CRUSTACEA AND MAN 1 59 



So far we have been dealing with Crustacea as the direct food of 

 man, but they are probably of more importance as indirect food, 

 because they form a substantial part of the food of fish, particularly 

 the young stages of marine fish. Here are a few examples, almost 

 at random, from the large number available. 



On the trawling grounds off Plymouth the young stages of the 

 poor cod, Gadus minutus, feed almost entirely on copepods, particu- 

 larly Pseudocalanus elongatus. The adults feed to a large extent 

 on decapods, though fish may form up to fifty per cent of the 

 food. 



Baby pike, Esox lucius, in Windermere, feed mostly on small 

 Crustacea, but once a pike reaches a length of an inch and a half 

 it begins to feed on small fish. 



A survey of the feeding habits of young fish of the herring family 

 from waters near Bombay showed that Crustacea formed at least 

 fifty per cent, often much more, of the food of nearly all the 

 sixteen species examined. 



Sometimes, for part of a year, a single crustacean species may 

 form the food of a fish. A striking example has been observed in 

 the Baltic. On the south-west coast of Finland the smelt (Osmerus 

 eperlanus) feeds on mysids for most of the year, but during the 

 period from mid-November to mid-December most of its food con- 

 sists of mature males of the amphipod Pontoporeia affinis. This 

 peculiar situation is explained by the fact that Pontoporeia is mainly 

 a bottom-dwelling creature, but during the short winter breeding 

 season the males become active swimmers and so attract the atten- 

 tion of the smelt. 



Other important benefits that man gains from the Crustacea are 

 the products of the whaling industry. The most important whales 

 are those which have whalebone strainers inside the edges of their 

 mouths and feed on medium-sized Crustacea which they strain out 

 of the water. In the Antarctic the most important of the whale 

 food species is Euphausia superba, which forms enormous shoals 

 in the summer. 



Some seals also feed on Crustacea, and then in turn provide food 

 or clothing for man. There are also records of terrestrial mammals 

 feeding on Crustacea, perhaps the most surprising of these is the 

 habit of the wild swine near Trincomali, Ceylon, which come down 

 to the shore each night and dig up crabs. 



Another use that man has found for Crustacea is to provide food 

 for his pet fish. Living Daphnia is sold by aquarium dealers, and 



