On a moonlit night last month, volunteers were on Pickering Beach along the Delaware Bay in Kent County counting horseshoe crabs. This is part of an annual effort to help the crab not only survive, but thrive.
In the 1990s, the horseshoe crab population plummeted to an all-time low due to overharvesting.
That prompted a federal response.
In 1998, the Atlantic States Marine Fisheries Commission took over management of horseshoe crab harvesting. Since then, there have been frequent updates to their interstate management plan.
Managing the horseshoe crab population is necessary. More than 20 species of birds rely on horseshoe crabs producing an abundance of eggs each year so they have a food source. That food source is particularly important for the migratory rufa red knot during its yearly 9,300-mile journey to the Arctic.
And humans rely on horseshoe crabs, too. Not for their eggs, but for their blood and bodies.
Jordan Zimmerman is a fisheries biologist with the Delaware Division of Natural Resources and Environmental Control’s Division of Fish and Wildlife.
“A lot of the focus on horseshoe crabs lately has been with harvest, that includes the biomedical industry, which takes their blood. They do that to produce a product called Limulus Amebocyte Lysate which is used to test just about every pharmaceutical and implant in the biomedical industry,” he explained.
Horseshoe crab blood doesn’t contain white blood cells, it contains amoebocytes. When it’s added to a substance that contains bacterial endotoxins, it congeals. Bacterial endotoxins can be deadly if they enter the human body, making it very important that they’re not in products like vaccines and implants.
“There’s also a commercial harvest,” said Zimmerman. “They’re a valued bait for both the American eel and the channeled and knobbed whelk fisheries, which are throughout the Atlantic coast of the United States.”
The number of horseshoe crabs allowed to be harvested for bait use changes annually based on the abundance of the crabs and of red knots.
“The management plan we have for horseshoe crabs establishes state quotas to limit the number of horseshoe crabs that can be harvested for bait... states are required to implement and abide by those quotas."Fishery Management Plan Coordinator Caitlyn Starks on the limitations in harvesting horseshoe crabs for bait.
“The management plan we have for horseshoe crabs establishes state quotas to limit the number of horseshoe crabs that can be harvested for bait. So the board decides what those quotas are, and then the states are required to implement and abide by those quotas,” explained Caitlin Starks, Fishery Management Plan Coordinator for Horseshoe Crabs for the Atlantic States Marine Fisheries Commission.
In Delaware, the 2023 quota was 164,364 male horseshoe crabs. The harvest of female horseshoe crabs has been banned in the state since 2013.
Delaware does not have a biomedical horseshoe crab fishery, but in Massachusetts, Rhode Island, New Jersey, Maryland, Virginia, and South Carolina, horseshoe crabs are collected for biomedical purposes. There are a total of five limulus amebocyte lysate, or LAL, manufacturers across the Atlantic Coast that these crabs go to.
Starks says that unlike with bait use, horseshoe crabs collected for biomedical use are released back into the ocean alive.
“The companies are required to report the exact numbers of horseshoe crabs that they collect, as well as the number of crabs that are bled, and observed mortalities up to the point that they’re released back to the water.”
The number of horseshoe crabs harvested for bait is typically higher than the number harvested for biomedical use, and has a 100% mortality rate. The biomedical fishery mortality rate is 15%.
The focus on numbers in the discussion of horseshoe crabs is increasingly prevalent, but not new.
In 1990, 8 years before horseshoe crab harvesting became federally and regionally regulated, a group of volunteers created the Delaware Bay Horseshoe Crab Spawning Survey to gather data on the numbers of male and female horseshoe crabs spawning on Delaware Bay beaches.
It’s one of over 25 data sources used to inform Atlantic horseshoe crab stock each year.
Starks says spawning surveys are not used for estimating overall abundance- that’s primarily done in trawl surveys.
“But they are used for an important piece of our management, which is understanding the sex ratio on the spawning beaches, so how many males there are versus females are there. And that data is input into some of our management models,” she explained.
Zimmerman has been involved in horseshoe crab spawning surveys for over 22 years – including last month’s count at Pickering Beach.
“So we’re doing 25 beaches between both sides of the bay, 12 nights a year. 4 lunar periods- the full moon and new moon in May, and the full moon and new moon in June,” said Zimmerman.
When high tide comes in, so do the horseshoe crabs. And counting them at the time of peak activity can be a heavy lift.
Zimmerman says there are roughly 160 volunteers a year, Baywide, who come from across the globe to participate.
Anyone over 13 years old can take part in it. They are given training on how to properly conduct the survey, and join an expert at one of the 25 survey beaches during high tide. On some nights, this could mean counting the crabs well after midnight.
The size of surveying teams can vary depending on experience. Zimmerman was part of a team of 3 on Pickering Beach.
“The experimental design of the survey is to sample a kilometer of the horseshoe crab spawning beaches,” Zimmerman explained. “We have 2 random starts. On number, or start, is in the first 10 meters. The second one is in the second 10 meters. And once we establish those, then myself and Ben can leapfrog down the beach 20 meters at a time. So we’re getting one meter squared sample, and the count from there, every 10 meters, for a total of 100 samples within the kilometer.”
Ben Staud, an Environmental Control Technician with the Delaware Division of Fish and Wildlife, was part of Zimmerman’s team. Despite the heavy lift, he says taking part in the survey is a great experience.
