Íslenskar landbúnaðarrannsóknir - 01.09.1976, Page 27

TAGGING EXPERIMENTS AT KOLLAFJÖRÐUR 25
3. The Adult Retum.
The return rate of one-year-smolts marked
in 1973 and a comparable group of two-
year-smolts is shown in Table 2. The one-
year-olds were either adipose-clipped or tag-
ged with plastic tags. The remrn rate of the
finclipped salmon should give a fairly good
idea about the return rate of untagged fish.
The percent remrns of the one and two-
year-smolts are very comparable, especially
when one considers that the one-year-fish
were divided into smaller treatment groups
that had a remrn rate ranging from 6,6 to
10,6 percent. The adipose-clipped one-year-
smolts gave the highest return rate ever ex-
perienced at the Fish Farm, close to 15%.
The results from the 1973 experiments
show that one-year-old Atlantic salmon
smolts of hatchery origin can have remrn
rates just as high as their two-year counter-
parts, if they get the proper treatment for a
period of 35 weeks before their release.
THE 1974 EXPERIMENT
1. Treatment of Smolts.
The one-year-smolts released in 1974 had
basically gotten the same treatment as their
counterparts in 1973. They did, however,
have a somewhat shorter winter-period with
respect to temperamre and higher tempera-
mre than the 1973 group during the rest of
the time. The number of degree-days from
the beginning of December until release
time was calculated, and it became evident
that the 1974 smolts had accumulated 30%
more degree-days by the end of April than
the 1973 group. The former were thus ap-
proximately one month ahead in their devel-
opment if temperature played a key role.
When the 1974 one-year-smolts were
tagged in early May they were more vul-
nerable and harder to handle than the 1973
smolts the previous year. Considerable mor-
tality occurred after transport to the tagging
facilities and during tagging. Mortality after
transfer to a salt-water pen was considerably
higher than the year before although the
results concerning photoperiod and non-
photoperiod-fish were essentially the same.
If one defines smoltification as the total
development of parr into a tender down-
stream migrant, the above findings indicate
that the total accumulation of degree-days
plays a vital role in the speed of smoltifi-
cation. This agrees basically with the findings
of Johnston and Eales (1970) although
they were primarily concerned with silvering
as a part of the smoltification process.
It is apparent that one should be very
careful not to subject the smolts to excessive
temperamre for faster growth at the expense
of proper smoltification timing.
Conclusion.
The production of viable one-year-smolts is
now practical in Iceland and this will make
the salmon smolts considerably cheaper and
decrease the cost/benefit ratio in salmon
production. In order to be able to have the
bulk of the production one-year-smolts it
will be necessary to hatch the salmon eggs
in heated water from the very first day on
which is now being done at the Kollafjörður
Fish Farm. The fry will start feeding in early
February about 3 months ahead of the
normal schedule. This wili enable the smolt
producer to have most of his parr over 12 cm
long in September when they should be sub-
jected to namral photoperiod.