GUNDERSON: POPULATION BIOLOGY OF SEBASTKS MA 77 s 



marized by stock and suggested that maturation of 

 both male and female Pacific ocean perch depends 

 more on the size of a fish than on its age. The 

 maturation length (where 509c of the fish in that 

 length group are sexually mature) showed statis- 

 tically significant differences between stocks, fish 

 from the WVI stock maturing at a smaller size 

 than those from the QCS stock. Males matured at 

 29.4 cm in the WVI stock and 30.5 cm in the QCS 

 stock, while corresponding values for females 

 were 34.2 and 36.3 cm. 



Estimates of the age at sexual maturation indi- 

 cated that WVI females release their first brood 

 when 9-10 yr old, while those in Queen Charlotte 

 Sound are 11 yr old. The results for males suggest 

 that males from the WVI stock mate for the first 

 time when 6 yr old, while this occurs at age 7 in the 

 QCS stock. 



Measurement of oocyte diameters from a series 

 of ovaries collected over the complete extent of the 

 reproductive cycle suggested that any estimates of 

 fecundity must be regarded as tentative, owing to 

 the uncertain significance of auxiliary modes of 

 oocytes. Incomplete fertilization of oocytes also 

 complicates the situation, and there is very little 

 known about the relation between the number of 

 developing oocytes and the number of viable lar- 

 vae that will result from them. 



Length (L)-fecundity (F) data were summarized 

 by stock and were described by the relation: F = 

 aL b . Analysis of covariance showed that there 

 were significant between-area differences in the 

 length-fecundity relationship, females from Wash- 

 ington-Oregon being more fecund than Queen 

 Charlotte Sound females of comparable length. 



The effect of fishing on stocks of Pacific ocean 

 perch was examined through an approach similar 

 to the yield per recruit analysis that is commonly 

 used in stock assessment. However, the model and 

 computer program developed for this study differ 

 from conventional methods in that they allow for 

 estimation of exploitable biomass and population 

 fecundity as well as yield per recruit. Data re- 

 quired included age-specific schedules of instan- 

 taneous natural mortality, vulnerability to 

 fishing, mean weight, and fecundity. Annual yield 

 to the fishery, annual production of larvae, and 

 average exploitable biomass on hand during the 

 year were then calculated for a population based 

 on a constant number of female recruits, assuming 

 different combinations of instantaneous fishing 

 mortality (F) and age of recruitment to the fishery 

 (t p ). 



The results showed that different levels of t,„ or 

 between-stock differences in the input parameters 

 had very little effect on the relative trends in yield, 

 population fecundity, and exploitable biomass 

 with increasing F. 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 population fecundity showed a recip- 

 rocal trend, decreasing sharply as F increased 

 from 0.0 to 0.2, and declining more gradually forF 

 greater than 0.2. 



The value of M used in the calculations had a 

 pronounced effect on the results. If M = 0.1, the 

 costs of letting F reach 0.2 are quite high, since 

 exploitable biomass and population fecundity 

 would be reduced to about 407c of their virgin stock 

 levels. If M = 0.2, however, the costs of letting F 

 reach 0.2 are somewhat lower, with exploitable 

 biomass and population fecundity declining to 

 about 509^ of their level in the virgin stock. 



This preliminary analysis provided some esti- 

 mates of the reductions in population fecundity 

 that could be expected under different levels of 

 fishing intensity, but gave no insight into the ef- 

 fects of this reduced fecundity on future recruit- 

 ment. As a result, the analysis was carried one 

 step further and it was assumed that, at reduced 

 levels of population density, all compensatory 

 changes in recruitment are mediated through in- 

 creases in growth. Attendant changes in fecundity 

 at age and age at sexual maturation would then 

 tend to increase the level of population fecundity 

 and recruitment, since both fecundity and mat- 

 uration are related to size. 



Three hypothetical levels of compensatory 

 growth and sexual maturation were considered, 

 and none of these were effective in restoring 

 preexploitation levels of population fecundity 

 when F = 0.2. This was true even when mean 

 length at each age increased 57c and sexual mat- 

 uration occurred a year earlier than normal. Even 

 when F is restricted to 0.1, Pacific ocean perch 

 would have to undergo significant compensatory 

 changes in growth to restore population fecundity 

 to virgin stock levels, when the stock was presum- 

 ably near the replacement point (P r ) on the 

 spawner-recruit curve. 



Since Pacific ocean perch stocks are poorly 

 adapted to extensive displacements from P r , it was 

 suggested that drastic action will probably be re- 

 quired to return them to their former levels of 

 population fecundity, beginning perhaps with a 



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