INTERNATIONAL FISHERY INVESTIGATIONS 593 



expanded greatly in a north-easterly direction and touches 

 the coasts of Africa and Southern Europe, but not those of 

 Britain. In the following March the stream again contracts to 

 its former area. 



It is these variations of the Gulf Stream current which are 

 the causes of corresponding variations in the water circulation 

 of the North Sea, the Norwegian Sea, and the Baltic. There 

 is always a drift of relatively warm and dense water passing 

 over the Wyville-Thomson ridge — the elevated ridge of sea- 

 bottom which joins the Shetlands to the Faroe Isles. But 

 the volume of Atlantic water which passes over this ridge is 

 not always the same, but increases during the winter months 

 and attains a maximum in the spring. Although the water 

 in the " Norwegian branch of the European Stream" is heavier, 

 owing to its greater salt contents, than the water normally 

 present in the Norwegian and North Seas, it is, by reason of 

 its higher temperature, lighter, and so it floats on the surface. 

 Increasing in volume during the winter, it gradually covers 

 the greater part of the central area of the North Sea, attaining 

 its maximum expansion in March. Not only does it affect 

 the hydrographic condition of this area, but just the same thing 

 occurs in the outlying areas ; the Skagerak, the Baltic, the seas 

 round Iceland, and the remote White Sea become annually 

 invaded by a warm-water current, or heat wave, which at 

 different seasons, according to the locality, attains a maximum 

 and then subsides. It is the study of these seasonal variations, 

 their periodic and unperiodic changes, which has now become 

 so very important a department of marine research in relation 

 to the changes in the condition of the sea fisheries. A hydro- 

 graphic picture of the condition of the seas of North Europe 

 would be as follows : an increasing flow of warm and dense 

 water from the Atlantic during the winter months, covering 

 the sea to a variable extent and attaining a maximum according 

 to the topographical relations of the land and sea; then sub- 

 siding and becoming replaced in the North Sea by less dense 

 and much colder water from the land and from the Baltic; 

 and in the Norwegian and White Seas by cold and relatively 

 light water from the Polar Sea. 



Interesting as these changes are from the point of view 

 of the oceanographer, they possess much greater interest for 

 the student of meteorology and fisheries, for it is now beyond 



