Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 153

HYDROACOUSTIC ASSESSMENT 151
caught. So the midwater trawl is not an
adequate sampling gear in Thingvalla-
vatn. Because the fish in Skorradalsvatn
are known to be similar, although some-
what smaller, no test fishing was attemp-
ted there. But, reliable test fishing in the
two lakes will be necessary before heavy
exploitation of the fish populations can be
properly monitored, and controlled. The
present commercial catch from
Thingvallavatn is not agood sampleofthe
population, as the majority is taken on the
spawning grounds where only a relatively
small segment is present. Still little is
known of the age composition or life his-
tory ofthe pelagic char in Thingvallavatn.
RESULTS
Large populations of char (Salvelinus al-
pinus) were found in Thingvallavatn and
in Skorradalsvatn. Concentrations of fish
throughout the lakes ranged from light to
\ er\ heavy, with the a\rerage being higher
iti both lakes than was expected prior lo
the surveys. Lish were detecled at all
depths in both lakes, but the majority was
stratified between 10 and 25 m. Mosl of-
ten, lish were not detected in areas where
the water depth was much less than about
20 m in Thingvallavatn and about 10 m in
Skorradalsvatn wliich is shallower.
Thingvallavatn
Lish densities ranged from only a few to as
many as 140 fish/100 m2 lake surface, with
the mean density being 24.7 fish/100 m2.
A population estimate, based on the ex-
pansion of this density over the lake sur-
face area for depths greater than 5 m,
amounted to 17.6 X 106fish. Noinforma-
tion is available as to the size or species
composition of these fish, however.
FISH DENSITY (fish/lOOOm3)
0 10 20 30 40 50
Fig. 9. Vertical distributions of fish densities for the
Skorradalsvatn hydroacoustic survey.
Two echograms produced during the
survey showing the diíference in daytime
and nighttime distributions of the fish are
given in Lig. 4. The nighttime areal and
depth distributions are shown in Ligs. 5
and 6, respectively. In general, more fish
were found in the northern and southern
parts of the lake than in the central por-
tion.
Skorradalsvatn
The densities of fish found in Skor-
radalsvatn were much higher on the aver-
age than those found in Thingvallavatn.
Densities from a low of about 10 fish/100