180 THE RATE OF GROWTH [ch. 



fact, 90 cm. is, or was till lately, the median size of cod* in our 

 Scottish trawl-fishery; one-tenth are over a metre long and the 

 largest are in the neighbourhood of 120 cm., with an occasional 

 giant of 150 cm. or even more. But it has come to pass that fish 

 of outstanding size are seen no more save on the virgin fishing 

 grounds; a Greenland halibut, brought home to Hull in 1938, 

 weighed four hundredweight, was nearly two feet thick, and must 

 have been of prodigious age. 



There are other ways of determining, or estimating, a fish's age. 

 The Greek fishermen shewed Aristotle f how to tell the age of the 

 purple Murex, up to six years old, by counting the whorls and 

 sculptured ridges of the shell, and also how to estimate the age of 

 a scaly fish by the size and hardness of its scales ; and Leeuwenhoek 

 saw that a carp's scales J bear concentric rings, which increase in 

 number as the fish grows old. In these and other cases, as in the 

 woody rings of a tree, some part of plant or animal carries a record 

 of its own age; and this record may. be plain and certain, or may 

 too often be dubious and equivocal. 



The scales of most fishes shew concentric rings, sometimes (as in 

 the herring) of a simple kind, sometimes (as in the cod) in a more 

 complex pattern; and the ear-bones, or otohths, shew opaque 

 concentric zones in their translucent structure. The scales are 

 ''read" with apparent ease in herring, haddock, salmon, the otohths 

 in plaice and hake; but the whole matter. is beset with difficulties, 

 and every result deserves to be checked and scrutinised §. 



* As distinguished from "codling." 



t Hist. Animalium, 5476, 10; 6076, 30. 



X The carp-breeder is especially interested in the age of his fish; for, like the 

 brewer with his yeast, his profit depends on the rate at which thej'^ grow. 

 Leeuwenhoek 's and other early observations were brought to light by C. HoflFbauer, 

 Die Alterbestimmung der Karpfen an seiner Schuppen, Jahresber. d. schles. 

 Fischerei-Vereins, Breslau, 1899. 



§ Thus, for instance, Mr A. Dannevig says (On the age and growth of the cod, 

 Fiakeridirektorets Skrifter, 1933, p. 82): "as to the problem of the determination 

 of the age of the cod by means of scales and otohths, all workers agree that the 

 method is useful. But on a number of fundamental points there are just as many 

 divergences of opinion as there are investigators." 



