270 



SALMON OF THE KARLUK RIVER, ALASKA 



respectively, of three-fresh-water fish in the samples. While the percentages of three- 

 fresh-water fish in the small samples taken from the runs of those years are not 

 exactly comparable to the data under consideration, it is evident that the three-fresh- 

 water age group was dominant. 



The change in age composition might be due to any one, or a combination, of 

 the following causes: (1) An increase in the ocean mortality of the 3-year seaward 



FniiTRE ; 



O 20 40 60 80 



PERCENTAGE OF THREE-OCEAN FISH IN ESCAPEMENT 



100 



-Percentage of tbree-ocean fish in the return plotted acainst percentage of three-ocean fish in escapement for tlifi years 

 1922 and 1024 to 1B29, inclusive. The straight line represents a ratio of 1 to 1. 



migrants or a decrease in the ocean mortality of the 4-year seaward migrants; (2) an 

 increase in the fresh-water mortality of the 3-yeai seaward migrants or a decrease in 

 the fresh-water mortality of the 4-ycar seaward migrants; (3) an increase in the 

 length of fresh-water residence. 



The ocean mortality of the 4-year seaward migrants, as determined by the marking 

 experiments reported in a later section, is less than that of the 3-year seaward migrants. 

 This might be expected as they arc larger at the time of migration than the 3-year 



