Along Delaware Bay’s shores, something extraordinary occurs every May. One of nature’s most unlikely arrangements, horseshoe crabs crawling ashore to spawn and leaving behind a thick carpet of pale green eggs just beneath the sand, attracts thousands of shorebirds to the beaches. This stop is essential for the red knot, a robin-sized shorebird finishing one of the longest migrations on the planet. It’s survival.
But that arrangement is collapsing. Furthermore, New Jersey Audubon’s Vice President of Research and Monitoring, ecologist Dr. David Mizrahi, has spent decades closely observing it.
It’s difficult to look away from the numbers. Over 94,000 red knots were observed congregating on Delaware Bay during spring migration in 1989, according to aerial surveys. That number had fallen to about 6,800 by 2021. In 2022, there was a slight increase to slightly over 12,000, which was still less than half of the peak count of 30,000 in 2019. These are not positive trends for a species that has been classified as “threatened” under the Endangered Species Act since 2015. They are cautionary indicators.
Red knots travel to breeding grounds in the Arctic from as far south as Tierra del Fuego, at the tip of South America. Almost precisely in the center of that journey is Delaware Bay. After arriving weary and underweight, the birds must almost double their body mass in a few weeks before continuing northward. This is made possible by the horseshoe crab egg, which is calorie-dense, plentiful, and historically so abundant that eggs spilled out of the sand onto the surface. In the 1990s, researchers found almost 50,000 eggs per square meter on Bay beaches. That density had dropped to about 7,000 by May 2022, falling well short of even the lowered benchmarks that scientists had anticipated.
According to Mizrahi, the management of horseshoe crabs over the past 20 years has “adversely affected the entire near-shore tidal ecosystem of Delaware Bay.” That’s cautious language from a cautious scientist, but it’s not subtle.
Horseshoe crab populations have been reduced by decades of overharvesting, mainly for use as bait in the conch and eel fishing industries. As a result, the spawning events that once produced a consistent feast for migratory birds are now spottier, shorter, and unpredictable. During lunar tides, spawning sometimes lasts only a few days instead of weeks as it once did.
Research reports and survey data give the impression that the window is closing more quickly than the policy responses can keep up. Red knots equipped with satellite transmitters have revealed that some birds completely avoid Delaware Bay, eschewing the stop that evolution incorporated into their migration. This is probably because the food reward no longer warrants the detour. That signal is concerning. There is nothing comparable to what Delaware Bay used to offer at other stopover locations.
Nor is the threat limited to the past. In 2022, the Atlantic States Marine Fisheries Commission reviewed a proposal that would have overturned a long-standing prohibition on using female horseshoe crabs—the very crabs whose eggs support this entire system—for bait. Defenders of Wildlife and Earthjustice, among other conservation organizations, cautioned that the action would most likely make the situation worse. In the words of Christian Hunt of Defenders of Wildlife, “the red knots arriving in Delaware Bay are starving to death, and loosening protections now would push a threatened species closer to a point where recovery becomes impossible.”
Whether the combined pressure of public awareness, research, and advocacy will be sufficient to change management choices at the rate the birds need is still up for debate. Over the course of more than 20 years, Mizrahi and his associates have developed a strong, data-rich case. There is no question about the science. Whether the laws governing the harvesting of horseshoe crabs can change fast enough to be significant is still up for debate.
Delaware Bay is not an isolated wilderness. Every spring, birdwatchers and researchers travel to this location, which is near major cities between New Jersey and Delaware. Satellite tags are being used to document, map, and track the red knot’s crisis as it occurs in plain sight. The idea that a species might be on the verge of extinction on a bay that millions of people live close to and could, in theory, decide to protect rather than in some remote part of the world is unsettling.
According to Dr. Mizrahi’s research, the biology may still be salvageable. The part that no one can model is whether politics are timely.
