SEAWARD MIGRATION OF CHINOOK SALMON. 47 



points and that individuals may remain here for some time feeding heavily on the offal 

 and as a consequence growing with unusual rapidity. The greater size of the fish from 

 the Clackamas River, as compared with those of groups i and 2, may be due to a racial 

 difference characteristic of the fish in this tributary or to the fact that many of these 

 fish have, in all probability, been reared for a part or all of their lives in the hatchery. 



The rate of growth in the estuary, and especially under the canneries at Ilwaco 

 and Astoria, is distinctly more rapid than in the higher waters. The increase in length 

 is especially rapid during June, July, and August, by which time the fry in the estuary 

 have far outstripped those in the upper part of the stream — in fact, have reached a 

 greater size than will be attained during the remainder of the year by those individuals 

 that do not migrate early. The growth in the estuary during September and October 

 is positive, but much slower than that which took place during the three months just 

 preceding. After the month of October the data pertaining to fish from the estuary is 

 very scanty, but apparently a period during which little or no growth takes place follows, 

 this coinciding with a similar condition in the regions upstream. 



It mil be noted on the graph that the final tendency of each of the curves is down- 

 ward. This seems conclusive evidence that the larger indi%'iduals migrate earlier. 

 Gilbert (1915) has found this to be true of young, seaward migrants of the sockeye 

 salmon. The present author's conclusion that the young fish in the tributary streams 

 tend to migrate shortly after beginning the new growth, if not before, also indicates that 

 the larger specimens migrate earlier, since it has also been shown that the specimens 

 which have begun the new growth invariably average larger than those which have not 

 done so. 



The single collections from Seufert, Little White Salmon River, and the McKenzie 

 River do not offer any basis for estimation of the actual rate of growth during successive 

 months, but it will be seen that they agree in general with the growth of fish in the main 

 river, averaging somewhat more than the fish in group i, but less than those of group 2. 



In the case of fish taken from the Columbia River proper it may not be strictly 

 correct to speak of the increasing size as growth. In all probability fish that have 

 once entered the main river continue, more or less steadily, their migration to the ocean. 

 We would thus be dealing, in successive months, with entirely different lots of fish. In 

 a general way, however, our figures should show the main features of the growth. 



At the time the fry become free swimming they are between 35 and 40 mm. in 

 length. During March, April, and May the average length does not exceed 50 mm. 

 Above the estuary the growth is quite regular from the time the fry first appear until 

 October or November, by which time an average length of between 90 and 100 mm. has 

 been attained. For the next several months no particular growth is recorded. The 

 collections of yearlings made in April and May from the Clackamas River indicate that 

 a new period of growth has been initiated, but because of the fact that about this time 

 the last of the fish leave the tributary on their downward migration no further data are 

 available. The rate of growth as indicated by these data has undoubtedly been modi- 

 fied by the migraUon of part of the fish. As has been shown, the larger fish tend to 

 migrate earlier than the smaller ones. This would tend to slow up the growth curve 

 and to obscure the sharp rise during the early summer so conspicuous for the curves 

 for the other groups. 



