14 PACIFIC SALMON FISHERIES. 
and king salmon, the most important commercially of the five species, 
by means of marking artificially reared fry, usually by clipping 
one of their fins before they are liberated, as noted elsewhere in this 
report, but with unsatisfactory results. 
Fortunately, certain experiments carried on in Tomales Bay, Cal., 
and in New Zealand, where king fry were planted in streams not 
frequented by the species in question and the return of the adults 
noted, have yielded some interesting and accurate information on 
the subject. These indicated that the age was four or more years, 
as no run was reported until the fourth year. 
A more certain method of determining the age of salmon has been 
developed in recent years through the adaptation by American 
scientists of the discovery by European investigators that the ridges 
observed on the scales of certain fishes indicated a period of growth 
of the animal itself. 
Dr. Charles H. Gilbert, of Stanford University, as early as 1910, 
applied this method to the determination of the age of the various 
species of Pacific salmon. As to its application to the Pacific salmon 
and the general method followed, Dr. Gilbert has the following to say: 
While the method is new as regards Pacific salmon, it has been experimentally 
tested and fully approved by the Fisheries Board of Scotland in the case of the Atlantic 
salmon, and is now universally accepted as furnishing reliable data as to the age and 
many other facts in the life history of that fish. It has been shown to be applicable 
also to various species of trout, and its value has been demonstrated in fishes as widely 
divergent as the carp, the eel, the bass, the flounder, and the cod. Descriptions of 
this scale structure and its significance have appeared in a large number of papers, 
both scientific and popular. It will suffice here to repeat that the scale in general 
persists throughout life, and grows in proportion with the rest of the fish, principally 
by additions around its border. At intervals there is produced at the growing edge 
a delicate ridge upon the surface of the scale, the successive ridges thus formed being 
concentric and subcircular in contour, each representing the outline of the scale at a 
certain period in its development. Many of these ridges are formed in the course of 
a year’s growth, the number varying so widely in different individuals and during suc- 
cesstve years in the history of the same individual that number alone can not be 
depended on to determine age. For this purpose we rely upon the fact that the fish 
grows at widely different rates during different seasons of the year, spring-summer 
being a period of rapid growth and fall-winter a season when growth is greatly retarded 
or almost wholly arrested. During the period of rapid growth the ridges are widely 
separated, while during the slow growth of fall and winter the ridges are crowded 
closely together, forming a dense band. Thus it comes that the surface of the scale 
is mapped out in a definite succession of areas, a band of widely spaced rings always 
followed by a band of closely crowded rings, the two together constituting a single 
year’s growth. That irregularities occur will not be denied, and this is natural, 
inasmuch as growth may be checked by other causes than the purely seasonal one. 
Also a considerable experience is requisite for the correct interpretation in many 
cases, and a small residue of doubtful significance has always remained. ‘This element 
is too small to affect the general results, and further investigation will almost certainly 
eliminate the doubtful cases altogether. 
a Age at maturity of the Pacific coast salmon of the genus Oncorhynchus. By Charles H. Gilbert. Bul- 
letin U. S. Bureau of Fisheries, vol. xx Xu, p. 4, 5. 
