2;U PHYSICAL AND BIOLOGICAL CO^'DITIONS IN THE NORTH SEA. 



the Baltic and the North Seas." Professor Heiucke has discussed the 

 fish fauna of Heligoland, its composition and sources, in an interesting 

 paper iu the series issued under the title of " Wissenschaftliche Meeres- 

 untersuchungeu," by the staff of the Biological Station at Heligoland, in 

 association with the Commission for the Investigation of the German 

 Seas, at Kiel. Professor Heincke's paper is contained in End. I., Hft. 1 

 of this series (1894), and in the same volume are a number of papers 

 dealing on similar lines with other divisions of the marine fauna of the 

 Heligoland Bight. 



It will be most convenient and logical to start the present discussion 

 with a consideration of the results of Professor Pettersson's work. He 

 found that the Skagerack and Cattegat were filled with layers of water 

 distinguished from one another by differences of salinity, and that the 

 lower layers entered the channel as under - currents, and could be 

 recognised at the surface somewhere in the North Sea, 



The different waters he distinguishes are the following : 



1. Ocean-water of 35 per thousand salinity or more. 



2. Water of from 34 to 35 per thousand salinity. On account of its 

 extension over a great part of the North Sea, this is called North Sea 

 water. 



3. Water whose salinity is 32 to 34 per thousand. This forms a 

 broad edging along the coasts of Holland, Germany, Denmark, and 

 Norway, and is named by Pettersson "bank-water." I shall prefer to 

 distinguish it for the present purpose as coast water, 



4. Water from 30 to 32 per thousand salinity, or less, belonging to 

 the outflowing stream from the Baltic, 



The numbers of course signify the parts of salt by weight in a 

 thousand parts of the water. 



Now, the oceanic water fills the central part of the North Sea as far 

 as the Dogger Bank from bottom to surface. Towards the east it does 

 not reach to the surface, but fills the bottom of the deep channel which 

 extends along the Norwegian coast and into the Skagerack. 



The North Sea water is found in the North Sea south-east and west 

 of the Dogger Bank, and along the coasts on each side of the North 

 Atlantic Ocean. In the Skagerack it lies over the oceanic water, and is 

 not found at the surface, except in a band along the north coast of 

 Denmark. The North Sea water flows into the Skagerack principally 

 in spring and summer. 



The coast water flows into the Skagerack most abundantly in 

 autumn and winter, when it reaches a considerable thickness, and 

 predominates at the surface in the central part of the channel. In 

 summer time the quantity of this water present is very much reduced. 

 It is then displaced by Baltic water. 



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