STEVENS AND ARMSTRONG: CANCER MAGISTER IN GRAYS HARBOR 



Growth:Width 



Crabs in the 0+ age group (1980 year class) 

 increased in width by a factor of 4, from 10 mm in 

 May to 40 mm in October 1980, but growth slowed 

 from then to the following April 1981 when they 

 were about 50 mm wide (Fig. 7). The same pattern 

 of rapid growth during spring and summer was 

 evident among the 1+ and 2 + age groups (1979 

 and 1978 year classes, respectively), although 

 measurable increases in carapace width were re- 

 corded into winter 1981 for these older crabs. Crabs 

 of the 1979 year class grew from 45 to 73 mm 

 between May and October 1980 (factor of 1.6), 

 while the 1978 year-class crabs grew from 84 to 118 

 mm during the same period, an increase of 1.4. 

 Females had slightly greater mean widths than 

 males up to about 125 mm cw, but the differences 

 were minor (Table 3). 



Growth: Weight 



Regression equations for log 10 dry weight (g) on 

 log 10 carapace width (mm) were derived sepa- 

 rately for male and female crabs, but were not 

 significantly different. Therefore, a pooled regres- 

 sion equation was calculated for both sexes 

 combined: 



LL 

 < 



a. 

 < 



HO 



120 



100 



80 



60 



40- 



20 



1979 



1980 



1981 



J J 



A S 

 1980 



N D 



J F M A M 

 1981 



J J 



FIGURE 7. — Mean width of four age groups of crabs (0+, 1 + , 2+, 

 3+ ) in Grays Harbor, Wash., 1980-81. Mean width ( + 1 SE) shown 

 for each sampling period was determined by graphical analysis 

 of width-frequency data or interpolation (see Table 3). 



or 



Log 10 Weight (g) = -4.064 + 2.832 (Log 10 



Width, mm) 

 Weight (g) = (8.63 x 10" 5 ) Width 2 832 



(r 2 = 0.985, P = 0.0001; Fig. 8). 



Differences in width/weight and width/age rela- 

 tionships between male and female crabs would 

 probably increase at sexual maturity, which oc- 

 curs about 2 yr after metamorphosis, and at 

 widths of 93-122 mm for males and 100-105 mm for 

 females (Butler 1960, 1961; Poole 1967). Growth 

 data presented herein are probably valid only for 

 male crabs <132 mm and female crabs <115 mm 

 width. 



Changes in mean weight with time (Fig. 9) 

 probably represent a continuous curve, but there 

 appeared to be an inflection point in late August 

 that separated spring-summer and fall-winter 

 growth stanzas. Therefore, k values were calcu- 

 lated for the periods May-August and 

 September- April . 



Monthly weight-specific growth rates were 

 greater in spring-summer than in fall-winter for 

 all age groups but decreased with size (Fig. 9). 

 Specific growth rates were greatest for 0+ age 



group crabs in their first summer during which the 

 average monthly weight increase was 206% (Table 

 4). Growth decreased to an average 15.8% per 

 month during the winter. Growth rates increased 

 again for age 1+ crabs in their second summer 

 (31% per month), but were lower than experienced 

 in their first year. This pattern was found for all 

 age groups. Crabs in the 2 + age group (probably at 

 sexual maturity) increased in weight 25% per 

 month in the summer of 1980, but only 6.5% per 

 month during the following winter (Table 4). First 



TABLE 4. — Weight-specific growth rates (k) and percent weight 

 increase of three year classes of Cancer magister in Grays 

 Harbor, Wash. Weight calculated from mean carapace widths of 

 each year class by regression equation (see Figure 8). 

 Growth per month calculated for spring-summer (May-August) 

 and fall-winter (September- April) growth stanzas (see also 

 Figure 9). 



477 



