AGE AT MATURITY OF THE PACIFIC COAST SALMON OF THE 
GENUS ONCORHYNCHUS. 
By CHARLES H. GILBERT, 
Professor of Zoology, Stanford University. 
INTRODUCTORY. 
During the past two summers (1910 and 1911) the writer has been engaged on 
behalf of the United States Bureau of Fisheries in determining the age at maturity of 
the five species of Pacific coast salmon, together with such other facts of their life history 
as can be inferred from the structure of their scales. The problem has a peculiar interest 
as concerns these species because of the fact, now sufficiently demonstrated, that 
all individuals perish when they have attained sexual maturity. They spawn but 
once and then die, whatever the age or size at which sexual maturity has been reached “ 
and whatever may be their physical condition at the time of spawning. This peculiarity 
in their life history renders the question of their age at maturity an unusually important 
one, both from the scientific fish cultural and the purely economic standpoints. 
Many attempts have been made to solve the problem for the two species commer- 
cially most important — the king salmon and the sockeye — usually by marking the 
artificially reared fry by clipping one or other of their fins before they are liberated, 
in the hope of recognizing the adult fish on their return at maturity. Unfortunately, 
these important experiments have lacked adequate supervision, and have furnished 
somewhat discordant and uncertain results. These have been supplemented, however, 
by the experiments in Tomales Bay, Cal., and in New Zealand, where fry were planted 
in streams not frequented by the species in question and the return of the adults was 
noted. Also, in the case of the sockeye, we have had as additional evidence the 
quadrennial increase in the Fraser River, which has been very generally accepted as 
demonstrating a four- year cycle for this species. 
All the evidence available was considered by F. M. Chamberlain, in his highly 
valuable “Observations on Salmon and Trout in Alaska” (Bureau of Fisheries Docu- 
ment No. 627, 1907), with the resulting conclusion (p. 66) : 
There seems to be sufficient reason for believing four years to be the usual term of life for the sockeye 
and the king salmon, but experiment has pretty conclusively shown that they may mature in less time or 
may be retarded beyond that term. 
a With the possible exception of certain male king salmon fingerlings, which mature precociously in the streams during their 
6ist year, at a length of 3 to 7 inches. The fate of these has not as yet been determined. 
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