INTERNATIONAL FISHERY INVESTIGATIONS 597 



here refer to the investigations of the Russians in the Barentz 

 Sea, and to the corresponding work prosecuted by the Nor- 

 wegians in the seas round Iceland and in the Norwegian Sea 

 itself. Before 1902 fishery investigations were practically re- 

 stricted to such work as could be carried on in the immediate 

 neighbourhood of marine laboratories in inshore waters, and by 

 small vessels. Much indispensable knowledge was acquired by 

 these means, but it is obviously essential that if the conditions 

 of the sea fisheries are ever to be thoroughly understood, work 

 over the whole of the fishing grounds must be undertaken. 



Returning again to our typical fish, the plaice, we find that the 

 investigations carried out by the fishing steamers of the inter- 

 national organisation during the last five years have enabled us to 

 form a picture of the distribution of this fish which we could not 

 previously have constructed. The fishing experiments and the 

 study by the naturalists of the various countries of the statistics 

 afforded by the commercial fisheries have shown us how the 

 plaice is related, as regards its size and abundance, to the depth 

 of water in which it is to be found. We have found that, with 

 a few exceptional cases, the fish is smallest and most abundant 

 in shallow waters, and largest and least numerous in the deeper 

 parts of the North Sea. So constant is this relation that it is 

 almost always possible to state what is the size of plaice on any 

 fishing ground, given a knowledge only of the general depth 

 of water. On the Dutch coast it has been possible to draw 

 contour lines round the coast, and within each zone there are 

 only to be found plaice of a certain size. Expressed in language 

 borrowed from the mathematicians, we can say that the size of 

 the plaice on any area of varying depth of water is a function 

 of the depth. 



A method of investigation first developed by the international 

 naturalists on a large scale, that of marking and liberating living 

 fishes, has enabled us to form some ideas as to the ordinary 

 migrations made by the fish at different stages in its life history. 

 The study of the deposition of limy matter in the bones and 

 in the ear-stones has given us a convenient means of deter- 

 mining the age of the fish, so that by making fishing experi- 

 ments on a large scale, and by the examination of the ear-stones 

 or otoliths, and supplementing these observations by a method 

 of statistically grouping the fishes caught according to their 

 sizes, we are now able to map out the sea with reference to 



