Náttúrufræðingurinn - 1998, Page 41

young lava fields are a characteristic feature in
the topography of the area. The hahitat of the
dwarf charr is characterized by this topography,
as well as the special interaction between spring
water and sea water. The spring water has rela-
tively constant temperature, round 4°C through-
out the year. The sea water is introduced at high
tide to the habitat of the dwarf charr, both in the
littoral zone of the sea and in the ponds, as it
flows through the coarse lava field beneath the
freshwater column. This is reflected by tidal
shifts in the ponds where sea water is found in
the deeper parts ol' ponds and/or their habitat in
the substratum where it has the strongest influ-
ence at flood tide.
The charr where observed in high densities, up
to 64 indiviuals/18m2. Captured charr were from
one summer old (0+) up to six years old (6+) and
ranged from 3.7 to 13.9 cm in length. The charr
could mature more than once and the first sexual
maturalion among males occurred at lower age
(>1+) and lesser size (> 6 cm fork length) than
among females (>3+ and > 10 cm fork length).
The females had a total of 50-75 eggs (3.8-5.0
mm in diameter) that in weight ranged between
6.8-17.4% of their body weight. The individuals
maluring were almost fully mature, so the
spawning starts in late September-October.
At all sitcs, the majority of charr were eating
and proportion of feeding fish and their stomach
fullness increased with increased fish size. The
prey types were invertebrates from both sea and
freshwater, except in the pond Brunntjorn where
invertebrates living in saline water were not
found in the stomachs. Tliis reflects special fea-
lure of the Brunntjom pond, where the freshwa-
ter rarely mixes with the sea layer beneath, al-
though it reflects the tidal lluctation. The overall
main prey types according to volume were
amphipods and insects, which consisted mainly
of chironomids larvae and pupae and trichoptera
larvae. In the sea littoral zone the charr were also
eating polychaets and salmon lice (Lepeopíheir-
us salmonis Krpyer) that had fallen off salmon
(Salmo salar L.) during their migration along the
shoreline. Importance of the prey types accord-
ing to volurne varied in relation to fish size. The
smallest prey, thc ehironomids larvae, were the
main food bulk of one-summer-old charr, while
the larger fish were eating more diverse prey,
whereof proportion of large insects (trichoptera)
and food of sea origin increased with increased
fish size.
The small size of the charr enables both imma-
ture and mature individuals to utilize the same
habitat. Additionally, the dark colouring of
dwarf charr reflects their adaption to their habi-
tat, as does their blunt snout; the shorter lower
jaw compared to the upper jaw refers to their
benthivorous feeding behaviour. Dwarf charr
haven't been reporled previously in habitat di-
rectly influenced by the sea. That, together with
their ability to utilize simultaneously prey of
sea origin and prey from freshwater, reflects
once again how adaptive this species is. In addi-
tion, dwarf morphs of charr coexist in general
with other charr morphs, but in the ponds at
Straumsvik no other charr morphs are found and
in fact no other fish species were observed.
These unique circumstances are therefore of spe-
cial interest, in relation to evolutionary as well as
ecological aspects.
PÚST- OG NETFANG HÖFUNDA/
AUTHORS'ADDRESSES & E-MAIL
Jóhannes Sturlaugsson
johannes.sturlaugsson@itn.is
Ingi Rúnar Jónsson
ingir@itn.is
Sigurður Guðjónsson
siggud@itn.is
Veiðimálastofnun/
Institute of Freshwater Fisheries
Vagnhöfða 7
IS-112Reykjavík
Stefán Eiríkur Stefánsson
stefan@itn.is
Stofnfiskur hf
Laugavegi 103
P.O. Box5166
IS-125 Reykjavík
199