INVESTIGATION OF THE PLANKTON OF THE IRISH SEA, 143 
OBJECTS AND METHODS. 
The objects of the investigation were stated in the first Report (1907) to 
be :—(1) to study the distribution of the plankton as a whole and of its 
various constituents during the year, and (2) to arrive at some estimate of 
the representative value of the samples collected in the plankton nets. Other 
problems were taken up from time to time, but these two remained the chief 
objects during the whole investigation; the results obtained in regard to 
them will be the main points discussed in this paper. 
During the years preceding this work much attention had been directed, 
mainly as the result of the elaborate quantitative investigations of the Kiel 
School of Planktologists and the German Plankton Expedition in the 
Atlantic in 1889, to the supposed uniform distribution of the plankton 
organisms in sea areas under constant conditions; far-reaching con- 
clusions were arrived at in regard to the amount of food-matters in the sea, 
and the numbers of floating fish-eggs and of the fish-populations—all based 
upon the assumptions of a uniform distribution over wide areas and of the 
validity of a comparatively small number of samples taken at considerable 
distances apart. Therefore it became obvious that a fundamental point in 
the investigation was to determine, if possible, the catching power of various 
nets, not in the laboratory, but under working conditions at sea, and to make 
comparisons between the catches of two exactly similar nets worked 
simultaneously and also successively at short intervals apart in space and 
time. 
It is impossible to determine exactly how much water is strained by a net 
towed behind a ship. Even in traversing a measured distance at a known 
rate of towing with a net which has been measured and tested in the 
laboratory, there remain many other factors of unknown effect—such as 
uncaleulated currents in the water and irregular movements of the boat, and 
also the unknown degree of clogging of the meshes according to the amount 
and nature of the organisms caught *—which prevent accurate conclusions 
being drawn as to the number of diatoms, &e., per gallon of water or per 
area of sea-surface. But we can compare two hauls of the same net taken 
in rapid succession, or the hauls of two precisely similar nets towed side by 
side over the stern, or again one at each side of the ship ; or one may com- 
pare dissimilar nets and find one is consistently more effective than another 
either in the size of the catch or in catching some special type of organism. 
All these and many other experiments have been tried at Port Hrin, and 
tried over and over again, and the results are recorded on our tabular forms ; 
and although I do not attach importance to minor details f, still the wide 
* See also W. E. Allen, of the Scripps Institution, California, on the same subject, in 
‘Ecology,’ vol. ii. July 1921, p. 216. 
T For example, when diatoms are present in millions per haul it is only millions that 
matter, and when Copepoda are present in thousands I pay no attention to the odd hundreds. 
M2 
