SEAWARD MIGRATION OP CHINOOK SALMON. 



57 



in a constantly increasing percentage of fish whose scales show an intermediate band pre- 

 ceded by a band of distinctly narrowed rings. The fry migrating in August and Septem- 

 ber will contain a relatively small percentage of specimens whose scales are of this type. 

 As the season advances this percentage gradually increases, as the percentage of fish 

 entering the estuary from above and having begun the slower winter growth, 

 increases. When fish which have not begun the slower growth 

 enter the estuary, the vigorous intermediate growth may begin 

 immediately, so that no distinctly narrow rings will intervene 

 between the growth of the first summer and the intermediate 

 growth. All possible gradations between these two types of scales 

 may be seen among the fall migrants. The migrating yearlings 

 taken in the spring all have the band of narrow winter rings pre- 

 ceding the intermediate growth. 



The question arises : Is there any criterion whereby fry migrat- 

 ing seaward in the fall and yearlings migrating seaward in the 

 spring can be distinguished ? It has been shown that there is an 

 average difference in the following respects: (i) Spring yearling 

 migrants show a larger average amount of intermediate growth, 

 both on the basis of ring counts and scale measurements; and (2) 

 the intermediate band in the case of fall migrants is less frequently 

 preceded by a band of narrower rings. Although these average 

 differences in the scale growth of the fall fry and the spring year- 

 ling migrants are well enough established they are not diagnostic, 

 and it would be impossible in many cases to determine from the 

 scales the time at which migration took place. 



Owing to the practical importance of determining, if possible, 

 any discernible difference between the scales of fish migrating at 

 such -(videly separated times, a series of each group was photo- 

 graphed in order to see whether some criterion, independent of the 

 data presented in the tables, might be established by means of 

 which the fish could be identified. The necessity for such series 

 of photographs was discussed on page 6. For this purpose there 

 were selected, at random, 50 specimens collected at Point EUice, 

 October 16, 191 5, as representative of the fall migrants, and 50 

 specimens of the spring, yearling migrants collected on the lower 

 Columbia River during March and April, 191 6. A careful study 

 of these series of photographs has disclosed no such criterion as 

 was sought, and the conclusion is forced that, so far as the nuclear 

 growth alone is concerned, it can not be hoped to distinguish in 

 all cases adult fish which have migrated as fry in the fall from 

 those which have migrated as yearlings in the spring. Plate II, figures i to 4, and 

 Plate III, figures i to 4, were selected from these photographs as examples of the scales 

 of the fall and spring migrants. 



The available data regarding the scale growth of the Sacramento River fish do not 

 indicate that there is as much variation as has been shown to exist in the case of the 



