Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 23

THE ATLANTIC SALMON IN ICELAND 2 1
Fig. 6. Diurnal variation in the temperature meas-
ured in centigrades at Reykjavik (R) and Akureyri
(A) from March through November 1936 to 1945 by
Eythorsson and Sigtryggsson (1961).
LIFE HISTORY
Time of ascent
The salmon ascent the rivers in Iceland
from May into October with the largest
number in July. The time of first entry
varies in the different rivers. In some riv-
ers that flow into the Faxa Bay in south-
western Iceland a few salmon may ascend
already in late May. So do the salmon in
the rivers MidQardará and Laxá in Adal-
dalur in northern Iceland. In other rivers
the salmon begin their ascent in June or
even as late as in early July. The general
tendency is that the salmon ascend rivers
in southwestern Iceland early in the sea-
son and gradually later as the location of
the rivers are farther to the north and
toward northeast.
The time of ascent may vary from one
river to the next in the same district. Thus
the salmon run about 2 weeks later in the
river Grímsá, one tributary to the river
Hvítá in BorgarQördur, than it does in two
other ones; the rivers Nordurá and Thv-
erá, although the junctions of these three
rivers to the river Hvítá are within a dis-
tance of a few kilometers. The ascent of
salmon in the river system Ölfusá-Hvítá
in southern Iceland begins about the mid-
dle ofjune and may extend into October.
There are various factors in initiating
the ascent of salmon. The gratest numbers
of salmon enter during high tides. Ascent
is stimulated by fioods in the rivers and is
delayed in seasons with low discharge,
especially in small streams. Low water
temperature in rivers delays ascent. Men-
zies (1931) has shown that there is little
upstream migration ofsalmon past obsta-
cles as long as the water temperature is
under 5.6°C. Andersen (1977) maintains
that lively upstream movement of salmon
is associated with water temperature of
8°-10°C in parts with a heavy current, and
cites as an example that passage of salmon
on a large scale through the fishpass at
Málselvfoss in northern Norway does not
3*