Recovery of Shorebird May Require Further Restrictions on Bait Use of Horseshoe Crabs


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Recovery of Shorebird May Require Further Restrictions on Bait Use of Horseshoe Crabs

By Jonathan Bart and Lawrence Niles (Conserve Wildlife Foundation of New Jersey)
April 2009

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Above: Red knot (Calidris canutus rufa) on the shore of Delaware Bay, May 17, 2007. Photograph copyright © by Mark Peck, biologist, Royal Ontario Museum, Toronto, Canada; used with permission. [larger version]

Above: Red knots migrate annually from Arctic breeding grounds to the southern tip of South America and back, covering more than 30,000 km (18,600 mi). Each May, red knots and other shorebirds stop at Delaware Bay on the U.S. east coast, where they feed on the eggs of spawning horseshoe crabs. [larger version]

Above: Red knots (red breasted), ruddy turnstones (black breasted), and sanderlings (tan speckling) on the shore of Delaware Bay, May 17, 2007. Photograph copyright © by Mark Peck, biologist, Royal Ontario Museum, Toronto, Canada; used with permission. [larger version]

Declining numbers of a shorebird called the red knot have been linked to the harvest of horseshoe crabs from Delaware Bay, a large estuary between New Jersey and Delaware that is an important stopover for red knots migrating from South America to the Arctic. This link was investigated by scientists from various federal, state, and nongovernmental entities in several countries, who published their results in the February issue of BioScience in an article titled "Effects of Horseshoe Crab Harvest in Delaware Bay on Red Knots: Are Harvest Restrictions Working?"

Long-term surveys show that the average weight of red knots when they leave Delaware Bay has declined significantly since their primary food source, the eggs of horseshoe crabs, has been reduced. The study also revealed that red knot survivorship is related to departure weight and that the population size of red knots has declined by more than 75 percent.

"We concluded that the increased harvest of horseshoe crabs led to a reduction in the food supply for red knots at a critical period in their annual cycle, and this led to a dramatic decline in population size," said U.S. Geological Survey (USGS) scientist, Jonathan Bart, one of the authors of the study.

There is a long tradition in Delaware Bay of harvesting horseshoe crabs for use as bait in various fisheries. In the years 1992-1997, reported harvest of crabs grew twentyfold, from about 100,000 individuals harvested to more than 2 million. The newly released study shows that this increase in horseshoe crab harvest has led to a dramatic decrease in the number of spawning crabs and a 90-percent decline in crab eggs available for shorebirds to eat.

Delaware Bay is globally recognized as an important feeding stopover for migrating shorebirds, especially red knots. Each year, red knots migrate from Arctic breeding grounds to the southern tip of South America and back, covering more than 18,600 miles. In May, large numbers of red knots migrating northward congregate in the bay, where they gorge on horseshoe crab eggs in preparation for their continued migration to the Arctic. For red knots, Delaware Bay is the final stop before a single direct flight to Arctic breeding grounds, where, on arrival in early June, weather is uncertain and feeding conditions are poor. Body reserves gained on Delaware Bay, therefore, are crucial for both the flight to the Arctic and survival and successful breeding.

Concern over red knot populations led to restrictions in horseshoe crab harvest starting in 1997, but as Lawrence Niles, biologist with the Conserve Wildlife Foundation of New Jersey and senior author of the new study, says: "Despite restrictions, the 2007 horseshoe crab harvest was still well above that of 1990, and no recovery of knots was detectable. Recovery of both horseshoe crabs and red knots may require more restrictions on horseshoe crab harvest, possibly even a complete moratorium for some period. We've proposed a program of adaptive management, including monitoring, that should result in the information that managers need to find the right balance."

Finding the right balance is a challenge faced by natural-resource managers across the United States, where numerous conflicts have arisen over multiple uses of resources. Examples include the spotted owl and the forest-products industry in the Western United States, wolves and sport hunters in Alaska, Pacific salmon and water resources in the Pacific Northwest, and red knots and horseshoe crab harvesting on Delaware Bay. The recent article calls attention to an educational resource that uses the conflict over red knots and horseshoe crab harvesting to teach students about the complexities of resource conflicts and the methods used to resolve them. To learn more about the curriculum, called "Green Eggs and Sand," visit URL http://www.dnr.state.md.us/education/are/ges.html.

The full reference for the article is:

Niles, L.J., Bart, J., Sitters, H.P., Dey, A.D., Clark, K.E., Atkinson, P.W., Baker, A.J., Bennett, K.A., Kalasz, K.S., Clark, N.A., Clark, J., Gillings, S., Gates, A.S., González, P.M., Hernandez, D.E., Minton, C.D.T., Morrison, R.I.G., Porter, R.R., Ross, R.K., and Veitch, C.R., 2009, Effects of horseshoe crab harvest in Delaware Bay on red knots; are harvest restrictions working?: BioScience, v. 59, no. 2, p. 153–164 [URL http://www.bioone.org/toc/bisi/59/2].