Native Fishes and the Restoration
of Scott Creek
Michael Hammer, Native Fish Australia (SA) Scientific Officer
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.
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.
New Discovery for Scott Creek Consation Park!
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.
Given the dominance of predatory trout within Scott Creek, the presence of mountain galaxias is a very exciting find. However, the two are not found together, rather the native species has been excluded to a tiny patch of habitat above a waterfall isolated form the creek.
It is amazing to find mountain galaxias surviving in such marginal habitat and it seems a very precarious position for long term survival. The discovery provides the evidence behind previous suggestions that mountain galaxias should be thriving in Scott Creek and indeed many other local waterways currently stocked with trout. The protection of the species flags an ideal research project to study the local biology of the species, with a view to forming a management program within the park to begin to restore the range of the species - if your interested, please get in touch with Tom Hands or Michael Hammer.
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