SEAWARD MIGRATION OF CHINOOK SALMON. 25 



located. The average length is 106.4 ^™- The males, 24 in number, average 107. i 

 mm. in length; the 28 females, 106 mm. 



A particularly interesting feature of this collection is the fact that a considerable 

 proportion of the specimens have scales which show a distinct widening of the marginal 

 rings. Fourteen (27 per cent) of the specimens have scales of this character. The 

 other specimens all have scales whose marginal rings are of the narrow, winter type. 

 The series of collections from the upper regions of the Columbia River basin is not 

 complete enough to allow conclusions to be drawn regarding the character of this widen- 

 ing of the marginal rings, but it can be showTi on material from the Sacramento River 

 that the new growth of the second year usually begins during the fall. Previous to 

 beginning this "new growth" there has been formed a more or less distinct band of 

 narrower rings, the winter band. This is unquestionably the same phenomenon which 

 is evident in the present case, namely, the beginning of the vigorous new growth which 

 will continue during the growing season of the following year. 



This question naturally presents itself: If this widening of the marginal rings in the 

 case of the fish from the upper parts of the stream is to be interpreted as the new growth 

 belonging to the second year, is it certain that the similar widening which has been found 

 on the scales of the young fish in the estuary is not, in reality, the same thing which has 

 merely been hastened by the migration to the brackish water in the estuary ? In other 

 words, why give different interpretations to the two phenomena? 



Similar physiological causes are, in all probablity, behind the accelerated growth in 

 each instance. The intermediate growth, however, is directly the result of changes 

 brought about by the migration into brackish water, while the "new growth" is a 

 response to environmental changes which are independent of any special acti\ity on 

 the part of the fish. The changes resulting in new growth are seasonal and affect all 

 of the fish in any particular locality at neariy the same time of the year. The stimulus 

 is probably not a simple one but is a complex of several factors, such as temperature, food 

 supply, degree of maturity, etc. Racial differences in different localities may also enter 

 as modifying factors. 



The change brought about by migration is the more profound as is indicated by 

 the fact that the rings of the intermediate growth are usually heavier and more widely 

 spaced than those composing the new growth accomplished before migration. The 

 difference between the two types of rapid growth is not, however, diagnostic, and 

 it is usually impossible to distinguish in individual cases between intermediate bands 

 and bands of new growth. Many of the fish taken in the upper part of the stream and 

 which have begun the new growth could not be distinguished by the scales from fish 

 taken in the estuary whose scales show the intermediate growth. From October on, 

 therefore (and probably for some weeks previous to this time), one is likely to encounter 

 fish in the estuary whose scales would be practically identical — having a marginal band 

 of wider rings — but some of which will have formed the marginal band as a result of 

 migration into brackish water, while others will have formed the marginal band in the 

 upper parts of the stream previous to migration. Undoubtedly as the season advances 

 the percentage of fish which have formed this band in response to the migration will 

 decrease, while the percentage of fish which have started the new growth of the second 



