel, The Nature Conservancy, New Jersey Division of Fish and Wildlife, Sea Grant, U.S. Fish and Wildlife Service, The Wetland Institute