Fate of Migrating Red Knots Tied to Horseshoe Crabs

Submitted by Deborah Cramer

Fate of Migrating Red Knots Tied to Horseshoe Crabs
Red knots and spawning horseshoe crabs, Mispillion Harbor, Delaware Bay
© Barrie Britton

Red knots, (Calididris canutus rufa) sandpipers weighing little more than iphones, may be in danger of extinction. Their fate depends on how many migrating birds and spawning horseshoe crabs gather each May on the beaches of Delaware Bay.

The birds arrive from Tierra del Fuego, where, in the southern hemisphere’s summer, they winter, feeding on small clams from beaches where the highest tides flow more than four miles across mud and sand. They stop in Delaware Bay en route to their breeding grounds in the Canadian arctic, a journey of 9300 miles.

Nesting red knot on Southampton Island, Canada
Nesting red knot on Southampton Island, Canada
© Mark Peck, Royal Ontario Museum

The arctic nesting season is short and harsh; there is little to eat on the barren wind-swept tundra. Whether the birds can survive their grueling journey, and arrive in the arctic strong and healthy enough to breed depends upon how well they can refuel along the way.

Horseshoe crab eggs – soft, easily and rapidly digestible, high in lipid – are essential. Red knots, cued to a mysterious call scientists have yet to understand, arrive in Delaware Bay at the last full or new moon in May, when America’s largest population of horseshoe crabs begins to spawn.

Horseshoe crabs spawning in Delaware Bay
Horseshoe crabs spawning in Mispillion Harbor, Delaware Bay
© Barrie Britton

In the 1980s, there may have been as many as 20 million horseshoe crabs living in Delaware Bay, and 100,000 to 150,000 red knots wintering in Tierra del Fuego. When migrating birds arrived in Delaware Bay, the beaches were packed with eggs. In one of the world’s most intense feeding frenzies, the birds easily doubled their weight during the 10-14 day layover.

Since then, populations have plummeted. Millions of crabs have been killed as bait for whelk and eel fisheries, and bled for the essential pharmaceutical Limulus ameboycte lysate. These crabs are returned to the water; some die. Between 1990 and 2005, the number of horseshoe crabs plummeted by 88%. Egg densities have thinned by 98%, down from 226,000 per square meter to 3400 per square meter, leaving the birds without enough food. The number of red knots passing though Delaware Bay has decreased by 70%.

Horseshoe crab eggs in surf
Horseshoe crab eggs in surf, Delaware Bay
© Jay Fleming

Regulators reduced the crab take, but by enough and soon enough? Horseshoe crabs mature in 11 – 17 years. The population appears to be stabilizing, but the number of breeding female crabs hasn’t yet increased, and neither has the egg density on Delaware Bay beaches. For now, the future of the red knot hangs in abeyance.

References and more information

Shuster, Carl N. Jr., Robert B. Barlow, and H. Jane Brockmann. 2003. The American Horseshoe Crab. Cambridge: Harvard University Press.

Mizrahi, David S. and K. A. Peters. 2009. Relationships between sandpipers and horseshoe crab in Delaware Bay: a synthesis. In The Biology and Conservation of Horseshoe Crabs, John.T. Tanacredi, Mark L. Botton and David R. Smith, eds. 65-87. New York: Springer.

Niles, Lawrence J., et al. 2009 Effects of horseshoe crab harvest in Delaware Bay on red knots: are harvest restrictions working? BioScience 59:153-164.

Niles, Lawrence. J., et al. 2008. Status of the Red Knot in the Western Hemisphere. Studies in Avian Biology No. 36. Cooper Ornithological Society.

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