34 



BULLETIN OF THE BUEEAU OF FISHEBIES 



The effect of age on such precocious maturing is shown by the fact that none 

 of the individuals that belong to the two youngest groups of seaward migrants — 

 those in their first and those in their second years — mature thus precociously, and 

 they do not appear in the run until their third year in the sea. It is demonstrated 

 further by the fact that the 3-year migrants rarely mature during their first sea 

 year. For although the 3-year migrants greatly outnumber those that seek the 

 sea in their fourth year, the latter group produces over 90 per cent of the No. 1 

 grilse. It would seem that in this case age alone must be the determining factor, 

 for although the 4-year migrants average a little larger than do those in their third 

 year, the amount of overlap is so great as virtually to eliminate size as a factor of 

 any considerable importance. 



The two factors that by their interaction seem largely to control the phenomena 

 of early maturing in the sea life of the fish are age and sex. The effect of the two 

 working concurrently is shown in illuminating fashion in the history of the No. 2 

 grilse, which mature during their second year in the sea. Descending the river as 

 fingerlings the previous year, they had spent the summer and winter, and more or less 

 of the second season, feeding vigorously in the sea. A few of them appear in the 

 early portion of the run with the first fish to enter in May, and there are some strag- 

 glers that accompany the run into late June or early July. Then comes an interval 

 of several weeks in which no grilse appear, to be followed late in the season by a 

 period in which they occur in much greater numbers than at any previous time. In 

 certain seasons they constitute, numerically, a very considerable proportion (as much 

 as 30 per cent) of the fish that form the latter part of the run. They are still obviously 

 undersized fish, and although the larger individuals overlap in size the smallest of the 

 next older group, the greater part of them are easily segregated by their small size 

 and general appearance. 



Two separate year classes are represented among these No. 2 grilse — those in 

 their fourth and those in their fifth year. They were fellow migrants from lake to sea 

 during the same spring, when one class was in its third year and the other in its 

 fourth. They have spent the same length of time in the sea, where their principal 

 growth has been made, and they differ but little in average size. The reaction of 

 these two classes to the influences that cause early maturity is most instructive. In 

 both classes the early part of the run, during spring and early summer, is composed 

 exclusively of male fish. In the latter part of August and in September mature 

 females appear in both classes, but in widely different proportions, the 5-year class 

 containing a much larger relative number of females than does the 4-year group. 

 This becomes obvious in the following table, which gives separately for 1924, 1925, 

 and 1926 the numbers of the sexes in No. 2 grilse taken by random sampling early 

 in September. 



Table 4. — Sex distribution in grilse 



Year 



Four-year grilse 



Five-year grilse 



Males 



Females 



Per cent 

 females 



Males 



Females 



Per cent 

 females 



1924 



325 

 190 

 41 



43 

 35 

 7 



12 

 16 

 15 



229 

 80 

 92 



174 

 56 

 38 



43 

 38 

 29 



1925 - 



1926 — 



