A Closing, High-speed Plankton Catcher for Use in Vertical and 
Horizontal Towing 
B. M. Bary , 1 J. G. de Stefano , 2 M. Forsyth , 2 and J. van den Kerkhof 2 
A feature common to conventional conical 
nets is the speed at which they may be towed 
— one to two or three knots (1 kt. equals 1.85 
km. /hr.)— which is necessary to safe working 
of the gear and an undamaged condition of 
the catch. Increasing the speed of towing is 
believed to set up a zone of pressure which 
advances before the mouth of the net, pos- 
sibly warning or otherwise causing animals 
to avoid capture. 
Modifications in attempts to improve the 
efficiency of this basic type of net, include 
methods of opening and closing nets under 
water (Kemp, Hardy and Mackintosh, 1929; 
Marr, 1938; Barnes, 1953; Currie and Foxton, 
1957) ; alteration of its shape, as in the Hensen 
and Apstein nets (Svetdrup, Johnson and 
Fleming, 1942), and as made by Sheard 
(1941), and Barnes (1953); addition of flow 
meters to determine the volume of water 
filtered (Harvey, 1934; Clarke and Bumpus, 
1940; Arnold, 1952; Currie and Foxton, 
1957); and means of taking series of discrete 
samples (Hart, 1935; Motoda, 1952). 
A different approach involves the type of 
sampler which can be towed horizontally at 
higher speeds. The mouth in this type usually 
precedes the towing point (unlike conical 
nets) so that the sample is collected from 
undisturbed water; when combined with high 
speed through the water, this contributes to- 
wards collecting a wide range of organisms. 
In general, high-speed catchers incorporate 
a type of conical net, either in a rigid frame 
1 N. Z. Defence Science Corps, % Navy Office, 
Wellington; now at Oceanographic Laboratory, 78 
Craighall Road, Edinburgh 6, Scotland. 
2 Dominion Physical Laboratory, Lower Hurt, N. Z. 
Manuscript received June 12, 1957. 
(Cassie, 1956), or in a rigid (metal) container 
(Arnold, 1952; Gehringer, 1952). Usually the 
area of the mouth is much restricted relative 
to the area available to filtering the water, but 
in the Gulf Sampler III (Gehringer, 1952) the 
diameter of the mouth (1.6 inches) is almost 
that of the contained net. Flow meters may 
be incorporated as in the tail of the Scripps 
High Speed Sampler (together with a depth 
recorder); in the Gulf Sampler IA (Arnold, 
1952); and in mouth and tail of the Gulf 
Sampler III (Gehringer, 1952). To the best 
of our knowledge, no high-speed sampler can 
be closed when towing is completed. 
The Continuous Plankton Recorder (Hardy, 
1935) operates on the principle of a continu- 
ally renewed filtering area, and, although it is 
a high-speed sampler, is in a different category 
from those already discussed. 
Less attention has been paid to more rapid 
sampling by vertical tows. In one method 
(Hart, 1935) flights of small, conical nets are 
evenly spaced on a wire and are towed over 
a vertical distance equivalent to the spacing; 
they are then closed and hauled in. Samples 
are thus collected simultaneously from several 
levels. Motoda (1952) incorporates a series 
of collecting buckets in a frame; unused 
buckets successively lock on to the cod end 
of the net during a step-by-step rotation of 
the frame. Each bucket collects material only 
over a particular range of depth. None of this 
style of nets is towed faster than about one 
metre per second (two knots). 
Nets for vertical towing are usually con- 
structed somewhat differently from those for 
horizontal or oblique tows (Kemp, Hardy, 
and Mackintosh, 1929). To convert from the 
one use to the other may be impracticable, 
46 
