456 



Abstract — Estimates of the abun- 

 dance of American horseshoe crabs 

 (Limulus polyphemus) are important 

 to determine egg production and to 

 manage populations for the ener- 

 getic needs of shorebirds that feed 

 on horseshoe crab eggs. In 2003, over 

 17,500 horseshoe crabs were tagged 

 and released throughout Delaware 

 Bay, and recaptured crabs came from 

 spawning surveys that were conducted 

 during peak spawning. We used two 

 release cohorts to test for a tempo- 

 rary effect of tagging on spawning 

 behavior and we adjusted the number 

 of releases according to relocation 

 rates from a telemetry study. The 

 abundance estimate was 20 million 

 horseshoe crabs (909c confidence 

 interval: 13-28 million), of which 6.25 

 million (90% CI: 4.0-8.8 million) were 

 females. The combined harvest rate 

 for Delaware, New Jersey. Virginia, 

 and Maryland in 2003 was 4^ (90* 

 CI: 3-6%) of the abundance estimate. 

 Over-wintering of adults in Delaware 

 Bay could explain, in part, differences 

 in estimates from ocean-trawl sur- 

 veys. Based on fecundity of 88,000 

 eggs per female, egg production was 

 5.5x10" (90% CI: 3.5x10", 7.7x10"), 

 but egg availability for shorebirds also 

 depended on overlap between horse- 

 shoe crab and shorebird migrations, 

 density-dependent bioturbation, and 

 wave-mediated vertical transport. 



Abundance of adult horseshoe crabs 

 (Limulus polyiphemus) in Delaware Bay 

 estimated from a bay-wide mark-recapture study 



David R. Smith^ 



Michael J. Millard^ 



Sheila Eyler^ 



' United States Geological Survey Leetown Science Center 

 11649 Leetown Road 

 Kearneysville, West Virginia 25443 

 E-mail address ((or D R Smith) drsmitli g'usgs gov 



^ U.S. Fish and Wildlife Service 

 Northeast Fishery Center 

 308 Washington Avenue 

 Lamar, Pennsylvania 16848 



3 US- Fish and Wildlife Service 

 Maryland Fisheries Resources Office 

 177 Admiral Cochrane Drive 

 Annapolis, Maryland 21401 



Manuscript submitted 29 March 2005 

 to the Scientific Editor. 



Manuscript approved for publication 

 21 October 2005 by the Scientific Editor. 



Fish. Bull. 104:456-464(2006). 



The American horseshoe crab (Lim- 

 ulus polyphemus), having persisted 

 largely unaltered for millions of years, 

 is now central to a modern manage- 

 ment debate of conflicting interests by 

 commercial watermen, birdwatchers, 

 biopharmaceutical companies, and 

 eco-tourists (Odell et al., 2005). The 

 management controversy is most acute 

 in the Delaware Bay region where the 

 high abundance of horseshoe crabs 

 has resulted in the dependence of 

 migrant shorebirds on horseshoe crab 

 eggs to fuel their northern migration 

 to nesting grounds (Botton and Har- 

 rington, 2003). 



Because management goals have 

 linked the horseshoe crab fisheries 

 to the viability of other species, such 

 as migrant shorebirds (ASMFC), it 

 is not sufficient to manage on the ba- 

 sis of typical reference points, such 

 as maximum sustainable yield, be- 

 cause sustainable harvest is not the 

 primary issue. In Delaware Bay, the 

 viability of shorebirds has taken pre- 

 cedence in decision making (Botton 

 and Harrington, 2003; Baker et al., 

 2004). Effective management should 

 reference a critical threshold of horse- 

 shoe crab abundance that provides 

 sufficient eggs and should sustain 



that threshold. Thus, abundance be- 

 comes a critical parameter because 

 abundance estimates are useful for 

 predicting the egg biomass that is 

 available to migrant shorebirds and 

 for assessing harvest rate. 



Horseshoe crabs bury their eggs in 

 beach sediment, typically 15 to 20 cm 

 deep (Shuster and Sekiguchi, 2003). 

 Eggs are exhumed to the beach sur- 

 face and become available to forag- 

 ing shorebirds through a process of 

 entrainment in activated sediment, 

 followed by vertical transport into 

 surface sediments. Nest disturbance, 

 which precedes entrainment of eggs, 

 is predominantly due to bioturba- 

 tion; whereas wave energy is only a 

 contributing factor because typical 

 estuarine waves do reach nest depth 

 (Jackson et al., 2005). Given a rela- 

 tionship between egg exhumation and 

 spawning density, egg availability 

 could be predicted from current and 

 projected horseshoe crab abundance. 



ASMFC (Atlantic States Marine Fish- 

 eries Commission). 1998. Interstate 

 fishery management plan for horseshoe 

 crab. Fisherv Management Report No. 

 32, 58 p. ASMFC, 1444 Eye Street, NW, 

 Sixth Floor. Washington, DC 20005. 



