SEA-FISHERIES LABORATORY. 229 
actual determination of the number of any species in the 
sea, that, as a solitary fact, has little scientific impor- 
tance, and must not be taken as the aim of quantitative 
work. It is the comparison of daily or weekly catches 
for a long period showing the seasonal fluctuations in 
production, and how this varies according as variations 
in the environment may take place (even the relative 
“cloudiness” of the atmosphere appearing to affect 
plankton production), that should provide a rich field for 
future research.* ‘There is, further, to be noticed, the 
inter-relation between one form and another, the phyto- 
plankton and the zoo-plankton; and lastly the diurnal 
oscillation from surface to deep waters, and vice versa, 
must be determined, and taken out from the other factors. 
Owing to the approximate nature of the calculations, it 
appears to me that whenever possible, a standard net 
should be used. ‘The Hensen and medium Apstein nets 
fulfil practically all conditions, and their coefficients have 
been carefully corrected, though it will be seen that this 
is not sufficient, and an improvement would be the calcu- 
lation of several coefficients to be adopted according to 
the various types of plankton being fished. The dimen- 
sions of the Apstein net and the coefficients are given at 
the end of this paper. 
The following is a summary of the theoretical method 
of determining the coefficient which was employed by 
Hensen.f ‘The filtration experiments are too compli- 
cated to be of general use, but by means of the resulting 
tables and the formulae, the method could be apphed to 
other nets, though to my mind the empirical methods to 
be discussed later are more satisfactory. 
* Kofoid. Bull. Illinois State Laboratory, Vol. VI, 1903, Art. IT, 
p. 483. 
+ Hensen. Methodik. Ergebnisse der Plankton Expedition der 
Humboldt-Stiftung, Ba. I, B. 1895. 
