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Over the
last three years I have spent much time investigating the interesting
aquatic environment and the fishes that can be found around Scott Creek,
in the Onkaparinga River System. Mostly, this has involved the conservation
park surrounding Mackereth Cottage, where flow seems to continue year
round. This may be a small trickle in summer or a raging torrent after
heavy rain. The waters are clear and cool, and aquatic vegetation such
as Ranunculus amphitrichus, Potamogeton crispus and Triglochin are lush
in some areas. The creek contains large numbers of introduced fish and
willows, although it is encouraging to see the restoration of the creek
that is currently occurring. Below I have briefly summarised my observations
and thoughts towards a greater understanding and the restoration of
Scott Creek. One native species, the climbing galaxias Galaxias brevipinnis
and three introduced species, brown trout Salmo trutta, redfin Perca
fluviatilis and eastern gambusia Gambusia holbrooki, can be found.
The climbing
galaxias Galaxias brevipinnis ( seen above) reaches around 25
cm. It is a unique and amazing fish, having a modified body structure
with a flattened ventral (bottom) surface and large fleshy pelvic and
anal fins. Consequently it is able to climb moist surfaces to facilitate
feeding and migration. Populations in Scott Creek are very patchy. A
limited number of pools in the conservation park seem to house quite
large adult fish with an attractive brown and dark mottled colouration.
Monitoring one population regularly, they seem to be in the one pool
for most of the year but disappear in the middle of winter coinciding
with the strongest flows. Where they go is a mystery. Juveniles can
be found near the oval towards the top of the creek, which may indicate
a spawning migration. The reported biology for this species suggests
that after adults spawn the larvae are swept to sea, later migrating
back to freshwater. Mt Bold certainly prevents this cycle and I'm not
sure if this juvenile migration occurs at all locally. This makes an
ideal area for research and highlights a generally poor knowledge of
the biology of our local species.
The
climbing galaxias Galaxias brevipinnis
Much more suitable habitat exists for galaxias in Scott Creek but
their distribution may be limited due to the impacts of the introduced
species, particularly trout and redfin. These species grow much larger
than the native species and are aggressive predators. It is interesting
to note that trout are still released into this waterway (which includes
a conservation park) despite the documented evidence regarding their
impact on native species, particularly galaxias. A further two species
may be expected in Scott Creek, the mountain galaxias Galaxias olidus
and the river blackfish Gadopsis marmoratus but a combination of environment,
habitat and predators may have lead to their local extinction. Introduced
species, especially the Gambusia, also compete for food and resources
with native fish and have been implicated with the decline of tadpoles.
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The
removal of willows
This seems to have helped the aquatic vegetation and it is good
to see the riparian growth and diversity developing so quickly in
place of the willow monoculture. The lack of alternate shading is
a short term problem with warmer temperatures and algal growth,
where the replanting of natives such as ti tree and gums will help
greatly. Further gradual willow and blackberry control along with
replanting are the future focus of restoration works. |
Under the
water, sediment seems to have been washed out of pools in the cleared
areas to reveal a solid gravel bottom. Similarly the ongoing efforts
to remove large trout and redfin (by NFA(SA) and Scott Creek Friends
of Park members) seems to have increased the abundance and diversity
of invertebrates and hopefully the galaxias can spread out a bit. The
trout seem to keep recolonising some cleared pools but if we keep at
it, there should be a noticeable improvement in the waters of the conservation
park to provide a more holistic approach to the aquatic restoration
work in the area.
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An exciting addition to the known list of aquatic fauna for the park
has recently been made, with the discovery of the mountain galaxias
Galaxias olidus. This species is an active small species, reaching around
100mm, and is cigar shaped (long and skinny).
It prefers
flowing stream environments and is particularly susceptible to predation
by exotic fish.
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