PLANKTON 273 



directed primarily to the question whether these fluctuations 

 depend upon differences in the plankton production in differ- 

 ent years. It was then proposed to begin systematic investi- 

 gation of the fish larvae and the plankton in spring, and to 

 determine more definitely the food of the larval fish at various 

 stages — aU of which was interrupted by the war. 



About the same time Dr. Hjort made the interesting 

 suggestion that possibly the great fluctuations in the number 

 of young fish observed from year to year may not depend 

 wholly upon the number of eggs produced, but also upon 

 the relation in time between the hatching of these eggs and 

 the appearance in the water of the enormous quantity of 

 Diatoms and other plant plankton upon which the larval 

 fish, after the absorption of their yolk, depend for food. He 

 points out that, if even a brief interval occurs between the 

 time when the larvse first require extraneous nourishment 

 and the period when such food is available, it is highly 

 probable that an enormous mortahty would result. In that 

 case even a rich spawning season might yield but a poor 

 result in fish in the commercial fisheries of successive years 

 for some time to come. So that, in fact, the numbers of a 

 " year-class " of fish may depend not so much upon a favour- 

 able spawning season as upon a coincidence between the 

 hatching of the larvae and the presence of abundance of 

 phy to -plankton available as food.^ 



The curve for the spring maximum of Diatoms corresponds 

 in a general way with the curve representing the occurrence 

 of pelagic fish eggs in our seas. But is the correspondence 

 sufficiently exact and constant to meet the needs of the case ? 

 The phy to -plankton may still be relatively small in amount 

 during February and part of March in some years, and it is 

 not easy to determine exactly when, in the open sea, the fish 

 eggs have hatched out in quantity and the larvae have 



^ For the purpose of this argument we include in " phyto- 

 plankton " the various groups of Flagellata and other minute 

 organisms which may be present with the Diatoms. 



T 



