Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 88

86 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Differences between smolt types.
The survival of the various types of smolts
is shown in Fig. 7.
If we compare the survival of 2-year-
photoperiod and 2-year-outdoor smolts in
medium and small categories in the 1974
experiment, it is clear that the photo-
period smolts have significantly higher
survival. The same difference can be
noted between one-year-photoperiod and
2-year-outdoor srnolts in the medium
category in the 1975 experiment. These
differences presumably can be attributed
to fin erosion ofpectoral and ventral fins in
the outdoor smolts due to inferior hygiene
in the outdoor ponds. The large outdoor
smolts have survival comparable to the
photoperiod fish because of better condi-
tions when released.
This information is much in agreement
with the information obtained from the
fin-clipped smolts. It is quite clear that the
microtag gives about 12% survival in the
small 2-year-photoperiod smolts, a survi-
val rate which would mean a 9.5% return
rate for Carlin-tagged smolts, using a ratio
of 1.6 microtagged to each Carlin-tagged
smolt found earlier. This return rate for
Carlin tags, in that size class (12 cm aver-
age) is much too high, and past tagging
experiments show a 2% return rate for 13
cm smolts as being exceptional (Isaksson
1976). Thus, thereisindicationfrom these
data that a conversion factor of 1.6 is too
low for small smolts and the microtag is a
satisfactory tool for studying the survival
rate of small smolts where Carlin tagging
would provide meaningless results.
Furthermore, there are indications that
the increases in survival with increases in
lenght are at least partly artifacts of the
Carlin tag.
The 1-yearsmolts releasedin 1974were
overly smoltified and very sensitive to
handling and tagging. It is very clear from
the figure that this had considerable effect
on survival. Although the 1-year smolts in
the 1975 experiment do not appear to be
doing any better, they did have 14% sur-
vival when released in June, as discussed
later in this report.
Weight at return
Growth Effects of Carlin Tags.
The weight at return for Carlin-tagged
and microtagged smolts is shown in Fig. 5.
Due to small sample sizes in the 1975 ex-
periment, only the 1974 experiment gives
valid comparisons. There are significant
differences in weight, the microtagged fish
being 100-300 grams heavier upon return.
l'he greatest difference is found in the
small smolt. This shows that the Carlin
tag reduces growth as well as survival, and
has the greatest iníluence on small smolts.
Lenght at return was also checked for the
groups and was found to show the same
differences. Saunders and Allen (1967)
studying Canadian Atlantic salmon
grilse, found similarly that Carlin tags and
fin qlips affected the lenght at return.
Fig. 5 shows how the Carlin tag affects
both growth and survival the same way in
all size classes, although the effect on
weight at return is less pronounced than
survival effects in the largest smolts. With
respect to the microtagged smolts, there is
reason to believe (see section on growth
and smolt types) that some of the reduc-
tion in survival in smaller smolts is caused
by poor fin condition. This same factor
might be affecting weight, although most
of the weight differences between the size
categories are due to a direct relationship