24 PRESIDENT S ADDRESS. 



to 1920 inclusive, shows that in most of these years the Diatoms were 

 present in abundance in the sea a few days at least before the fish 

 larvae fi'om the hatchery were set free, and that it was only in four 

 years (1908, '09, '13, and '14) that there was apparently some I'isk of 

 the larvae finding no phyto-planktcn food, or very little. The evidence 

 so far seems to show that if fish larvte are set free in the sea as late as 

 March 20, they are fairly sure of finding suitable food ; ^^ but if they 

 are hatched as early as Febiniary they run some chance of being 

 starved. 



But this does not exhaust the risks to the future fishery. C. G. 

 JoR. Petersen and Boysen-Jensei\ in their valuation of the Limfjord"' 

 have shown that in the case not only of some fish but also of the larger 

 invertebrates on which they feed there are marked fluctuations in the 

 number of young produced in different seasons, and that it is only at 

 intervals of years that a really large stock of young is added to the 

 population. 



The prospects of a year's fishery may therefore depend primarily 

 upon the rate of spawning of the fish, affected no doubt by hydrographic 

 and other environmental conditions, secondarily upon the presence of a 

 sufficient supply of phyto-plankton in the surface layers of the sea at 

 the time when the fish larvae are hatched, and that in its turn depends 

 upon photosynthesis and physico-chemical changes in the water, and 

 finally upon the reproduction of the stock of molluscs or worms at the 

 bottom which constitute the fish food at later stages of growth and 

 development. 



The question has been raised of recent years — Is there enough 

 plankton in the sea to provide sufficient nourishment for the larger 

 animals, and especially for those fixed forms such as sponges that are 

 supposed to feed by drawing currents of plankton-laden water through 

 the body ? In a series of remarkable papers from 1907 onwards Putter 

 and his followers put forward the views (1) that the carbon require- 

 ments of such animals could not be met by the amount of plankton 

 m the volume of water that could be passed through the body in a 

 given time, and (2) that sea-water contained a large amount of dis- 

 solved organic carbon compounds which constitute the chief if not 

 the only food of a large number of marine animals. These views 

 have given rise to much controversy and have been useful in stimu- 

 lating further research, but I believe it is now admitted that Piitter's 

 samples of water from the Bay of Naples and at Kiel were probably 

 polluted, that his figures were eiToneous, and that his conclusions 



^- All dates and statements as to occurrence refer to the Irish Sea round 

 the south end of the Isle of Man. For further details see Eeport Lanes. Sea- 

 Fish. Lab. for 1919. 



-^ Report of Danish Biol. Station for 1919. 



