Horseshoe Crabs on seaweed
horseshoe crabs on seaweed on Block Island—a great habitat since 43% of the Island is protected. © Karine Aigner

Stories in Delaware

Horseshoe Crabs in the Delaware Bay

The annual spawning of horseshoe crabs is an ancient spectacle—and an opportunity to understand and protect the population.

Each spring, the Delaware Bay becomes the site of one of the most spectacular and ancient phenomena in the natural world: the spawning of horseshoe crabs and the flocking of ravenous shorebirds that depend on them. 

On moonlit nights in May and June, thousands of the crabs crawl ashore on Delaware Bay beaches at high tide to lay eggs, as they have for 450 million years. The crabs are sometimes so dense that the water’s edge looks like a road paved with brown shells.


wash up along the shore of Block Island in Rhode Island.
Horseshoe Crabs Young horseshoe crabs wash up along the shore of Block Island in Rhode Island. © Karine Aigner

Crabs are Critical for Birds and People 

Horseshoe crab eggs are critical in sustaining birds that migrate up to 10,000 miles each year. The threatened red knot, for example, flies from wintering grounds in South America to the Arctic each spring, stopping in Delaware along the way to fuel up on horseshoe crab eggs. Without the food, the red knot cannot complete its journey. 

While birds like the red knot and ruddy turnstone rely on horseshoe crab eggs for a critical food source, humans also utilize adult horseshoe crabs for two primary purposes. Watermen harvest horseshoe crabs to use as bait for eel and whelks. The crab is also harvested for medical use. Its unique blue blood is used to make a chemical that detects bacterial toxins.

A handful of horseshoe crab eggs, which the red knots and other birds feed on.©Erika Nortemann/The Nature Conservancy(CC BY 2.0)
Horseshoe crab eggs A handful of horseshoe crab eggs, which the red knots and other birds feed on. © Erika Nortemann/TNC

Why Count the Crabs?

Some evidence suggests that the crabs are over-harvested in North America, which has led to a harvesting ban in New Jersey and restrictions in Delaware. Knowing how many horseshoe crabs there are helps fisheries managers develop effective policy for managing populations. 

Every May and June, during the evening full and new moon high tides, volunteers count horseshoe crabs at 23 beaches along the shores of the Delaware Bay in both Delaware and New Jersey. This data is used to create the annual Delaware Bay Horseshoe Crab Survey which is utilized by the Atlantic States Marine Fisheries, state agencies and scientists. In New Jersey, The Nature Conservancy participates in the annual census and reports its findings as part of the collective effort. In Delaware, TNC's Milford Neck Preserve provides nearly a mile of undeveloped beachfront that hosts thousands of spawning crabs and hungry shorebirds.

Horseshoe Crabs Spawning horseshoe crabs gather on the beach to lay eggs at the Milford Neck Preserve. © The Nature Conservancy/Katherine Marro

Pitch In

TNC continues to support the horseshoe crab count. However, due to COVID-19, the 2020 Horseshoe Crab surveys have been cancelled. If the situation allows, some government agency and partner staff members may collect data on a few beaches, including at Big Stone Beach, on a limited number of nights.