FISHERY BULLETIN: VOL. 74, NO. 4 



Table 2.-Sex ratios of the monthly collections of rock crabs 

 taken intertidally in East Boothbay, Maine, 1972-75. Sex ratios 

 that deviated significantly {P = 0.05) from 1:1 are marked '. 



Mo. 



1972 1973 1974 1975 

 M:F M;F M:F M:F 



Mo. 



1972 1973 

 M:F M:F 



1974 



M:F 



1975 

 M:F 



Jan. 



1.27:1 — 0.96:1 Sept. 0.89:1 



1.57:1 



Mar. — — 

 Apr. — 0.60 

 May — 1.07 

 June 1.02:1 0.95 

 July 1.25:1 1.55 

 Aug. 1.01:1 0.71 

 0.87 



0.76 

 1 

 1.03 

 0.82 

 0.86 



1.23:1 

 1 1.08:1 

 1 — 

 1 — 

 1 — 

 1 — 



0.71:1 

 0.68:1 — 



Oct. 0.68:1 — 1.23:1 — 

 Nov. — — 0.96:1 — 



Dec. — — 0.93:1 — 



Total 0.98:1 0.99:1 0.95:1 1.08:1 



Growth 



Carapace width prior to shedding was plotted 

 against the new carapace width after shedding for 

 45 crabs that molted while captive in the labora- 

 tory. This relationship was fitted by the method of 

 least squares using the simple linear equation 

 Y = Si + bX, where 7 = postmolt CW, 

 X = premolt CW, and a and b were constants. 

 Analysis of covariance, which was used to test 

 homogeneity of regression coefficients, revealed 

 no statistical differences between growth in- 

 crements of males and females, so all data were 

 pooled. The calculated equation for crabs ranging 

 from 9 to 48 mm CW was Y = 0.566 + 1.247X. 

 This relation was similar to Cleaver's (1949) con- 

 stants (a = 0.57; b = 1.23) calculated for Dunge- 

 ness crab, C. magister, juvenWes (5-91 mm CW). 



Based on the relationship between premolt vs. 

 postmolt and measurements of cultured post-lar- 

 val crabs (stages I-V) obtained from Herbert C. 

 Perkins, formerly of the National Marine Fish- 

 eries Service, West Boothbay Harbor, Maine, I 

 estimated sizes for instars I-XIII (Table 3). Sizes 

 for instars above XIII were not computed because 

 of the inherent uncertainties of extrapolating 

 beyond the data range. If we assume that Maine 

 rock crabs begin to attain maturity about 60 mm 

 CW (Krouse 1972; Scarratt and Lowe 1972) and if 

 as suggested by Butler's (1961) work with C. 

 magister the premolt vs. postmolt relationship 

 changes with the onset of sexual maturity, then 

 sizes for instars beyond XIII (53 mm CW) are 

 inadequately described by the aforementioned 

 regression. 



Because the increments of growth (24.3-28.3%) 

 for post-larval crabs (instars III-V) cultured in the 

 laboratory were appreciably less than those 

 growth increments (29.2 to 30.6%) for instars 

 VI-VIII of the captive wild crabs, widths for 

 instars II-XIII were estimated by the empirical 



value (2.6 mm CW) for stage I and then the 

 subsequent stages were calculated with the linear 

 regression (Table 3). Instar sizes calculated by this 

 procedure were larger (about one instar size 

 greater) than those sizes based on empirical data 

 for stages I-V and predicted by regression for 

 instars VI-XIII, e.g., instar V (estimated by 

 regression) = 9.5 mm and instar VI (other 

 method) = 9.6 mm. For purposes of this study, 

 those instar sizes calculated with the empirical 

 post-larval data were favored. 



Table 3. -Comparison of instar sizes of rock crabs. For one 

 group, instars I-V represent actual measurements and instars 

 VI-XIII are calculated by the relationship Y = 0.566 + 1.247Z; 

 for the other group, instar I is an actual measurement and the 

 remaining instars are estimated from the aforementioned 

 equation. 



An attempt was made to objectively assign size 

 with age by correlating instar size with the 

 monthly width-frequencies (Figure 2). As men- 

 tioned previously, post-larval crabs (2-5 mm CW) 

 first entered the sampling area in August or 

 September after having hatched in late spring or 

 summer and having developed through the larval 

 stages during the remainder of the summer. Size 

 distributions for April 1974 through April 1975 

 revealed first entry of young-of-the-year crabs in 

 August followed by subsequent growth of young 

 crabs until about January when growth ceased and 

 this modal group stabilized at about 4-20 mm CW 

 (instars III-IX). This wide size range is best 

 explained by varying hatching and settling dates 

 whereby some crabs entered the population per- 

 haps 1 to 2 mo later than the rest. These crabs 

 settled to the bottom when temperatures were 

 likely to be declining; thus these individuals 

 experienced little growth until the following 

 spring. 



952 



