180 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



suggestion that, if on occasions the larvae are hatched out 

 before their food, the phyto -plankton, is present in sufficient 

 abundance, there may then be an enormous mortality of larvae 

 which will affect the young fish population of that year and 

 greatly reduce the numbers of that particular year-class of 

 that fish in the commercial fisheries of successive years for 

 some time to come (depending upon the average age of that 

 species of fish). 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.* 



In a general way, the curve for the spring maximum of 

 pelagic fish eggs in the Irish Sea begins to rise late in February 

 and remains high throughout March and April, and occasionally 

 later. The Diatom curve also starts towards the end of 

 February and remains high throughout March, April, May and 

 June. There is evidently a general correspondence between 

 the two maxima, but is it 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. Although the 

 present Biological Station was erected in 1902, the first stock 

 of adult plaice could not be received into the spawning pond 

 until the winter of 1903-04 and the number of fertilised eggs 

 skimmed from the pond and of larvae set free from the hatchery 



* 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. 



