636 DEPTHS OF THE OCEAN 



of harling, like the mackerel fishery in the North Sea. It is 

 carried on only during the day, some of the fish weighing over 

 thirty pounds. This is the only fishery I know of in the open 

 ocean over deep water and away from the coast banks, and the 

 species captured visit the coast banks, at all events, some time 

 during the year. 



Among pelagic fishes, however, the sardine is the most 

 important to the fisheries on the Atlantic coast banks, and it is 

 captured in the same area as the Atlantic bottom fish, i.e. from 

 the Channel along the coasts of Spain and Portugal and Africa. 

 The sardine, the bonito, and the tunny are here probably the 

 only Atlantic pelagic species of economic importance. 



B. The Northern Pelagic Communities 



In the ocean we find no sharply defined border between the 

 animal-communities belonging respectively to the tropics and 

 the polar seas ; on the contrary, there are numerous transitions 

 between the extreme conditions of life peculiar to the tropics 

 and the polar regions. It is therefore difficult to classify the 

 communities, and this difficulty is intensified by the fact that 

 most records note merely the occurrence or non-occurrence of 

 certain organisms and not their quantitative occurrence — a vital 

 point in discussing questions of distribution. If I attempt to 

 separate the genuine Atlantic from the northern pelagic animal- 

 communities, it is because I feel that in this way we shall 

 actually gain a better conception of their main features. I 

 believe that a division of this kind will coincide generally with 

 the limit drawn between the areas of distribution peculiar to 

 the southern and northern bottom fish on the Atlantic coast 

 banks, viz. the isotherm of 10° C. at 100 metres, running from 

 the Channel, south of Ireland, skirting the south coast of Iceland, 

 and thence to the United States. 



Among northern communities it is impossible to separate 

 oceanic and coastal communities so sharply as among Atlantic 

 communities, probably because northern communities are chiefly 

 restricted to comparatively small areas, and the substances 

 carried from the land vary in quantity and quality, giving rise 

 to corresponding variations in the food supply. Neither is the 

 vertical distribution so easily defined as in the Atlantic, certain 

 species having a very different vertical distribution in different 

 areas. 



It is extremely important for a true conception of the 



