AGE AT MATURITY OR THE PACIFIC COAST SALMON 
21 
HUMPBACK SALMON (Oncorhynchus gorbuscha). 
[Fig. 17. pi. x; fig. 29, pi. xvn.J 
Available data concerning the habits of young humpbacks are derived almost 
wholly from Chamberlain’s observations in Alaska. Like the dog salmon, the hump- 
back young seek the sea as soon as they are able to swim. No yearlings have ever been 
reported from fresh water. In accordance with this habit, the nuclear area is con- 
sistently of the sea type, as in the dog salmon, being large in size and consisting for the 
most part of rings widely spaced. An inner nuclear core or region of narrowed rings is 
not infrequently present, and may here also simulate a close-ringed small nucleus of 
stream type. But the examination of a large series, taken in connection with the known 
history of the young, satisfies that such is never the correct interpretation. 
No young humpbacks in their first year have come under our observation. Those 
listed by Chamberlain (op. cit., p. 55 to 57) were taken during the summer months and 
ranged from about 40 to 115 millimeters, the larger individuals being those of late 
summer. All were undoubtedly in their first year, and would yield interesting results 
on examination of their scales. 
The possibility that the humpback salmon reaches maturity in less time than other 
species is considered by Chamberlain, who states: “The rapid growth of the young and 
the biennial occurrence of the species in Puget Sound may be noted in behalf of this 
belief.” This suggestion that the species matures in its second year is wholly sub- 
stantiated by the evidence of the scales (pi. x, fig. 17 and pi. xvii, fig. 29). A wide- 
ringed nuclear area of sea type, with its outer portion consisting of a definite winter band, 
is followed by the band of widely spaced rings representing the second summer’s growth. 
Numerous individuals have been examined, representing all sizes readily secured in a 
spawning run, and all were uniformly in their second year. Statistics containing sizes 
of males and females have not been prepared. 
Prof. McMurrich’s inference from the humpback scales is correct, except perhaps 
that portion which recognizes in the central area of the scale a record of life in fresh 
water. 
SUMMARY OF RESULTS. 
The following conclusions can be drawn from data here presented: 
1. The sockeye spawns normally either in its fourth or fifth year, the king salmon 
in its fourth, fifth, sixth, or seventh year, the females of both species being preponder- 
atingly 4- year fish. 
2. The young of both sockeye and king salmon may migrate seaward shortly after 
hatching, or may reside in fresh water until their second spring. Those of the first type 
grow more rapidly than the second, but are subject to greater dangers and develop 
proportionately fewer adults. 
3. Coho salmon spawn normally only in their third year. The young migrate 
either as fry or yearlings, but adults are developed almost exclusively from those which 
migrate as yearlings. 
