OCEAN CURRENTS 



185 



and less sait water from the Baltic the herring disappear into 

 deeper and warmer water, where they are immune from the 

 local fishermen. 



The Baltic Current has a yearly periodicity depending on 

 the drainage of fresh water from the land into the Baltic. The 

 maximum is in summer. Even in May the whole surface of 

 the Skager-Rack is covered with a thin laver of Baltic water. 

 In autumn this begins to recède, and at the end of winter the 

 Baltic Current is only felt as a narrow coastal stream, along 

 the Bohuslan and Norwegian shoals. Hère, again, the pre- 

 vailing winds can cause marked déviations from the normal 

 direction and strength of the current. 



FiG. 39. — ISOHALINES IN THE NORTH SeA. (FrOM KxUDSEX. 



Section south-west from Ekersund. 



With regard to the deeper currents in the North Sea their 

 prevailing direction is an easterly one. This is shown by the 

 water of high salinity found in the Norwegian Deep. The 

 gênerai direction of the isohalines shows that the bottom 

 current must run in a south-easterly direction. 



Experiments hâve been carried on by the Scottish Fishery 

 Boardwith "bottom trailers," strongglass bottles so weighted 

 as to drift along or just clear of the bottom of the sea. From 

 the North of Scotland and the Orkney Islands there is a drift 

 of the bottom water to the eastward to the North Sea banks. 



The currents of the Baltic Sea dépend to a large extent on 

 the wind, but also the increase and decrease of fresh water 



