Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 59

ÍSL. LANDBÚN.
j. agr. res. icel. 1978 10,2: 57-66
Design and calibration
of a salmon counter
BjöRN KRISTINSSON
Rafagnatœkni, Reykjavík
and
MARÍANNA Alexandersdóttir
Institute of Freshwater Fisheries, Reykjavík.
ABSTRACT
A resistivity counter is described. A novel feature is the counter sensor which is a mat made offive parallel,
uninsulated steel ropes placed directly on the river bottom. Calibration tests against a mechanical counter
in Ellidaár gave comparable results as long as the salmon were actively migrating across the sensor mat.
Data obtained in Ellidaár show a distinct diurnal migration pattern, with migration heaviest in the period
from sunset to sunrise.
DESCRIPTION OF THE COUNTER
Sensor unit
The flat mat electronic salmon counter is
intended for use in open shallow streams.
The sensor proper is of the linear resistive
type with the sensor mat made of five
parallel, uninsulated steel ropes kept
apart by plastic tubing. The mat is usually
produced in 10 m long units and is
stretched across the river bed supported
by the plastic tubing (Fig. 1).
The center rope carries a zero potential
and together forms, with the ropes on each
side, the main electrodes that sense the
presence of fish by an increase in conduc-
tivity. The outmost electrodes have the
function of compensating for conductivity
changes due to surface wave motion that
otherwise might give rise to false counts in
shallow water. The connection to the
electronic circuitry is shown in Fig. 2.
The unit is normally powered by stor-
age battery. The electrodes are energized
by an approximately 500 Hz current of
less than 1 volt potential. An initial ad-
justment is required to balance and com-
pensate the mat circuitry before use.
When a fish swimming upstream arrives,
its presence is sensed in sequence first by
the lower main electrodes and then the
upper main ones.
A spacing between the electrodes equal
to the average size of the fish to be counted
will give maximum sensitivity. The auxil-
liary electrodes are, however, so con-
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