416 U. S. BUREAU OF FISHERIES 



extensive plants made during the last 10 or 1 1 years the range has been 

 much extended on the Pacific coast as well as elsewhere in this country. 

 It seems to be found in the rivers during the greater part of the year. 

 In the Columbia River the spawning season is from February to May, 

 in Puget Sound in the spring, and in southeast Alaska in May and 

 June. The best commercial fishing is in January, February, and 

 March. In California the catching of this species is restricted to hook 

 and line fishing. 



AGE OF SALMON AT MATURITY 



As practically all salmon which have the opportunity spawn but 

 once and then die, knowledge of the age at which this occurs is of great 

 interest both from an economic and scientific standpoint. Many 

 attempts have been made to solve the problem with the sockeye 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, Calif., 

 and in New Zealand, where king fry were planted in streams not 

 frequented hj the species in c^uestion 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 yeAV. 



A more certain method of determining the age of salmon has been 

 developed in recent years through the adaptation by American scien- 

 tists 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. 



The late Dr. Charles H. Gilbert, of Stanford University, as early as 

 1910, apphed this method to the determination of the age of the 

 various species of Pacific salmon. As to its appHcation to the Pacific 

 salmon and the general method followed, Doctor Gilbert has the fol- 

 lowing to say: 



While the method i.s new as regards Pacific salmon, it has been exijerimentally 

 tested and fully approved by the Fisheries Board of Scotland in the case of the 

 Atlantic salmon, and is now universally accepted as furnishing reUable data as to 

 the age and many other facts in the Hfe history of that fish. It has been shown 

 to be applicable also to various species of trout, and its value has been demon- 

 strated 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 outhne 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 difi"erent individuals and during successive years in the his- 

 tory of the same individual that number alone can not be depended on to deter- 

 mine 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, \<4iile 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 fuit 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 



