Íslenskar landbúnaðarrannsóknir - 01.09.1978, Síða 15

THE ATLANTIC SALMON IN ICELAND 13
warm up considerably as they flow long
distances through lowland areas in the
summer. During the winter they may
freeze over.
The direct runoff rivers have a flow
which is gratest in the spring, when snow
is melting, and in the autumn when heavy
rain falls. The smallest discharge is in the
winter and a secondary minimum is ob-
served in the summer. Floods may occur
in all seasons. The water temperature is
governed mostly by the air temperature
above the freezing point, but below, an-
chor ice is formed which after a time drifts
to the surface and the rivers become co-
vered with ice while the flow diminishes.
The flow of small streams may be seri-
ously affected in the winter by drifting
snow.
The spring-fed rivers have an under-
ground source. The flow is even, and the
water temperature is 3°—5°C at the origin
of the rivers. The river beds and banks are
usually stable. Their temperature may in-
crease in the summer, when these rivers
flow long distances from their headwaters.
In winter they never freeze over near their
source.
The three groups of rivers described
above do not always maintain their
characteristics from source to estuary.
The main river may start out as a glacial
river receiving tributaries of runoff
streams and spring-fed streams along its
watercourse. The river system of ölfusá-
Hvítá in southern Iceland is such a mix-
ture of the three types.
Species of freshwater fishes
Five species of fish are native to Iceland,
Atlantic salmon, Salmo salar, brown trout,
Salmo trutta, Arctic char, Salvelinus alpinus,
European eel, Anguilla anguiUa, and
threespined stickleback, Gasterosteus
aculeatus. Two varieties of brown trout and
char are found; some stay in fresh water all
through life, whereas others migrate to the
sea to feed.
Since 1960 strayers of pink salmon, On-
corhynchus gorbuscha, have been caught
most years in Icelandic rivers all over the
country, the largest catch being made in
the summer of 1975. Rainbow trout, Salmo
gairdneri, was introduced from Denmark
into two fish hatcheries during the early
sixties but is now found only in one of
them.
The Atlantic salmon
Among the freshwater fishes of Iceland,
the Atlantic salmon is of the greatest
economic value and the one most sought
after. It ascends about 80 rivers and
streams, counting separately the large
tributaries to the glacial rivers. Salmon
occur mostly in lowland areas up to about
200 m over sea level in the rivers but in a
few rivers they are found at altitudes ap-
proaching about 400 m above sea level.
Most often the salmon ascend only short
distances iland from the coast, the
maximum known distance being about
100 km. Since Iceland is a mountainous
country, many unsurpassble waterfalls
are found in the watercourses, for some of
which fishpasses have been built.
DISTRIBUTION AND ABUNDANCE
Distribution of salmon
Salmon are found in greatest abundance
in the rivers of the southern (Sudurland),
southwestern (Sudvesturland), and west-