SEAWARD MIGRATION OF CHINOOK SALMON. 



19 



winter growth, as is indicated by the narrower marginal rings. The following table (13) 

 gives the data regarding this collection. No attempt is made to segregate the few 

 specimens whose scales do not possess this primary check. 



Table 13. — Young Chinooks from Lake at Seufert, Oreg., Sept. 2, 1915. 



Length. 



Total. 



With 

 check. 



Scale record. 



Number of rings — 



To check. Total 



Length of anterior 

 radius — 



To check. Total 



Average 

 estimated 

 length of 

 fish at 

 time of 

 formation 

 of check. 



106 to 110 mm. 

 loi to 105 mm. 

 96 to 100 mm.., 

 91 to 95 mm — 

 86 to 90 mm... 



gi to 8s mm 



76 to 80 mm. . . 



71 to 75 mm 



66 to 70 mm 



61 to 6s 



Total. 



Av., 80.9 mm. 



8.S 



9S 



14.0 

 17-0 



25- S 

 330 



4S-S 

 SS-5 



63. 

 63. 



S-6 

 8-3 

 6.7 

 6.1 

 5-6 

 6.4 

 4.0 



13-6 

 14.6 

 13.9 

 12.3 

 II. 4 

 II. 2 

 II. o 



21-3 



28.8 



17-3 

 180 

 13- o 



41-3 

 44-4 

 39-2 

 37- S 

 32-5 

 310 

 28.0 



49- S 

 54- S 

 46.0 

 4S o 

 41.0 

 39- o 

 330 



46 



6.7 



The almost exact correspondence between the estimated length at the time of the 

 formation of the primary check and the actual observed length at the time of planting 

 proves conclusively that in this particular instance the altered rate of growth following 

 the formation of the check was in response to the changed enviromnental conditions 

 resulting from the removal of the fish from the hatchery at Bonneville to the lake at 



Seufert. 



Sixty-nine specimens were collected September 15, 1916, at Crandall's seining ground 

 on Grims Island. In several respects this is an unusual collection. The average length 

 is but 74.4 mm., the smallest recorded since June. The proportion of specimens whose 

 scales show the intermediate growth is also very small, only three in the entire collection. 

 None of the other collections made at this point are remarkable for the small size of the 

 fish as compared with other collections made at the same time of year in other localities, 

 so that it is unlikely that selection has taken place here as was evidently the case with the 

 collection made within the mouth of the small stream near Point ElHce. A possible 

 explanation may be that we are dealing here with a series composed largely, if not 

 wholly, of fish migrating seaward from some particular tributary or region of the Co- 

 lumbia River watershed, in which the fry do not attain, before migration, as large a 

 size as is common for other parts of the watershed. Gilbert (1912a) has described such 

 differences among the young migrating sockeyes in different tributaries of the Fraser 

 River system. This explanation seems, therefore, plausible in the case of these young 

 chinooks, although admittedly unproved. 



The three specimens which show a band of intermediate rings are among the largest 

 taken and average 89.3 mm. in length. The average number of rings preceding the 

 intermediate growth is 7. The number of intermediate rings averages 9, and the 

 average total number of rings is, therefore, 16. The average length of the anterior 

 radius of the scale is 21.3 to the beginning of the intermediate growth and 47.3 to the 



