280 SALMON OF THE KARLUK RIVER, ALASKA 



amputation of both ventral fins and the posterior half of the dorsal. From this 

 marking, 1,340 fish, or 1.34 percent of the number marked, were recovered at the 

 counting weir below Cultus Lake; and these were the total number of marked fish 

 that returned to Cultus Lake from this experiment. Of the unmarked seaward 

 migrants, 3.2 percent returned to Cultus Lake. 



In 1930 (Foerster, 1936b), 104,061 seaward migrants were marked by the ampu- 

 tation of both ventral fins. A total of 3,821 fish, or 3.67 percent of the number 

 marked, was recovered from the commercial fishery and at the counting weir below 

 Cultus Lake. It was considered that the recovery was at least 90 percent of the total 

 number of marked fish returning from the experiment, so that the actual return ". . . 

 probably lay somewhere between 3.67 and 4.1 per cent." 



During 1931 (Foerster, 1936b), 365,265 seaward migrants were marked by the 

 amputation of the adipose and both ventral fins. A total of 12,803 fish, or 3.51 per- 

 cent of the number marked, was recovered from the commercial fishery and at the 

 counting weir below Cultus Lake. The recovery was at least 95 percent of the total 

 number of marked fish returning from the experiment so that the actual return 

 ". . . lies between 3.5 and 3.7 percent." 



In Foerster's experiments of 1927 and 1928 a greater survival was found among 

 the unmarked fish than among the marked fish. Three factors were considered in an 

 endeavor to account for the disparity. 



. . . infiltration of unmarked adults from other areas, the straying of marked individuals 

 to other spawning regions or a definite differential mortality among mar ked groups. 



Evidence was produced to show cause for ruling out the first two factors, and it 



was concluded that — 



There remains, therefore, only the factor of differential mortality among the marked indi- 

 viduals, and on the data available this is held to be the one largely r esponsible for the lower 

 return of marked adults when compared with that for the unmarked. 



The differential mortality was calculated to be 65 percent for the 1927 experiment 

 and 58 percent for the 1928 experiment, and the probable value was considered 

 to be the mean of the two values or 62 percent. Thus there was a 186 percent greater 

 survival among the unmarked fish than among the marked fish of the first experiment, 

 and a 138 percent greater survival among the unmarked fish than among the marked 

 fish of the second experiment, and the probable value was considered to be approxi- 

 mately 163 percent. 



Based on the information on differential mortality between marked and unmarked 

 fish derived from the 1927 and 1928 marking experiments and on the data collected 

 from the marking experiments of 1930 and 1931, Foerster considered that the survival 

 of Cultus Lake red salmon during the time spent in the ocean ranged between 3.5 

 percent (his lowest percentage return uncorrected for differential mortality) and 11.7 

 percent (his highest percentage return, 4.1 percent, multiplied by 2.86 to correct for 

 differential mortality). The most probable value was considered to be 9.9 percent 

 (the mean probable value of the recoveries, 3.75 percent, multiplied by a mean value, 

 2.63, to correct for differential mortality). 



