Management of horseshoe crab harvests is based on population numbers. But when it comes to saving an endangered species that depends on them, should fisheries managers consider more?
Kathleen Doyle
During the first full moon of summer, the largest concentration of Atlantic horseshoe crabs on Earth crawls ashore along the Delaware Bay to spawn. The spectacle of thousands of mating horseshoe crabs—an unbroken band of domed shells at the water’s edge—is unique in the world.
Each female lays up to 50,000 eggs in the wet sand. Within their spawning window, six species of shorebirds pause their migrations from South America to gorge on the nutritious eggs. The birds, including the endangered Red Knot, can’t complete their 9,500-mile flights to the Arctic without the fuel. If the crab population were to collapse, so would the birds.
Like the birds seek the crabs, so do commercial fishermen and the pharmaceutical industry. Collecting horseshoe crabs for bait and for biomedical testing drove a plunge in their numbers in the 1990s. That decline, coupled with concern for the Red Knot, caused the Atlantic States Marine Fisheries Commission (ASMFC) to set limits on the catch in 2013.
Horseshoe crab numbers have since stabilized. The ASMFC now believes the population is robust enough to allow higher harvests that include females, yet many experts contend that numbers aren’t enough, that fisheries managers must consider other factors. So what is the best management model for protecting the crabs and saving the birds?
Ancient Creatures, Modern Problems
Horseshoe crabs are a living link to a very distant past. They have existed in some form for more than 450 million years—since before the dinosaurs—and they have managed to survive four mass extinction events. The modern horseshoe crab emerged about 250 million years ago.
Three species of horseshoe crab live in Asia: the mangrove horseshoe crab (Carcinoscorpius rotundicauda), the Indo-Pacific horseshoe crab (Tachypleus gigas) and the Chinese or tri-spine horseshoe crab (Tachypleus tridentatus). The American horseshoe crab, (Limulus polyphemus) lives in the Atlantic Ocean and Gulf of Mexico. The greatest concentration spawns in the Delaware Bay.
Many experts are concerned that humans may ultimately be responsible for the demise of this ancient creature.
Beginning in the 1970s, the pharmaceutical industry began using horseshoe crab blood to create the Limulus amebocyte lysate (LAL) test, a test for contaminants in injectable drugs, vaccines and implanted medical devices. The biomedical industry is not required to make their data public, so an accurate number of horseshoe crabs harvested for this purpose is not available.
In the early 1990s, commercial fisheries began using horseshoe crabs as bait for American eels, whelks and conch, all of which are sold mainly in Asian markets. All are overfished. The American eel is considered endangered by the IUCN, as is the three-spined horseshoe crab, which had previously been used as bait in Asia.
By the mid-to-late 1990s, the combined impacts of medical and fishery harvesting resulted in the dramatic decline in the population of both horseshoe crabs and migrating shorebirds. The International Union for the Conservation of Nature (IUCN) has classified the Atlantic horseshoe crab as vulnerable. Within five years of the switch to horseshoe crabs as bait, the average horseshoe crab harvest went from about 100,000 a year to more than 2.5 million. Five decades of surveying shorebirds that stop along the Delaware Bay also revealed a sharp decline—from more than 1.5 million to 200,000. In 1989 about 94,000 red knots were counted. In 2014, the red knot was listed as threatened under the Endangered Species Act. By 2022, the number was barely over 12,000.
Should Management Change?
In 2013, ASMFC began setting limits on horseshoe crab harvest of no more than 500,000 males (no females), though each of the 15 member states could adopt stricter limits. New Jersey banned all harvesting of horseshoe crabs in 2006. The governor of New York recently vetoed a bill that would have implemented a similar ban. Delaware allows the limit established by ASMFC, which was 173,014 for 2024.
Though the limits have stabilized the horseshoe crab population, their numbers have not yet rebounded. The limits have had no impact on the Red Knot.
A strong case can be made for discontinuing the use of horseshoe crab blood by the pharmaceutical industry. A synthetic alternative has been available since the late 1990s. The alternative to LAL is the Recombinant factor C (rFC). The governments of Japan and China, as well as the European Union, have approved use of the synthetic, which has been commercially available since 2003.
