■10- 



growth only, and in the fourth experiment fishing affected both spawning stress 

 mortality and biomass growth rate. 



The results of the numerical experiments in the first series of computer runs 

 are presented in Figures 1 to 16 as biomass change in eight years with the 

 corresponding experimental conditions as described above. (The four experiments 

 are presented on two figures each, corresponding to the two different, above- 

 mentioned fishing set ups.) 



Experiment 1 (Figures 1 to 't) , no effect of fishing on spawning stress mortality 

 or on growth rate ("uncompensated") . 



In case of constant catch (curve 1), pollock biomass decreases less (Figure 1) 

 than the yellowfin biomass (Figure 2) with the same amount of catch. The larger 

 percentage of the biomass that is exploitable and the higher growth rate for 

 pollock are responsible for this difference. With constant catch plus density 

 dependent fishing (curves 2 to '*) , the pollock biomass decreases faster (Figure 1) 

 than the yellowfin biomass (Figure 2) at approximately the same numerical fishing 

 mortality coefficient (F) . However, the corresponding biomass based fishing 

 mortality coefficient (f ) is about twice as high for pollock as compared to 

 yellowfin (due to specified input increments of f ,) • Therefore, a higher 

 proportion of biomass would be removed in the case of pollock. The reason for 

 this is that the fraction of exploitable biomass in a virgin pollock population 

 is considerably higher (about 70^) than in yellowfin (about kS%) . Some of the 

 effects of above-mentioned differences in biomass parameters (growth rate and 

 relative size of exploitable biomass) are more clearly shown in Figures 3 and ^4 , 

 which show the dynamics of "uncompensated" and unfished biomasses (curve 1) and 

 biomasses fished at different rates (curves 2 to M • 



