Delaware Bay Horseshoe Crab management on the verge of an historic decision

Posted by Conor McGowan


?? Conor P. McGowan

The story is widely known in shorebird circles. Shorebirds, especially the rufa subspecies of Red Knots, stopover in the Delaware Bay during their epic migration from South America to arctic Canada to feast (or at least try to) on horseshoe crab eggs during the massive annual crab spawning of late May. The crabs crawl up the beach by the millions, dig nests, and leave behind a mess of billions of edible and easily digestible tiny green eggs. Unregulated harvest of these crabs for use as bait in eel and whelk fisheries during the late 1980???s and much of 1990???s resulted in steep declines of crab eggs on Delaware Bay beaches, which was concurrent with observed declines in Red Knot abundance. The wintering population in Tierra del Fuego fell from up to 90,000 birds to just 15,000 over just a few years, raising dire concerns for the birds, the stopover in the Bay and for the crab populations themselves. In 1998 the Atlantic States Marine Commission, the governing body over many interstate fisheries regulations on the Atlantic coast of the US, began applying harvest restrictions in large part because of concerns over shorebird populations. The regulations became increasingly more restrictive throughout the 2000???s and for the last few years, Delaware and New Jersey have been allowed to harvest only 100,000 male crabs and no female crabs, down from millions of crabs in 1997.

Since 2007 a team of scientists from New Jersey, Delaware, Virginia Tech University, the US Fish and Wildlife Service, and the USGS have been work with various stakeholders (the states, environmental groups, etc.) to develop a management plan for Horseshoe Crab harvest that explicitly incorporates population size targets for Red Knots. The team has developed an adaptive management plan that uses models of population dynamics for both species with explicit ecological and demographic links between the two species. Working with the stakeholder states the team devised a central objective statement, a list of possible harvest actions, and a set of alternative hypotheses of system function (alternative models). The Adaptive management modeling uses the population dynamics models to evaluate each management action and determine which management actions are most likely to achieve the stated objectives given our uncertainty about system function. The objectives call for a maximization of Horseshoe Crab harvest as long as a minimum Red Knot population threshold has been exceeded. A major benefit of this approach is that the explicit modeling allows managers and researchers to acknowledge key uncertainties about how the ecosystem functions (i.e., species interactions between crabs and knots). With uncertainties exposed risk averse management actions can be enacted while working to reduce those uncertainties. By comparing model predictions to the observed species abundance under management actions (in this case crab harvest) scientists and managers can learn about the quality of the models they???ve developed, potentially improve those models and make better management decisions in the future with those improved models.

The use of two linked population models for making predictions about the effects of harvest management actions is unprecedented.?? ??It marks the first time, under the Atlantic States Marine Fisheries Commission, that the needs of a shorebird population are being given equal status as the commercially harvested species and if the plan is enacted, this should be considered a major victory for wildlife conservation, especially shorebird conservation. The ASMFC Horseshoe Crab Management Board will vote to give final approval, or not, to the adaptive harvest management plan for the Delaware Bay at their August meeting in 2010.??


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