70 BUIvtrETlN OP THE BUREAU OF FISHERIES. 



2. The average length of the youngest fry is between 35 and 40 mm. The rate 

 of growth is especially rapid during the first five or six months, by which time the 

 average stature for the first year has been attained. The average length of yearlings 

 is approximately 100 mm. (4 inches), both in the Columbia and Sacramento Rivers. 



3. The length of the scales and also the number of rings formed on the scales par- 

 allel quite closely the increasing length of the fish. Many of the youngest and smallest 

 fry have not developed scales before migrating seaward. 



4. Migration into the brackish water of the estuary is usually accompanied by an 

 increase in the rate of growth, which is recorded on the scales as a marginal band of 

 wide rings — the intermediate band. No intermediate band has been demonstrated on 

 the migrating fry of the Sacramento River, but this is undoubtedly due to the lack of 

 material collected at the right time and place. 



5. The scales of fry remaining in fresh water develop a marginal band of narrow, 

 winter rings during the latter part of the summer. The new growth of the second 

 year begins soon thereafter. 



6. The normal time for seaward migration among Columbia River chinooks is 

 during the summer next succeeding the fall in which the eggs are laid. Seaward mi- 

 grating Chinook fry are, however, found throughout the year in the Columbia River, 

 and the collections taken in March, April, and May include also migrating yearlings. 

 There is, therefore, for each brood of fish, a period extending over about 18 months, 

 during which the young may migrate seaward. 



7. In the Sacramento River there is a distinct migration of fry lasting from 

 January to June, inclusive. Although definite proof is lacking, it is probable that 

 there is another period of seaward migration during the late autumn. 



8. In the younger migrants of the Columbia, including practically all the fish 

 migrating previous to June, the intermediate band is not to be distinguished from the 

 preceding scale growth, due to the fact that the first few rings formed on the scales 

 are always somewhat wider than the latter ones. After the first of June the intermediate 

 band may be (but not always) distinguished as a marginal band of distinctly wider 

 rings. Begimiing in August or September this intermediate band may be preceded 

 by a more or less distinct band of narrow rings that correspond to the winter band 

 forming on the scales of upstream fish. The percentage of fish, whose scales show such 

 a narrowing preceding the intermediate band, increases through the autumn and winter, 

 so that by the following spring this narrowing becomes characteristic of the scales of all 

 the yearling migrants. Although there is this average difference in the scales of fish 

 migrating as fry during the fall and those migrating as yearlings in the spring, it is 

 impossible, with our present knowledge, to distinguish in many individual cases between 

 fish migrating at these two periods. 



9. A sudden change in environmental conditions, such as removal from hatchery 

 to wild conditions, may result in modified growth, recorded on the scales as a distinct 

 break or check in the scale growth. This we have designated as the "primary check." 

 Characteristically this appears as a more or less distinct narrowing of the rings succeeded 

 by a series of wider rings. 



10. There is apparently a distinct tendency for the larger specimens among the 

 fish of any particular tributary to migrate earlier than the smaller specimens. It is 



