OUR FOOD PROM THE SEA 199 



Apart from these and many other experiments in practical 

 fisheries exploitation and cultivation — in which the United 

 States of America have certainly led the way— all modern 

 scientific fisheries research is directed towards finding out the 

 conditions under which our food-fishes live, feed, migrate 

 and reproduce their kind, so as to determine the possibilities 

 and methods of preserving them from destruction, increasing 

 their number and even predicting when and where profitable 

 fisheries may take place. Marine zoologists and other men 

 of science in practically all civilised countries are gradually 

 building up by their observations a science of the sea — Oceano- 

 graphy—which, although it involves physico-chemical prob- 

 lems, is largely a matter of biological investigation with very 

 important bearings on fishery questions. 



As an example let us take the investigation of the ultimate 

 food-matters of the sea, a very large subject in itself. The fishes 

 that we eat, such as cod, plaice and herring, feed upon other 

 animals, and these upon still smaller living things. The plaice 

 is a bottom feeder hving chiefly upon cockles and other molluscs, 

 which in their turn feed upon Diatoms, lo\^ly microscopic plants 

 which are present in enormous profusion in sea-water at certain 

 times and places. The cod is almost onmivorous, but a 

 favourite food is evidently the larger Crustacea, which again 

 feed upon smaller invertebrates, and these— with perhaps some 

 intermediate stages — ^upon the Diatoms.^ The herring feeds 

 mainly upon Copepoda, small, somewhat shrLmp-lilce Crustacea, 

 which, like the Diatoms, may be present in the water in 

 enormous abundance ; and there is some evidence to show 

 that the shoals of herring which constitute a fishery may be 

 following and feeding upon special swarms of Copepoda. At 

 the time of the summer herring fishery off the Isle of Man in 

 August 1917 the fishermen themselves noticed that they 

 were catching the fish in greatest abundance in the neighbour- 

 hood of large discoloured patches of water containing in- 

 numerable reddish specks. They brought a bucket of this 

 water ashore to the Port Erin Biological Station, where the 



^ There are other still smaller organisms in sea-water, but I take the 

 Diatoms as a type of all the mioro-phyto-plankton. 



