AS AN INDEX OF AGE. 105 



Lility, the result of seasonal variation in the temperature and food- 

 supply. Of these two causes I am more inclined to give preponderance 

 to the lattar. 



After an examination of thousands of scales from these Gadidse 

 I hold that in ninety-eight cases out of a hundred one would arrive 

 at a very closely approximate idea of the age of the fish from an 

 examination of three or four well-developed scales taken from the 

 median region of the Hanks near the lateral line. Other areas of the 

 body show annual rings in the scales, but in the area mentioned 

 they are more easily determined than elsewhere. The percentage 

 given would be less in the case of fish more tlian four or five years 

 of age, for reasons already stated in a previous part of this paper. 

 In this connection, however, it has to be remembered that the deter- 

 mination of age for younger is of more practical importance than for 

 older fish. 



Corroboration of the truth of this hypothesis, that the ages of certain 

 marine fishes may be determined by means of annual rings on the 

 scales is afforded by the fact that the ages ascertained liy my method 

 agree in the main with the results calculated out by other workers who 

 have worked at the subject of the age of fish from a different standpoint. 

 In this connection I have quoted repeatedly from Cunningham and 

 Fulton, the latter of whom has worked out the subject in a very com- 

 plete manner after Petersen's method {Scottish Fishery Board, 1900 and 

 1901). 



Allowing for difference of locality of capture, my results agree in the 

 nu\in with those of Fulton, and they also afford many points of agree- 

 ment with Cunningham's results for fish of the English Channel. As 

 I have already stated, I had little previous knowledge of Mr. Cunning- 

 ham's and Dr. Fulton's results on the probable ages of fish, and it was 

 only after I compiled my own statistics on age-determination that I 

 compared them with those of other workers. 



It is almost impossible to acquire direct proof of this hypothesis, 

 the conditions of life in tank and aquarium being so unlike the natural 

 haunts, yet even with this, I have already mentioned that in the case 

 of a whiting which lived from shortly after hatching for thirteen and 

 a quarter months in a tank, the number of growth-lines formed on the 

 scale during that period roughly agreed (after allowing for a slower 

 scale growth under captive conditions) with the number of growth-lines 

 in the scales from sea whiting calculated to be about the same age. 



The labelling of Gadidte as adopted for other fish by the International 

 Sea Fisheries Scheme along with an examination of their scales would, 

 I believe, furnish a direct proof of this hypothesis. 



