TY 2a, eee eo 
OF DIATOMS AND COPEPODA IN THE IRISH SEA. 201 
at a time when they are present in great abundance, are feeding on some 
prevalent form of Copepod, 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 Biological 
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 Piitter, 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-tishes. 
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 impinges 
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 
