FISHERY BULLETIN: VOL. 85, NO. 4 



fort is concentrated between Virginia and New 

 Jersey (Botton and Ropes in press). 



MATERIALS AND METHODS 



NEFC bottom trawl surveys ranged from Cape 

 Fear, NC, north to the Scotian Shelf; clam sur- 

 veys cover the Middle Atlantic region north to 

 Georges Bank. Both are general purpose survey 

 programs, collecting data for standing stock as- 

 sessments for many finfish and shellfish species, 

 as well as collecting specimens for age determina- 

 tion, dietary analysis, and many other purposes 

 (Grosslein 1969; Clark 1979). 



During the period covered by this study (1975- 

 83), the #36 and #41 Yankee otter trawls were 

 used as sampling gear during bottom trawl sur- 

 veys. Both nets have a mesh size of 5 inches 

 throughout the wings and body, and 4.5 inches in 

 the cod end. A liner of 0.5-in nylon mesh is em- 

 ployed at the aft end of the top belly and the cod 

 end. Trawls are equipped with roller gear to facil- 

 itate use over rough bottoms. All tows were 30 

 minutes in duration at a vessel speed of 3.5 knots; 

 stations were located using loran. An average of 

 1,129 stations per year was sampled (range, 711 

 stations in 1975 to 1,547 stations in 1979). 

 Cruises were conducted during fall (September 

 through early December) and spring (March 

 through May) in all years, with five additional 

 surveys during the summers (July through Au- 

 gust) of 1977-81 and two during the winters (Jan- 

 uary through February) of 1978 and 1983. Sam- 

 pling was conducted both day and night. After 

 sorting the catch, personnel recorded the number 

 of horseshoe crabs taken in each tow and their 

 total wet weight to the nearest 0.1 kg. 



A stratified random sampling design was used 

 in the surveys (Grosslein 1969). The region was 

 divided into several strata based primarily on 

 depth. Stations were allocated to strata roughly 

 in proportion to the area of each stratum and as- 

 signed to specific locations within strata at ran- 

 dom (Clark 1979). For the purpose of this paper, 

 stations of 9-27 m (5-15 fm) depth were defined as 

 "inshore" and those deeper than 27 m as 

 "offshore". Preliminary inspection of the catch 

 data compiled by Ropes et al. (1982) showed the 

 bulk of the horseshoe crab population to be lo- 

 cated between northern New Jersey and southern 

 Virginia. Within this region, there were 27 in- 

 shore and 16 offshore strata, based on depth and 

 location. Stratified mean number per tow and 

 biomass per tow of horseshoe crabs were con- 



verted into estimates of standing stock by using 

 the "area swept" by a standard survey trawl in 

 relation to catch as an estimate of minimum abso- 

 lute density. Tows within strata were used to cal- 

 culate variances around the means. Total popula- 

 tions for the inshore and offshore regions were 

 estimated by expanding the average stratified 

 mean catch per tow by the ratio of total area sur- 

 veyed to the area sampled by an average tow. 

 Further details of statistical methods are found in 

 Clark (1979). 



Ocean clam surveys used commercial style hy- 

 draulic dredges, towed for 5 minutes at 1.5 knots. 

 The design and performance of this sampling gear 

 is discussed by Meyer et al. (1981). Cruises from 

 1965 to 1978 used either a 30-in (91 cm) or 48-in 

 (122 cm) knife-width dredge; cruises from 1979 to 

 1983 used a 60-in (152 cm) dredge. There were no 

 ocean clam surveys in 1968, 1971-73, and 1975. 

 Station locations were selected using a stratified 

 random sampling design; the average number of 

 stations sampled per year was 370 (range, 139 in 



76 



74 72 



70 



42 



40 



38 



36 



34 



Figure l. — Mean number of horseshoe crabs per tow, 



806 



