Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 92

90 ISLENZKAR LANDBÚNAÐARRANNSÓKNIR
research tool in an attempt to improve the
release methods at the Kollafjördur Fish
Farm.
It is fairly well established that size of
parr in the fall before migration along with
photoperiod and water temperature in the
spring, are the most important factors
controlling smoltification. Elson (1954)
concluded that Atlantic salmon parr
reaching 10 cm in the fall would smoltify
but those under 10 cm would not. Hoar
(1965) pointed out that photoperiod af-
fected the smoltification timing in sal-
monids and simultaneously influenced
salinity tolerance. He suggested that
smoltification could be advanced by about
2 months by subjecting the presmolts to
unseasonally long day lenghts. Baggerman
(1960) showed that Pacific salmon de-
velop a saltwater preference at the time of
seaward migration. This was accom-
panied by increased thyroid activity
which started before the onset of migra-
tion and is probably one of the causes of
migration.
Saunders and Henderson (1970) found
that salmon smolts exposed to reciprocal
photoperiod had decreased sensitivity to
external stimuli suggesting reduced
thyroid activity. These flsh grew slower
when put in saltwater than fish ex-
perimencing normal photoperiod.
Isaksson (1976) found that smolts which
were exposed to continuous light for 30
weeks before release did not migrate to sea
and had no returns. Ifsubjected to natural
photoperiod for 30 weeks, the smolts mig-
rated out readily and had adult returns up
to 10-15%.
Silvering has often been used as an in-
dex of smoltification. Johnston and Eals
(1970) studied silvering as a part of the
smoltiflcation process and concluded that
temperature was more important in in-
fluencing the process than photoperiod.
Icelandic studies support these findings
(Ísaksson, 1976). Foerster (1937)
suggested that sockeye smolt migrations
are delayed by cold weather during the
last few months before seaward migration.
From this literature review it seems
fairly certain that correct photoperiod is
the main factor which determines whether
migration will occur in the spring. Temp-
erature seems to be a factor which deter-
mines how quickly the various physiologi-
cal processes necsessary for migration
take place and thus determines when the
smolts are ready to migrate. Iclandic ex-
perience shows that 1-year smolts kept in
artificially heated water in the spring may
grow supersensitive to handling and very
silvery before their counterparts in cold
water (Ísaksson, 1976). It is very likely
that these fish would be ready to migrate
out much earlier than smolts in natural
temperatures. No one knows what kind of
survival such smolts would have if re-
leased in April as opposed to May. The
oceanic environment might be very un-
favorable at such an early time of the sea-
son, and the smolt, even if ready to adapt
to saltwater, might have trouble finding
the right food. If successful, however, one
could theorize that an extra month in the
sea might result in increased weight at
return.
Since Kollafjördur Fish Farm had con-
sistently released smolts in late May, ir-
respective of climatic conditions, it was
considered opportune to test different re-
lease times by the use of the microtag
technique which allowed the tagging of