Fewer nesting horseshoe crabs linked to red knot decline?
07/29/2009
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Red knots depend on the eggs of spawning horseshoe crabs after their migration from South America. Here a red knot in breeding plumage dines on eggs along the shores of Delaware Bay/ |
Every spring, millions of Atlantic horseshoe crabs (Limulus polyphemus) clamber onto the beaches of Delaware Bay to lay eggs. Though horseshoe crabs spawn elsewhere, the Delaware Bay– which borders New Jersey and Delaware – supports the largest spawning population in the world. They’re weird-looking for crabs, and that’s because they’re not really crabs, but an ancient marine arthropod more closely related to ticks and spiders than crabs, and have been around some 540 million years, before most other life evolved. Today, their eggs provide a critical food source for migrating shorebirds, many of which have come from as far as the tip of South America – Tierra del Fuego – to gorge themselves on the overabundant eggs. Unfortunately, numbers of horseshoe crabs arriving to spawn have declined over the past two decades, though numbers appear to have stabilized, but research shows that at least one migratory bird, the red knot, has declined in concert.
The subspecies of red knot (Calidris canutus rufa) that winters in Tierra del Fuego and breeds in the Arctic have declined dramatically over the past twenty years, from 100,000 to 150,000 down to an estimated 18,000 to 33,000 today. The U.S. Fish & Wildlife Service listed them as a candidate endangered species in 2006. Scientists believe several factors have contributed to their decline, and one overriding fact they can pinpoint is a decline in horseshoe crab eggs, which have also declined substantially since 1990 according to several studies, including a recent study by Nigel Clark, an ornithologist with the British Trust for Ornithology, and his colleagues. Eggs eaten by the shorebirds have been churned up by waves or other spawning females, so bird predation on the eggs does not affect the crab population. Two other subspecies of red knot exist, but genetic studies so far show that they most likely do not interbreed with the other population in decline.
Clark joined an international team that has monitored the shorebirds feeding on horseshoe crabs 12 years ago, the Delaware Shorebird Project, where volunteers and biologists have tagged several thousand birds, including red knots, ruddy turnstones and sanderlings on several beaches in Delaware and New Jersey every May and June. “We normally obtain about 20,000 records of them each year and estimate that we see over 60% of those that are alive each May,” says Clark. Based on the number of birds and their condition, and the number of crabs and eggs observed, they make recommendations about the level of Horseshoe crab harvest to the Atlantic States Marine Fisheries Commission (ASMFC). Apparently they’re used as bait for conch and eel fishing.
The long-lived crabs don’t mature until they’re over nine years old and live to be twenty years or so, and that means any recovery of their numbers will take time, as well. A single female will come to shore three or four times per tidal cycle laying up to 4,000 eggs, and then return until she's laid almost 100,000 eggs! One thing visitors to beaches with spawning horseshoe crabs can do is “Just Flip ‘Em” – the name of a campaign spawned by the Ecological Research and Development Group to teach people that up to 10% of the crabs die every year just because they’re unable to right themselves after getting turned upside down by waves. The telson, or tail, does not sting, contrary to what some people fear. So just flip 'em!
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