Horseshoe Crab Harvest and Red Knot Conservation

USGS Cooperative Research Unit Corner

Horseshoe Crab Harvest and Red Knot Conservation

Since 2010, the US Geological Survey (USGS) Alabama Cooperative Fish and Wildlife Research Unit, with expertise in shorebird ecology and structured decision making/adaptive management, has been supporting state and federal efforts to manage horseshoe crab harvest in a sustainable manner. The unit has developed predictive models that link red knot populations to horseshoe crabs, formalizing and quantifying the hypothesized relationship.

Every spring, on the beaches of Delaware Bay, one of nature's great marvels occurs as shorebirds from across the Western Hemisphere converge on the Bay to rest and recuperate during their epic northward migration to their Arctic breeding grounds. The timing of shorebird stopover coincides with the massive spawning events of the Atlantic horseshoe crabs which spawn in Delaware Bay in much greater densities than anywhere else in the world. On a full moon night, out on Slaughter Beach, Pickering Beach, Ted Harvey Beach, or dozens of other places in Delaware Bay, there is a virtual carpet of spawning horseshoe crabs, with sometimes more than 50 horseshoe crabs per square meter in a 10-meter-wide spawning mass that stretches a kilometer or more down the beach. The horseshoe crabs leave behind billions of eggs on the sandy beaches and the shorebirds come to the Bay each spring to exploit these food resources during migration.

In the early 2000's one shorebird species, the red knot (Calidrus canutus rufa), exhibited steep apparent population declines in the winter grounds and in Delaware Bay, and many scientists and conservationists hypothesized that the population collapse was caused by unregulated horseshoe crab harvests in the Delaware Bay region. To effectively manage horseshoe crab harvest sustainably and subsequently to help support migrating shorebird populations, the states of New Jersey and Delaware began implementing harvest restrictions and the Atlantic States Marine Fisheries Commission (ASMFC) began to regulate and monitor harvest in 1998. However, the core management hypothesis, that red knot and other shorebird populations and demographics are regulated by horseshoe crab egg availability on Delaware Bay beaches was not fully studied or effectively quantified.

The Alabama Coop Unit has been working in collaboration with the US Fish and Wildlife Service, USGS Leetown Science Center and Patuxent Wildlife Research Center, New Jersey Department of Environmental Conservation, Delaware Department of Natural Resources and Environmental Conservation, Maryland Fish and Game, Virginia Tech University, and the Atlantic States Marine Fisheries Commission with consultation from the British Trust for Ornithology. Together the team devised analyses of red knot banding data to estimate red knot weight gain during stopover in Delaware Bay and then annual survival based on a bird's weight upon departing the Bay, linking those to horseshoe crab abundance. The team used the results and other studies in scientific literature, to build computer models that compared the effects of different harvest actions on the horseshoe crab and red knot populations. The team also worked with stakeholders (e.g., fishermen representatives, conservation groups) to elicit specific and measureable management objectives.

With the data they gathered, the USGS Alabama Coop Unit then used the process of structured decision making to determine which harvest action best achieved the stated objectives. Because uncertainty persisted about the magnitude of the interaction between red knots and horseshoe crabs despite extensive data analysis, the team worked to design and implement a formal adaptive management strategy. An adaptive management plan, like the one developed for Delaware Bay horseshoe crab harvest, provides managers with the set of management actions that will most likely achieve all the management objectives even though there is still uncertainty about how the managed system functions or will respond to those management actions. The analysis and modeling recommended a male only harvest of at most 500,000 horseshoe crabs.

After extensive scientific peer review, public review and public comment, the ASMFC accepted and adopted the adaptive management plan in 2012 and implemented the first harvest recommendation from the plan in the fall of 2013. The Alabama Coop Unit continues to be involved with annual decision process because there are ongoing needs to analyze monitoring data to assess the abundance of both species then make harvest recommendations and also to continue conducting research on the crabs and birds in Delaware Bay and their ecological interactions. More detailed results from the Alabama Coop Unit's data collection and adaptive management planning process will be published in the November 2015 edition of the journal Biological Conservation, the abstract is available online.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Conor McGowan, Assistant Unit Leader, USGS Alabama Cooperative Fish and Wildlife Research Unit at Auburn University.

September 14, 2015