Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 91

AN EVALUATION OF TWO TAGGING METHODS 89
large male smolts if mature, may not want
to leave freshwater, which magniíies the
number of females returning as grilse.
Since the 1-year-photoperiod smalts do
not have an abnormally high male ratio
compared to the other smolt groups, the
weight differences in Fig. 7 are unaílected
by sex ratio.
Tag Retention.
The tag loss af the dangler tags was meas-
ured by double tagging with dangler tags
and microtags. This phase was not in-
tended for microtag loss because the mic-
rotag procedure using an adipose clip with
each microtag inherently provides a dou-
ble tag scheme, which in this instance in-
volved far greater numbers than the Car-
lin double-tagging and thus a potentially
more effective measure. The tag loss for
microtags was originally designed to be
measured by addition of a ventral fin clip
to a microtag in case erosion of adipose
fins on untagged fish would eliminate use
of untagged adipose-clipped adults as a
measure of microtag loss. This scheme
was abandoned when it turned out that
the 2-year-outdoor smolts used in the ex-
periment had badly frayed, sometimes
missing ventrals, but few adipose fins were
eroded. It also turned out that the ventrals
had varying degrees of regeneration from
no fin to an almost complete one. Tag loss
for microtags was finally achieved by
counting the number of adipose-clipped
salmon returning without a tag.
The tag loss information is shown in
Table 2. We see that the tag loss of Carlin
tags averages about 10% but has a very
wide confidence band due to a small
number returning.
The tag loss for plastic tags atlached
with polyethylene thread is twice as high
as for the Carlin tags, averaging about
25%. These tags have only been used once
in Iceland (Isaksson, 1976), al which time
they gave very satisfactory results. They
have been extensively used in other con-
tries such as Canada, Britain'and Ireland.
In the present experiment they were calib-
rated only with microtags.
Microtag loss for both years is about
1.7%. This is a very low tag loss and is
consistent with the tag loss for Paciíic sal-
mon reported by Jefferts et. al. (1963) for
Pacific salmon. It must be kept in mind,
although we have no data in this instance,
that the adopose fin is sometimes missing
on salmon from natural causes. Also,
adipose-clipped salmon were released into
salmon rivers less than 50 miles away from
the Fish Farm, and straying is known to
occur. Consequently, these tag loss figures
for microtags must be considered as
maximums.
The tag loss studies for dangler tags
were intended to allow calibration of the
tags in order to adjust previous experi-
ments. This may not be possible with ac-
curacy due to wide confidence bands, but
nevertheless it has been shown that tag
loss is an important factr in any tagging
experiment with dangler tags.
COMPARISONS OF TIME AND LO-
CATION OF SMOLT RELEASE AT
KOLLAFJÖRDUR FISH FARM
Along with the test of the microtag versus
other tagging techniques, it was decided
to use the microtag on a large scale as a
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