The U.S. Pharmacopeia, an independent, scientific, nonprofit organization, establishes standards for all drugs marketed in the United States. About 140 countries recognize USP standards. Pharmaceutical companies have been slow to make the change, but there is increasing pressure for them to switch to rFC.
Dr. David Mizrahi, vice president for research and monitoring at New Jersey Audubon and co-founder of the Horseshoe Crab Recovery Coalition, says the pharmaceutical industry is expected to incorporate the 3Rs of research: replacement, reduction and refinement, a framework for reducing animal suffering. He is hopeful that the ethical expectations described in the 3Rs, along with the synthetic alternative, public attention and visits to the coast will finally bring an end to biomedical harvesting. To encourage drug companies, Mizrahi has invited industry reps to witness the magic of the summer’s first full moon on the Delaware Bay.
Mizrahi is less optimistic—though not hopeless—about ending the bait harvest any time soon. ASMFC recently considered increasing the number of horseshoe crabs that may be harvested for bait in the Delaware Bay based on results from a new data collection system that was implemented in 2021. That system considers numbers of both horseshoe crabs and shorebirds.
Using the new data collection process, ASMFC calculated that in 2022, Delaware Bay had about 40 million mature male and 16 million mature female horseshoe crabs. It concluded that “mature female horseshoe crabs have been steadily increasing in the region” since 2012 and that the numbers now support harvesting not only the usual 500,000 males, but also up to 175,000 females.
A huge public outcry—34,000 opposed to the new rules, five in favor—led ASMFC to rescind the suggested female harvest. “Acknowledging public concern about the status of the Red Knot population in the Delaware Bay, the board elected to implement a zero female horseshoe crab harvest as a conservative measure and maintained that decision for 2024 and 2025,” says Tina Berger, communications director for ASMFC.
Mizrahi says that if the population of horseshoe crabs were robust, he would not be opposed to using them as bait, but he and other experts argue that the sampling the new management model is based on is flawed. The new model relies heavily on numbers generated by two trawl surveys and a shorebird count.
Dr. Kevin Shoemaker, associate professor at the University of Nevado, Reno, analyzed ASMFC’s framework for data collection. He concluded that one of the flaws in the new model is that it overestimates the lifespan of Red Knots as 15 years, whereas the actual life span is five years.
Another flaw he has noted: Counting horseshoe crabs based on trawl numbers isn’t helpful because Red Knot survival “is strongly sensitive to horseshoe crab egg-density, indicating that persistent degradation of the horseshoe crab egg resources could have dire consequences for the Red Knot.”
Will Harlan, Southeast director and senior scientist at the Center for Biological Diversity, agrees that the most useful metric for determining Red Knot survival is horseshoe crab egg density: the number of eggs laid on beaches where they nest. Decades of egg density survey data are available, he says, noting, “Horseshoe crab egg density correlates directly with red knot abundance.” One study in New Jersey found that egg numbers have declined by 90 percent since the 1980s.
Harlan adds that the measures that ASMFC used “artificially and inaccurately inflate the numbers of horseshoe crab populations” and have “large data gaps that have been artificially filled with inflated assumptions.” He claims that “very few to no juvenile female horseshoe crabs are being observed in surveys—so few that it is breaking the model. As a result, ASMFC is artificially and astronomically inflating the number of juvenile females in the model to arrive at its desired outcomes for recommending a female horseshoe crab harvest.”
In July 2024, ASMFC convened stakeholders to make recommendations about horseshoe crab management in the Delaware Bay. The consensus was to continue with the current management framework but prohibit female horseshoe crab harvest while the ASMFC board considers possible changes. No date has yet been set for implementation.
About the author: Kathleen Doyle is a teacher, environmentalist and award-winning author of “Blues River” and other books for children.
Learn More
The latest on the horseshoe crab management plan: Horseshoe Crab Draft Addendum IX – Atlantic States Marine Fisheries Commission