Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 25

THE ATLANTIC SALMON IN ICELAND 23
start until the water temperature is over
8°C. A similar experience was noted at the
fishpass at Laxfoss in the river Nordurá in
Iceland (Fig. 7).
Information on the ascent of salmon in
Icelandic rivers has been gained mainly
from catch records during the early part of
the salmon fishing season. In a few in-
stances the salmon have been counted by
fish counters, the most reliable one being a
mechanical counter which has been in op-
eration since 1960 at the river Ellidaár.
This fish counter is mounted in a trap of a
weir about 800 m above the tidal zone of
the river.
The counts at the weir from 1962-1971
and 1973 (Eiríksdóttir 1974) showan ob-
vious trend for the peak of the run to be
during the first or second week of July.
The run then gradually tapers out in Au-
gust and even into September (Fig. 8).
The ascent varies in time from year to
year. Thus the difference in time when
10% of the run has been counted can be
about 1 month, at 50% 5V2 weeks and at
90% more than 6 weeks.
The average cumulative catch curve
from 1949-1973 (Fig. 9) (Eiríksdóttir
1974) shows that there was not as much
difference in time at the 10% catch level as
in the average cumulative curve of ascent
for 1962-1971, and 1973, it being only IV2
weeks. At 50% it was almost 2 weeks and
at 90% a little over 2 weeks.
Spawning
Jones (1959) states that salmon will spawn
at depths from 15-120 cm, the ideal depth
being 60 cm or more and that the most
favorable rate of flow is 30-45 cm per sec-
ond at the surface. Spawning proceeded to
completion when the water temperature
was between 2°C and 6°C in his tank ex-
periments, but he has observed spwning
in the river when temperature was as high
as 10°C. Jones states furthermore that at
water temperature below 1°C the cutting
of beds is languid and not completed, and
orgasm does not take place.
In Icelandic salmon rivers there are
usually large areas with gravel bottom at
various depths suited for spawning.
Spawning takes place at low tempera-
tures. As water temperature falls first in
the fall in the north and northeastern parts
of the country, spawning starts there ear-
lier than in the southern and western
parts. Spawning time in the colder rivers
would be in September and early October;
salmon have even been caught in northern
rivers with ripe ova in late August. In the
rivers in western and southern Iceland
spawning generally takes place from late
September into November, but salmon
have been observed to spawn as late as
December in the river Sog. This river
drains Thingvallavatn, the largest lake in
Iceland, which is deep and it takes a long
time to cool the water temperature in the
autumn.
The spawning beds of the river Ulfarsá
near Reykjavík were surveyed in detail for
six autumns during the years 1955—1963
(Gudjónsson 1964). The river is 10.4 km
long and has an average discharge of 800
liters per second. It discharges from lake
Hafravatn, which is 68 m above sea level,
with a surface area of 1.02 km2 and into
which flows a small stream, the Sel-
jadalsá. In Ulfarsá spawning took place
mostly during the first half of November.
In many parts of the river there was gravel
bottom where the salmon, mostly grilse,