Náttúrufræðingurinn - 1985, Page 44

(Table 1, Fig. 8). Similar changes occurred
further downstream, at Helluvad and
Thverá, but the production was lower than
at Midkvísl.
Considerable variation in the summer
temperatures in the river during 1977—
1983 (Fig. 4) dit not seem to affect the
production of S. vittatum.
The drift of benthic animals is consi-
dered to be related to discharge of the
rivers (Elliott 1967). The discharge of the R.
Laxá is very stable and floods rarely occur.
A study was made in Midkvísl on the drift of
S. vittatum at different densities of larvae
and a constant discharge. Observations
were made at 2—4 hours interval (2—3
replicates) during each 24 hour sampling
period to cover diel fluctuations. The
mean proportion of the population found
drifting in each period was very small and
stable (0.003-0.005%) (Table 2). From
early July to late August, there was a
proportional increase in the drift at nights
(Fig. 13), but in mid-summer when
nights were light, similar proportions of
larvae were found in the night and day
drift. No difference was found in the size
of larvae in the drift and in the benthos.
(Fig. 12).
The larvae of S. vittatum is the main
food of trout (Sahno trutta L.), harlequin
ducks (Histrionicus histrionicus (L.)) and
Barrow's goldeneye (Bucephala islandica
(Gmelin)) in Laxá. The changes in the
population of S. vittatum affected the
populations of these predators (Fig. 8).
The annual catch of trout fell from about
3000 to 1000—1500, and other studies have
shown that the numbers of harlequin duck-
lings produced on the first 10 km of the
river in the autumn fell from about 120 to
10—60 and the Barrow's goldeneye
changed its feeding habits, moving from
the river to L. Mývatn. With increasing
production of S. vittatum, the number of
harlequin ducklings produced on the
upper part of the river started to increase,
and was 231 in 1984.