-8- 



The percentage of biomass that is exploitable (E) was computed with the 

 assumption that recruitment (and the biomass of prefishery juveniles (A)) remains 

 constant : 



E :. A - hf^^ (9) 



Pollock, A = 70 

 Yellowf in, A = ^45 

 In the first set of runs (juvenile recruitment not compensated), the values 

 for h were: pollock 1200, yellowfin 2l60. In the second set of runs juvenile 

 recruitment was affected by fishing and the values for h were: pollock 700, 

 yellowfin l800. (Values derived from data in Laevastu I983). 



6. EFFECTS OF FISHING ON THE DYNAMICS OF WALLEYE POLLOCK AND YELLOWFIN SOLE 

 BIOMASSES WHEN KNIFE-EDGE RECRUITMENT TO FISHERY IS USED, AND FISHING 

 DOES NOT AFFECT RECRUITMENT TO JUVENILES 



Three different series of computer runs were made, each consisting of four 

 numerical experiments (see Table 1). Each experiment contained two sets of 

 different fishing. The first fishing set contained constant annual catch only 

 (384 t/year, curve 1 on Figures 1 and 2), and constant catch plus three different 

 density dependent fishing mortalities (curves 2 to ^4 on Figures 1 and 2). Second 

 set contained zero fishing and three different density dependent fishing mortalities 

 without a constant annual catch. These different fishing mortalities operated 

 on the whole biomass. The corresponding number based annual fishing mortality 

 coefficients (F) , operating on exploitable part of the stock would have been: 0.08'», 

 0.198, and 0.38 for walleye pollock and 0.075, O.I6, and O.3O for yellowfin sole. 



In the first experiment growth rate and spawning stress mortality were assumed 

 not to be affected by fishing. Fishing was made to affect spawning stress mortality 

 only in the second experiment. In the third experiment fishing affected biomass 



