46 Fortieth Annual Report 



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Chart showing the eastern boundary of the area invaded by Salps in August 1921. 

 The position of the herring trawling fleet is indicated by the hatched area. 



Now, as a rule the annual increase in the volume or flow of the 

 Atlantic Stream penetrating the North Sea causes a gradual displace- 

 ment of the water of the previous year. The resultant water masses 

 are then cooled down during the winter months and fill the bottom 

 layers of the northern North Sea, to be replaced again the following 

 year. But only on three occasions since the inception of the 

 International Investigations, viz. in 1905, 1920 and 1921, have the 

 magnitude and direction of flow of this invading warm water of high 



