FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



275 



migrate seaward. At the end of the migration period a part of this population is 

 present in the lake. These fish remain for another year and obtain more growth. In 

 the fourth year, the largest individuals remaining in the lake proceed seaward, the 

 time of appearance in the migration being correlated with their size. The slowest 

 growing individuals of the entire progeny which have not migrated remain in the lake 

 for another year and then migrate seaward in their fifth year. 



The older fish are of a larger average size than those of lesser age and their larger 

 size is due to the longer growing period that precedes migration. Fish in the older 

 age groups are usually the slower growing fish of the progeny from a particular spawn- 

 ing. Thus, the urge to migrate seaward is related to the size and growth rate of 

 fingerlings, and it appears that environmental conditions that affect the growth of the 

 fish during the time spent in the lake also affect the time at which the fingerlings 

 migrate to the ocean. 



The data on the percentage of males in the samples of migrants which were ex- 

 amined to determine sex are presented in tabic 28. The males and females were 

 equally represented. Grouping the 3- and 4-year fish, it was found that the total of 

 11,080 fish examined consisted of 5,557 males and 5,523 females. The slight varia- 

 tions in the sex ratios from year to year are probably due to chance because there is 

 no significant statistical difference in the ratios. 



SEX RATIOS OF ADULT FISH 



The sex ratio of the adult fish is in marked contrast to that of the seaward migrants. 

 Data on the percentage occurrence of males in the samples for the years 1922 and 1924 

 to 1936 are presented in table 29, arranged according to the length of time spent in 

 the ocean. The percentage occurrence of the males decreases with increased ocean 

 residence. All of the zero-ocean fish 8 are males. The average percentages of males 

 in the one-ocean fish, of varying periods of fresh-water residence, range from 100 

 percent to 75 percent. The average percentages of males in the two-ocean fish range 

 from 62 percent to 32 percent, while the average percentages of males in the three- 

 ocean fish of varying fresh-water residence range from 38 percent to 35 percent. 



» Fish which spend only a few months in the ocean and return as mature fish in the fall of the same year in which they migrated 

 seaward. 



