Íslenskar landbúnaðarrannsóknir - 01.09.1976, Page 24

22 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
photoperiod returned in 1972 at a rate af
0,3%. This is a very low return rate but 30
times better than the year before. It should
be noted that the two-year-smolts subjected
to the same treatment only had a remrn rate
of 0,5% indicating íhat age of smolts by
itself is a very minor factor where return
rate is concerned. The return rate of tvo-year-
smolts kept in outdoor ponds for 35 weeks
before release was 4,6%, a satisfactory re-
mrn rate. As expected the groups, that had
high recapture rate in the electric fishing,
had a poor remrn rate from the sea.
THE 1972 EXPERIMENT
1. Treatment.
The smolts used for this experiment were
subjected to elevated temperamres during the
egg stage and were hatched about one month
earlier than a control group in namral tem-
peratures. They were fed Ewos dry feed in the
beginning and a mixmre of Ewos and Ice-
landic dry feed in later stages. In early Octo-
ber these fish had reached a length of 10 cm
and were transferred from intensive rearing
to the photoperiod experimental set-up.
The set-up consisted of several 250 watt
mercury light bulbs spuspended ca 1 meter
above the water surface. The amount of light
at the water surface was found to be on the
average 13 —1400 lux ranging from 200 to
2300 Iux depending on location in the race-
way. The lights were turned on in the mor-
ning and off at night automatically using an
electric clock. The clock was set every 3 days
to match sunrise and sunset according to the
almanac.
The fish were kept in fairly warm water
(9—10°C) until early December. Then the
temperature was Iowered to 2—3°C to imi-
tate winter. The temperature was raised again
to 10 °C on the lst of February.
The fish were kept in compartments in two
concrete raceways. The one-year-fish were in
one, and two-year-'fish in the other. The two-
year-fish got the same treatment as the one-
year-old except for temperamre that was
natural all through the winter. The compart-
ments could be expanded as the fish grew.
Both these groups were Carlin tagged in
late April and released with other smolts
from the dirt ponds in late May. It was
noted at the time of tagging that the one-
year-olds that were subjected to elevated
temperature were more silvery than their
two-year counterparts that were kept in na-
tural temperamre. This supports the findings
of Johnston and Eales (1970) who sug-
gested that temperamre was the primary
factor responsible for silvering of salmon
smolts.
2. The Electric Eishing.
During early August the outlet creek from
the Fish Farm was fished with electricity to
find out which groups, if any, did not migrate
to sea. In contrast to earlier years there were
no marked or tagged fish in the outlet at all.
It thus seemed that both groups in the photo-
period experiment had been ready for sea-
ward migration.
3- The Adult Returns.
The returns in 1973 of the experimental
groups and the combined returns of viable
two-year-smolts are shown in Table 2. The
returns of the one-year-smolts are about 2%
or seven times higher than the preceding
year. The two-year-smolts from the photo-
period experiment have somewhat higher re-
mrn rate than one-year-smolts but considera-
bly lower rate than the two-year-smolts from
outdoor ponds. This group strayed about
10% in contrast to only 3% that were re-