BARENTS SEA IN AUGUST, 1907. 83 



of warmest water and most rapid growth in the North Sea) the dark 

 ring is formed. In winter the growth of the otolith, as of the fish, 

 ceases. 



Immermann* has shown that these rings on the otoliths of the 

 plaice are purely optical effects, explicable by the regular changes in 

 the life conditions of the fish. 



Avoiding technicalities, the reason for the occurrence of apparent 

 white and dark rings is as follows : The whole otolith is composed of 

 layers of chalk substance deposited regularly as the growth of the 

 fish is in progress. Kestrained growth, as when cold water conditions 

 prevail, has the effect of crowding together these layers. As the 

 temperature rises, and the growth rate of the fish increases, so must 

 that of the otolith, and thus is effected a wider expansion of the layers 

 of chalk substance. The optical effect of this is that the crowded 

 rings, not permitting the passage of light, appear white, whilst the 

 expanded translucent layers appear dark by contrast. 



The reason for the narrowness of the dark ring, and the comparative 

 great width of the white ring in the otolith of the Barents Sea plaice, 

 is thus afforded. For the greater part of their annual growth- period 

 these fish are subjected to very low temperature (at the time of my 

 visit the bottom temperature was only two degrees Fahrenheit above 

 the freezing point), so that for only a short period would rapid growth 

 appear to take place, and it is not unlikely that this period coincides 

 with the culmination of the Atlantic flood. 



As has been pointed out before (p. 72), this expansion of Atlantic 

 water is in progress in the month of August ; in other words, biological 

 spring has commenced. Correlated with this physical phenomenon 

 the otoliths of the plaice slioio the commencement of a white ring at the 

 edge. In the North Sea by this time, according to investigations 

 (Wallace, Maier, etc.), the dark ring has commenced. Thus in the 

 two regions of the ocean, many hundreds of miles apart, it can be 

 seen on the otoliths of the plaice that biological spring in the one 

 region coincides with midsummer in the other, as we know to be the 

 case from hydrographic observations referred to above. 



For suggestions and help in the investigation of the small collec- 

 tion of otoliths, I am greatly indebted to Dr. Wallace, who also kindly 

 undertook an independent investigation of a number. Our two re- 

 sults were in close agreement, the chief discrepancies being in the 

 case of otoliths on which certain of the rings appeared to split in a 

 doubtful manner. 



* Beitrdge zur Alter shedimm.ung der Fische II. Die innere SlruMur dcr Sclwllen 

 Otolithcn. Arb. d. wisseii. Koni. f.d. Intern. Meeres. No. 6. Bd. vi., 1907. 



