•19- 



The long-term mean year class strengths (normalized to 100^) of yellowfin 

 sole, flathead sole, pollock, and herring from the Bering Sea are shown in 

 Figure 12 (data from Niggol 1982). These data allow the computation of the 

 rate of change of mortality with reference to previous year class strength, 

 which is graphically given in Figure 13- From the age where the stock is known 

 to have reached 80!^ maturity, a dashed line is drawn, representing 10^ increase 

 of mortality per year. This trend line coincides in all four species relatively 

 well with the trend of increased mortality during the first five years after the 

 maturation. The difference in the position of the 10^ increase trend line 

 from the actual values for yellowfin sole is apparently caused by incomplete 

 data on maturation. If the maturation occurs one year earlier (age 8), a 

 good correlation is obtained. The scattering of the data at older ages is due 

 to the previously mentioned "tail end" in long-term mean age composition, caused 

 by some strong year classes which have occurred during the period of the data 

 used in the summation. 



The mean rate of increase of the spawning stress mortality in the four 

 species in Figure 13 is slightly less than ]0% per year during the first five 

 years after maturation. On the other hand, Beverton (I963) found an increase of 

 senescent mortality slightly over 9^ per year, using a different approach. In 

 the present study a 3% per year increase of spawning stress mortality was used, 

 (This lower value also compensates for the values which are caused by the "tail 

 end".) 



