SEA-FISHERIES LABORATORY. DAI 
be caught accurately by the pump and tube. Lohmann* 
states that with the speed of current attained in the tube 
in his experiments, this could not occur, and certainly 
the pump catches of copepoda are greater, apparently, 
than net catches, yet at the same time there must be an 
area round the pump-tube opening where the current is 
very slow, and here, certainly, such animals could move 
away. 
Probably the constant motion of the tube up or down, 
diminishes this factor, since the pump opening is con- 
tinually being brought into new regions. Species of 
Ceratium or fish eggs are reliably caught by both methods. 
In any ease, these organisms will be counted in the pump 
catch filtered by paper, and also in the net catch. 
Knowing as before the volume of water brought up by 
the pump, which the number counted in the pump catch 
represents, it is easy to calculate what volume of water 
was filtered by the net to give the number of the 
organisms chosen and counted in the catch of the latter. 
Before any new net is tested as above described, it should 
be thoroughly washed with soap and water and pressed. 
This causes a considerable shrinkage which would other- 
wise take place during the first catches made with the 
net. A net should, further, not be kept in use for too long 
a time unless the coefficient is again tested, and whilst 
in use should be well washed down after each catch. On 
some occasions a very heavy plankton may be present so 
that the walls of the net are soon covered with a layer. 
When this is the case most coefficients will be disturbed, 
unless a net is used which has a very large filtering area 
and a narrow mouth, and very short hauls are made. 
Finally, when making volumetric estimations of the catch 
*Lohmann. Wissen, Meeresuntersuch, Kiel Komm., N.F., 
Bd. X, Abt., Kiel, 1908, 
