June 19, 2019 | 65° F

Rutgers faculty release thousands of horseshoe crabs into the bay to rebuild dwindling population

Over the last few decades, the number of horseshoe crabs in the area has fallen from 2 million to 500K

Photo by Rutgers.edu |

After the number of horseshoe crabs in the Delaware Bay declined from 2 million to 500,000 a group of Rutgers professors, students and interns came together to introduce a batch of new crabs into the ecosystem.

Rutgers professors and interns released about 10,000 3-month-old horseshoe crabs into the Delaware Bay earlier this month. The release was a public outreach event in a larger research project toward rebuilding the horseshoe crab population in that area.

Thomas Grothues, a professor in the Department of Marine and Coastal Studies, is currently working at Rutgers' Aquaculture Innovation Center (AIC) in Cape May and is one of the professors leading the project.

Grothues said that there used to be approximately 2 million horseshoe crabs in the Delaware Bay, but due to various factors related to human involvement in the ecosystem, the population has been on the decline since the early twentieth century. There are now about 500,000 horseshoe crabs in the bay.

The horseshoe crabs provide a necessary source of food for birds migrating from Tierra del Fuego on the southernmost tip of South America to the northern tundra of Canada, he said. The species of birds that make that annual trip have evolved so that their migration coincides with the horseshoe crabs' egg laying. The birds fly without rest for six days, then stop on the beaches around the Delaware Bay to forage for the eggs, which are oily and fatty enough to sustain the birds for the rest of their journey.

But when the horseshoe crab population is as diminished as it is now, their eggs are less abundant and less accessible to the migrating birds. The birds then do not reproduce at their normal rate, and their population suffers as well, Grothues said. The Red Knot, one of the birds that makes this migration, is now, especially at risk.

The horseshoe crab population is also necessary to a number of human enterprises. Fisheries around the Delaware Bay provided horseshoe crabs as ingredients in pet foods, fertilizers and fishing bait. Most critically, a chemical used in testing certain pharmaceutical products can only be made from horseshoe crab blood, Grothues said. Even as the other fisheries have collapsed, the pharmaceutical industry has remained in the area out of necessity.

Due to predation and other environmental pressures, it is perfectly natural that most horseshoe crabs die within a few days of hatching. But when the population decreases below a certain threshold, it becomes increasingly difficult for it to maintain its numbers. Grothues called this a “predation pit,” where the population is just large enough to feed its predators. After the predator base is satiated, the remainder of the horseshoe crabs will comprise the new population. If this remainder is too small, the population will not grow, or it may have a net decrease year over year.

“Imagine a farmer who begins eating his corn because he's close to starving,” Grothues said. “You're paying for maintenance, and there's no growth there.”

Several factors have contributed to the decline in population, he said. For one, the horseshoe crabs were overfished.

There are photos of walls of these things stacked up,” Grothues said. “They're very easy to harvest, so people would go and gather them, and they would be ground up for fertilizer and stuff like that.”

At the beginning of the 20th century, the horseshoe crabs were thought to be a public nuisance, and so a bounty was offered for them, he said. Kids were incentivized to turn in horseshoe crab tails to local authorities for a payout of a few cents per tail.

Human developments along the shoreline have altered the beaches, which are the breeding grounds for the horseshoe crabs. Sand on beaches is naturally circulated by the ocean currents. In the summer, currents deposit sand on the beaches. Then in the winter, currents strip sand from the beaches, Grothues said. 

Human developments interrupt this process, he said. After the sand is removed in the winter, the beaches cannot move inland. The horseshoe crabs require beaches with deep sand deposits in order to lay their eggs, and these environments are becoming increasingly rare in the Delaware Bay area.

The AIC is still at an early phase in its research towards rebuilding the horseshoe crab population, Grothues said. Each year, the AIC releases a few thousand horseshoe crabs and keeps a small portion for further study. The crab hatchlings to be released are kept for a few months before the release, which is meant to reduce their mortality rate. Yet at this stage in the research project, releases serve mainly as public outreach.

“A facility like the AIC, we can afford to make mistakes while we learn the best way to do things, then pass that on to the commercial sector,” Grothues said. “We're very, very early in this process, and we don't know how to do it yet — not right. We're learning.”

Max Marcus

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