936 



Fishery Bulletin 101(4) 



Abundance estimates by stage class provided additional 

 information. Multiparous males were estimated at 2.40 

 million for day sampling and 4.23 million for night sam- 

 pling. Multiparous females were the next most abundant 

 group, estimated at 1.63 million for day sampling and 2.25 

 million for night sampling (Table 3). 



Primiparous males were uncommon during daylight 

 sampling, estimated at only 84,000 during the day, as com- 

 pared to 307,000 at night. In contrast, primiparous females 

 were estimated at 338,000 and 361,000 for day and night 

 sampling, respectively. 



The estimated abundance of mature males (primiparous 

 and multiparous combined) exceeded that of mature fe- 

 males: 2.48 million to 1.97 million for sampling during the 

 day and 4.54 million to 2.61 million for night sampling. Es- 

 timates of immature horseshoe crabs showed that the op- 

 posite trend with greater numbers of females than males, 

 1.34 million to 0.38 million, respectively, for day sampling 

 and 2.31 million to 1.19 million, respectively, for night sam- 

 pling. With both mature and immature horseshoe crabs, 

 estimates derived from night sampling were higher than 

 those derived from day sampling (Table 3). 



Confidence intervals for the estimates were wide, but in- 

 formative. Confidence limits for total horseshoe crab abun- 

 dance were 2.29 million to 11.33 million for day sampling 

 and 5.95 million to 16.85 million for night sampling. The 

 lower confidence limits provide useful reference points for 

 conservative, risk-averse management schemes. 



Discussion 



The study does not estimate actual population size, but 

 rather the total number of horseshoe crabs available to 

 the survey gear. Horseshoe crabs remain at the beaches 

 where they were spawned for the first one to two years of 

 life and gradually disperse offshore as they grow (Rudloe, 

 1981; Shuster, 1982). Crabs of these early age classes were 

 undoubtedly in shallow shelf waters and coastal embay- 

 ments beyond the reach of the vessel. Even if they were 

 present, crabs of early age classes may have been too small 



to be caught in the gear The study also excluded adults 

 that may have been in shallow waters and embayments. 

 It is also unlikely that 100% of the horseshoe crabs under 

 the gear were in fact captured because some may have 

 been buried too deep in the substrate to have been dug 

 out by the gear For all of these reasons, abundance esti- 

 mates can legitimately be considered minimum population 

 estimates. Results can be used as abundance indices for 

 comparison between years, if the study is continued in the 

 future. 



The differences between day and night estimates may be 

 the result of horseshoe crabs burying themselves during 

 the day. Alternatively, the horseshoe crabs may be able to 

 detect and avoid the trawl during the day. Night and day 

 collections at individual locations were correlated (r=0.71) 

 suggesting that both were a true reflection of horseshoe 

 crab abundance at that site, although at different levels of 

 efficiency. If the catches were uncorrelated, it would not be 

 possible to determine which, if either, sample accurately 

 represented true abundance. The larger catches and lower 



