PLANKTON 271 



which was caught in the tow-net at the rate of about 6,000 in a 

 five-minutes' haul, and 6,000 was also the average 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 herriag and the mackerel 

 should not be able to select particular species of Copepoda 

 or other large organisms from the macro-plankton, and we 

 have evidence that they do. Thirty years ago (in 1893) the 

 late Mr. Isaac Thompson showed me that young plaice at 

 Port Erin were selecting one particular Copepod, a species of 

 JonesieUa, out of many others caught in our tow-nets at 

 the time. H. Blegvad in Denmark showed in 1916 that 

 young food fishes, and also small shore fishes, pick out certain 

 species of Copepoda (such as Harpacticoids) and catch them 

 individually — either lying in wait or searching for them. A 

 couple of years later Dr. Marie Lebour published a detailed 

 account of her work at Plymouth on the food of young fishes, 

 proving that certain fish undoubtedly do prefer certain 

 planktonic food. 



These Crustacea of the plankton feed upon smaller and 

 simpler organisms — the Diatoms, the Peridinians, and the 

 Flagellates — and the fish themselves in their youngest post- 

 larval stages are nourished by the same minute forms of the 

 plankton. Thus it appears that our sea-fisheries ultimately 

 depend upon the living plankton, which no doubt in its turn 

 is affected by hydrographic conditions. A correlation seems 

 to be estabhshed between the Cornish pilchard fisheries and 

 periodic variations in the physical characters (probably the 

 salinity) of the water of the English Channel between Ply- 

 mouth and Jersey. Apparently a diminished intensity in 

 the Atlantic current corresponds with a diminished fishery 



