Shorebirds Face Extinction Due to Crab Decline

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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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