The accuracy of volume estimation in filling a marked tank by means of a plankton pump has been determined. The method even in its simplest form is satisfactory under good conditions. Further refinements, in which the plankton was filtered off, after filling a modified tank (for example with an overflow device) would no doubt increase the accuracy.

A device is described by which, using an ex-N.F.S. pump, a constant (but variable by selection) volume of water can be taken without the use of large tanks.

A small number of sets of samples taken with adequate volume control indicate the same order of variability as net hauls. Comparison of the distribution of x2 for paired samples with nets and pumps also indicates variation of a similar type.

These results suggest that the variability of net hauls is not entirely due to the variations in the volume of water filtered. Further work is in hand to elucidate this question.