•12- 



The most astonishing fact on the last four figures is that curve h, presenting 

 the highest level of fishing, shows the highest growth of biomass -- "the more 

 you fish, the more there is to fish". This pronounced compensation is scarcely 

 expected in reality, as the recruitment changes due to total spawning biomass 

 changes (decrease) would counteract the compensation resulting from intense 

 f i sh ing . 



7. DYNAMICS OF WALLEYE POLLOCK AND YELLOWFIN SOLE BIOMASSES UNDER DIFFERENT 

 FISHING INTENSITIES WHEN FISHING AFFECTS RECRUITMENT 



The increase of stocks of fish as a result of compensating mechanisms caused 

 by increased fishing to the extent computed in the previous section of this 

 paper, seems somewhat unrealistic. However, some of the observed concomitant 

 increases of stocks and landings might be due to the same mechanisms as applied 

 in the previous computations, such as the increase of roundfish stocks and 

 landings in the North Sea in the 1960's and 1970's, and the increases of many 

 heavily exploited pelagic stocks a few years prior to the total collapse of these 

 stocks due to excessive fishing (recruitment overfishing) on shoaling species. 



The reasons for the "overcompensations" were considered to be mainly caused 

 by two assumptions made in the computations of the changes of biomass parameters 

 due to fishing (Laevastu 1983): 1) the knife-edge recruitment to exploitable 

 stock, and 2) recruitment (and juvenile biomass) remains constant when fishing 

 increases. These assumptions are probably unrealistic. Thus, new numerical 

 computations of changes of biomass growth rate and spawning stress mortality at 

 different levels of fishing were carried out, whereby year classes prior to 

 full recruitment to fishery were subjected to partial fishing, and the recruitment 

 from juveniles to exploitable biomass was made a function of fishing (Laevastu 

 1983). The new pertinent numerical coefficients are given in Section 5 of this 

 pa pe r . 



