Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 65

DESIGN OF A SALMON COUNTER 63
TABLE 1.
Results of calibration experiments in Ellidaár 1976
Date Trap Hand counted Mech. counter Released Electr. counter Difference DifT. Diff./No. count
Aug. 6—9. Open 47 78 -31 0.66
Aug. 10—13. Closed 87 134 60 57 + 3 0.05
Aug. 14—16. Open 158 77 + 81 0.51
Aug. 17—20. Closed 89 93 59 71 -12 0.20
Aug. 21—13. Open i 40 55 -15 0.38
Aug. 24—27. Closed 182 132 126 115 + 11 0.09
Sept. 31—3. Closed 45 38 29 63 -34 1.17
Sept. 4—6. Open 42 87 -42 1.00
Sept. 7—9. Closed 3 2 1 14 -13 13.00
tained in 1976, it was determined that the
resistivity counter needs additional test-
ing and calibration.
Diurnal activity.
The counter records the time of each
signal, making it possible to study the
diurnal activity of the fish in the river.
During the 1975 season the migration in
the Ellidaár was monitored continuously
from July 1 to September 8. The counts
were summarized over each weekly period
(Fig. 6). The salmon moved mainly at
night, with maximum activity occurring
just after sunset. In July, the peak month,
when there are no darkness hours in
Reykjavík, there was some activity during
the day, with a maximum at midnight.
During August and September the fish
were hardly active at all except between
sunset and sunrise. In 1976, a similar
diurnal pattern was evident in Grimsá,
another Icelandic stream which we
monitored (Fig. 7).
The diurnal activity is related to light
intensity. Huntsman (1948) reported that
in the Margaree River salmon ran chiefly
for an hour after dusk in shallow water.
Stewart (1973) found salmon activity was
greater during darkness hours in rivers
monitored in Lancashire.
DISCUSSION
Resistivity counters have been in use in
the U. K. since the 1950’s (Vibert 1967;
Hellawell 1973). Hellawell (1973)
tested an open channel resistivity counter
using photographic recording and esti-
mated that salmon caused 95% of the
counts.
Most Icelandic streams have low con-
ductivity (Table 2), making them suitable
for a resistivity counter. Ther mat counter
has already been installed in six Icelandic.
streams in West Iceland, in addition to
several tunnel counters in fish ladders.
The mat counter has the advantage of not
needing a supporting structure. Two
years experience with the Icelandic mat
counter have indicated some of the prob-
lems involved in the use of this type of
counter: