Íslenskar landbúnaðarrannsóknir - 01.09.1976, Síða 26

24 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Table 3
Salinity Tolerance of Various Groups of One- and Two-Year-Smolts
Age of smolts Treatment before planting Number put into floating pen Mortality within 3 days % Mortality after 4 1/2 months Mean length when planted Mean length after 4 1/2 months Salinity in pen Type of mark Comments
1 Smolts subjected to intensive indoor rearing 1000 21 16-17 cm 27.2%o Carlin tag Removed from pen after 4 days
1 Smolts subjected to intensive indoor rearing 200 27 45 13-14 cm 19.0 cm 27.2%o Adipose clip
1 35 weeks of natural photoperiod through transparent roof 200 0 5.5 13-14 cm 25.5 cm 27.2%© Treated as one group after two weeks.
2 35 weeks in outdoor ponds before release 200 0
year-smolts were adipose clipped but the ot-
hers were unmarked. The results from these
experiments are in Table 3.
In the following discussion the smolts sub-
jected to intensive indoor rearing will be re-
ferred to as non-photoperiod-fish but the
others as photoperiod-fish. It is evident that
the non-photoperiod-fish cannot adapt to the
saline environment. Both of these groups had
a heavy mortality (20—30%) within a very
short period even if they differed considera-
bly in mean length which shows that length
does not play an important role in salinity
tolerance if the fish is not properly smoltified.
The larger non-photoperiod-fish stayed a
very short period in the pen but the other
group stayed there for 4V2 months. It had
45% mortality in that time compared to an
average mortality of 5,5% in the other two
groups that had gotten namral photoperiod
for 35 weeks.
There was a considerable difference in
growrh rate between the photoperiod and
non-photoperiod-fish. The latter had only
reached an average length of 19 cm after
AVi months compared to 25,5 cm in the
photoperiod-fish. This is in good agreement
with the findings of Saunders and Hen-
derson (1970) which found inferior marine
growth in smolts subjected to inverse spring-
photoperiod. There was an outstanding diffe-
rence in appearance through the whole periad.
The non-photoperiod-fish had a strange
greenish-yellow appearance but the others
had a silvery appearance. All the fish were
tagged with plastic tags and released in Octo-
ber of 1973 and remrned the following
summer. They had an overall return rate of
3,5% but none of the returning fish had an
adipose clip in addition to the tag so it was
quite clear that the non-photoperiod-fish were
not returning at all.