FISHERY BULLETIN: VOL. 75. NO. 2 



perch stocks being examined and the effects of 

 varying the age at entry into the fishery (t' p ) were 

 ignored. 



In addition, the analysis was restricted to the 

 female portion of the stock. Over the long term, the 

 population will be far more sensitive to removals 

 of females and reduced population fecundity than 

 it will to removals of males, and the optimal har- 

 vest rate for females will determine the level of F 

 that should be applied to the stock as a whole. 



The input data used to describe the QCS and 

 WVI stocks are shown in Table 17. The values 

 used for mean weight at age, vulnerability 

 coefficients, and fecundity at age were derived 

 from the information in Tables 3 and 6 and Figure 

 22. Natural mortality was assumed to be the same 

 for all age groups concerned and computations 

 were carried out for both M = 0.1 and M = 0.2. 



Assessment of Immediate Response 

 to Fishing 



Only a small fraction offish less than 8 yr old are 

 recruited to the fishing grounds and, for the pur- 

 poses of this study, it was assumed that recruit- 

 ment begins at age 9 (t p = 9). It is possible that 

 significant quantities of 9-yr-olds are discarded by 

 fishermen, however, making it difficult to esti- 

 mate their vulnerability coefficient from market 

 samples. For this reason, t„ = 10 was also consid- 



ered, so that the sensitivity of the results to 

 changes in t p could be evaluated. 



The results (Figure 23) showed that different 

 values of t p had very little effect on the relative 

 trends in yield, population fecundity, and exploit- 

 able biomass with increasing F. In fact, the rela- 

 tive levels of each followed almost identical trends 

 for both stocks and both values of t p considered. 

 However, the value of M used in the calculations 

 had a pronounced effect on the results. 



In all cases examined, there was a sharp rise in 

 yield as F increased from 0.0 to 0.2, and a more 

 gradual increase for F-values greater than 0.2. 

 Relative levels of exploitable biomass and popula- 

 tion fecundity showed a reciprocal trend, decreas- 

 ing sharply as F increased from 0.0 to 0.2, then 

 declining more gradually for F greater than 0.2. 



Relative changes in population fecundity were 

 almost identical to changes in exploitable bio- 

 mass, indicating that changes in CPUE can be 

 used directly to estimate the magnitude of 

 changes in population fecundity. During 1966-68, 

 then, population fecundity for stocks in the 

 Oregon-Queen Charlotte Sound region must have 

 declined in the same manner as CPUE and is cur- 

 rently only about 50% of what it was prior to in- 

 tensive fishing. 



Preliminary examination of the data (Figure 

 23) shows that the most significant changes in 

 yield, exploitable biomass, and population fecun- 



o 0.1 



0.2 Q3 0.4 Q5 0J6 07 



F (WVI,lp=IO) 



0.2 0.3 0.4 0.5 

 F (0CS,tp=l0) 



600- 



400- 



"5200' 



■s 



■J 200 



I io 



0.2 0.3 0.4 0.5 6.6 0.7 

 F (WVI,tp=9) 



FIGURE 23.— Population fecundity (es- 

 timated number of larvae released an- 

 nually), exploitable biomass. and an- 

 nual yield for hypothetical populations 

 based on 1,000 recruits per year. Re- 

 sults are presented by stock, for two dif- 

 ferent ages at recruitment it p ) and two 

 different levels of instantaneous 

 natural mortality (A/). 



0.2 0.3 0.4 0.5 

 F (QCS, tp=9) 



396 



