BOTTON and ROPES: POPULATIONS OF HORSESHOE CRABS 



catch. Mean abundance and biomass were nearly 

 an order of magnitude higher at the inshore 

 strata than the offshore strata, and horseshoe 

 crabs were at least twice as likely to be caught 

 inshore than offshore during all seasons (Table 2). 

 The operation of the hydraulic clam dredge was 

 limited to waters <80 m, and within this range, 

 crabs were again most abundant at shallower 

 depths. Sixty-two percent of the total catch was 

 found shallower than 20 m, and 90% was found 

 shallower than 30 m (Fig. 3). 



Ninety-six animals were caught in 19 tows 

 from 100 to 199 m depth, while 53 animals were 

 taken in 7 tows below 200 m (Table 3). These 

 deep-water individuals were caught between 

 34°21'N and 37°42'N (North Carolina to southern 

 Virginia), with the majority from Cape Hatteras, 

 south. 





o 



Trawl 

 Dredge 



150 



>200 



Depth (m) 



Figure 3. — Bathymetric distribution of horseshoe crabs, as per- 

 cent of total catch, based on bottom trawl data (solid line) and 

 ocean clam dredge data (dashed line). 



DISCUSSION 



Horseshoe crabs on the northwestern Atlantic 

 continental shelf were most abundant between 

 Virginia and New Jersey. On this section of the 

 shelf, the population was estimated to be some 

 2.3-4.5 million individuals and was relatively 

 constant between 1975 and 1983. The estimate of 

 inshore abundance was necessarily conservative, 

 because large survey vessels could not operate in 

 shallow water. Horseshoe crabs may be abundant 

 in areas inshore of the NEFC surveys; for exam- 

 ple, in New Jersey State surveys, stations <1.8 

 km from shore frequently have over 10 horseshoe 

 crabs/5-min hydraulic dredge tow in the Cape 

 May area (Botton and Haskin 1984), and in areas 



of Narragansett Bay, 20-min otter trawls con- 

 ducted by the State of Rhode Island have caught 

 up to 40 horseshoe crabs (R. Sisson'^). Both in- 

 shore and offshore stock estimates may also be 

 conservative because the trawls, which are 

 equipped with roller gear, may be <100% effi- 

 cient in sampling horseshoe crabs, particularly if 

 the animals are burrowed. 



Seasonal surveys of the middle Atlantic conti- 

 nental shelf indicate a decline in horseshoe crab 

 abundance during summer (July-August), which 

 is consistent with the hypothesis that the shelf 

 animals have seasonal inshore spawning migra- 

 tions (Shuster and Botton 1985). More animals 

 were caught during spring (April-early May) and 

 fall (September-early December) periods, repre- 

 senting the prespawning and postspawning sea- 

 sons, respectively. Therefore, the Virginia to New 

 Jersey shelf population probably consist largely 

 of individuals which spawn during the spring and 

 early summer in the Chesapeake and Delaware 

 Bays, and disperse offshore. 



The range of horseshoe crabs on the continental 



3R. Sisson, Division of Fish and Wildlife, Rhode Island De- 

 partment of Environmental Management, Wakefield, RI 02879, 

 pers. commun. December 1983. 



Table 3. — Occurrence of all horseshoe crabs below 100 m 

 depth on the continental shelf in groundfish surveys from 

 1975 to 1983. 



809 



