Horseshoe crab decline threatens shorebird species

Each year, the red knot, a medium-sized shorebird, makes a 20,000-mile round-trip from the southern tip of Argentina to the Artic Circle – one of the longest migrations of any bird. And each year from April to June, the red knot stops over in the Delaware Bay to feed on horseshoe crab eggs resulting from the largest spawning of horseshoe crabs found on the East Coast of the United States.

Researchers from Virginia Tech and the New Jersey Division of Fish and Wildlife have documented a reduction in the number of red knot birds throughout the Delaware Bay tied to a decline in horseshoe crabs.

The research will be reported in The Journal of Wildlife Management, in the article, “Horseshoe Crab Eggs Determine Red Knot Distribution in Delaware Bay Habitats,” by Virginia Tech fisheries and wildlife research scientist Sarah Karpanty, professor Jim Fraser, and associate professor Jim Berkson, New Jersey Division of Fish and Wildlife biologists Lawrence Niles and Amanda Dey, and Virginia Tech statistics professor Eric Smith. The article provides scientifically defensible information for wildlife management officials as well as for other members of the scientific community.

During their Delaware Bay stopover, the red knot nearly doubles its body mass as it gorges itself almost exclusively on horseshoe crab eggs. The purpose of this feeding frenzy is to ensure that the shorebirds have enough energy to complete the trip north to their breeding ground in the Artic.

However, due to horseshoe crab’s popularity as bait used by fishermen, the crabs appear to be in serious decline. At the same time, there has been a great reduction in the total population of red knots, the report notes. “The number of horseshoe crab eggs was the most important factor determining the use of the beaches by red knots. The availability of horseshoe crab eggs was even more influential than the presence of human disturbance, predator occupation, and availability of other types of food,” says Karpanty, a post-doc in the College of Natural Resources.

The red knot’s dependence on the horseshoe crab for survival has attracted the interest of local, state, and international wildlife management officials and researchers. Due to the red knot’s unusual migratory and eating behaviors, scientists from as far away as Australia frequently travel to the Delaware Bay to study this rare species.

“Biologists with the Delaware and New Jersey divisions of fish and wildlife have been very helpful during this project, and they welcome researchers from all over the world,” says Fraser. “We hope to see collaborative efforts like this continue so that we can learn how to better manage wildlife resources like the red knot and horseshoe crab.”