computer programme BIODIS (see Laevastu and Larkins I98I; Laevastu 1979)- The mean 



biomass growth rate changes when the distribution of biomass with age is changed by 



fishing. This change can be numerically computed in a numerical programme FISHMO 



(Laevastu 1982). The dependence of growth rate (g) on fishing mortality was computed 



assuming knife-edge constant recruitment, using long-term mean age composition of 



exploitable biomass (although the computations can be made on any known age 



composition) and assuming that the spawning stress mortality increases 3% per 



year after maturation; fishing mortality is assumed to be constant in fully exploited 



year classes (see further Appendix). 



The relations between the fishing mortalities in numbers of exploitable 



population (F ) and in terms of weight of total biomass (F ) and monthly growth 

 ^ "^ en ^ tw 



rates are given in Figures 6 and 7 for walleye pollock and yellowfin sole. These 



figures show that the growth rate changes are species specific and must be evaluated 



as such. 



The quantitative relations between fishing mortality (expressed in three 



different terms as described above, F , F , and F^ ), and corresponding changes 



en ' ew tw ^ ^ ^ 



in spawning stress mortality with reference to exploitable biomass, are shown in 

 Figures 8 and 9 for walleye pollock and yellowfin sole, respectively. As fishing 

 mortality increases, spawning stress mortality decreases. This change is different 

 in different species in terms of biomass and is largely dependent on the growth 

 rate of the species, but depends also on the age of maturity (see further Appendix). 



In numerical biomass based ecosystem models, both fishing mortality and 

 spawning stress mortality are expressed on the basis of total biomass of the 

 species. The relations between the mortalities expressed on this basis are 

 shown in Figure 10. This figure also demonstrates that these relations vary from 

 species to species and consequently they must be determined for individual species 

 and populations. 



