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A Talk on the Wild Side.

Delaware: Betting on Survival in Delaware Bay

A close-up of a red knot

A red knot tagged for research rests and refuels in Mispillion Harbor, Delaware. Climate changes such as sea-level rise and increasing storm intensity are adding challenges to red knot survival. Photo: Gregory Breese/USFWS.

Camera iconMore photos: Delaware Bay Estuary Project photoset on Flickr

Video iconRelated video: Crash: A Tale of Two Species on PBS.org

Not far from the casinos of Atlantic City, a different kind of wager takes place each May along the shores of Delaware Bay. 

That’s when red knots, birds the size of a coffee mug, stake their future on the eggs laid by tens of thousands of horseshoe crabs. Without enough crab eggs to fuel them, the long-distance fliers may not survive their 10,000-mile spring trek from the southern tip of South America to their Arctic breeding grounds.

In recent years, the red knots’ bet on the crab eggs has been more of a crapshoot. First, the over-harvesting of horseshoe crabs for bait caused an egg shortage. Now, scientists also point to a wild card.

“The peak of horseshoe crabs spawning in Delaware Bay has not always been aligned with the migration of the red knots,” said Gregory Breese, the U.S. Fish and Wildlife Service’s project leader for the Delaware Bay Estuary Project. “That could be related to climate change.”

Changing water temperatures in Delaware Bay and more frequent and intense storms appear to be disrupting the synchronization between the spawning of the crabs and the arrival of the red knots. When waters warm, the crabs lay their eggs earlier, and other creatures may beat red knots to the feast.

Piles of horseshoe crabs wade into water

Delaware Bay hosts the largest concentration of spawning horseshoe crabs in the world. Photo: Gregory Breese/USFWS.

Other climate-related influences are also likely affecting crabs and red knots. Rising sea levels have eroded beach and shrunken crab spawning areas. Red knots spend most of their time in regions of the world already experiencing effects from climate change. When Arctic snow cover is less than average during their summer breeding season, models suggest that red knot survival rates drop, Breese said. In the winter, red knots are arriving one week later than they did a decade ago in Tierra del Fuego, an archipelago on the tip of South America. 

Such climate impacts are expected to increase with time. But, said Breese, “When and if a threshold will be reached and causing a dramatic change in the red knot population is guesswork at this point.”

In the 1980s, the rufa subspecies of red knot took a severe hit from overharvesting of the crabs in Delaware Bay. The rufa population, estimated at between 90,000 and 150,000 birds at the start of the decade, fell to between 15,000 and 20,000, according to Kevin Kalasz, shorebird project coordinator for the Delaware Division of Fish and Wildlife. Mid-Atlantic Coast states imposed strict limits on crab harvests, starting in the year 2000.  (Human health concerns helped drive the action: Crab blood plays a key role in biomedical research, helping to detect bacterial contaminants in drugs and implanted devices.)

Green-blue eggs in the cup of someone's hand

Horseshoe crab eggs in hand, Mispillion Harbor, Delaware. Photo: Gregory Breese/USFWS.

Rufa numbers have held steady since 2003, said Kalasz. The bird is a candidate for federal protection under the Endangered Species Act. 

With crab harvests restricted, Kalasz expects climate change to factor more strongly in the red knots’ survival.

Once scientists have a more precise understanding of climate change impacts on the red knot, they will be better able to devise counteracting management actions, Kalasz said. He called such research “one of our top priorities.” One possible response, he said: replenishing beach area where the crabs spawn to counteract rising sea levels.


Author: Stacy Shelton, USFWS


Related Websites:

Two of NCTC's recent video productions touch on these topics --the new visitor video on Bombay Hook National Wildlife Refuge entitled, "America's Gem on the Delaware," and an upcoming video for Prime Hook National Wildlife Refuge.
# Posted By David Klinger | 5/3/11 10:15 AM


Thank you for sharing. If available, please post links to the videos mentioned here.
# Posted By Leith Edgar, USFWS | 5/4/11 12:28 PM

A link to the Fish and Wildlife Service's new visitor video for Bombay Hook National Wildlife Refuge in Delaware:
# Posted By David Klinger | 5/11/11 5:28 PM

I lived in Southern New Jersey for years and remember the time of year when the horseshoe crabs would show on the Bay. I would love to see the 2 <a href="http://janisdigitalmedia.com">video productions</a> that the previous poster was talking about. Definitely would be worth watching...
# Posted By John B. | 8/3/11 10:31 PM

I used to live in Delaware. Its a great place that seems to be forgotten by most people.

# Posted By Jared | 5/30/13 6:31 PM
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