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1 July 2009 Response from Niles and colleagues
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These crab harvest data confirm the scenario we described, though the increase started earlier (1989, not 1991). Mean annual landings on the US Atlantic coast from 1970 to 1988 (211 metric tons [mt]) were less than 2006 landings in four states (352 mt), indicating that recent harvest exceeds historic harvest. However, pre1998 data may be underestimates because of nonmandatory reporting.

We presented egg-density data for New Jersey to show the long-term trend. Data for Delaware are not sufficiently long term to reflect changes caused by the increased crab harvest. Recent surveys show egg densities are higher in Delaware than in New Jersey. We cannot explain this, as it is not reflected by spawning crab densities (Niles et al. 2008).

We agree that several surveys have documented horseshoe crab population trends. Overwhelmingly, they show the same decline as the Delaware 30-foot trawl survey did, which is most relevant as it measures trends within the Delaware Bay.

Several measures have been taken to control the horseshoe crab harvest; however, especially in the 1990s, these proved insufficient to arrest the decline because they failed to take adequate account of the species' long maturation. While the crab population may be on the cusp of recovery, as of 2008 only the offshore trawl showed an increase; spawning females on Delaware Bay beaches (most relevant to egg densities and shorebirds) have not increased (Michels et al. 2009). Moreover, the Delaware Bay 30-foot trawl survey showed a decline in 2008 (Stewart F. Michels, Division of fish and Wildlife, Delaware Department of National Resources and Environmental Control, Dover, personal communication, 3 June 2009). Recovery of shorebirds requires a sustained increase of the crab population to mid-1990 levels.

Laursen and colleagues (2008) studied aerial counts in 850 square kilometers of a highly fragmented wetland. Five species showed no statistical difference between ground and aerial counts, so Laursen and colleagues are generally supportive of the accuracy of aerial counts. Delaware Bay and Tierra del Fuego have simple linear coastlines that allow accurate counting, especially by the experienced personnel involved.

In response to a petition to list the rufa subspecies of the red knot under the Endangered Species Act, the US Fish and Wildlife Service (FWS) commissioned an assessment of all threats to its population. The review by 22 authors (Niles et al. 2008) identified factors that could have an impact on rufa breeding, wintering, and stopover sites. Nevertheless, the FWS concluded, “The primary factor threatening the red knot is destruction and modification of its habitat, particularly the reduction in key food resources resulting from reductions in horseshoe crabs” (73 CFR 238). It is unreasonable to suggest that 22 authors and FWS staff should be so influenced by a “ruling theory” as to reach an erroneous conclusion.

We agree red knots may not increase immediately even if horseshoe crabs are restored, but we expect they will. Niles and colleagues (2008) considered the impacts of climate change, suggesting a potential short-term benefit from a longer Arctic breeding season; however, major habitat loss may result as vegetation zones shift northward.

We mentioned the adaptive management initiative. We agree this effort has since made progress, but it is, as yet, unfinished. In the meantime, we reiterate our recommendation for a risk-averse approach to the management of the horseshoe crab harvest.

References cited

1.

K Laursen, J Frikke , J Kahlert . 2008. Accuracy of ‘total counts’ of waterbirds from aircraft in coastal waters. Wildlife Biology 14: 165–175. Google Scholar

2.

SF Michels , D Smith , S Bennett . 2008. Horseshoe Crab Spawning Activity in Delaware Bay: 1999–2007: Report to the Atlantic States Marine Fisheries Commission's Horseshoe Crab Technical Committee. Atlantic States Marine Fisheries Commission. Google Scholar

3.

LJ Niles , et al. 2008. Status of the red knot (Calidris canutus rufa) in the Western Hemisphere. Cooper Ornithological Society. Studies in Avian Biology 36. (4 June 2009;  myfwc.com/docs/Conservation/FBCI_StatusoftheRedKnotpdfGoogle Scholar
Published: 1 July 2009
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