The Science
A long-term lack of effective monitoring has resulted in a data deficit for populations in parts of the East Coast. Delaware Bay's population has the benefit of years of monitoring and conservation work, and can provide a clearer picture of how a population can be destroyed.
We often use Delaware Bay as an example of how things can go wrong, but make no mistake: the horseshoe crab faces an existential threat all along the Atlantic Coast.
Below is a primer of the scientific evidence that the horseshoe crab, and the Atlantic Coast ecosystem that depends on it, are in trouble.
Atlantic Coast Horseshoe Crab Landings, 1970-2006
The 1990s saw an explosion in the horseshoe crab harvest, going from an annual harvest of 100,000 in 1991 to 2.5 million by 1998.
Because of the dramatic reduction of these crabs from the breeding population, their numbers crashed along the Atlantic Coast.
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In the early 1990s, horseshoe crabs became a popular bait for commercial fisheries, notably that of whelk and eel. The whelk fishery was booming as a domestic and international food, while eel was a popular bait choice for striped bass. Horseshoe crabs were found to be very effective bait for these species, and their demand skyrocketed. With fishers using 1-2 crabs per pot and a preference for gravid (egg-bearing) female crabs, the heavily increased harvest devastated horseshoe crab populations.
1) Overharvesting of horseshoe crabs led to a crash in their population.
Weakfish landings in Delaware Bay declined concurrently with horseshoe crabs.
Agency biologists discount the connection but fail to provide alternative explanations. Fish and migratory birds depend on horseshoe crab eggs to survive.
The severe drop in the crabs' breeding population caused a shortage in available eggs for foraging species, and resulted in a crash in their populations as well.
The Red Knot is an IUCN red-listed shorebird that uses Delaware Bay as a stopover to feed during their migration. During their week-long feeding frenzy, they eat enough to double their body weight so they have sufficient fuel to complete their northbound journey to the Canadian Arctic. Their weights are statistically linked to horseshoe crab surface egg density, as seen in the graph to the right.
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Poor egg availability on Delaware Bay beaches leaves emaciated birds with no help to complete the last stage of their intricate migration, increasing the risk of death during the journey or failure to breed once they reach their destination in the Canadian Arctic.
2) The dramatic decrease in horseshoe crab egg availability caused shorebird and fishery populations to crash.
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1991 egg density, pre-overharvest
Due to overharvest, horseshoe crab egg densities dropped from 40,000/m2 to an average of 5,000/m2. They have shown little sign of recovery in the last 25 years.
Although harvests have since been curtailed, the current population of Delaware Bay horseshoe crabs is still only at 37% of carrying capacity, according to ASFMC ARM models.
Numbers of female crabs are estimated at about 4.5 million but the carrying capacity of the bay is estimated at over 12 million.
arrow_back_iosSurface eggs in the top 5 cm of sand have not shown a substantive or sustained increase and remain an order of magnitude below historic densities observed prior to crab overharvest.
The lack of increase in egg resources is consistent with the lack of substantive or sustained increase in mature female horseshoe crabs during the same period.
3) Despite harvest regulations, horseshoe crab populations have not increased.
Click to view an infographic explaining why the failure of horseshoe crab eggs to reach carrying capacity is a problem for other species.
Horseshoe Crab Egg Densities in Top 5cm of Sand, 1991-2023
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Increase from improved productivity
Overharvested horseshoe crabs can recover in less than 12 years according to an analysis published in by Tan and Jardin in 2017.
Most of the recovery would occur in less than 8 years with 2 million new adult female crabs annually entering the population.
Implementing and enforcing best management practices for crabs used by the biomedical industry will help reduce bleeding mortality to zero. Adopting the use of synthetic rFC will lessen the demand for LAL and reduce the overall impact to horseshoe crabs from the bleeding harvest.
Restoring and protecting beach habitat along the Atlantic Coast will supply adequate breeding grounds to support a healthy horseshoe crab population.
4) We can fix this.
Recovery of our horseshoe crab populations is possible, if we change the way we manage them.
Ending the overharvest of horseshoe crabs will allow an immediate increase in populations as more adults enter the breeding population.
Horseshoe Crab Abundance Over Time If ARM Model is Enforced
Increase from ending harvest
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Click here to take action.
The scientific evidence supports our campaign.
We need your help to ensure the horseshoe crab's future.
