president's address. 21 



In many oceanographical inquiries there is a double object. There 

 is the scientific interest and there is the practical utiUty — the interest, 

 for example, of tracing a particular swarm of a Copepod like Calanus, 

 and of making out why it is where it is at a particular time, tracing it 

 back to its place of origin, finding that it has come with a particular 

 body of water, and perhaps that it is feeding upon a particular assem- 

 blage of Diatoms ; endeavouring to give a scientific explanation of every 

 stage in its progress. Then there is the utility — the demonstration 

 that the migration of the Calanus has determined the presence of a 

 shoal of herrings or mackerel that are feeding upon it, and so have 

 been brought within the range of the fisherman and have constituted 

 a commercial fishery. 



We have evidence that pelagic fish which congregate in shoals, 

 such as herring and mackerel, feed upon the Crustacea of the plankton 

 and especially upon Copepoda. A few years ago when the summer 

 herring fishery off the south end of the Isle of Man was unusually near 

 the land, the fishermen found large red patches in the sea where the 

 fish were specially abundant. Some of the red stuff, brought ashore 

 by the men, was examined at the Port Erin Laboratory and found to 

 be swarms of the Copepod Teviora longicornis ; and the stomachs of 

 the herring caught at the same time were engorged with the same 

 organism. It is not possible to doubt that dm'ing these weeks of the 

 herring fishery in the Irish Sea the fish were feeding mainly upon this 

 species of Copepod. Some ten years ago Dr. E. J. Allen and Mr. 

 G. E. BuUen published '^ some interesting work, from the Plymouth 

 Marine Laboratory, demonstrating the connection between mackerel 

 and Copepoda and sunshine in the English Channel; and Farran''' 

 states that in the spring fishery on the West of Ireland the food of the 

 mackerel is mainly composed of Calarms. 



Then again at the height of the summer mackerel fishery in the 

 Hebrides, in 1913, we found ^^ the fish feeding upon the large Copepod 

 Calanus finmarchicus , which was caught in the tow-net at the rate of 

 about 60(X) in a five-minutes' haul, and 6000 was also the aver-age 

 number found in the stomachs of the fish caught at the same time. 



These were cases where the fish were feeding upon the organism 

 that was present in swarms — a monotonic plankton — but in other cases 

 the fish are clearly selective in their diet. If the sardine of the French 

 coast can pick out from the micro-plankton the minute Peridiniales in 

 preference to the equally minute Diatoms which are present in the sea 

 at the same time, there seems no reason why the herring and the 



'^ Journ. Mar. Biol. Assoc, vol. viii. (1909), pp. 394-406. 



" Vonseil Internat. Bull. Trimestr. 1902-8, ' Planktonique,' p. 89. 



'^ ' Spolia Runiana/ iii. Linn. Soc. .Journ., Zoology, vol. xxxiv. p. 95, 1918. 



