GUNDERSON: POPULATION BIOLOGY OF SEBASTES ALUTUS 



TABLE 18. — Relative yield (Y/Y max ), population fecundity 

 V2/E max ), and exploitable biomass (B7B ' ma x ) atF = 0.1 and 0.2. l 

 The range of values obtained by taking t p = 9 or 10, for two 

 different stocks of Pacific ocean perch is presented. 



1y max = y' e 'd wnen F = .7; £max and S' max = population fe- 

 cundity and exploitable biomass when F = 0.0. 



dity occurred when F = 0.1 andF = 0.2, and the 

 results for these two levels of fishing intensity 

 have been summarized in Table 18. All data were 

 presented in terms of the range of values obtained 

 when considering different stocks and t p values. 

 The ranges were always quite narrow, attesting to 

 the fact that consideration of different stocks and 

 t p values had little influence on the results. 



The conclusions that can be drawn from Table 

 18 depend to a large degree on what is considered 

 to be the best estimate of M. If M = 0.1, the costs of 

 letting F reach 0.2 are quite high, since exploit- 

 able biomass and population fecundity would be 

 reduced to about 40% of their virgin stock levels. 

 From this consideration alone, it would seem ad- 

 visable to limit F to 0.1. 



IfM = 0.2, however, the costs of letting F reach 

 0.2 are somewhat lower with exploitable biomass 

 and population fecundity declining to about 50% of 

 their level in the virgin stock. Limiting F to 0.1 

 would reduce the yield to only 45% of the level 

 attainable at F = 0.7, while population fecundity 

 and exploitable biomass would undergo reduc- 

 tions of about 30% from virgin stock levels. 



On the basis of this analysis, then, there is a 

 reasonable possibility that if M = 0.2, the optimal 

 level of F could be as high as 0.2. From a biological 

 point of view, however, a central question still 

 remains unanswered, since we have not yet 

 evaluated the consequences of reducing popula- 

 tion fecundity. It is one thing to point out the 

 degree to which population fecundity will be re- 

 duced by various levels of fishing intensity and 

 quite another to determine the impact this reduc- 

 tion will have on future recruitment. 



Effects of Fishing on 

 Future Recruitment 



Variability in egg and larval survival is ex- 

 tremely high for marine teleosts. Larvae grow 

 rapidly during the planktonic phase and require 



large quantities of food. For example, haddock 

 larvae initially grow at rates of about 12% per day, 

 increasing in weight by a factor of 10 5 during their 

 first year of life (Jones 1973). When food is not 

 plentiful, available supplies can be exhausted 

 rapidly, resulting in starvation and high rates of 

 density-dependent mortality. Even if larval mor- 

 tality is not directly due to starvation, density- 

 dependent mortality could easily result from slow 

 growth and prolonged exposure to predators 

 (Cushing 1974). 



Density-independent mortality, such as that 

 suffered when eggs or larvae are swept into un- 

 favorable nursery areas, can also be quite vari- 

 able. Ketchen (1956) and Ketchen and Forrester 

 (1966) found that in the case of English sole and 

 petrale sole, mortality of this nature seems to ac- 

 count for a high proportion of the variability in 

 year class strength. 



Marine fish have evolved three basic ways of 

 adapting their life history to cope with the highly 

 variable survival of their progeny: 1) iteroparity 

 (repeat spawning), 2) high fecundity, and 3) com- 

 plete elimination of the egg and/or larval stage 

 through ovoviviparity or viviparity. Murphy 

 ( 1968) has shown that iteroparity is favored under 

 conditions of high variability in larval survival 

 and relatively constant adult mortality. This line 

 of evolution leads to the existence of a large 

 number of adult age-groups — a common situation 

 in marine fishes. With several adult age-groups in 

 the population, the size of the adult stock is buf- 

 fered somewhat against variations in the strength 

 of individual year classes. 



High fecundity and elimination of the 

 planktonic phase offer two divergent means of cop- 

 ing with variable larval mortality and are typified 

 best by the gadoids on one hand and by elasmo- 

 branchs on the other. Atlantic cod commonly pro- 

 duce several million eggs per adult, and Cushing 

 and Harris (1973) have shown that the spawner- 

 recruit relation for this species is distinctly convex 

 or dome-shaped (curve a in Figure 24). This rela- 

 tionship implies that eggs are "overproduced" at 

 high parental stock densities, with attendant de- 

 clines in larval survival. At stock densities below 

 the replacement point (P r ), the high fecundity al- 

 lows for great resilience and rapid return to P r . 



The development of most elasmobranchs is 

 characterized by the elimination of the larval 

 stage found in the majority of teleosts and the 

 young are fully developed when born. Fecundity is 

 extremely low, with 2-108 young being produced 



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