22 . president's address. 



mackerel should not be able to select particular species of Copepoda 

 or other large organisms from the ma<;ro-plankton, and we have 

 evidence that they do. Nearly thirty years ago the late Mr. Isaac 

 Thompson, a constant supporter of the Zoological Section of this Asso- 

 ciation and one of the Honorary Local Secretaries for the last Liver- 

 pool meeting, showed me in 1893 that young plaice at Port Erin were 

 selecting one particular Copepod, a species of Jonesiella, out of many 

 others caught in our tow-nets at the time. H. Blegvad '^ 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 established between the Cornish pilchard fisheries and periodic 

 variations in the physical characters (probably the salinity) of the 

 water of the English Channel between Plymouth and Jersey." Appa- 

 rently a diminished intensity in the Atlantic current corresponds with 

 a diminished fishery in the following summer. Possibly the connection 

 in these cases is through an organism of the plankton. 



It is only a comparatively small number of different kinds of 

 organisms — both plants and animals — that make up the bulk of the 

 plankton that is of real importance to fish. One can select about half- 

 a-dozen species of Copepoda which constitute the greater part of the 

 summer zoo-plankton suitable as food for larval or adult fishes, and 

 about the same number of generic types of Diatoms which similarly 

 make up the bulk of the available spring phyto-plankton year after 

 year. This fact gives great economic importance to the attempt to 

 determine with as much precision as possible the times and conditions 

 of oc-currence of these dominant factors of the plankton in an average 

 year. An obvious further extension of this investigation is an inquiry 

 into the degree of coincidence between the times of appearance in the 

 sea of the plankton organisms and of the young fish, and the possible 

 effect of any marked absence of correlation in time and quantity. 



Just before the war the International Council for the Exploration 



*' Bep. Danish Biol. Stat. xxiv. 1916. 



" JouTii. Mar. Biol. Assoc. May 1918. 



" See E. C. Jee, Hydrografliy of the English Channel, 1904-17. 



