SEAWARD MIGRATION OP CHINOOK SAI^MON. 67 



There is apparently no significant difference in the length of immature males and 

 females, but the mature males are distinctly larger than the immature specimens. 

 This indicates either that they are slightly older or that they have, for some unknown 

 reason, grown more rapidly than the other individuals of the same brood. 



PRECOCIOUSLY MATURE MALES. 



The precocious maturing of young chinooks has been noted by Rutter and is a 

 phenomenon well known to many hatchery men. Most of these precocious males are, 

 without question, the same age as the immature fish taken at the same time, but, as we 

 have just shown, average distinctly larger (about 16 per cent). The time for maturing 

 corresponds with the normal spawning time for the adult fish, late summer and autumn, 

 although, as shall presently be shown, they may be found during the winter and spring, 

 long after the normal spawning season is past. In addition to the precocious males 

 from the McCloud River they have been also found in various collections from the 

 Columbia River system, as follows: 



1. Clackamas River, August 30 and 31, 1916: Four out of 10 specimens. Length of the mature 

 males is 105 per cent that of the immature specimens. 



2. Seufert, Oreg., September 2, 1915: Three out of 52 specimens. Length of mature males is 116 

 per cent that of immature specimens. 



3. McKenzie River, September, 1916: Eight out of 11 specimens. Length of mature males is 123 

 per cent that of immature specimens. (These were collected from a pond used for holding spawning 

 fish and have not previously been considered because of the small size of the collection and the great 

 irregularities in size and scale growth.) 



4. Hatchery ponds at Bonneville, Oreg. : Specimens of mature males were not infrequently found 

 here diuing the spring of 1915, while the author was engaged in marking a series of yearling fish. 



It will be noticed that the mature males are only recorded in collections from 

 tributaries fairly well upstream. 



In appearance these precocious fish are, when fully mature, strikingly different 

 from the immature specimens. In addition to the greater size, the head is relatively 

 larger, the body is deeper and thicker, the skin covering the entire body and fins is 

 thickened so that the scales appear smaller, and the coloration is distinctly modified. 

 The general color is a dark yellowish brown, becoming distinctly yellow ventrally. The 

 color of the spots is deepened so that they are conspicuous even against the darkened 

 background. There is also a tendency toward the development of bright yellow or 

 rose-colored borders to the fins. The testes are large and white, in every respect resem- 

 bling the testes of normal, mature, sea-run males. The scales are normal and show no 

 absorption along the edges, as is so characteristic of the scales of spawning sea-run adults. 



The habits of these fish do not apparently differ greatly from the habits of the 

 immature fish with which they are associated. They feed regularly and are, to all appear- 

 ances, fully as well conditioned as the others. This probably accounts for the fact that 

 the scales are not absorbed at the margins. Rutter ( 1 903) reports that they do not seem to 

 beattracted by the females as are the sea-run adult males. Our observations are, however, 

 to the contrary. The fish contained in the collection from the McKenzie River, made in 

 September, 191 6, were taken from a pond used for empounding spawning fish and the 

 percentage of mature males is much higher than in any of the other collections. Rutter 

 reports that the milt from such males will fertilize eggs normally. 



