1. EFFECT OF FISHING ON THE POPULATION, ITS BIOMASS, AND ITS AGE STRUCTURE 



Fishing removes a given fraction of older, larger fish from a population. 

 If recruitment is not appreciably reduced by this removal, the rest of the 

 population left in the sea would have a younger average age as a result of fishing. 

 The younger fish have higher growth rates than older, exploitable fish (see Fig. 1). 

 Thus, the average growth rate of the biomass of a given population would increase as 

 a result of the "rejuvenation" of biomass by fishing. 



The "natural mortality" in older fish increases with age. This increase of 

 senescent mortality seems to be related to maturity; i.e., after maturation, 

 mortality increases in each year about 10^ above the mortality of the previous 

 year (Beverton 1963; Laevastu and Larkins I98I). This mortality is called here 

 spawning stress mortality. The term senescent mortality is also used for this 

 age-specific mortality. When increased fishing removes older fish which would 

 have died from spawning stress mortality later, the latter mortality is expected 

 to decrease (in relation to total population) with increasing fishing. 



The above described dynamical fishing dependent changes in a population can 

 be quantitatively (numerically) computed given a few simplifying assumptions, 

 which would not restrict the use of the results in more complex, real conditions. 



In traditional fisheries population dynamics computations, the "natural 

 mortality", which encompasses spawning stress mortality, is assumed to be constant 

 with age. In most cases the magnitude (value) of this mortality is unknown and 

 guessed at. In some cases, where age composition data of catches from a given 

 stock are plentiful and the stock is reasonably fished, "natural mortality" and 

 fishing mortality are computed as total mortality and partitioned. Fishing 

 mortality has usually been made to increase with age, and in some species to 

 decrease again in the few oldest year classes. 



