-10- 



3. SOME POSSIBLE EFFECTS OF FISHING ON RECRUITMENT, AND CONSEQUENT CHANGES IN 



GROWTH RATE AND IN BIOMASS AGE STRUCTURE 



The numerical results of growth rate and spawning stress mortality changes due to 

 fishing (as described in previous sections) were applied in a single species, 

 biomass based Schaefer type model to investigate the biomass dynamics with time 

 (Laevastu and Marasco I983). These studies showed that the changes of growth rate 

 and spawning stress mortality "overcompensated" the removal of biomass of fishing; 

 i.e., the biomass increased with increased fishing. Two possible causes for this 

 "overcompensation" might arrse from simplifications in the model, described in a 

 previous chapter: a) the knife-edge recruitment to exploitable biomass is not fully 

 realistic, as one- or two-year classes younger than the first fully recruited year 

 class are partially subjected to fishing which will affect the age structure and 

 consequently the growth rate and spawning stress mortality rate; and b) when the 

 biomass of spawners decreases due to fishing, the larval and juvenile recruitment 

 could decrease. Consequently the juvenile biomass would decrease, which would 

 have two effects: the growth rate increase due to fishing would be less than with 

 constant juvenile biomass, and the recruitment to exploitable biomass would be 

 decreased. 



The fishing of year classes which are not fully recruited was simulated by 

 assuming that ^2% of the year class prior to the full recruitment was subject to 

 fishing. The fully recruited year class was reduced by the amount of fishing 

 mortality in the previous year class. Any other scheme, based on actually observed 

 distribution of year classes, as well as observed year class strengths, could also 

 be introduced in the model. However, a well defined general case serves better 

 for investigation and demonstration of the changes in biomass parameters caused 

 by f ish ing. 



