Delaware Bay’s crabs contributing to dip in already threatened red knot population

Decades-long changes in the Delaware Bay region may be contributing to the declining numbers of a medium-sized  shorebird known to generations of Delawareans and New Jersey residents.

The red knot population that winters in Tierra del Fuego, Chile, fell to below 10,000 birds this year, according to results of an annual survey. That’s a huge drop from 2000, when scientists counted more than five times that number in Chile.

A type of sandpiper, red knots winter at the tip of South America, then fly more than 9,000 miles to the Arctic to breed. It has one of the longest migration routes in the animal kingdom.

Climate change, development, and increasing storm surges along the way are making this bird’s life more difficult.

And “one of the key concerns is in the Delaware Bay,” said Rutgers University professor Rick Lathrop.

The birds stop at the bay in late spring.

“That’s where they feed heavily on horseshoe crab eggs, they double their weight. That weight gain is critical in terms of their ability to migrate successfully to the Arctic, and then especially their ability to breed successfully,” he said. “They need that as energy stores.”

But horseshoe crab numbers are down, and there aren’t enough eggs for the birds to fatten up before their trip north. What’s more, the eggs are available for a shorter time than in the past.

“Instead of crabs spawning over a long period of time and charging up the sand with lots and lots of eggs, it’s just a brief period of intense spawning so that the egg resources, they’re ephemeral,” said biologist Larry Niles, who led the research team.

The birds’ short stopover in the Delaware Bay may not correspond with the crab’s short-lived spawning, Niles said. The knots “have no idea what’s happening in Delaware Bay before they get here.”

In the last five years, the count has oscillated between about 10,000 and 13,000 birds.

Red knots are considered a threatened species in the United States.