Íslenskar landbúnaðarrannsóknir - 01.09.1976, Síða 10

8 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
are, however, very much lower than those
found by Guðjónsson, and Carlin’s re-
sults are intermediate between the two. As
an example we can see that the return-rate
for a 15 centimeter fish according to the
1971 experiment is ca. 4%. Carlin (1969)
has a value of 7 % for this length group but
Guðjónsson (1972) ca. 12%. These data
are compared in graph 1. It has been taken
into account that Carlin (1969) measured
the total length of his smolts. It does not
seem to make much difference for this pur-
pose. More recent information from the 1973
experiment suggests very pronounced varia-
tion between years.
The 1972 Tagging Experiment.
1. Groups Tagged.
Groups tagged and the major experiments
conducted are shown in Table 4.
As in 1971 there was a great deal of
tagging of two-year-old smolts to get a
better idea of the year-to-year variation in
returns from these releases on a production
basis. One group of two-year-old smolts was
tagged with a modified Carlin tag using
locally made polyethylene thread (group 5).
In October of 1971 a group of prospective
one-year-old smolts and a control group of
two-year-olds were subjected to normal photo-
period, using artificial light. The lights consi-
sted of several 250 watt mercury light bulbs
suspended ca. 1 meter above the water surface
The amount of light at the water surface
was measured in lux, and was found to be
on the average 1300—1400 lux ranging
from 200 lux to 2300 lux. The lights were
turned on in the morning and off at night
automatically, using an electrical clock. The
clock was set every 3 days to correct sunrise
and sunset times. At the beginning of the
experiment the one-year-olds were only 10
cm long. It was therefore necessary to keep
them in fairly warm water (9— 10°C) until
the beginning of December. They were then
kept in cold water (2—3°C) until lst of
February 1972 when they were again kept
in warmer water (9—10°C). As a result of
the warming up they were up to 12,7 cm ave-
rage length by mid-March. When they were
released they had been about 30 weeks under
artificial light.
The two-year-old control group was in
natural temperatures all through the winter.
A group of relatively small two-year-old
smolts from outdoor ponds was fin-clipped
with a left-ventral mark. This was done to
get an idea of the possible returns of un-
tagged two-year smolts.
2. The Electric Fishing.
During the first week of August the outlet
creek from the Kollafjörður Fish Farm was
fished with electricity to estimate the tend-
ency of the smolts not to migrate to sea. In
contrast to earlier years there were no marked
or tagged fish in the outlet at all. This was
not unexpected for the two-year-old smolts
but very encouraging with respect to the one-
year-olds. It seemed that all the fish in the
photoperiod experiment had been ready to
migrate to sea. The real validity of this as-
sumption remained to be substantiated by the
return-rate of these fish from the sea.
3. Adult Returns.
The remrns of tagged fish to the Fish Farm
are presented in Table 5.
The remrns of the various groups of two-
year-old smolts vary from 4,1—9,8% ave-
raging about 6,0%, Of these, 96% returned
after one year but 4% after two years. This
remrn-rate is somewhat higher than the re-
mrn-rate for the 1971 tagging experiment,
but it must be considered that the smolts