216 



Fishery Bulletin 104(2) 



the mortality from the bleeding process was estimated 

 to be 7.5% (Walls and Berkson, 2003). Currently, there 

 is no substitute for LAL that offers comparable speed 

 and sensitivity. 



Horseshoe crabs also play an important role in ma- 

 rine and terrestrial food webs (Botton and Shuster, 

 2003). Shorebirds migrating from South America to 

 Arctic breeding grounds stop in the Delaware Bay to re- 

 build depleted energy reserves (Botton et al., 1994). The 

 time and place of their stop-over coincides with that of 

 annual horseshoe crab breeding, when the crabs arrive 

 en masse to spawn on sandy beaches during high tides 

 of May and June (Botton and Harrington, 2003). An 

 adult female horseshoe crab lays approximately 88,000 

 eggs per year (Shuster, 1982), and a single red knot 

 (Calidris canutus) can consume an estimated 18,000 

 crab eggs daily (USFWS-^). 



Horseshoe crabs account for substantial economic 

 value in the Delaware Bay region. Regional economic 

 contribution of the eel and conch fisheries is approxi- 

 mately $2.2 to $2.8 million annually. The regional eco- 

 nomic value of the horseshoe crab biomedical industry 

 is $26.7 to $34.9 million annually. Ecotourism related 

 to migrating shorebirds has become increasingly im- 

 portant to the economy of the Delaware Bay region. An 

 estimated 6,000 to 10,000 recreational bird watchers 

 visit the Delaware Bay in spring and contribute $6.8 

 to $10.3 million to the regional economy. 



Recently, there has been concern that the horseshoe 

 crab population can fulfill the current needs of these 

 user groups. These concerns led the Atlantic States 

 Marine Fisheries Commission (ASMFC) to develop a 

 fishery management plan for horseshoe crabs. Unfortu- 

 nately, very few abundance data are available for this 

 species. Many state and federal trawl surveys record 

 horseshoe crabs caught during sampling, but gear and 

 sampling methods are not designed for horseshoe crabs 

 and catches are not common. Only recently have sta- 

 tistically robust horseshoe crab-specific surveys been 

 initiated: a Delaware Bay spawning survey (Smith et 

 al., 2002) and an offshore trawl survey (Hata and Berk- 

 son, 2003). 



Because of the limited data available, previous stock 

 assessments of the Delaware Bay population have been 

 restricted to trend analyses to determine whether a 

 single survey identifies a significant change in the pop- 

 ulation or whether there is a consensus among data 

 sets. However, many Delaware Bay surveys have high 

 variability and low power to detect population change 

 and would therefore only be able to identify dramatic 

 changes in population size. Also, trend analyses do not 

 provide estimates of stock status (Caddy, 1998) such as 

 relative biomass (B/Syt,.y) and relative fishing mortality 

 iF/fusY^- 'I'^^ ultimate goal for horseshoe crab assess- 

 ment employs a stage-based catch-survey methodology 



(Collie and Sissenwine, 1983; HSC-SAS^), incorporating 

 data from harvest and surveys. It will be a number of 

 years before this modeling approach can be implement- 

 ed, however, since stage-class data from commercial 

 harvest are not currently being collected. 



The surplus production modeling approach (Prager, 

 1994) used in our study is an appropriate bridge be- 

 tween these two methods. Production models allow for 

 the incorporation of harvest data and multiple surveys, 

 improving predictive power over that of a single survey. 

 This technique does not include a stage-structure in the 

 model; instead it focuses on the dynamics of the popula- 

 tion as a whole. Similar methods have been successfully 

 applied to horseshoe crab data from Rhode Island (Gib- 

 son and Olszewski'') and production models have been 

 widely used for assessments of other species (Booth and 

 Punt, 1998; Cadrin and Hatfield, 2002; Vaughan and 

 Prager, 2002). Surplus production models assume a low 

 population growth rate at small population sizes and as 

 the population nears the carrying capacity (Quinn and 

 Deriso, 1999). In the logistic growth form of the model 

 used in the present study, maximum growth rate (and 

 maximum surplus production) occurs at one-half of the 

 carrying capacity. At this point, the maximum surplus 

 population growth can be harvested while still main- 

 taining a stable population size. Surplus production 

 models provide estimates of maximum sustainable yield 

 (MSY; the largest harvest that can continuously be 

 removed from a stock), population biomass, and fishing 

 mortality, as well as allow for the estimation of effects 

 of future management. 



We fitted a regional-scale production model to Dela- 

 ware Bay horseshoe crab data in order to quantify the 

 current status of the Delaware Bay population and 

 to estimate impacts of future management actions. 

 The results from this production model will allow the 

 ASMFC and member states to manage the Delaware 

 Bay population of horseshoe crabs more effectively with 

 the goal of providing a sustainable resource for com- 

 mercial harvest, the biomedical industry, and migrating 

 shorebirds. 



Methods 



Production model 



We used an age-aggregated production model with the 

 Prager (1994) form of the Graham-Schaefer surplus-pro- 

 duction model (i.e., logistic population growth). 



^ USFWS( U.S. Fish and Wildlife Service). 2003. Delaware 

 Bay shorebird-horseshoe crab assessment report and peer 

 review. Migratory Bird Publication R9-03/02, 107 p. USFWS, 

 4401 N. Fairfax Dr., MBSP 4107, Arlington, VA 22203. 



^ HSC-SAS (Horseshoe crab stock assessment subcom- 

 mittee). 2000. A conceptual framework for the assess- 

 ment of horseshoe crab stocks in the mid-Atlantic region, 

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

 ington, DC 20005. 



^ Gibson, M., and S. Olszewski. 2001. Stock status of horse- 

 shoe crabs in Rhode Island in 2000 with recommendations 

 for management, 13 p. RI Division of Fish and Wildlife, 

 4808 Tower Hill Rd., Wakefield, RI 02879. 



