Íslenskar landbúnaðarrannsóknir - 01.09.1978, Síða 33

THE ATLANTIC SALMON IN ICELAND 3 1
it „has often been observed that periods of
high water reduce invertebrate fauna in
streams.“ He also states that „streams
which are liable to spates have less abun-
dant and less varied fauna than others.“
The runoíf rivers in Iceland as well as the
glacial ones can vary greatly in discharge,
so the wetted perimeter changes a great
deal. This may have a detrimental effect
on the food supply and cover, especially
during the summer. Floods are also fre-
quent and may occur at any time of the
year. It can be expected, that the fluctua-
tion in discharge of rivers in Iceland is a
limiting factor for survival of salmon in the
nursery area.
There are several predators of salmon
in Iceland. Among fish there are brown
trout and eels. Several birds take salmon,
such as the black-backed gull, black-
headed gull, Arctic tern, and goosander.
With reference to mammals, there are
minks, occasionally foxes, and in the sea
and estuaries the common seal (Phoca vit-
ulina) and thegrey seal (Halichoerusgrypus).
Size at smoltification
Salmon smolts were caught in a fyke net in
the river Ulfarsá, at short distance above
its estuary, for over 20 years (Gudjónsson
1978). The length of the smolts (forked
length) was measured in the spring of
1947, and 1949-1959. The average length
of the smolts for these years was from
11.0-13.3 cm, but most often between
12.4-12.9 cm. Poe (1975) measured the
length of 927 smolts caught in the river
Ellidaár in thespring of 1975. The average
lenth was 12.47 cm. Jóhannsson (1978)
back-calculated the smolt length from
scale samples taken from salmon netted in
the river ölfusá in 1976. He found that the
smolt size was 12.0-12.5 cm.
Age at smoltification
The age at smoltification of Icelandic sal-
mon varies from 1—5 years. This may be
seen from scale samples of 1,799 fish col-
lected from ten rivers from 1946-1965 and
analyzed by the staff of the Institute of
Freshwater Fisheries. Most of the salmon
had smoltified after 3 years in the rivers
(Table 3); 93.2-96.8% in six streams and
86.9% in a seventh one. In two rivers in
the south, 71.7 and 79.4% of the fish had
spent 3 years in fresh water respectively.
In these two rivers 12.6 and 21.2% spent 2
years in the rivers and 7.1 and 7.8% 4
years, while in the first seven rivers
0.8-9.5% spent 2 years in fresh water and
1.6-6.0% 4 years. Age analysis of salmon
from the river Midfjardará in northern
Iceland differed markedly from that of the
other rivers in that 53.7% spent 3 years in
fresh water and 41.1% 4 years.
Isaksson (1977) has reported that some
very small fry were found in September
1977, during electrofishing in the river
Langá in western Iceland, the smallest
ones measuring about 3.0 cm in length.
The fry is about 2.5 cm long when the yolk
sac is absorbed, and as stated by Fort and
Brayshaw(1961), abont 3.2 cm when
the central scale platelet is laid down. It
can thus be expected that when the fry
grow slowly through their first summer in
the colder salmon rivers, and also in the
generally warmer ones after a cold spring
or during an unusually cool summer an
annulus will not form on the scales during
their first year in the river. Therefore, it is
possible that an unknown number of
smolts from rivers in western, northern,