2. EFFECTS OF FISHING ON THE BIOMASS PARAMETERS 



Traditionally, annual fishing mortality has been computed in terms of the 

 number of fish removed from the exploitable part of the population. It can be 

 computed alternatively in terms of biomass removed from either the exploitable 

 or total biomass. Time intervals other than a year (e.g., on monthly basis) 

 may also be used. The relations between the different fishing mortality 

 coef f icients for two species are given by Laevastu (1983)- In the present 

 computations a monthly total biomass based fishing mortality and a fixed rate of 

 (constant in weight) fishing were used. 



Fishing removes older fish; thus, if recruitment remained constant, the 

 exploitable portion of the biomass would decrease and the portion of prefishery 

 juveniles would increase. This is called rejuvenation of population. Since the 

 growth rate of the juvenile biomass is higher than that of the exploitable biomass 

 (see e.g., Niggol 1982), the mean growth rate of the whole (total) biomass will 

 increase as fishing intensity increases (Laevastu 1983). 



Spawing stress mortality (also called senescent mortality) increases about 

 9 to 10^ (number based) per year after 80^ of the population has reached maturity 

 (Beverton 1963, Gushing 1973, Laevastu and Larkins I98I). Fishing removes 

 mature fish, some of which would have died as a result of spawning stress mortality. 

 Consequently, this mortality decreases with increasing fishing, thus partially 

 counteracting the effects of fishing on stock biomass. 



Fishing would decrease the numbers of fish in the population and the size of 

 the biomass is expected to decrease if recuitment remains constant. However, this 

 decrease of stock biomass is not related in a linear manner with the decrease of 

 spawning stress mortality, because the increase in population growth rate caused 

 by the relative increase of faster growing juvenile biomass is expected to 

 compensate for losses due to fishing. 



