Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 128

126 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
yearly catches are 5-12 tons of brown tr-
out and char; 5-15 tons of char are har-
vested in lake Mývatn.
Most other lakes in Iceland today are
used by the owners, the farmers, to pro-
duce food for home consumption. But
there is an increasing trend in leasing of
the lakes for recreational fishing, perhaps
more to native than to foreign fishermen.
As the fishing pressure continues to in-
crease, the need for active and purposeful
management policies aries. But these
policies cannot be formulated without es-
timation of vital parameters such as
population size, growth rates, age at first
fecundity, natural mortality, and others.
Clearly it is impossible to obtain meas-
urements of all these parameters in every
lake. Therefore representative lakes or
classes of lakes have been chosen for
study, especially for study of the growth
rates of both trout and char.
BROWN TROUT IN LAXÁ
Lake Mývatn in northest Iceland, with a
surface area of 28 km2, is located 277 m
above sea level and discharges through
the Laxá. The river is 58 km long with a
flow of45 m3/sec. Salmon ascend the river
for 27 km until they reach impassable
power company dams. Above these dams
the river is inhabitated by trout only. This
area of the Laxá is divided into an upper
section, 15 km long, and a lower section
the next 15 km. The current is stronger
(0.7 — 1.5 m/sec) in the upper section than
in the lower (0.3 - 1.0 m/sec), and the
river is wider here. The total area of the
Laxá above the power dams is about 150
ha. The bottom consists mainly of lava,
large stones and sand and there is a lack of
suitable gravel. Since the damming of the
lake (in connection with the power sta-
tions), the waterflow is stable. Earlier in
the spring floods cleaned out the sand,
deposited in Laxá by the tributary Kráká.
Growth
The fish were aged from otoliths and
scales, and the growth was back-calcu-
lated using the Lea-Dahl method. The re-
sults are shown in Table 1. Differences
observed between the 2 years are incon-
sequential, but growth in the lower section
is decidedly better than in the upper sec-
tion, and also the fish are older there. This
is clearly shown in Fig. 1, where the per-
centage distribution by length groups has
been plotted for the two sections. There is
a 7-8 cm difíerence between the means of
the two groups.
The results have been combined and
used to calculate a Bertalanffy length-in-
growth curve of the form: lt = la> (1 —
e-K<t_t°)). By use ofa Walford plot the coef-
ficients were estimated to be:
loo = 70.0 cm
K = 0.215
to = 1.708
The length/weight relationship varies
through the summer season and the con-
dition factor reaches a maximum in Au-
gust, with an average value of about 1.4
for medium-sized fish. The extremely
good condition of these fish can be seen in
the photograph in Fig. 2.
The reason for the exceptionally rapid
growth in Laxá is due primarily to the
organic material flowing out of lake
Mývatn. This highly eutrophic lake has a