Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 89

AN EVALUATION OF TWO TAGGING METHODS 87
between the size at release and wéight at
return. This has been found previously to
be true (Isaksson, unpublished) and is not
surprising since the weight diíferences
between a 12-cm and 17-cm smolt are
enormous, the latter being approximately
3 times as heavy as the former.
Growth Effects of Ventral Fin Clip.
Fig. 6 shows the adult retuiyi weights of
2-year-outdoor smolts in three size clas-
ses. There are no differences between the
weights in the small and medium groups,
but there is a significant difference at the
0.05 level in the large smolts. This is in
agreement with the survival in the same
figure. The reason for this (as was the case
for survival) is that the small and medium
smolts are already in poor condition due
to eroded fins, so additional clipping does
not hurt them; but the large smolts are
seriously aífected, both with respect to
weight and survival. We can again note
the close relationship between survival
and weight at return.
Growth Differences Between Smolt Types.
The weight at return for three diíferent
smolt types in three size categories are
shown in Fig. 7. It is interesting to note
that the 1-year-photoperiod smolts in the
1974 experiment are significantly larger at
return than either the 2-year-photoperiod
or 2-year-outdoor smolts in the same size
class. There is also a significant difference
between the 2-year-outdoor and 2-year-
photoperiod smolts. The difference be-
tween the 2-year groups is in agreement
with the survival information in the figure
and can be explained by differences in
smolt condition when released. The survi-
val of the 1-year-photoperiod smolts has
already been discussed earlier, but in or-
der to enderstand the good growth of these
fish in the sea, we need to look into the
operation of the Kollafjördur Hatchery.
Each fall the fastest-growing fingerlings
at the station are taken and put into a
raceway where they are subjected to
fiatural photoperiod through a transpar-
ent roof throughout the winter. Fingerl-
ings that do not meet this deadline as fa'r
as size is concerned are either sold (the
larger ones) or kept for an extra year in the
hatchery, thus becoming 2-year smolts.
Whether these go into outdoor ponds or
raceways during the second winter de-
pends on their size in the fall. One might
thus say that the 2-year-photoperiod
smolts are the slowest-growing fish in the
station.
With this background information it
seems very reasonable to consider the
good growth rate of the 1-year photo-
period smolts in the sea as a continuation
of their good growth in freshwater.
Donaldson (personal communication) has
found this to be true for coho salmon re-
leased at the University of Washington,
Seattle. Whether this is actually inherited
can be debated, but it is known from ani-
mal husbandry in general that a stunted
growth in a population is difíicult to over-
come.
Due to small sample sizes in the returns,
the 1975 experiment did not provide much
information on the 2-year-outdoor smolts,
but the 1-year-photoperiod smolts do
have significantly greater growth in the
large smolt category.
One parameter that could easily affect
the comparison of weight between differ-
ent groups of smolt is the sex ratio. It is