Plankton Catcher — Bary ET AL. 
47 
thus making various sorts of nets almost ob- 
ligatory. Material thus collected could pe 
compared only with difficulty (e.g., see Winsor 
and Walford, 1936; Winsor and Clarke, 1940; 
Barnes and Marshall, 1951). More important, 
perhaps, is the limitation a multiplicity of gear 
imposes on assessment of relative efficiencies 
of vertical and horizontal tows as methods of 
quantitatively sampling a water column. 
The following account is of a plankton 
catcher designed for both vertical and hori- 
zontal sampling, and requiring a minimum 
of conversion from the rigging for one type of 
tow to that for the other. Its rigid outer casing 
is of fibreglass; it has a comparatively wide 
mouth, contains a conical net of stainless 
steel, and has been towed successfully at 
speeds up to 10 kt. horizontally, and 5 to 
6 kt. vertically. A valve, which can be closed 
by a messenger, stops the flow of water into 
the net, and there is a depth-flow meter in the 
tail. The diameters of mouth and valve differ, 
and it is the narrower aperture of the valve 
that controls the flow into the catcher. It 
accepts 89 per cent of a column of water, of 
an equivalent area of cross section, at speeds 
between 3 and 10 kt. The result encourages 
us to publish details of the catcher and its 
performance. The principles of its construc- 
tion appear sound, but some modifications are 
being considered in plans for a second in- 
strument. 
REQUIREMENTS FOR THE CATCHER 
The requirements on which the design of 
the catcher was based are: 
1. That collections made may be profitably 
subjected to quantitative analysis. 
2. That speeds of 6 kt. or faster, both ver- 
tically and horizontally, may be achieved. 
3. That more than one unit may be attached 
to a single, vertical wire. 
4. That no encumbrances precede the mouth 
during either vertical or horizontal towing. 
5. That it be versatile in catching power. 
To be a "quantitative” sampler, the mouth 
must be closable during vertical and hori- 
zontal towing. Second, the volume of water 
passing through the filter must be measured. 
Third, development of a pressure zone pre- 
ceding the mouth must be minimised by 
making flow through and around the catcher 
of low impedance. 
A low resistance to flow inside the catcher, 
together with a long streamlined body, con- 
tributes towards fast stable towing with least 
drag. During vertical towing the unit is at- 
tached by its side to a weighted wire, and its 
ability to tow in a stable manner is relied on 
to reduce the angle at which it hangs (on the 
vertical wire). Excessive drag would act to 
increase the angle. The inherent stability also 
ensures steady true towing at the higher speeds 
in a horizontal direction. 
In horizontal tows hauling from a point 
behind the mouth eliminates bridles, and 
other attachments, preceding the mouth. The 
advance of such attachments possibly warns 
or scares at least the more agile zooplankton. 
In vertical tows, the wire is to one side of the 
mouth. It may possibly become a relatively 
constant feature and therefore not be so dis- 
turbing to organisms. The mouth remains 
otherwise unobstructed. 
Fast towing, both vertically and horizon- 
tally, and the wide unobstructed mouth are 
aimed at providing collections which are 
widely representative of the organisms en- 
countered in a column of water. 
DESIGN AND CONSTRUCTION 
In Figures 1, 2, and 3 the construction and 
relationships of parts of the catcher are 
shown; Figure 4 is a photograph of the dis- 
mantled catcher, and Table 1 is a schedule 
of parts. 
The outer shell is rigid, 7 feet long, and can 
be dismantled into three major sections, nose-, 
body-, and tail-pieces (Figs. 1 and 4, A, B, 
C). The nose incorporates the opening of the 
mouth and the closing mechanism, the body 
contains the conical net, and the tail the 
depth-flow meter and stabilizing fins. Nor- 
mally the nose is bolted to the body, but the 
