ABSTRACT 



When fishing (fishing mortality) increases in a given population, spawning 

 stress mortality (senescent mortality) decreases. Thus the change of a population 

 (biomass) is not a linear function of recruitment minus catch and a constant 

 "natural mortality". Furthermore, fishing removes the older slower growing 

 fish. Consequently, the mean growth rate of the remaining biomass increases. 

 These two fishing dependent changes (called "rejuvenation of population" by some 

 earlier researchers) must be considered in biomass based fishery models because 

 the removal of biomass by fishing is compensated by these changes to a considerable 

 extent if recruitment remains constant. The magnitude of the "rejuvenation" 

 effect varies from species to species, and depends on the growth rate of the 

 species, age of full recruitment, and quantitative relation between prefishery 

 juvenile and exploitable biomasses. The effects of fishing on "uncompensated" 

 biomass dynamics, as well as biomass dynamics compensated for fishing (i.e., 

 compensating for concomitant changes of growth rate and spawning stress mortality) 

 are demonstrated with numerical examples for walleye pollock ( Theragra cha 1 cogramma) 

 and yellowfin sole ( L imanda aspera ) . 



The predation mortality must be estimated in a single species dynamic 

 computation. In this study the predation mortality is assumed to consist of a 

 constant fraction simulating the predation by mammals and predation on shoals 

 in general, and a biomass density dependent fraction of predation mortality. 



Fishing yield can be computed with an exploitable biomass density dependent 

 fishing mortality coefficient, as well as with a constant annual catch plus a 

 biomass density dependent fishing mortality coefficient which simulates the 

 incidental catch (or bycatch) . The meaning of these types of computation of 

 fishing and the effects of fishing in general are explored in this paper. 



