Smith et al.: Abundance of Limulus polyphemus in Delaware Bay 



457 



Thus, studies to estimate abundance are 

 an important step in the process of man- 

 aging horseshoe crabs to meet the ener- 

 getic needs of shorebirds. 



We used results from a bay-wide mark- 

 recapture effort during spring 2003 to 

 estimate horseshoe crab abundance. Un- 

 derlying assumptions of mark-recapture 

 methods were accounted for in our study 

 design and were evaluated during analy- 

 sis. Recapture effort was distributed over 

 Delaware Bay beaches by involving par- 

 ticipants of a bay-wide spawning survey. 

 We related our abundance estimates to 

 reported landings to assess the recent 

 harvest rate and used published fecun- 

 dity estimates to predict egg production. 



Materials and methods 



We captured, tagged, and released horse- 

 shoe crabs from boats during two peri- 

 ods in 2003; a prespawning-season period 

 (abbreviated to "preseason" in this article) 

 from 26 March to 8 May and a prepeak- 

 spawning period ( abbreviated to "prepeak" ) 

 from 28 to 30 May 2003. The numbers of 

 crabs tagged were 7221 and 10,322 during 

 the preseason period and prepeak period, 

 respectively. The target population was 

 adult horseshoe crabs that were present in 

 Delaware Bay to spawn. One boat fished 

 throughout the bay during the preseason 

 period, and three boats fished nearshore 

 during the prepeak period (Fig. 1). Pre- 

 peak period captures took place within 

 strata of equal length along the Dela- 

 ware Bay shoreline (Fig. 1). During the 

 prepeak period, two boats fished in New 

 Jersey, but only one boat fished in Dela- 

 ware because of a lack of funding. Because 

 of the additional boat in New Jersey, fish- 

 ing effort and the number of crabs tagged 

 were higher in New Jersey. Any animal 

 injured during capture was culled and not tagged. Adult 

 males and females were tagged with standard button 

 tags. Tags were 4.4 cm in diameter, bore a unique tag 

 number and carried instructions on the tag for report- 

 ing a captured tag. Further detail on tags and tagging 

 methods are described in Brousseau et al. (2004). 



Recaptured tags came from the Delaware Bay spawn- 

 ing survey during the peak period of spawning (29 

 May, 31 May, and 2 June 2003). Although the spawning 

 survey was conducted during spring tides in May and 

 June, the recapture period for abundance estimation 

 was limited to the peak spawning period to help satisfy 

 the assumption of population-closure during the time of 

 mark-recapture study (Borchers et al., 2002). Spawning 

 survey volunteers were instructed to count all horse- 



\ 



38 45'0"N- 



New Jersey 



39 15'0"N 



•3845'0"N 



75°15'0"W 



75"0'Q"W 



Figure 1 



Locations in Delaware Bay where horseshoe crabs {Limulus 

 polyphemus) were tagged and released. Size of symbols are 

 graduated to reflect size of catches, and symbol style reflects 

 whether releases occurred during the prespawning season 

 period (26 March to 8 May 2003: triangle) or prepeak-spawning 

 period (28-30 May 2003; circle). White lines perpendicular 

 to the shoreline indicate strata within which catch effort 

 during prepeak-spawning period was equally distributed. 

 Black lines along the shore indicate approximate locations 

 of spawning areas. 



shoe crabs in sample quadrats and record tags that they 

 encountered inside and outside of the sample quadrats. 

 Flashlights were used when the survey occurred after 

 dark. During the spring tide period around the new 

 moon (29 May to 2 June 2003), 23 beaches were sur- 

 veyed throughout Delaware Bay (Smith and Bennett'^; 



Smith, D. R., and S. Bennett. 2004. Horseshoe crab 

 spawning activity in Delaware Bay: 5 years of a standard- 

 ized and statistically-robust survey, 10 p. Report to the 

 ASMFC Horseshoe Crab Management Board, Atlantic States 

 Marine Fisheries Commission, Washington DC. Website: 

 http://www.lsc.usgs.gov/aeb/2065/isa%20report%2005.pdf 

 [accessed on 21 October 2005]. 



