INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 167 
is of real importance to fish. About half-a-dozen species of Copepoda 
constitute the greater part of the summer zooplankton suitable as food for 
larval or adult fishes, and about the same number of generic types of diatoms 
similarly make up the bulk of the available spring phytoplankton year after 
year. This fact gives great economic importance to the attempt to determine 
with as much precision as possible the times and conditions of occurrence of 
these dominant factors of the plankton in each year. An obvious extension 
of this investigation is an enquiry into the degree of coincidence between 
the times of appearance in the sea of the plankton organisms and of the 
young fish, and the effect of any marked want of co-relation in time and 
quantity. 
Most of the food-fishes in our seas produce floating (pelagic) eggs which 
hatch out as larve in spring at periods varying from February to May, 
according to the kind of fish and the temperature of the water—a low 
temperature retarding the spawning and subsequent development. The 
marked increase in the number of diatoms in the water which causes the 
vernal plankton maximum begins to show at the very period when the fish 
larvee are produced in greatest quantity, viz., March and April, in the Irish 
Sea. We have seen that the diatoms vary in their abundance and date of 
first appearance from year to year, and the question arises—Are they also, 
like the fish-larvæ, retarded in development by the low temperature, or the 
want of sun, in a late season, so that there comes to be some correspondence 
in date between the larvæ and the natural food upon which they are 
dependent after the absorption of their food-yolk ? 
Dr. Johan Hjort * has made the suggestion that if on occasions the larvee 
are hatched out before their food is present in sufficient abundance, there may 
then be an enormous mortality of larvæ, 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. 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 larva and the presence of 
abundance of phytoplankton t available as food. 
Ina 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. The diatom curve also starts towards the end of February 
and usually 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 
* Conseil Internat. Explor. de la Mer--Rapp. et Proc, Verb. xx., 1914. 
+ Including in “ phytoplankton " the Flagellata and other minute organisms which may 
be present with the diatoms. 
