Horseshoe Crab Red Knots Delaware Bay


Delaware Bay is home to one of nature’s great wonders: the spring spawning of millions of horseshoe crabs and shorebirds that migrate thousands of miles to feed on the nutritious eggs of crabs en route to the Arctic.

But that wonder is slipping away, as a depleted population of horseshoe crabs can no longer provide the abundance of eggs shorebirds need to fuel their epic migratory journeys. One such shorebird species, the red knot, has been designated a threatened species under the Endangered Species Act, largely due to a major population crash due to the overexploitation of horseshoe crabs. In recent years, the number of Red Knots stopping in Delaware Bay during their spring migration has reached an all-time high.

Now, a regional fisheries commission has a plan that would make this situation even worse. While bait harvesting of female horseshoe crabs has been banned in Delaware Bay since 2012 due to dwindling crab numbers and its important connection to red knots, the Atlantic States Marine Fisheries Commission proposes to change the harvesting rules in a way that would allow resumption of a female harvest. The commission is advancing the controversial proposal despite growing public opposition and plans a critical vote in November.

The commission says sophisticated computer modeling supports the counterintuitive conclusion that harvesting more crabs, including females, would not harm the horseshoe crab population or the red knots that rely on crab eggs.

However, new independent scientific assessments show how erroneous and risky this proposition is. Scientists who evaluated the commission’s methods and findings determined that the modeling ignored red flags that the horseshoe crab population remains precarious, and they caution against increasing the harvest, especially of females. Among other warning signs, the scientists found that, despite a decade without female bait harvest, the recruitment of young females into the population has not rebounded, the proportion of females in the total population has not increased. and the average size of mature females has decreased, reaching its lowest level in the last three years of survey data. These results are the opposite of what a female harvest ban should accomplish and suggest that female crab mortality remains stubbornly high. These are strong warnings that it would be risky to reopen a female bait crop and add further mortality to the population.

Delaware alphabet: R for red knots.  These birds are attracted to Delaware by the abundance of protein-rich horseshoe crab roe.

The new scientific studies also revealed serious flaws in how the commission’s proposal predicts the impacts of increased harvest. Despite the well-established correlation between overexploitation of horseshoe crabs and the collapse of the knot population in the 1990s, the commission’s model assumes that there is little or no relationship between the fate of these two species. In fact, the commission’s computer model predicts that even if horseshoe crabs disappeared entirely today, red knot abundance would remain stable or even increase over the next 50 years. The model also inappropriately inflates its projections of crab population size: it fills gaps in survey data with absurd estimates that are well above the maximum level ever supported by direct measurements. Also, the population estimates from the model do not agree with actual trawl survey data for the horseshoe crab population. In fact, the model shows no correlation with the most reliable survey, the annual trawl conducted by Virginia Tech that measures the number of horseshoe crabs around Delaware Bay.

Based on these findings, independent scientists who reviewed the available information on the commission’s proposal concluded that it was not supported by the best available science. But the full computer model that underpins the new crop proposal has never been made public, despite repeated requests. The scientists’ analyzes revealed serious concerns about the model based on the available information, demonstrating that public verification of the full model is essential.

It is not in keeping with sound science or good government for the commission to approve a major expansion of horseshoe crab bait harvesting using a model that has been found to contain key flaws and omissions. – and which has never been fully disclosed to the public. Instead, with the red knot population at an all-time low and the crab population well below recovery from past overexploitation, the commission is expected to reject the proposal to expand the harvest.

David Mizrahi is vice president of research and monitoring at New Jersey Audubon. Ben Levitan is Senior Counsel for Earthjustice’s Biodiversity Advocacy Program. Christian Hunt is a senior federal land policy analyst at Defenders of Wildlife.


Comments are closed.