■11 



The definitions of the model conditions and the consequent computations of 



the parameters described above included assumptions which were based on simple 



and known conditions. However, hypotheses must be established for simulation 



of the effects of fishing on recruitment and on change of juvenile biomass. The 



hypotheses in this model must be limited by the model restraint -- i.e., what can 



be computed with the parameters available in the model. One of the factors 



affecting the recruitment to exploitable stock is the variation of predation on 



juveniles. This process is quantitatively simulated in large ecosystem models 



such as DYNUMES. In the present model it was assumed that the juvenile biomass 



decreases in direct relation to the quotient of total biomass (as affected by 



fishing) divided by unf i shed total biomass (B^/B ). This computation utilizes an 

 ^ t e 



iterative procedure. 



The results of the computations with fishing affecting not fully recruited year 

 class and with juvenile biomass adjustments to depict recruitment changes are given 

 in Tables 2 and 3, Columns B. The corresponding values with the "unadjusted" model 

 are given in Columns A for comparison. 



The percent adults is higher in the adjusted model because of the decrease of 

 juvenile biomasses. Spawning stress morta 1 i t ies , which refer to total biomass, are 

 also correspondingly higher. The growth rates have decreased in "adjusted" model. 

 The growth rate decrease is different in the two species -- the growth rate decreased 

 considerably more in the pollock because the juvenile biomass in the pollock is 

 smaller than in the yellowfin. 



The computed changes in the biomass parameters and rates of processes in the 

 biomasses caused by different fishing intensities in two different fish species 

 demonstrate the necessity of using species-specific data in any fisheries 

 population dynamics computations. 



