It can be argued that a relatively intense fishery on a given stock removes an 

 equal fraction of fully recruited year classes (i.e., is nonselective in respect 

 to size/age, and is directly proportional to the respective densities (strengths) 

 of the year classes present). This consideration is not true in respect to year 

 classes which are not fully recruited to fishery (re size selection by gear). The 

 few oldest year classes in demersal and semidemersal species (such as Pacific cod, 

 Gadus macrocephalus ) , might also be subject to lesser fishing pressure, due to 

 greater depth distribution of these year classes. 



In this study the fishing mortality (F) is assumed to remove the same fraction 



of numbers of fish from each age group of fully exploited population (termed here 



as F ). Consequently, the fraction by weight removed by fishery (F ) is different 

 en -1 / > / 3 ' ' ew 



than the fraction of numbers, depending on growth of the species. This is 



illustrated in Table 1 with different age groups of walleye pollock, Theragra 



chalcogramma , and yellowfin sole, Limanda aspera . This difference between the 



fraction of numbers and fraction of weight removed from exploitable population 



(F and F , respectively) of walleye pollock and yellowfin sole is shown in 

 en ew r / i r 



Figure 2. The effect of fishing can also be referred to total biomass of the species 



(corresponding coefficient being F ). This relation in the two above-mentioned 

 "^ 3 tw 



species is shown in Figure 3. 



The three different fishing mortalities are compared on the abscissas of 

 Figures ^4 and 5- These two figures show the changing proportions of juveniles and 

 exploitables under the varying levels of fishing pressure. 



2. NUMERICAL COMPUTATIONS OF THE EFFECTS OF FISHING ON THE GROWTH RATE OF 

 BIOMASS AND ON THE SPAWNING STRESS MORTALITY 



The mean growth rate (in terms of weight) of the biomass of any species can be 

 computed if we know the distribution of biomass with age and have weight at age 

 data at hand. The growth rate computations are carried out in a separate numerical 



