FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 267 



in the water of the same streams, on the same dates, above the area where spawning 

 and spawned-out salmon were found. Furthermore, a part of the salmon spawn 

 along the beaches of the lake and eventually die, and the carcasses, together with the 

 carcasses which drift downstream into the lake from the tributaries, decompose and 

 the phosphorus contained therein becomes available to the phytoplankton. A 

 shortage of phosphorus in the lake water would inhibit the growth of all forms of 

 phytoplankton. 



It is apparent from a study of the chemical analyses of the lake water and of the 

 stream waters that both phosphorus and silica are being absorbed, during the sum- 

 mer months, by the phytoplankton as fast as they become available, for otherwise 

 the concentration of these chemicals in the lake water would approach that found in 

 the streams. Since the concentration of those chemicals in the lake water during 

 most of the summer was less than a measurable amount, it is evident that they must 

 he limiting factors in the production of the phytoplankton and may possibly be 

 affecting indirectly the growth and survival of the red salmon fingerlings of Karluk 

 Lake. 



CHANGE IN AGE COMPOSITION OF THE POPULATION 



The percentage occurrence of the various age groups in the population, as deter- 

 mined from a study of the scale samples (tables 3 to Hi), appears to be changing 

 from year to year. Eowever, a direct, comparison of one year's data with another 

 cannot truly represent the change, if any, since a given year's run is composed of 

 the progeny from the escapements of several years. 



To determine whether or not a change lias been taking place in the age composi- 

 tion of the population, it is necessary to compare the age composition of the escape- 

 ments with the age composition of the fish returning from the respective escapements. 

 The age compositions of the escapements for a scries of years are presented in table 

 18, and the age compositions of the returns from the escapements appear in table 25. 



The percentage of . r > 3 fish in the escapements for the years 1922 and 1924 to 1929, 

 inclusive, was 59.3, 7fi.O, 66.S, 81.1, 70.8, 56.9, and 34. S while the percentage of 5 a 

 fish in the returns from these escapements was 50.0, 49.3, 41.2, 52.5, 45.2, 39.5 and 

 42.0, respectively. There was a lower percentage of 5 3 fish in the return than there 

 was in the escapement for every year with the exception of 1929. A similar condi- 

 tion is found to exist if the returns from the spring and fall escapements arc consid- 

 ered separately. 



The pairs of percentages for the f> 4 age group for the years 1922 and 1921 to 1929, 

 inclusive, are as follows (the first figure being the percentage of the (i« group in the 

 escapement for a given year and the second figure being the percentage of tin' 6, 

 group in the return from the escapement): 1.0:11.3; 10. 5:22. S; 15.8:39.3; 7.6:33.2; 

 6. 1:29.4; 9.0:20.3; 35.1 : 27.7. In all years except 1929 there was a greater percentage 

 of the f> 4 group present in the return from the escapements than there was in the 

 escapements. 



In considering these two major age groups there appears to be a decrease in the 

 relative abundance of one, and an increase in the relative abundance of the other. 

 It thus becomes of interest to determine if a change is taking place in the length of 

 ocean residence, and in the length of fresh-water residence of these fish. 



