Stehlik et al.: Distribution and abundance of brachyuran crabs on northwest Atlantic shelf 



477 



ROCK CRAB 



Ci_-At_^— 



Figure 3 



Rock crab distribution from winter dredge and winter trawl surveys. 



(1973) reported that rock crabs enter Chesapeake Bay 

 in the fall, when temperatures are below 12 °C, and exit 

 in the spring before they rise to 12° C. Although rock 

 crabs have been caught at temperatures as high as 

 25°C (Williams and Wigley 1977), our data show that 

 they tend to avoid temperatures above 18°C (Fig. 6). 

 They were most abundant in spring in temperatures 

 of 4-7°C, and in fall at 8-18° C. In previous trawl sur- 

 veys, most rock crabs were caught at 0-14°C (Musick 

 and McEachran 1972, Haefner 1976), but less sampling 

 was done at higher temperatures. 



Mean catch-per-tow by number and weight in fall 

 varied among years (Fig. 7). The year of peak catches, 

 1986, produced nearly 20 times more rock crabs than 

 in the lowest catch year, 1978. However, the decadal 

 data set revealed no consistent trend in abundance. The 

 highest catches by weight were in 1981 and 1986; the 

 year 1986 was unusual in that 60% of the crabs were 

 < 5 cm carapace width. The mean weight per crab was 

 35 g in 1986, but averaged 63 g in all other years 

 combined. 



Size frequencies of rock crabs differed by subarea 

 and sex (Fig. 8). Modal carapace widths of males and 

 females in the Gulf of Maine and middle-Atlantic 



peaked at 7-9 cm. The largest crabs in both subareas 

 were males. On Georges Bank, they were smaller; 

 widths of both sexes peaked at 4 cm. 



Weighted mean catch-per-tow of rock crabs by 

 trawls was highest at night in spring and at twilight 

 in summer and fall (Table 3). We assume that crabs 

 on the bottom can be caught by trawls, but crabs 

 buried in sediments can be captured only by clam 

 dredges. Nocturnal activity in rock crabs was re- 

 ported by MacKenzie (1981), who observed that rock 

 crabs are buried at times during the day, but not at 

 night. Fogarty (1976) and Rebach (1987) found that 

 rock crabs exposed to artificial photoperiods are more 

 active in dark than in light periods. Dredge catches 

 of rock crabs also fluctuated by time of day, but the 

 effect of light upon catchability of crabs by dredges is 

 unknown. 



Abundances of males and females were significant- 

 ly different for all gears and seasons (Table 4). Males 

 outnumbered females in spring and summer trawl sur- 

 veys, whereas females outnumbered males in fall trawl 

 and all dredge surveys. In contrast, Shotton (1973) and 

 Haefner (1976) found that males usually outnumbered 

 females in late-spring and fall surveys off Virginia. 



