September 9, 1920] 



NATURE 



49 



Age and Growth Determination in Fishes. 

 By Rosa M. Lee. 



FOR centuries past it has been known that the 

 scales of fishes showed concentric rings, and 

 later observers have concluded that from the 

 number of these rings the age of the fish could 

 be ascertained. 



This discovery was, however, not developed to 

 any great extent until the last ten or twenty years, 

 during which several scientific observers defin- 

 itely traced the connection between the number of 

 rings on the scales and the number of years of 

 life of certain fishes, the carp amongst fresh- 

 water fishes and certain Gadidae amongst salt- 

 water fishes having been intensively examined and 

 providing a first proof of the correctness of the 

 general theory. The salmon is another fish the 

 economic value and the intrinsic interest of which 

 led to particular attention being paid to its scales 

 in connection with its life-history by many ob- 

 servers, both of professional and amateur status. 



But it was the activities of the International 

 Council for the Exploration of the Sea that gave 

 the greatest impetus to the investigation of the 

 age of fishes, and most systematic researches have 

 been carried out with the purpose of correlating 

 the knowledge gained from scale investigations as 

 to the life-history of fishes with that derived from 

 other methods of investigation undertaken by the 

 Council. Amongst the workers of all the nations 

 who have investigated this subject, the Nor- 

 wegians stand pre-eminent for the very ex- 

 haustive researches they have carried on and for 

 the original developments they have made in the 

 methods used, both practical and theoretical. 

 They have concentrated largely on investigating 

 this question in the herring and salmon races, and 

 their contributions to the knowledge of the age 

 distribution and growth of these species have 

 been very valuable. 



A great deal of work on the same lines has 

 been carried on by the other countries (including 

 Great Britain) participating in the work of the 

 Council, not only in Europe, but also in America, 

 and the data now collected in regard to various 

 species are extensive enough to enable us to judge 

 the validity of the methods used and the import- 

 ance of the results obtained. 



The theory of age determination, briefly enunci- 

 ated, is that the periodic quickening and slacken- 

 ing of growth in the fish brought about by the 

 annual changes in the external conditions of their 

 life, viz. summer and winter temperatures, with 

 their accompanying abundance and .scarcity of 

 food, or of appetite for food, are reflected in the 

 formation of the scales, and are mainly evident as 

 rings marking the winter growth when develop- 

 ment of the scale either slackens or wholly ceases. 

 Such rings can be seen easily with the naked eye 

 or with a low-power lens on the scales of many 

 fishrs, and those on the salmon scale illiistr.itpd 



NO. 2654, VOL. 106] 



in Fig. 1 furnish an example of the general 

 appearance of winter rings. 



Such rings are caused by the definite patterns 

 into which the lines on the upper surface of the 

 scale are formed under changing conditions. The 

 arrangement varies for every species of fish, those 

 of the same family being somewhat alike. Indeed, 

 fish can be identified by their scales alone, as has 

 been done on tinned fish purporting to be salmon. 



There are striking differences in the appearance 

 of the winter rines in the scales of certain well- 

 known fishes. In the herring scale the striae 

 (surface lines) are close and equidistant; they pass 

 from side to side, and appear to be broken by 

 narrow, concentric, transparent rings. In the 

 haddock there are small quadrilaterals arranged 

 concentrically, occurring in zones of wide and 

 narrow growth. In the smelt and the eel this state 



Fig. t. — Winter rings on a salmon scale- 



of wide and narrow zones can be traced, but only 

 with difficulty, and the winter growth is indicated 

 by clear concentric spaces, where the markings 

 are either incomplete or wholly absent. In certam 

 river fish also the winter growth can be traced 

 by the unfinished lines rather than by the narrow- 

 ing or closing in of the lines, which often marks 

 winter growth. 



Underlying all the differences in the patterns on 

 the scales of the various fish a common principle 

 of formation can be traced, and this is undoubt- 

 edly due (in normal circumstances) to the annual 

 slackening or cessation of growth associated with 

 winter conditions. 



A study of many scales at all times of the year 

 has shown that the discontinuity or incomplete- 

 ness of the markings (striae or platelets) may be 

 a temporary characteristic of nearly all stages of 

 growth, and that when growth is active (summer 

 growth) the lines ultimately become complete, but 

 th.Tt in the winter thcv remain incomplete, and 



