ZOOI.OCJICAI, SOCIETY BULLETIN 



RKCAPTURINO THE TORPOISRS IN THE SALT-WATER POND 



111 the matter of name there is some latitude. 

 All ])ori)oises and dolphins belong to that fam- 

 ily of tlie order of whales called Dclphinidar, 

 or dolphins, of which there are at least fifty 

 different species, and the names porpoise and 

 dolphin are to some extent interchangeable. 



The former is, however, usually applied to the 

 short-jawed kinds. The name "bottle-nose" is 

 inapt in the case of such animals, as the nose 

 or nostrils of all dolphins and porpoises is on 

 top of the head. 



The name dolphin is also applied to a fish 

 {Coryphaena), celebrated for its changing 

 colors. 



THE SCALE AS AN INDEX TO THE AGE 



OF A FISH, AND THE AGE OF 



THE PACIFIC SALMON. 



A .MEANS of determining definitely the age 

 of a fish at any time has long been sought 

 by ichthyologists and fish culturists. This 

 is of importance in finding out the ages at 

 which fishes reach sexual maturity, as well as 

 the lengtii of life of the various species. In 

 the case of the Pacific salmons of the genus 

 Oncorhynrhus the problem has a special sig- 

 nificance, since in all cases tiiese fishes die after 

 spawning,* and much discussion has been waged 

 over tile question of the age at which these 

 fisiies return to tile fresh waters to breed. 



It lias been discovered that the scales bear 



*.\ possible exception occurs in the case of certain 

 younjr male chinook snlmon, which mature precocious- 

 ly at 3 to 7 in. long, without having gone to sea. The 

 fate of these is not yet known. 



marks which indicate not 

 only the length of life, but 

 also the relative rate of 

 growth in different years. 

 This has been thoroughly 

 tested on the Atlantic sal- 

 mon in Scotland, and has 

 been shown to apply also in 

 the case of the trout and to 

 other fishes as widely separ- 

 ated as the carp, eel, bass, 

 cod and flounder. 



The eminent ichthyolo- 

 gist. Professor Ciias. H. 

 (jilbert of Stanford Uni- 

 versity, has recently pub- 

 lished the results of studies 

 on the Pacific coast sal- 

 mons** in a paper from 

 which the substance of the 

 present article is drawn. 

 \Miile the general character of the scale and its 

 markings are well enough known to the ichthy- 

 ologist, it may be well here to quote from Pro- 

 fessor Gilbert : "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 pro- 

 duced at the growing edge a delicate ridge upon 

 the surface of the scale, the successive ridges 

 thus formed being concentric. . . . each repre- 

 senting the outline of the scale at a certain pe- 

 riod in its development. Many of these ridges 

 are formed in the course of a year's growth, the 

 number varying so widely in difl'erent individu- 

 als and during successive years in the history of 

 the same individual that number alone cannot be 

 depended on to determine age. For this pur- 

 pose we rely upon the fact that the fish grows 

 at widely different rates during different sea- 

 sons of the year, spring-summer being a period 

 of rapid growth and fall-winter a season when 

 growth is 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 

 bv a band of closely crowded rings, the two 

 together eoiistituting a single year's growth." 

 (See the accompanying figure.) 



The matter is not quite as simple as it might 

 appear, however, for, as Professor Gilbert 



**Age at maturity of the Pacific Coast Salmon of 

 the Genus Oncorhvnohus. — Bull. V. S. Bureau of 

 Fisheries, Document No. 76T, March 20, 1913. 



