30 THE FISHING INDUSTRY 



and nitrites are available in larger quantities for the 

 nourishment of the plankton. 



All the great fisheries of the world are prosecuted in cold 

 or temperate seas ; as examples of this we have the Banks 

 of Newfoundland, the cod fisheries of Norway, and the great 

 trawling grounds of the North Sea and the North Atlantic. 



All fish, during the larval stage of their development, 

 feed first upon the contents of the yolk sac which, 

 when they are hatched, is attached to their ventral 

 surface. When the yolk is absorbed, the larvae feed 

 upon the microscopic plankton that abound in the 

 water on every side. The surface water, with its 

 warm temperature, high plankton content and sunlight, 

 forms an ideal nursery for the very young fish of all 

 species. Demersal fish, as they complete the larval 

 stage of their development and descend into deeper 

 water, have to rely for their food either upon the various 

 species of young shellfish and Crustacea that drop from 

 the surface water as they develop, or hunt for their food 

 amongst the small fish, mollusca, Crustacea, worms and 

 seaweeds of the sea-bottom. Plaice feed chiefly upon 

 cockles and other mollusca, which in their turn feed upon 

 diatoms. The cod is almost omnivorous, greedily 

 devouring small fish, Crustacea, worms or mollusca ; 

 its favourite food, however, is shrimps and prawns. 

 These, in their turn, feed upon smaller invertebrates, 

 for example small jellyfish and larval molluscs, and 

 these upon microscopic plankton. 



Pelagic fish, herrings and mackerel, feed almost 

 entirely upon the larger plankton, mainly copepoda 

 (small, shrimp-like Crustacea). These may be present 

 in the surface water in enormous quantities at certain 

 times. In many cases, shoals of herring or mackerel 

 probably follow special swarms of copepoda. Mackerel 

 also feed upon young fish, hermit crabs, and prawns. 



