Náttúrufræðingurinn - 1985, Síða 66

SUMMARY
The Biology of Arctic Charr
(Salvelinus alpinus (L.))
in Lake Þingvallavatn,
Iceland.
I. Feeding ecology.
by
Hilmar J. Malmquist,
Sigurður S. Snorrason
and Skúli Skúlason.
Institute of Biology
University of Iceland
Grensásvegur 12
108 Reykjavík
The main aim of the present study is to
describe and compare the feeding habits of
four different charr morphs inhabiting
Lake Þingvallavatn, a deep oligotrophic
lake in the south west of Iceland (area:
83,7 km2, mean depth: 34,1 m., max.
depth: 114 m.).
The diet of each morph is quite distinct
(Fig. 3). Dwarf and snail charr feed on
zoobenthos in the littoral zone. Snail charr
feed almost entirely on the pulmonate gas-
tropod Lymnaea peregra. Dwarf charr also
feed heavily on Lymnaea, but the diet is
more heteroegeneous, frequently includ-
ing chironomid larvae and pupae. Pelagic
charr feeds largely on zooplankton (Cyc-
lops abyssorum and Daphnia longispina)
and to some extent on chironomid pupae.
Piscivorous charr feed almost solely on
three spined sticklebacks (Gasterosteus
aculeatus).
The diet changed during the study
period (Fig. 4). In spring there is some
overlap in the diet of all charr morphs j
when they make use of emerging chirono-
mid pupae, and in late autumn when they.i
utilize a superabundance of charr eggs
following the spawning of pelagic charr.
The diet is not influenced by age and sex
but some difference was revealed between
sampling stations.
Frequency (%) of low stomach filling
was significantly higher in dwarf and pisci-
vorous charr than in the other two morphs
(Fig. 5). A good correlation was found
between low stomach filling and high
gonadal development within all charr
morphs, indicating low feeding activity
during spawning (Fig. 6).
The morphology and colour patterns of
the charr morphs seem to reflect their
feeding specializations and habitat
segregation. The two bottom morphs are
generally dark in body colour with a
rounded snout and subterminal mouth,
seemingly adapted to feeding on hard bot-
tom. Pelagic charr has the typical appear-
ance of a pelagic zooplankton feeder, the
body is slender and silvery, the mouth is
terminal. Piscivorous charr is rather light
in colour, the body is streamlined and the
large evenly matched jaws result in a
relatively big mouth opening width.
The morphs show clear differences in
growth pattern (Fig. 7) and life history.
Piscivorous charr and snail charr grow
relatively fast up to an age of 6+ — 9+,
when they mature. After maturation they
continue growing in a rather slow phase.
Pelagic charr mature at an age of 4+ - 6+
after which growth is extremely slow.
Dwarf charr mature even earlier, 2-i— 4+
years old. After maturation they grow very
slowly and rarely reach the length of fully
grown pelagic charr.
It is not known exactly at what age the
morphs segregate into their food niches.
Young pelagic charr may enter the pelagic
habitat at a very early stage, even as 0+
juveniles. The growth pattern of the pela-
gic morph may be strongly influenced by
the minute size of the food particles and
the actual feeding method. As the fish
reach 18—21 cm in forklength (70—100 g.)
their feeding method may not meet the
' fncreased energy demand required to keep
up the relatively high growth rate of juve-
nile fish. Inevitably piscivorous charr
experience a transformation in energetic
terms when they shift from invertebrate
food to sticklebacks. The immense
increase in food particle size is probably
crucial in allowing further growth and
delayed maturity in these fish. The
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