president's address. 23 



of the Sea ''^ arrived afc the conclusion that fishery investigations indi- 

 cated the probabiUty that the great periodic fluctuations in the fisheries 

 are connected with the fish larvae being developed in great quantities 

 only in certain years. Consequently they advised that plankton work 

 should be dix'eoted primarily to the question whether these fluctuations 

 depend upon differences in the plankton production in different years. 

 It was then proposed to begin systematic investigation of the fish 

 larvae and the plankton in spring and to determine more definitely the 

 food of the larval fish at various stages. 



About thei 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 larvae first 

 require extraneous nourishment and the period when such food is 

 available, it is highly probable that an enormous mortality 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 may depend not 

 so much upon a favourable spawning season as upon a coincidence 

 between the hatching of the larvae and the presence of abundance of 

 phyto-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 phyto-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 

 absorbed their food-yolk and started feeding on Diatoms. 



If, however, we take the case of one important fish — the plaice — we 

 can get some data from our hatching experiments at the Port Erin 

 Biological Station which have now been carried on for a period of 

 seventeen years. An examination of the hatchery records for these 

 years in comparison with the plankton records of the neighbouring sea, 

 which have been kept systematically for the fourteen years from 1907 



" Eapports et Proc. Verb. xix. December 1913. 



=>» Rapports et Proc. Verb. xx. 1914, p. 204. 



^1 For the purpose of this argument we may include in ' phyto-plankton ' 

 the various groups of Flagellata and other minute organisms which may be 

 present with the Diatoms. 



