SEA-FISHERIES LABORATORY. 179 



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. 

 "It is this fact that gives great economic importance to the 

 attempt to determine with as much precision as possible the 

 times and conditions of occurrence of these dominant factors 

 of the plankton in an average year '' (Report No. XXVII, 

 p. 32). An obvious further extension of this investigation 

 is an enquiry 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. 



Most of the food-fishes in our seas produce floating (pelagic) 

 eggs, which hatch out as larvae in spring at periods varying 

 from February to May* according to the kind of fish and the 

 temperature of the water — a low temperature in general 

 retarding the spawning and subsequent development. The 

 marked increase in the number of Diatoms (phyto-plankton), 

 especially Chaetoceras, which causes the vernal plankton 

 maximum, begins to show at the very period when the fish larvae 

 are produced in greatest quantity, viz., March and April, in the 

 Irish Sea. The Diatoms vary somewhat in their abundance 

 and date of first appearance from year to year, and the question 

 arises — are they also, like the fish larvae, retarded in develop- 

 ment by the low temperature in a late season so that there comes 

 to be some correspondence in date between the larvae and fche 

 natural food upon which they are dependent after the absorption 

 of their food-yolk ? 



Dr. Johan Hjortf has recently made fche interesting 



*A few exceptional occurrences extend the range from January bo 



September. 



fConseil Internal. Explor. de hi Met. Rapports <' Proch-Vtrbaux. XX, 

 p. 205, 1914. 



