Íslenskar landbúnaðarrannsóknir - 01.09.1976, Side 25

TAGGING EXPERIMENTS AT KOLLAFJÖRÐUR 23
captured outside the Fish Farm in other two-
year-groups.
From the above information it can be
deduced that the photoperiod treatment had
considerable positive effect on the return
rate of one-year-smolts but it stili was not a
satisfactory substitute for nature.
THE 1973 EXPERIMENT
1. The Treatment.
The smolts used for this experiment were
from a batch of eggs that were exposed to
elevated temperatures during hatching after
they reached the eyed stage. These fish were
feeding in the middle of March about 1,5
months earlier than fish from eggs hatched
at normal temperature. The fish were basi-
cally divided into two lots, one of which
was fed Ewos salmon feed, but the other
with Icelandic dry feed being experimentally
produced by Dr. Jónas Bjarnason of the
Icelandic Fisheries Laboratories. These fee-
ding experiments were discontinued in early
September and by that time the fish were
10—11 centimeters in length. Both groups
were now combined to give one large group
of one-year-smolts for the photoperiod ex-
periment. The fish were fed Icelandic dry
feed for the remaining period until their
release in early June 1973. In contrast to
earlier releases of one-year-smolts these fish
were not released from the ususal dirt ponds
but were transported at release time directly
from the experimental facilities to a large
raceway located very close to the sea. This
has probably facilitated the seaward migra-
tion of the fish but its impact on return rate
is not clear at the present time but will be
investigated in 1975. The extra handling at
this critical time might cancel out any posi-
tive effect.
During the summer of 1972 one of the
rearing houses at the KollafjörSur Fish Farm
was modified in the way that a corrugated
iron roof was replaced with a transparent
corrugated plastic roof. This eliminated the
necessity to use artificial light for the photo-
period experiment and provided excellent
facilities for the production of 50—60.000
one-year-smolts if needed. The experimental
ponds were separated from the rest of the
rearing house by a black plastic divider to
eliminate any interfering light at night.
The fish were put into the experimental
unit, a 25 m2 concrete raceway, in the middle
of October. The temperature was kept high
(9°C) except for a period of 2 months in
January and February of 1973 when natural
temperature was used to imitate winter con-
ditions.
These fish were tagged with plastic tags
with polyethylene attachment in the middle
of May. The one-year-smolts were divided
into several subgroups that have been dis-
cussed in an earlier report (Isaksson 1976),
but will be treated as one group in this
report.
2. Salinity Tolerance.
In early 1973 a salt-water pond with tidal
interchange had been built at the Fish Farm.
This made it possible to do rearing and
salinity tolerance experiments on the smolts
as well as adult remrn rate.
In late May 1973 a floating pen was put
on the salt-water pond and several groups of
smolts were released into the pen. The first
group to be released into the pen consisted
of large Carlin tagged one-year-smolts which
had been subjected to intensive rearing. This
group was removed from the pen after 4
days due to excessive mortality. Subsequently
2 groups of one-year-smölts and one group
of two-year-smolts were simultaneously rele-
ased into the pen. The intensively reared one-