SEA-FISHERIES LABORATORY. 183 



from April 5th (1907) to June 15th (1915), and an average or 

 central date would be about the middle of May. 



It is evident then that in most of these years the Diatoms 

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

 the fish larvae from the hatchery were set free. Looking at 

 the records for the individual years, we find that out of the 

 13 years (1907-1919) in nine cases (1907, 1910-12 and 1915-19) 

 the phyto-plankton preceded the appearance of the larvae, 

 and that it was only in the remaining four years (1908, '09, '13 

 and '14) that there was apparently some risk of the larvae 

 finding no phyto-plankton food, or very little. 



The evidence so far seems to show that if the fish larvae 

 are set free in the sea as late as March 20th, they are fairly 

 sure of finding suitable food ;* but if they are hatched as early 

 as February they run some chance of being starved. 



It would be interesting to know whether the fishes of the 

 four years noted above, when the first larvae preceded the 

 phyto-plankton increase by from 10 to 16 days, have been 

 poorly represented in the fisheries of subsequent years. It may 

 be, however, that any variations in the abundance of the year- 

 classes due to the quantity of food available for the larvae is 

 counterbalanced by that periodicity in the productivity of the 

 plaice fisheries which has been demonstrated by Dr. Johnstone, 

 and which is dealt with in the case of the Mersey Estuary by 

 Mr. Daniel in the present report. 



Our records for the present year (1920) are not yet fully 

 made up, but so far as observations show it appears to be the 

 earliest year of our series. The plaice in the pond must have 

 commenced spawning in January as we found hatched larvae 

 as early as February 9th, and we put a first batch of larvae out 

 to sea on February 23rd. The plankton contained some 

 Diatoms (BiddulpJiia and Coscinodiscns) as early as January, 



* All our dates and statements as to occurrence refer to the Irish Sea 

 round the south end of the Isle of Man. 



