NOTE Hata and Berkson: Abundance of Limulus polyphemus in the Delaware Bay area 



937 



coefficients of variation from the night estimates suggest 

 that the night estimates are more efficient and are prob- 

 ably better estimates of true abundance. 



The results of the present study are intermediate be- 

 tween previous estimates of ocean abundance. Botton and 

 Ropes (1987a) estimated that between 2.3 and 4.5 million 

 adults occurred on the continental shelf between New Jer- 

 sey and Virginia from National Marine Fisheries Service 

 (NMFS) trawl surveys, in contrast to a mean of 7.1 million 

 adults (primiparous and multiparous combined) estimated 

 in the present study area. However, the trawl gear used in 

 the NMFS surveys was inefficient for capturing horseshoe 

 crabs, and the inshore extent was limited by the survey 

 vessel size (Botton and Ropes, 1987a; ASMFC^). Botton 

 and Haskin (1984) sampled within 5.6 km of the New 

 Jersey coast using hydraulic clam dredges and obtained 

 horseshoe crab densities of 14,600 to 23,000 per km'^. These 

 densities are much higher than our nighttime estimate of 

 7900 horseshoe crabs per km^ (weighted by stratum area) 

 within 5.6 km. The gear we used was probably more effi- 

 cient in capturing horseshoe crabs than that employed by 

 the NMFS survey but may have been less efficient than the 

 hydraulic dredge. Differing methods between the studies 

 do not allow for a comparison over time. 



It is interesting to note that in both the night-based and 

 day-based estimates, females made up the majority of the 

 immature animals, whereas males made up the majority 

 of the mature animals. This could be due to the commercial 

 fishery's preference for harvesting gravid females (Botton 



and Ropes, 1987b). The continual focused harvest of mature 

 females may reduce their population enough to cause this 

 change in sex ratios. Alternatively, mature females or im- 

 mature males may have been more abundant outside the 

 study area. 



Conclusion 



The continuation of annual trawl surveys could allow a 

 full stock assessment to be conducted. The Horseshoe Crab 

 Stock Assessment Subcommittee of the Atlantic States 

 Marine Fisheries Commission has developed a stock 

 assessment plan (HCSAS^) based on the catch-survey 

 method derived by Collie and Sissenwine (1983). Unlike 

 age-based stock assessment models, the catch-survey 

 method requires only abundance of primiparous and 

 multiparous horseshoe crabs (HCSAS^). The commercial 

 fishery is selective for gravid females (Botton and Ropes, 

 1987b), and effort is biased toward areas of high abundance 

 (Burke''; Eutsler''; Munson^); therefore commercial data are 

 of limited use for stock assessment. A fishery-independent 



HCSAS (Horseshoe Crab Stock Assessment Subcommittee). 

 2000. Stock assessment of Atlantic coast horseshoe crabs: a 

 proposed framework, 19 p. A report to the Horseshoe Crab 

 Technical Committee, Atlantic States Marine Fisheries Commis- 

 sion, 1444 Eye Street, NW, Sixth Floor, Washington, DC 20005. 



