OF DIATOMS AND COPEPODA IN THE IRISH SEA. 123 



at a time when they are present in great abundance, are feeding on some 

 prevalent form of Gopepod, such as Calanus or Temora. Other similar cases 

 could no doubt be quoted and are known to marine biologists. 



Then, as to demersal fish — young plaice, after their metamorphosis, feed 

 chiefly on the smaller Copepoda, while in younger stages the post-larval 

 plaice feeds upon Diatoms. We have noticed at the Port Erin Biolooical 

 Station the post-larval plaice with its stomach showing of a golden brown 

 colour from the Diatoms with which it was filled, and we have watched in a 

 shallow pond the metamorphosed young plaice darting backwards and for- 

 wards pursuing, catching, and devouring the individual Copepoda. It is 

 known that these Copepoda in their turn feed in part at any rate on Diatoms, 

 so our two main constituents of the plankton are undoubtedly concerned 

 in the nourishment of either young or adult fishes useful to man. 



The association of shoals of fish with abundance of plankton is the 

 result of the fact that, in order to get an adequate quantity of planktonic 

 food, the fish must seek out and capture the Copepoda. In other words, the 

 fish must go where the plankton is abundant and must in its movements 

 follow the movements of the shoals of Copepoda. It is the very poverty 

 of the plankton in some sea-areas, insisted on by Putter, Lohmann and 

 others, which makes it necessary for plankton-eating fish to move about 

 in search of more abundant supplies. 



Consequently it is of importance to show, as we now can, that in our 

 coastal seas at least, where the fisheries we are interested in take place, the 

 plankton is not uniformly distributed. Many of the Copepoda occur very 

 definitely in local swarms, and various localities and depths are characterised 

 at the different seasons by particular assemblages of plankton. It is 

 therefore reasonable to believe, in view of the facts given above as to the 

 association of fish and plankton, that these variations in the distribution 

 must have a marked effect upon the presence and abundance of at least 

 such migratory fish as herring and mackerel, and also of the shoals of post- 

 larval young of many of our other food-fishes. 



No less than three masses of sea-water of different origin and character 

 may enter or affect the British seas in varying quantity, viz. : — (1) Arctic 

 water such as normally surrounds Iceland and the east of Greenland, and 

 may extend further southwards and eastwards towards Norway, the Faroes 

 and Shetlands ; (2) Atlantic (" Gulf-stream Drift ") water which impinoes 

 on the western shores of Ireland, and may flood the English Channel and 

 extend round the Shetlands or down into the North Sea ; and (3) " Coastal " 

 water such as flows out from the Baltic and, mixed with the other waters 

 bathes the coasts of N.W. Europe generally, and to a large extent surrounds 

 the British Islands. 



The Irish Sea may be regarded as primarily an area of coastal water 

 which is, however, liable to be periodically invaded to a greater or less extent 



