KROUSE: SIZE AND GROWTH OF ROCK CRABS 



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Figure 2.-Width-frequency distribu- 

 tions for rock crabs collected monthly 

 by hand at an intertidal area in East 

 Boothbay, Maine, 1972-75. 



»;=Q25 



= 0.18 



CARAPACE WIDTH. MM 



dividuals <10 mm CW began to diminish progres- 

 sively until late summer when young-of-the-year 

 crabs once again settled to the bottom (Table 1). 



Rock crabs >40 mm CW were decidedly less 

 abundant during the fall and winter (Table 1). In 

 fact, not a single crab >50 mm CW was captured 

 from January through April (Figure 2). This 

 seasonal shift in size distribution suggested that 

 crabs >40 mm CW moved seaward from the inter- 

 tidal zone with declining temperatures. Jeffries 

 (1966) reported that C. irroratus moved from 

 Narragansett Bay, R.I., in winter to the deeper 

 and warmer ocean waters. Conversely, in southern 

 waters during the late fall and winter, rock crabs 

 moved into Delaware Bay (Winget et al. 1974) and 

 inshore waters of Virginia (Shotton 1973; Terretta 

 1973) as the water temperatures fell within a 

 preferred range. 



Aside from the apparent thermal effects and/or 



Table 1.- Percentage of rock crabs of two carapace widths in 

 monthly samples for 1972-75. 



behavioral changes on the seasonal displacement 

 of these large crabs from the intertidal zone, this 

 movement may also be associated with the larger 

 crabs' physical ability to emigrate with ease from 

 an area of low temperature. In addition, the size of 

 these crabs may inhibit their ability to find suit- 

 able burrows in the littoral zone necessary to 

 afford protection from the often tempestuous 

 winter sea. Jeffries (1966) stated that C. irroratus 

 was not well suited for burrowing into coarse 

 bottom. 



Sex Ratio 



Ratios of males to females for each of the 

 monthly samples ranged from 0.60:1 to 1.57:1 

 (Table 2). The chi-square test revealed that only 

 sex ratios of catches of July and August 1973 

 deviated significantly (P = 0.05) from a 1:1 rela- 

 tionship. Thus I concluded that sex ratios of the 

 intertidal catches approximated a 1:1 relationship 

 (1,353 males: 1,376 females); whereas, rock crabs 

 larger than 50 mm CW collected in traps near 

 Boothbay Harbor, Maine, showed disproportion- 

 ate sex ratios which varied by season and locality 

 (Krouse 1972). It appears that these disparate sex 

 ratios were primarily a function of the onset of 

 sexual maturity which subsequently altered the 

 growth rate and seasonal distribution of male and 

 female crabs. 



951 



