Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 28

26 ÍSLENZKARLANDBÚNAÐARRANNSÓKNIR
es were mkde for the emergence of salmon
from 1966-1975. Based on 622 temperat-
ure units the time varied from April 19 in
1972 to May 29 in 1966 and 1969.
The hatching time for salmon in nature
has not been studied in Icelandic rivers,
but it may be expected that salmon will
emerge in the river Ellidaár in May, the
spawning mostly taking place in
November. In the colder salmon rivers the
hatching time will be much longer, with
spawning in September and emergence
likely in late June or even in July in north-
eastern Iceland.
Temperature units have been measured
in the hatcheries in Iceland. A batch of
salmon ova was fertilized on November 3,
1977 at the Kollaíjördur Fish Farm. The
ova were hatched in heated water of
8°-10°C. The ova hatched around De-
cember 23, and feeding commenced on
January 23, 1978. The temperature units
were 509 to hatching time and an addi-
tional 207 units for the alevin stage, total-
ing 716 temperature units. When the
temperature is about 4°C the hatching
time will be after 110-115 days and 450
temperature units are needed (Isaksson
1973).
The size of ova in the Icelandic salmon
is small, 7,000-9,000 per liter, while there
are 4,000—6,000 salmon ova per liter and
about 1,200 ova per kg of females in Nor-
wegian salmon (Berg 1963). The number
and size of ova depend on the size of
females, the larger ones having a greater
number and larger ova. Jones (1959)
stated that the average number ofova per
kg of females ranges from 1,430-1,540. In
Iceland 1,600-2,000 ova can be expected
per kg of female salmon.
Density of juvenile salmon in the nursery areas
Density of juvenile salmonids in several
rivers in Iceland has been measured by
Karlström (1971), Tómasson (1975),
Ísaksson (1975 and 1977), and
Arnlaugsson (1976). The results of the
studies are shown in Table 2.
The variation is great in the number of
juvenile salmonids in various places of the
same river systems, and also from one
river to another. The densities are high
when compared with corresponding fig-
ures from other countries. Power (1973)
studied five streams in northern Norway
where the density was 0.4 to 69 salmonids
per 100 m2. Karlström (1972) gives fig-
ures for rivers in northern Sweden of 4—5
per 100 m2, and Jones (1970) gives data
from Great Britain as being 5-400 per 100
m2.
Food of juvenile salmon
Tómasson (1975) examined stomach sam-
ples of 78 parr of the age of 0+ to 2+ in the
river Ulfarsá near Reykjavík. In these
were pupae of nematocera, empididae,
and simulidae, and larvae of
chironomidae, simulidae, tricoptera,
tipulidae, and coleoptera, besides cladoc-
era, hirudinea, and plecoptera. Terres-
trial forms were represented mainly by
diptera, homoptera, collembola, coleopt-
era, and araneae. Besides these there were
hydracarina, oligochaeta, and gas-
tropoda.
There are comparatively few species of
invertebrate animals found in Iceland,
but the number of individuals of some of
the species is very great. Tómasson found
that the parr of 0+ group consumed
mostly chironomids, and the parr 1 + and
2+ groups consumed mostly trichoptera