Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 83

AN EVALUATION OF TWO TAGGING METHODS 8 1
TABLE 2.
Tag loss for three different tag types used in the 1974—1975 experiments.
Tag used Number returning Number returning without tag Percent tag loss 90% conf. interv. Experiment year
Carlin with steel wire 41 4 9.8 2.4—17.1 1974 and 1975 pooled
Plastic with polyethylene 62 15 24.2 16.1—33.9 1974
Microtag 2.960 49 1.7 1.3—2.1 1974 and 1975 pooled
groups to get a mean of microt&g loss. This
method later turned out to be ofno usedue
tofin erosion, and themeasure ofmicrotag
loss was achived by counting salmon re-
turning with a missing adipose fin and no
tag.
Since microtag loss is believed to occur
principally in the first days and weeks fol-
lowing tagging (Jefferts et al 1963), sev-
eral tag loss checks were made on the day
following tagging. Spot checks with a field
detector after diíferent lenghts of time in-
dicated that initial tag loss was neglible.
It should be noted that some of the
blocks in the experimental design are
empty, caused by the lack of smolts of
certain types or sizes. This also accounts
for the uneven sample sizes.
Results
In the analysis of the data, the following
parameters were examined in the three
smolt-size categories:
1. Survival.
2. Wheight at return.
3. Sex ratio at return.
4. Tag retention.
Survival is, of course, of utmost impor-
tance in any sea-ranching operation, and
a reduction in survival due to a tagging
method may well lead to the conclusion
that the decrease in survival is too great to
justify the tagging.
It has been established previously that
the survival of Carlin-tagged smolls in-
creases with increase in size (Carlin, 1969;
Isaksson, 1976). It is of great concern to
know whether this shows a true survival or
is in some way due to inability of small fish
to carry the Carlin tag. It is therefore oí
utmost importance to compare the micro-
and Carlin tags. It is therefore of utmost
importance to compare the micro- and
Carlin tags in several size classes. Since
size at return may be related in some way
to size at release, all weight comparisons
were also made in three size categories.
The weight of the salmon when it re-
turns has a very direct influence on the
economy of the hatchery operation, espe-
cially when the salmon are sold to the
market for a certain price per kg. This
problem, complicated by the relationship
between the size at release and size at
return, will be discussed in the following
section.
It is very interesting to know whether
the Carlin tag has any effect on the weight
of the returning salmon. One pararneter
we examined is the weight of salmon re-
turning from the ocean as related to tag
type. This weight may also be dependent
on the size at downstream migration as
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