Smith et al : Abundance of Limulus polyphemus in Delaware Bay 



463 



Mark-recapture studies have been frequently applied to 

 marine species for describing migration or estimating 

 mortality (Hoenig et al, 1998; Bacheler et al., 2005). 

 Mark-recapture methods are used infrequently for abun- 

 dance estimates of marine species, with the exception 

 of anadromous species, whose spawning migration con- 

 centrates the population and enhances opportunities 

 for recapture (Schwarz and Taylor, 1998). Similarly, 

 the unusual horseshoe crab spawning migration and 

 behavior concentrated the population and made them 

 assessable for recapture. The validity of our abundance 

 estimates is founded on a large number of tag releas- 

 es and animals checked for tags, a study design that 

 ensured population closure, an adequate number of 

 samples to represent the population, and an evaluation 

 of the underlying assumptions. 



The sex ratio in our fishery-independent catch (69% 

 M: 31% F), which we believe is a representative sample 

 of adults in the bay at the time of peak spawning, was 

 similar to the sex ratio in the landings (68% M: 32% F). 

 Hata and Berkson (2003) observed a similar sex ratio 

 among adults in an offshore trawl survey (63% M: 37% 

 F). Although, commercial landings in 2003 were not 

 skewed toward the harvest of females, harvest could 

 have selected females disproportionately in past years. 



The horseshoe crab harvest has been reduced through 

 a series of reductions mandated by ASMFC (ASMFC^). 

 Although there is evidence of stock decline coinci- 

 dent with increased landings in the past 10-20 years 

 (ASMFC"*), the estimates presented in the present study 

 indicate recent regulatory changes had achieved a low 

 harvest level by 2003. Based on the abundance esti- 

 mates reported here, harvest rate in 2003 was 0.024 

 (90% CI: 0.02 to 0.04) for Delaware Bay state landings 

 and 0.04 (90% CI: 0.03 to 0.06) for Delaware Bay area 

 landings. In 2004, additional regulations were enacted, 

 which capped landings at 150,000 per state for Delaware 

 and New Jersey and prohibited harvest during May and 

 early June when migrant shorebirds stopover in Dela- 

 ware Bay. As a result of the 2004 regulations, landings 

 dropped to 173,023 (males and females) for Delaware 

 and New Jersey combined, which is a 63% drop from 

 2003 landings. Sex ratio of the 2004 landings was 68% 

 M: 32% F, consistent with the 2003 landings. 



Estimating abundance is an important step in the 

 process of determining the current capacity for horse- 

 shoe crab egg production in the bay and for managing 

 for the energetic needs of shorebirds. We estimate that 

 egg production in 2003 was 5.5x10" (90% CI: 3.5x10" 

 to 7.7x10"). The U. S. Fish and Wildlife Service Shore- 

 bird Technical Committee (USEWS^) estimated that a 



•• ASMFC (Atlantic States Marine Fisheries Commission). 

 2004. Horseshoe crab 2004 stock assessment report, 

 87 p. ASMFC, 1444 Eye Street, NW, Sixth Floor, Wash- 

 ington, DC 20005. 



= USFWS (United States Fish and Wildlife Service). 2003. Del- 

 aware Bay shorebird-horseshoe crab assessment report and 

 peer review. U.S. Fish and Wildlife Service Migratory Bird 

 Publication R9-03/02, 99 p. USFWS, 4401 N. Fairfax Drive, 

 Arlington, VA 22203. 



population of 423,000 shorebirds would require 1.07x10" 

 horseshoe crab eggs as they migrated through Delaware 

 Bay. This represents approximately 20% of all egg pro- 

 duction, which would have to be available temporally 

 and spatially during the shorebird migration. 



Egg availability for shorebirds will depend also on 

 overlap between horseshoe crab and shorebird migra- 

 tions, density-dependent bioturbation, and wave-medi- 

 ated vertical transport. Some important aspects of egg 

 production and the process of making eggs available to 

 foraging shorebirds are not well understood and have 

 not been quantified. For example, if fecundity is found to 

 be age- or size-related, then age or size would need to be 

 incorporated in a calculation of egg abundance. Also, the 

 process of bioturbation, which releases buried eggs to 

 the beach surface, is known to be related to the density 

 of spawning females, but has not been parameterized 

 (Jackson et al., 2002). Determination of the dietary re- 

 quirements of migrant shorebirds in terms of horseshoe 

 crab eggs coupled with reliable estimates of abundance, 

 fecundity, and bioturbation rates, will set the stage for a 

 management of horseshoe crabs that takes into account 

 the trophic support it provides in Delaware Bay. 



Acknowledgments 



We thank many individuals involved in the tagging 

 study and the Delaware Bay spawning survey. Principal 

 among them were Stewart Michels, Lome Brousseau, 

 Mike Oates, Bruce Freeman, Carl Shuster, Susan Love, 

 staff from the DE Coastal Program and the Delaware 

 National Estuarine Research Reserve (DNERR), Benjie 

 Swan, Bill Hall, Glenn Gauvry, Sherry Bennett, and 

 representatives from Audubon, The Nature Conservancy, 

 the Slaughter Beach folks, and the USFWS. Fund- 

 ing came from the USGS, NJ FG&W, and Delaware 

 Department of Natural Resources and Environmental 

 Control (DNREC). We appreciate comments from Stew- 

 art Michels, Greg Breese, and John Young on an early 

 draft of the paper. 



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