Stein lik et al : Distribution and abundance of brachyuran crabs on northwest Atlantic shelf 



489 



south of Nantucket Island, and on the shallow portion 

 of Georges Bank. Catch-per-tow was highest on the in- 

 shore stratum of the middle- Atlantic, where it was over 

 ten times offshore catch-per-tow (Table 2). Northern 

 lady crabs have been observed in densities as high as 

 3-4/m 2 within 2 km of the New Jersey coast (MacKen- 

 zie et al. 1985). The sediments where northern lady 

 crabs are found are fine-to-medium sand or gravelly 

 sand (Uchupi 1963). Williams (1984) found this species 

 on a variety of sediments, mostly sand. 



This study supports the speculation by Musick and 

 McEachran (1972) that northern lady crabs may be 

 inactive or buried in winter. As stated above, we 

 hypothesize that dredges catch buried crabs that 

 trawls do not. In our 1978 winter trawl survey in the 

 middle-Atlantic, which included 78 stations, no north- 

 ern lady crabs were collected, but in the 1978 winter 

 dredge survey in the same area, which included 133 

 stations, 240 were collected. Moreover, in spring trawl 

 surveys, during which water temperatures had not 

 risen appreciably since winter, this crab was rarely 

 collected. 



Northern lady crabs were caught with trawls where 

 temperatures were 4-24°C; 96% were caught where 

 temperatures were 11.0°C and above (Fig. 6). Along 

 the New Jersey coast, Grant (1987) found northern lady 

 crabs were abundant in summer but absent when 

 temperatures dropped below 18°C. This crab was not 

 found in the Gulf of Maine; temperatures there are 

 mostly 4-10°C (Mountain and Holzwarth 1989). 



Mean catch-per-tow varied substantially among 

 years. The catch was relatively low in 1978; in contrast, 

 the mean weight of crabs was about 16 times higher 

 in 1981, the peak year (Fig. 7). In 1985, the mean 

 weight per crab was 29 g, when an unusually large 

 number of small crabs were present on Georges Bank 

 and in the middle-Atlantic. For all other years com- 

 bined, the mean weight per crab was 58 g. 



The modal carapace width of northern lady crabs was 

 6cm (Fig. 8). From winter dredge surveys, for which 

 sex information was available, modes were 7cm in 

 males and 6 cm in females. The ratio of abundance of 

 males to females was 1.1:1 (N 427) at that time. 



The abundance of northern lady crabs was signifi- 

 cantly different by time of day, i.e., highest at night 

 or twilight in trawl surveys (Table 3). Similarly, in a 

 laboratory study this species was most active at dusk 

 and night (Sponaugle and Lawton 1990). 



The abundance of northern lady crabs was negatively 

 correlated with depth in spring and summer in the 

 middle-Atlantic (Table 5). In summer and fall, it was 

 positively correlated with temperature. In fall, depth 

 had a weak, positive partial correlation with catches, 

 although if considered alone, depth correlated negative- 

 ly. On Georges Bank, the abundance of northern lady 



crabs was positively correlated with temperature and 

 depth in summer and fall. 



Coarsehand lady crabs Coarsehand lady crabs were 

 distributed across the middle-Atlantic from southern 

 New Jersey to Cape Hatteras in depths to 293 m (Fig. 

 13a-d). The range of this species was reported previ- 

 ously as from Accomac County, Virginia, to Biscayne 

 Bay, Florida (Wenner and Read 1982, Williams 1984, 

 Haefner 1985). The principal sediment from New 

 Jersey to Cape Hatteras is sand (Uchupi 1963). Wil- 

 liams (1984) also reported that this crab occurs on sand. 



The distributions of coarsehand and northern lady 

 crabs overlapped, but coarsehand lady crabs were 

 distributed farther offshore (Fig. 5), as Musick and 

 McEachran (1972) and Williams (1984) reported. Abun- 

 dance of coarsehand lady crabs peaked at depths of 

 27-40 m. Coarsehand lady crabs were less abundant 

 than northern lady crabs in the middle- Atlantic inshore 

 stratum (Table 2), as Dudley and Judy (1971) observed 

 off the North Carolina coast. Both species were about 

 equally abundant in the offshore stratum. 



Coarsehand lady crabs were collected at tempera- 

 tures of 6-23°C, but only about 5% occurred at tem- 

 peratures below 14 °C (Fig. 6). The temperature range 

 of this species is narrower than that of the northern 

 lady crab. 



As in the other crabs, interannual variations in mean 

 catch-per-tow of coarsehand lady crabs were large. In 

 the years 1979, 1981, and 1984, only about a tenth as 

 many crabs were collected per tow as in the years 1980, 

 1982, 1985, and 1986 (Fig. 7). 



Size frequencies of coarsehand lady crabs (sexes not 

 distinguished) peaked at 7 cm (Fig. 8). Wenner and 

 Wenner (1989) reported coarsehand lady crabs from 

 North Carolina to Florida, in depths of 4-20 m, with 

 average carapace widths of only 3-4.5 cm. Our data 

 showed that in the inshore stratum (<27m), 69% of 

 the crabs were >5cm. 



Catches of coarsehand lady crabs were highest at 

 night or twilight in all trawl surveys (Table 3), indica- 

 tive of nocturnal or crepuscular activity. 



According to regression analysis, only temperature 

 was significantly correlated with abundance of coarse- 

 hand lady crabs caught by trawl (Table 5). The correla- 

 tions were smaller than those of northern lady crabs, 

 possibly reflecting the more even distribution of the 

 coarsehand lady crab within its range. 



Conclusion 



The four most common crab species on the northwest 

 Atlantic shelf have spatial and temporal distributions 

 that are quite different. Cancer species occur at higher 



