11 - JOH.HJORTani)0. G. JOH. PETERSEN 



with these, great differences in the pliysical 

 conditions. One who knows the North Sea 

 fisheries will be able to conclude, from the com- 

 position of the catches on the market, from what 

 parts of the North Sea they come. It may be 

 added also, that the configuration and nature 

 of the bottom exercise a great influence on the 

 distribution and abundance' of fishes, apart from 

 the hydrographical conditions named. Many 

 fishes prefer stony, others sandy ground. Where 

 there is a depression in the bottom, the water- 

 masses often remain at rest, soft mud then collects 

 and the gaseous condition of the water becomes 

 unfavourable for animal life. On such places 

 accordingly, only small catches are made. 

 This is especially distinct in Swedish and 

 Norwegian fjords, but similar phenomena may 

 be found in the open North Sea. Thus for 

 example, hauls with the trawl have shown 

 that there is a barrier (Bressa shoal) in the 

 northern North Sea, which shuts off a more 

 southerly depression, about 40 m deep, on the 

 muddy bottom of which but few fish live, 

 whilst on the barrier itself and further north a 

 richer animal-fauna is to be found. 



5) In the foregoing, we have only con- 

 sidered the fishes which live on the bottom 

 and are caught there. Of even greater prac- 

 tical and scientific importance however, are the 

 migratory pelagic fishes. Carried along 

 by the various marine currents, these fishes also 

 distribute themselves on various bio-geographical 

 regions which are limited by the cur- 

 rents, not only vertically but also horizontally. 

 Thus, in the Norwegian Sea, the polar current, 

 the Atlantic current and the coastal streams; 

 in the North Sea, the Atlantic current, the 

 coastal streams of the western and southern 

 North Sea and the weakly saline Baltic stream 

 of the eastern North Sea, exercise different 

 influences on tlie geographical distribution of 

 the pelagic animals and fishes. 



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