SOME KESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 463 



The otoliths are the hard, calcareous stones which are found in the ears 

 of all animals, but which are unusually large in bony fishes. The method 

 was elaborated a number of years ago. It depends on the fact that the 

 growth of the ear-stone or otolith is not regular, but varies from season 

 to season. So also with the bones of the fish, as for instance the 

 vertebraj. If the otolith be examined, even with the naked eye, it will 

 be seen to be built up of concentric layers. Every year a new layer is 

 added to those already laid down, and by counting the number of 

 concentric rings in tlie otolith, or vertebra, the number of years of age 

 of the fish can be determined. Up to the fifth year of life both 



N' OF Plaice 

 360 



520 



260 



240 



200 



I 2 A 4 S 6 7 S 9 10 II 12 13 14 IS 16 )7 Id 19 20 21 22 23 Cm 



Fig. 6. 



methods are reliable, but after this period the results are somewhat 

 uncertain. By the application of these two methods, particularly the 

 first, results have been obtained in all the countries participating in the 

 International Investigations, and the plaice of diflerent localities are 

 now being investigated, not only with respect to their abundance 

 according to the deptli of w^ater, but also with regard to size and age. 



A third method of age determination is a direct one, and depends on 

 the marking and liberation of a plaice, and its subsequent recapture. 

 This brings us to the consideration of the fish-marking experiments 

 which have now been carried out on a very extensive scale in Sweden, 

 Denmark, Germany, Holland, and England. This method of investiga- 

 tion is, of course, an old one, and was practised in Scotland by the 



