Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 113

AN EVALUATION OF SMOLT RELEASES 1 1 1
lantic salmon with all the individuals en-
tering the river system. Information on
wild smolt survivals in Iceland has been
very scanty. Gudjónsson (1954) reported
5.6% survival for the river Úlfarsá which
is in the vicinity of Ellidaár based on the
actual recoveries. It is likely that a 15%
survival would be more realistic if the es-
capement is included.
Studies in Iceland háve shown that
good salmon streams produce about 1,000
smolts per hectare of nursery area. Using
the previous figure for smolts at Ellidaár,
we need only 14 hectares to produce a run
of the present magnitude in this river.
In Table 1 there is shown recovery of
tagged smolts in 1976 and 1977. It is in-
teresting to note that there is very little
increase in wild smolt survival when the
1977 season is included, but there is a very
pronounced increase in hatchery smolt
survival. This indica^es that the Koll-
afjördur hatchery stock has a greater ten-
dency to stay 2 years in the ocean than the
Ellidaár stock, which may be either
genetically or environmentally deter-
mined. It is intersting that the contribu-
tion of microtagged hatchery fish is only
38% of the wild fish contribution based on
numbers, but increases to 56% on a weigh
basis, primarily affected by the 2-year-
ocean salmon.
In addition to survival differences, pro-
nounced weight differences at return were
observed between hatchery and wild
smolts (Table 1). These differences could
all be related to different sizes at release
since small smolts come back lighter
(Isaksson and Bergman 1978).
Straying
The straying of salmon to Kollafjördur
and other neighbouring rivers in 1976 is
shown in Table 2. It is most interesting to
compare the stray of the direct plant
smolts which were not forced to become
adapted to the river, and the release pond
smolts, which were forced to do so. Curi-
ously, there is no more straying of direct
plants to Kollafjördur. l'his difierence in
survival must thus be explained by a
mortality which is associated with the
planting method.
Straying from Ellidaár to Kollafjördur
and vice versa is approximately equal
which certainly does not indicate exessive
desire of the hatchery smolts to go to Kol-
lafjördur contrary to the findings in Ar-
túnsá. There are two possible explana-
tions for this: first, there could be a vital
chemical difierence between the water at
Ellidaár and Kollafjördur which does not
exist between Kollafjördur and Ártúnsá,
drawing water from the same mountain;
on the other hand, pheromones might be
operating, making the salmon more apt to
stray when their home stream does not
have a smell of young salmon.