1 1 I 



ABSTRACT 



The effects of fishing on a given species biomass have been quantitatively 

 evaluated. A constant recruitment is assumed in this study, but the evaluation 

 can be computed on any known age distribution of exploitable biomass. Fishing 

 mortality is assumed to be constant with age (i.e., equal fraction of fish is 

 removed from each fully recruited year class); however, spawning stress mortality 



(often called senescent mortality) increases with age. 



When fishing (mortality) increases, the spawning stress mortality decreases 

 relative to total and exploitable biomasses. Increased fishing also causes 

 the increase of the fraction of prefishery juveniles in relation to exploitable 

 biomass, assuming recruitment remains quas i -constant , As juveniles have higher 



individual growth rates than exploited year classes, the growth rate of the whole 

 biomass increases when fishing increases. The above relations are dependent on 

 biomass distribution with age in a given population, and on the age of maturity; 

 therefore, they vary from species to species. These changes are quantitatively 

 shown for two species from the Bering Sea - walleye pollock, Theragra cha Icogramma , 

 and yellowfin sole, Limanda aspera . 



The computations of spawning stress (or senescent) mortality and subsequent 



derivation of long term mean age composition of fully exploited portion of 



population is described in the Appendix. 



