CH. X] THE CONDITIONS OF LIFE IN THE SEA 247 



(3) Line fisheries in the North Sea. If the reader will 

 refer to the chart of depths of the North Atlantic he will see that 

 north from Shetland the bottom of the North Sea slopes down 

 gradually into the depths of the Norwegian Sea. During most of 

 the year Atlantic water lies on this slope of the North Sea Plateau ; 

 at the bottom there is cold Arctic water ; while between this, and 

 the surface stratum of Atlantic water, there is a mass of water 

 which is intermediate in salinity to the two principal layers. This 

 stratum of mixed water is rich in copepods and other plankton, 

 and is inhabited by fishes such as the halibut and ling. Sometimes 

 the upper salt layer is thick and then the mixed layer lies far down 

 the slope ; sometimes it is thin, and then the intermediate layer 

 lies further up the slope. In June, when the Atlantic flooding is 

 at a minimum, the 35 per 1000 layer is thm and the fishermen 

 set their lines in comparatively shallow water. In the autumn it 

 is thick, and the lines are set in deeper water^. 



(4) Fisheries of the Barentz Sea. Every year the Barentz 

 Sea is flooded with relatively warm and salt water from the 

 European Stream, and every year a fishery for food fishes, such 

 as plaice, recurs in these remote seas. When the Atlantic water 

 sets in there is an immigration of food fishes into the Barentz 

 Sea. On the subsidence of this flooding of genial water the food 

 fishes desert the area, the trawling is a failure, and the benthic 

 fauna changes in character-. 



(5) Icelandic fisheries. Iceland is surrounded by water 

 which is part of the southerly-flowing polar stream. On its sea- 

 ward border this cold coastal water is mixed with water which is 

 warmer and denser, and which is part of the Irminger Current, an 

 offshoot of the European Stream. Just on the border region of the 

 two areas of water there is a mixed portion, where the salinity of 

 the Atlantic constituent is reduced slightly by the fresher Polar 

 water. Here the spawning cod is to be found, and this fish does not 

 inhabit the colder inshore zone. " In the course of the summer all 

 food fishes inhabiting this border region (of mixed water) pass to 

 the east with the general movement of the stream ^." 



1 Pettersson, Rappts. et Proc.-Verhauoc, vol. iii. Appdx. A, 1905. 



2 Breitfuss, Verhandl. Vth Internat. Zool. Congress. 



3 Schmidt, " Fiskeriundersogelser Island Faeroerne Sommer 1903," Skriften 

 Komm. Havundersogelser, i. Kjobenhavn, 1904. 



