-15- 



in pollock considerably less than in yellowfin. This difference might also be 

 interpreted as another indication that in case of comparable stocl< sizes, 

 pollock stock can be fished more heavily than yellowfin stock. 



The results of the studies in this paper indicate that expected changes in 

 biomasses due to fishing are counteracted to a considerable degree by concomitant 

 changes in biomasses. The change of the growth rate of a stock is largely 

 influenced by the change of biomass distribution with age. The latter is also 

 affected by variations in predation on juveniles (including cannibalism), which 

 in turn will affect the recruitment to exploitable stock. Numerical experiments 

 suggest that the change in predation as a linear function of the biomass density 

 in the environment has relatively little influence on biomass dynamics. However, 

 predation does not change as a linear function if there is considerable 

 selectivity of prey items. 



The compensation mechanisms which counteract the effects of fishing on the 

 stock will be affected by changes in predation of juveniles, which will also 

 affect recruitment to the exploitable part of the stock in a more complex manner 

 than presented in this single species model. Therefore, further study of the 

 effects of fishing and changes in stock biomass caused by it, must be carried 

 out in holistic ecosystem simulations, such as PROBUB and DYNUMES, which can 

 also explain the mechanisms of interactions and non 1 inear i t ies in them in a 

 more realistic manner than is possible with a single species approach. 



8. CONCLUSIONS 

 1. Given low to moderate fishing (F maximum 0.^) the removal of the exploitable 

 biomass by fishing is compensated for by the increase in the growth rate of the 

 population and by the decrease of spawning stress mortality. 



