Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 80

78 ÍSLENZKAR L.ANDBÚNAÐARRANNSÓKNIR
relaliveley large dangler tags and con-
sequently the. historic practice in Iceland
was to tag only smolts longer than 13.5 cm
in fork lenglh. Great number of smolts
smaller than 13.5 cm are being released
into Icelandic rivers each year with in-
creased emphasis on one-year smolt pro-
duction. It is known that wild populations
have smolts which average 12—13 cm in
length at migration. If hatchery smolts of
the same size would be shown to have
comparable survival as their larger coun-
terparts, there would be considerable
savings in space and food in Icelandic
rearing stations. The microtag appeared
to be useful for tagging smaller Atlantic
salmon smolts - it is commonly used on
Paciftc salmon 6 cm or less in length - so it
was examined here as a method for
studying the survival of various types of
smolts in diíferent size categories.
Two persons can easily microtag 4 000
smolts in a working day, including
adipose clipping, which is only 0.5 man-
days per 1 000 smolts, compared to 2
man-days for the Carlin method. It is thus
obvious that far greater number of smolts
can be microtagged with the same man-
power resources. This is practically a pre-
requisite in order to be able to justify the
purchase of this fairly expensive machin-
ery (Fig. 1).
Each salmon tagged with the microtag
in these experiments had its adipose fin
removed. This speeds up recovery since
the adipose-clipped salmon can be recog-
nized by this external mark. The adipose
fin does not regenerate and its loss appar-
ently does not have a major effect on sur-
vival under the conditions discussed here.
It should be pointed out that under cer-
tain circumstances there is evidence that
adipose excision can have an important,
deleterious effect (Nicole and Cordone,
1973), probably related to the handling or
to induced disease rather than to the loss
of the adipose/w se.
During recovery, all microtagged sal-
mon were taken from the salmon trap at
the Kollafjördur station. Inside the sta-
tion each salmon with a missing adipose
was checked for a magnetic tag with a
special detector (Fig. 3). If the salmon did
have a tag it was removed with a cork bore
#7—9. This was very efficient, and experi-
enced personnel would get the tag on the
first try 90% of the time. The cores were
put in small numbered vials and the tags
were excised and read as soon as possible
(Fig. 4).
TYPES OF SMOLTS USED
In the years between 1966 and 1970, the
1-year smolts produced at the
Kollafjördur Fish Farm did not smoltify
properly and had no oceanic survival as
reviewed by Isaksson (1976). This prob-
lem was solved by manipulating photo-
period and temperature (Isakssox 1976),
resulting in a 10-15% oceanic survival of
1- year smolts in the 1973 tagging experi-
ment. It has become clear that the age of
smolts has no decisive effect on survival,
and 1- and 2-year smolts can be used in-
terchangeably, provided they are both
adequately smoltified prior to release. In
the following pages we will be referring to
various types of smolts which will be de-
scribed in the following section under the
name used in the text.
2- year-outdoor.
These are 2-year smolts reared during the
first year indoors but moved to outdoor