SEAWARD MIGRATION OF CHINOOK SALMON. 



63 



As stated above, migration in the Columbia River takes place throughout the 

 year, but the data here presented indicate clearly that the chief period of migration 

 for the fry is during the months from June to October, inclusive. On account of the 

 source of error mentioned above it seems probable that the main period of migration 

 is actually somewhat shorter than is here indicated. The mode of the curve, showing 

 the height of the migration, would not necessarily be affected by error of this sort. 

 The migration of yearlings is completed by June. This wide range in the time of migra- 

 tion is not surprising in such a large river system as that of the Columbia, where 

 a great diversity of climatic conditions obtains in different regions. There are two 

 possible explanations for the wide extension of the migration period: (i) Fish from 

 each tributary may migrate gradually, a few at a time, through the year; (2) fish from 

 each tributary may all migrate at about the same time, but migration from different 

 tributaries takes place at different times of the year. 



No. 



Mch. Apr Ma^ il»«e Julij Auj. Sept. Oct- Nov. Dec. Jan F«b. Mch. Apr. Mou 



Graph 9. — "SmootJied " curve showing average number of spedmens taken in each collection 

 in the main Columbia River and the estuary for each month. 



There is some evidence to show that the young fish from particular tributaries 

 tend to migrate at the same time and, moreover, that they tend to school together 

 during the seaward migration. The collection made at Crandall's seining ground, 

 September 15, 1916, especially suggests this interpretation, as the fish are noticeably 

 smaller and the character of the scale growth different from other collections made 

 during the same time of year under approximately similar conditions. (See p. 19.) 



The time at which the fry leave the tributary streams for the main river and the 

 rate of downward migration have not been determined. Undoubtedly, the time of 

 leaving the tributary streams is subject to great variation. On purely a priori grounds 

 it seems certain that the earliest fry to migrate — such, for example, as those taken in 

 March and April— must have come from the lower tributaries. The spawning season 

 in the different tributaries does not differ more than a few weeks over the entire Columbia 

 system, but the much colder water of the higher streains delays development so markedly 