“I enjoy the survey a lot, and it's a really cool thing to be able to get out on the beach at night and just see the spectacle of all these horseshoe crabs coming up onto the beach,” said Staud. “It’s not really like anything else you know we see like anywhere in the world. So, it's just kind of a cool thing to be a part of, and to tell friends and family about it and show them cool pictures of all these horseshoe crabs. No one really knows that much about it, and they're like ‘wow you know I didn't even realize horseshoe crabs acted this way, or showed this type of behavior.’ It’s just a really cool thing to be involved with.”
They call out the number of males first, and females second.
Male and female horseshoe crabs look almost identical, but there are certain visual and tactile cues that distinguish them.
For example, females are typically bigger than males, and there is a slight difference in the curve of their shells.
Female horseshoe crabs are more widely protected due to their eggs serving as a source of food for shorebirds.
Data over time also indicates that despite that added protection, male crabs remain more prevalent.
But quantitative data isn’t the only thing that the spawning survey produces.
Zimmerman says the qualitative aspect of observing the crabs' habits in the wild is also important.
There are many things you can learn from being out in the field that data points can’t fully describe.
The physical traits of both the horseshoe crab and the beaches themselves can be indicative of different threats to the species.
In recent years, the beaches have been shrinking.
“We definitely see the impacts of climate change, either directly or indirectly on the population. And probably first and foremost, the destruction of spawning habitats,” said Zimmerman. “Beach replenishment occurs with greater frequency and greater volumes all the time now, and so the more sand we lose the more we have to replace. And it's kind of chasing your own tail. You put it on there, it goes away, you put it back on again. And all that sand makes for a great spawning habitat, but without a suitable spawning habitat, then we're decreasing the carrying capacity of the population, and essentially putting the ceiling on it.”
Better habitat equals more crabs, he says.
The greater frequency and intensity of storms along the East Coast have affected both spawning activities and surveying. Horseshoe crabs choose to place their eggs on beaches with gentle waves, as they lay them about 10 centimeters beneath the sand, and must be able to crawl onto the beach with ease.
During storms, spawning activities grind to a halt because they cannot reach the beach. In addition, heavy winds and rain threaten to unearth any eggs that have already been laid.
Human activity, like real estate development in coastal communities, can also affect the horseshoe crabs' spawning habitat. Considered in conjunction with the shrinking beaches, horseshoe crabs are facing threats from both sides of the sand.
Surveyors like Zimmerman, who have been visiting these beaches each year for decades, are able to see the effects of these changes with greater clarity.
They can also observe visual changes to the crabs themselves. Telsons, which are the tail-like appendages that horseshoe crabs use to flip themselves over, can break naturally or because of human activities, like fishing.
A greater volume of crabs with broken telsons or other physical damages may indicate a need for more protections across fisheries.
In recent years, horseshoe crab bycatch has been a growing topic of conversation.
“Another potential issue is bycatch in other fisheries. In our most recent stock assessment, we estimate the number of discard mortality in other fisheries, which is the number of horseshoe crabs that potentially get caught as bycatch and are discarded and die as a result of that. There’s a lot of uncertainty around the preliminary results, but it suggests discard mortality may be comparable or greater than the amount of mortality that comes from other sources,” Starks explained, referencing the 2019 Horseshoe Crab Benchmark Stock Assessment and Peer Review Report.
The assessment classified bycatch mortality as the most important uncertainty and research recommendation identified, and called for more efforts to be made to improve data access and use however possible.
Based on the data written in the report, there was an assumed discard mortality rate of 50% for both gillnet and trawl discards of horseshoe crabs resulting from being stuck in the gear for extended periods of time.
The Atlantic States Marine Fisheries Commission recently updated their “Best Management Practices for Handling Horseshoe Crabs for Biomedical Purposes,” which includes how to best handle, transport and store them to minimize injury (like broken telsons) and mortality.
Because bycatch is not an intentional collection, the chances of horseshoe crabs being mishandled when discarded are higher than the regulated industries. Next steps will be determined when more research is conducted.
Horseshoe crabs have survived for millions of years, and are one of the most resilient creatures on earth.
“You can never truly get the real feel and appreciation for this unless you witness it in person... it's just not the same as being there with an arm’s reach of it all.”Fisheries biologist Jordan Zimmerman on horseshoe crab spawning events.
Even horseshoe crabs that have potentially fatal injuries, like broken telsons or large chunks missing from their shells, can still be observed engaging in spawning activities.
Data collection aside, it is a sight to be seen.
Zimmerman says taking part in the spawning survey remains one of the most unique experiences one can have in Delaware.
“You can never truly get the real feel and appreciation for this unless you witness it in person,” said Zimmerman. “Everybody's probably seen the pictures of huge spawning events, but it's just not the same as being there with an arm’s reach of it all.”
The 2022 Delaware Bay Spawning Survey yielded positive results – part of an upward trend in horseshoe crab abundance and male-female sex ratios in the region. The estimate of female spawners was the fourth highest, and the estimate of males was the third highest since 1999.
While the final 2023 Delaware Bay Spawning Survey results have yet to be released, factors such as the fair weather, high volume of horseshoe crab eggs found on beaches, and overall density of horseshoe crabs observed during the surveys bring high hopes for another good year for the ancient crustacean.
