58 
haul, because of the falling time of the mes- 
senger. However, they do not greatly affect 
the very considerable saving of time accruing 
from the relatively rapid lowering, hauling, 
and recovery of the catcher from, and its 
return to, the water, during a series of vertical 
tows. 
Before quantitative investigations can ap- 
proach absolute values, as opposed to relative 
values, the efficiency and characteristics of the 
catching gear should be known. Currie and 
Foxton (1957) have provided pertinent data 
for their new quantitative (Nansen-type) net, 
but the equivalents do not seem to be avail- 
able for high-speed samplers. The meter for 
such a sampler requires calibrating for flow, 
but in a situation remote from influences of 
the gear which may interfere with its func- 
tioning (e.g., in a calibration tube), and over 
the speeds at which the sampler is likely to 
be used. Values obtained must be considered 
in relation to the flow through the sampler 
itself, with and without nets included, and 
again over the relevant range of speeds. The 
proportion of the column of water (of unit 
length and of a cross-section equal to that of 
the controlling orifice) accepted by the gear 
under conditions of differing rigs and speeds 
now can be estimated fairly accurately. That 
is, the efficiency of the catcher can be deter- 
mined. The optimum towing speed can now 
be established, together with the effects on 
flow of nets of differing meshes, or of clog- 
ging. With these data, one may reasonably 
study whether the catcher is collecting a rep- 
resentative sample of the organisms it en- 
counters, and thus make an appreciation of 
the true density of the plankton population 
being investigated. 
SUMMARY 
A plankton catcher is described which has 
been towed successfully at speeds up to 10 
kt. horizontally, and 5 to 6 kt. vertically. It 
can be closed during either tow. To convert 
from the rigging for one type of tow, to that 
for the other, is rapid and easy and is believed 
PACIFIC SCIENCE, Vol. XII, January, 1958 
not to alter the catching ^ability of the unit. 
A depth-flow meter is included in the 
catcher. It has been calibrated by towing first 
when mounted in a tube of equal diameter 
to the mouth of the catcher (9 in.), and 
second, in the catcher, at speeds between 3 
and 9 kt. The catcher was towed without a 
net included, and successively with nets of 
10 and 40 meshes to the inch. Data from these 
tests show that the valve of the closing mech- 
anism (714 in. diameter) controls the flow 
into the catcher; about 89 per cent of the 
water presented to the valve is accepted. A 
net of 10 meshes per inch further reduces 
flow by 11.5 per cent, and one of 40 meshes 
by 20.4 per cent. Frictional resistance to flow, 
offered by the meshes, is believed to be re- 
sponsible as the filtering area of either net is 
more than adequate to filter the quantity of 
water presented, at speeds to 10 knots. 
Plankton is mostly alive and undamaged. 
Indications are that a representative range of 
organisms is being captured, including small 
squid and pelagic fish and the larger pelagic 
Crustacea. 
ACKNOWLEDGEMENTS 
We are especially grateful to Mr. W. H. 
Ward, Director, Dominion Physical Labor- 
atory, D.S.I.R., Wellington, for his interest 
and active assistance at all stages of the de- 
velopment and trials of the catcher, and for 
permitting the work to be continued under 
the auspices of the laboratory. Thanks are also 
due to Mr. I. Dick, Dr. E. I. Robertson, and 
Mr. J. W. Brodie, D.S.I.R., for enabling the 
project to be carried out, and to Messrs. N. 
P. Croft Ltd., Wellington, for their attention 
and care with molds and the fibreglass 
castings. 
It is with pleasure that we acknowledge the 
assistance of the Royal New Zealand Navy 
through the Staff Officer and Ratings of the 
R.N.Z.N. Volunteer Reserves Division, Well- 
ington, for enabling numerous trials to be 
conducted from the naval motor launch, 
H.M.N.Z.S. "Olphert”; the Captain, Officers, 
