304 BULLETIN OF THE BUREAU OF FISHERIES. 



Mention should be made here of the \vork of the investigators of the Kommission 

 zur Untersuchung der deutschen Meere, Reibisch (1899), Jenkins (1902), Heincke (1905, 

 1908), Maier (1906), and Immermann (1908). These investigators worked chiefly on the 

 sole, cod, and turbot, while Wallace (1907, 1909, 191 1), in England, worked on the plaice, 

 employing the otoliths and bones as means of age determination. The result of this 

 work seems to show that not only are age indications to be found on scales, but on the 

 otoliths, opercula, and bones. Since these structures reveal age only after they are 

 prepared by special technique, it is evident that they can never be employed in the ex- 

 amination of large numbers of specimens, as can the scales. These structures have served 

 the useful purpose of verifying, to a certain extent, the evidences found on scales. 



In spite of all this work, there remain doubtful points. Heincke (1908) cites 

 numerous instances of fishes that were very old, but undersized, along v/ith examples 

 of variation in size among fishes of this same age, but of different sex or from different 

 localities. Yet he fails to show that these variations are in the number of age rings 

 rather than in age. He concludes that the number of age rings is normal and correct 

 and that growth in these cases is abnormal; but from his data he might as well have 

 concluded that the size was normal but the number of rings abnormal. 



APPLICATION OF SCALE CHARACTERS TO ACE DETERMINATION. 



The idea that the age and life history of fishes may be determined by their scales 

 has given rise to much investigation. Each method has been investigated and an 

 attempt has been made to find other indications of age on scales. 



The different means of determining age with more or less accuracy are: 



(i) A cotmt of the annuli aided by — 



(a) Polarized light. 



(b) The selective action of picrocarmine stains. 



(c) The origins of the radii. 



(2) Identification of year groups by measurements of length and weight. 

 These methods may be used in combination. 



COUNT OF ANNULI. 



It has been contended that, at least for some species, growth does not proceed 

 uniformlv, but that during the winter and in other seasons, because of lack of food or 

 because of injuries or other causes, growth takes place more slowly than in summer or in 

 seasons of more abundant food supply or when conditions are otherwise more favorable. 

 Such changes of conditions as well as certain peculiar habits are said to leave their 

 marks on scales. From the investigations of Johnston, Gilbert, and Dahl on salmon and 

 trout, and Hoffbauer on carp, it would seem that circuli appear at fairly regular intervals 

 of time while the growth of the scale in width depends on the growth of the fish. The 

 appearance of the circuli at regular intervals of time while the scale increases more 

 rapidly in size in summer than in winter would produce concentric areas in which the 

 circuli were close together alternating with areas in which they were farther apart. 

 The earlier investigators considered these areas in which the circuli are not widely 

 separated "winter bands," assuming that the fish grew less rapidly in winter than in 

 summer, thus producing rings analogous to the annual rings in tree trunks. 



