Náttúrufræðingurinn - 1985, Blaðsíða 67

segregation of the two bottom morphs is
much more subtle and in terms of diet it is
bearly detectable. The differences in sto-
mach filling between these fish could imply
that the dwarfs are inferior competitors for
bottom food resulting in early maturation
and slow growth.
In comparison with sympatric charr
morphs elsewhere in Scandinavia and Ice-
land (see e. g. Henricsson and Nyman
(1976), Klemetsen and Grotnes (1980),
Hindar and Jonsson (1982), Ranta-aho
(1983) the food segregation of the Ping-
vallavatn morphs seems to be more pro-
nounced and last for a longer period of the
year. Thus the Þingvallavatn morphs
behave as well defined ecological groups.
The actual mechanism of this ecological
segregation is not known, but in the case
of snail charr, dwarf charr and pelagic
charr it may result from competition
hierarchies on the nursery grounds, sub-
ordinate fish either being pushed into the
pelagic habitat (pelagic charr) or forced to
adopt a very precocious life history i. e.
extremely early maturation and slow
growth (dwarf charr). Compared to many
lakes in Scandinavia the fish community in
Þingvallavatn is very simple. Apart from
arctic charr only two other fish species
inhabit the lake; brown trout (5. trutta)
(now very rare) and three spined stickle-
back. Together with the lake mor-
phometry, i. e. extensive pelagic zone and
complex bottom habitats, this may allow
the pronounced food segregation of the
charr in the lake.
Two hypotheses, at least, may be put
forward regarding the nature of the charr
morphs in Pingvallavatn:
1) The morphs may represent more or
less separate populations or sub-
species.
2) The morphs may be phenotypic
variants of the same parent stock, the
segregation resulting, wholly or
partly, from genetic polymorphism
or from differential responses of a
general plastic genotype.
The two hypotheses do not exclude each
other. For example one could envisage
two separate stocks, each segregating into
two morphs (phenotypes).
Considering morphological differences
(Skúli Skúlason, 1983), as well as food-
and habitat segregation of the Þingvalla-
vatn charr morphs, we feel justified to put
forward a hypothesis stating that the lake
holds two populations of arctic charr; a
littoral bottom population, including snail
charr and dwarf charr, and a population
inclined towards the pelagic zone, i. e.
pelagic charr and piscivorous charr.
To test such hypotheses one would want
to know to what extent the morphs are re-
productively isolated. It is also important
to test if various characteristics distinguish-
ing the morps from each other are inher-
ited. Based on the background knowledge
provided in the present study such tests
may be carried out successfully by careful
genetic analysis and breeding experiments.
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