12 U. S. BUREAU OF FISHERIES. 



AGE OF SALMON AT MATURITY. 



As practically all salmon which have the opportunity spawn but 

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

 groat 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 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. Gdbert 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 



Eersists throughout life, and grows in proportion with the rest of the fish, principally 

 y 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 

 successive 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 

 gr(jws 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 re(iuisite 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 (Jenus (Jncorhynchus. By Charles H. Qilbert 

 Bulletin, U. S. liureau of [■'isheries, 1912, Vol. XXXIl, pp. 4, 5. Washington, 1913. 



