FISHERY PROBLEMS 169 



is based upon this principle. It is easy to select instances 

 of fish upon which their hydrographic surroundings pro- 

 duce a visible effect. 



The codfish spawns in winter and in comparatively 

 warm waters. In Iceland it finds them between Port- 

 land and Cape Skagen ; it also finds them in the fjords 

 to the west. Spawning over, it makes for the northern 

 shores, to regain the cold waters in which it lives. 

 According to the old Swedish chronicles, the herring 

 of the Kattegat are subject to periodical revolutions. 

 For some sixty consecutive years they show themselves 

 along the shores ; then they depart, and remain absent 

 for another sixty years. They return again, and again 

 depart, and so on through the centuries. This is per- 

 fectly true. M. Cligny has given us an excellent summary 

 of the investigations conducted in this connection by the 

 Scandinavian oceanographers : " The Skagerack is a 

 cross-roads in which three distinct currents meet with 

 a regular periodicity ; in spring, after the snows are 

 thawed, when the rivers are swollen, the Baltic over- 

 flows, and fills the straits with water which is noticeably 

 deficient in salt. Under a very thick layer of this Baltic 

 water we find a thin layer of average salinity, similar 

 to that of the North Sea, and lower still a very salt 

 layer, said to be oceanic water, which comes, there are 

 reasons for thinking, from the North Atlantic, between 

 Iceland and Scotland. 1 Matters remain in this condition 

 during the summer ; but in autumn the North Sea water 



1 Salinity : Oceanic water, -035 ; North Sea water, -034 ; water of 

 the inshore banks, -032 to -039. The plankton of the first and 

 second consists more especially of diatoms; that of the third of 

 crustaceae, entomostraceae, and ciliata. Duhamel du Monceau 

 states that the sardine deserted the neighbourhood of Saint-Jean- 

 de-Luz from 1759 to 1774. 



