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Shorebirds Face Extinction Due to Crab Decline

John Roach
for National Geographic News
April 18, 2006
 
The food web around the horseshoe crab—one of Earth's oldest
species—is beginning to unravel, scientists say.

Certain species of migratory shorebirds depend on excess crab eggs to fuel the final leg of their spring journey to the Arctic. Researchers are concerned the birds are in jeopardy.

"Right now we're in that real dangerous period where nobody can say what's going to happen for certain," said Larry Niles, a biologist with the New Jersey Division of Fish, Game, and Wildlife in Trenton.

The birds feed on the crab eggs in the Delaware Bay, an estuary sandwiched between New Jersey and Delaware (map) that harbors the world's largest horseshoe crab population. But human harvest of horseshoe crabs has reduced the egg surplus.

In the race to keep birds—especially the red knot, which feeds exclusively on the crab eggs—from extinction, scientists and fisheries managers are considering increased protections for the crabs.

The crabs, which look like a hoof of a horse, are more closely related to extinct arthropods than to other crabs. Horseshoe crabs date back at least 360 million years and perhaps longer.

"They're quite the ancient critters, and they haven't changed for millions of years," said Michael Haramis, a wildlife biologist with the U.S. Geological Survey (USGS) in Laurel, Maryland.

Mark Botton is a biologist who studies horseshoe crabs at Fordham University in New York. "The conservation concern is not so much for the horseshoe crab itself, but rather that the population is big enough to provide lots of eggs," he said.

Collapsing Populations

In 1992 the Atlantic cod fishery collapsed. With no cod to catch, fishers started to go after conch. "And the best bait for conch was horseshoe crab," Niles, the New Jersey biologist, said.

As the conch harvest expanded, so too did the horseshoe crab harvest, he added.

By the mid-1990s the annual horseshoe crab harvest increased to 2.5 million from a few hundred thousand. The horseshoe crab population plummeted. Density dropped 90 percent, according to Niles.

While the density drop may not doom the crabs—there are still tens of thousands of them, according to population surveys—it has a stranglehold on migratory shorebirds.

Horseshoe crabs engage in an annual mass spawning, an event that once so overwhelmed the beach with eggs that it all but ensured their survival.

The spawning event is timed to the spring lunar tide—generally the highest of the season. Thousands of horseshoe crabs descend on the shoreline to lay clusters of eggs, which they bury 6 inches (15 centimeters) deep.

Since the crabs bury their eggs, they are fodder for migratory birds only when other crabs dislodge the eggs as they attempt to bury their own, Niles explained.

With literally thousands of crabs spawning at once, dislodged eggs were guaranteed. All the shorebirds had to do was eat. But as crab density dropped, fewer eggs were dislodged.

"That had a catastrophic effect on the shorebird population," Niles said. "The Delaware Bay stopover fell from over a million to just a couple hundred thousand this year."

The red knot, a migratory bird that stops over on its way from South America to the Arctic, is taking the hardest hit. The population has fallen to just over 17,000 from more than 50,000 six years ago, according to annual surveys.

"The birds need the eggs to successfully reproduce and [Delaware Bay] is the last stop before they go to the Arctic to breed," USGS's Haramis said.

Crabbing Moratorium

According to Niles, the red knot could be extinct within five years.

"We are in a major effort to try to stop the [horseshoe crab] harvest for a couple of years until the crabs can recover and, over time, eventually restore the shorebird population," he said.

Last month the state of New Jersey approved a two-year moratorium on the horseshoe crab harvest. The ban must clear a few remaining regulatory hurdles before taking effect in May, according to the New Jersey Division of Fish, Game, and Wildlife.

The New Jersey ban should protect 80,000 to 90,000 crabs a year.

In addition, the Atlantic States Marine Fisheries Commission is considering further restrictions—possibly a moratorium on the entire fishery in the Delaware Bay—and new limits on the horseshoe crab fisheries in Maryland and Virginia.

Public comment on the restrictions closed today. If approved, the limits will be effective July 1, according to the fisheries commission.

Fordham University's Botton said any moratorium is welcome for the crab and shorebirds but cautioned that the crabs take about ten years to reach reproductive age.

"So even if there were a total moratorium this year, it's not realistic to expect to see the benefits of that for years to come," he said.

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