28 Jun Permanent pools along the Wannon River provide critical refuge through the tough times
Back in April, Liam and I spent a week exploring the southern Grampians in search of permanent pools along the Wannon River to identity key drought refuge sites (permanent pools) for native aquatic species. These surveys were part of a wider project (generously funded by the Victorian State Government) aimed at acquiring baseline information on the population status of little galaxias (Galaxiella toourtkoort – formerly called dwarf galaxias). We were particularly interested in understanding and documenting native fish responses to permanent hydrological restoration works completed last year for Brady swamp and Gooseneck swamp, situated north-east of Dunkeld.
When this large wetland complex receives seasonal flows from the Wannon River, it provides key floodplain habitat which facilitates the successful breeding and recruitment of native species, including little galaxias, southern pygmy perch and the western swamp crayfish (endemic to the Grampians). However, due to the dry spell over the past 18 months the river hasn’t flowed since 2014, and the hydrology of the floodplain wetlands has been affected as a result. Given the dry conditions, our Wannon River surveys focused instead on identifying key refuge pools upstream and downstream of the Brady swamp complex to improve our understanding of each species specific habitat requirements and the distribution of refuge sites that will enable recolonisation of the Brady Swamp complex when the River next flows (hopefully soon in 2016!). During sampling, we revisited sites previously surveyed in autumn 2015 and also searched for additional pools.
Upstream of Brady swamp, little galaxias were only detected at one site, which was a 5 m2 refuge pool. Other sites upstream were typically characterised by faster flowing water and in-stream woody debris that were better suited to other native species such as river blackfish, Glenelg spiny crayfish and mountain galaxias. At all sites, due to the dry winter and spring in 2015, the water level was far more concentrated this autumn compared to last year, which contributed to higher catch rates of river blackfish and spiny crayfish at some sites. The more concentrated habitats appeared to favour the introduced redfin, which was recorded for the first time at two of the five upstream sites. Where redfin were most abundant, we observed only low numbers or a total absence of key native species, reflecting the competition and/or predation exerted by this problematic introduced species of fish.
Refuge pools downstream of the wetland typically provided more suitable habitat for wetland specialist species (i.e. slower flows) such as little galaxias and southern pygmy perch. However, compared to 2015, abundance and area of occurrence had reduced for both species which no doubt reflects the decline in available habitat and water quality, with salinity at most sites being well over double that of last year. In addition, flathead gudgeon which is a generalist species and thus able to tolerable a wider range of conditions, increased in abundance in these downstream pools. As pools become more concentrated, the highly predatory flathead gudgeon are likely to out compete or even predate on other native species.
On a positive note, a new significant refuge pool (presumably fed by a spring) was discovered during recent monitoring and despite being very saline (>9000 EC), supported low numbers of little galaxias and southern pygmy perch. Comparatively, a smaller refuge pool nearby which was less saline (<4000) supported the highest number of little galaxias. The refuge pools along ‘Grampians retreat’ had maintained reasonable water levels and were the only sites to not show increased salinsation (remaining around 2000 EC). This proved to be a key determinant for the increased abundance of river blackfish, little galaxias and southern pygmy perch observed at the site.
Overall, despite the lack of flows along the Wannon River over the past 18 months, there are still a small number of suitable permanent pools which are providing critical refuge for key native species over these drier periods. Hopefully the long awaited flows from the Wannon River will arrive this winter and we will have the opportunity to assess how native fish communities respond to the expected increase in inundation of Brady and Gooseneck swamp following permanent hydrological restoration works.