lOOVtotal tissue wt. The body weight/shell volume 

 index was calculated as: (total wt - gonad wti/ 

 shell volume. The gonadal index reflects reproduc- 

 tive condition and the body weight/shell volume 

 index reflects nutritional condition (Giese and 

 Pearse 1974). Preliminary work showed that in- 

 dexes calculated from wet and then dry weights 

 did not have significantly different variances. 



Resulfi 



Mytiltis edulis and M. californiaru/s longer than 

 about 50 mm were sexually mature, and the 

 gonadal indexes of both species had large var- 

 iances. Because gonadal indexes of both species 

 were calculated for a large' size range (70-110 mm 

 shell length) in each population, the covariance of 

 gonadal index on shell volume was analyzed for 

 each species. However, regressions of the arcsine 

 transformation of the gonadal index on shell vol- 

 ume calculated for each monthly sample did not 

 have significantly different slopes for either 

 species (ANCOVA, P>>0.05): for M. califor- 

 nianus the common regression slope = 0.09, 

 ^(18.1901 = 0.206; for M. edulis the common regres- 

 sion slope = 0.08,F,.,,22o, =0.217. Therefore, mus- 

 sel size was ruled out as a significant source of 

 variability in gonadal index for this study. Rather, 

 the variability was probably a result both of the 

 difficulty in precisely dissecting the diffuse gonad 

 from the body tissues and of a large degree of 

 inherent reproductive asynchrony in the popula- 

 tions. 



The gonadal indexes of M. edulis from the out- 

 fall and from the control populations showed the 

 same distinct cycle of gonads increasing in size 

 during summer and fall and dropping to a low in 

 spring (Figure 3). However, gonadal weights of the 

 outfall population were lower than the controls, as 

 can be seen by the generally lower level of the 

 outfall gonadal index, particularly in the April 

 through November samples. Similarly, the body 

 weight/shell volume index for M. edulis showed 

 an annual cycle which peaked in summer and 

 dropped in fall and winter to a low in spring ( Figure 

 3). The phase of this body weight/shell volume 

 index was slightly in advance of the gonad cycle. 

 As with the gonad cycle, the outfall population had 

 the same basic body weight/shell volume cycle as 

 the control, but it showed a generally lower level 

 than the controls and indicated that the outfall 

 mussles were in poorer nutritional condition than 

 the controls. 



NDJFMAMJJASOND 

 1973 



M J J A S 

 1973 



N D 



FiGliRE 3. — Monthly mean values for the gonad index and body 

 weight/shell volume index for Mytilus edulis. Circles = control 

 population; dots = outfall population. Vertical lines are the 95'» 

 confidence limits of the means. Each sample was 12 mussels. 



In contrast to M. edulis. the gonadal index of M. 

 californianus did not show a distinct annual cycle 

 (Figure 4). The April and May control samples 

 probably represented a peak of reproductive activ- 

 ity, but the erratic fluctuations of the index made 

 this uncertain without histological information or 

 field observations of spawning. Except for this 

 brief spring peak, the outfall population showed a 

 consistently higher level of ripeness throughout 

 the year than the control mussels. The body 

 weight/shell volume index of A/, californianus ap- 

 peared to show a slight annual cycle with a low in 

 March and April and higher levels in late sum- 

 mer (Figure 4). However, this trend was not pro- 

 nounced and did not appear to correlate with the 

 gonadal index. Contrary to the trend shown by 

 gonadal index levels, outfall body weight/shell 

 volume indexes were consistently lower than the 



500 



