FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



271 



migrants. There is no evidence that a marked change has taken place in the ocean 

 mortality of either the 3-year or the 4-year seaward migrants. 



A change in environment that would increase or decrease the mortality of the 

 fingerlings in the lake should affect each age group of seaward migrants in a similar 

 manner. No data are at hand to indicate that environmental conditions have 



100 



5 



LU 



80 



z 



Z) 

 h— 

 Ul 

 Or: 



60 



IT 



in 



I 



40 



a 



JO 



r- 



20 



Lxl 

 2 







20 40 60 80 



PERCENTAGE OF THREE- FRESH-WATER FISH IN ESCAPEMENT 



100 



Figure 8. 



-Percentage of three-fresh-water fish in the return plotted against percentage of thrce-frcsh-water flsh in escapement for 

 the years 1922 and 1924 to 1929, inclusive. The straight lino represents a ratio of 1 to 1. 



altered in such a manner as to affect the mortality of one age group without affecting 

 the mortality of the other age group. 



It is probable that the shortage of phosphorus and silica in Karluk Lake during 

 the summer months, which acts as a limiting factor in the production of phytoplankton, 

 also indirectly affects the growth of the red salmon fingerlings. A decrease in the 

 growth rate of the fingerlings may well result m an increase in the length of time spent 

 in fresh water. Data presented in a later section indicate that the fastest growing 

 fingerlings migrate seaward sooner than do the slower growing ones. Consequently, 

 anything affecting the growth rate of the fish would probably cause a change in the 

 time of seaward migration. 



