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A ‘fishway’ project by the West Gippsland Catchment Management Authority (WGCMA) was completed in 2019, and native fish habitat conservationists are excited to see its results two years on!

The Thomson River is one of Victoria’s most significant rivers. The river is 170km in length and covers a wide range of altitudes and conditions, flowing from around 1000m to 1m above sea level. The Thomson River is home to many of Victoria’s native fish and aquatic flora and fauna.  The project involved the creation of a fishway at Horseshoe Bend on the Thomson River,  four km south of the historic town of Walhalla. The fishway reconnects the Thomson River’s northern stream, flowing out of Victoria’s Alpine Regions to its southern stream, which connects with the Gippsland Lakes.

Mining family on the Thomson River banks 1909 Photo credit: W.H. Lee,
Content Contributor: West Gippsland Regional Library Corporation

A heritage site

The Thomson River became an alluvial gold mining hotspot during the late 1800s, due to the high quantities of gold found in the riverbanks and riverbeds. Alluvial gold is gold deposited by water movement.

The nearby town of Walhalla was quickly settled with thousands of miners during this time. However, by 1911 the easily accessible gold in the river had all been mined, resulting in migration of many of the miners out of the region. The Thomson River Alluvial Gold and Tailings Recovery Company discovered gold was more easily accessible in dried up riverbeds, however, the Thomson River is a fast flowing river. The company decided to take matters into their own hands by drying up Horseshoe Bend (seen in the map below). They created a tunnel to divert the water to the other side of the bend (image below shows tunnel). Horseshoe Bend consequently dried up and the alluvial gold was mined in the bend’s river bed.

Map of the Horseshoe Bend, showing the dry riverbed and tunnel prior to construction of the fishway. Source: Rutherford, Flatley and Hardie, 2018 (River Channel Relocation: Problems and Prospects)

Since 2002, the tunnel has been considered a heritage figure in Victoria so any environmental recovery or flow work must accommodate for the continued protection of the tunnel.

Submerged rocks at the tunnel exit are designed to break up the water flow and help prevent fish being swept up in the turbulent flow out of the tunnel. Photo credit: Victorian Department of Environment, Land, Water and Planning.

Importantly, the Thomson River supports the life cycles of at least eight native fish species, as well as other migratory aquatic species, such as eels.

Australian grayling (Prototroctes maraena). Photo credit: NSW Department of Primary Industries.

In particular, the Australian Grayling (also known as the cucumber mullet) is known to have significant population numbers in the Thomson River, whilst becoming almost extinct in many Victorian rivers since the 1970s. The Australian Grayling is listed as vulnerable under the Federal Environmental Protection and Biodiversity Convention Act 1999, and further classified as threatened under Victorian laws.

The construction of the tunnel in the Thomson River in 1911 has meant the species is unable to pass through Horseshoe Bend, as the turbulent water in the tunnel provides a physical barrier, especially for fish moving upstream. This has isolated populations on either side of the tunnel, and reduced access to habitat and genetic diversity for the species. This migration is important for the Australian Grayling, as different stages of its life cycle require different habitats. The Grayling requires downstream estuary conditions to spawn, however, the adult Graylings require cool, clear freshwater, found upstream. Thus, the tunnel provides a significant barrier for adult Grayling to find adequate food and habitat.

The WGCMA summarises how connecting the waterways through the fishway is essential for fish and aquatic life of the river:

  • It allows species to thrive and succeed by improving genetic diversity, allowing fish populations to interbreed.
  • It provides greater access to food sources and habitat.
  • It reduces the risk of predator attack by providing more places to live.
  • It helps a species to return to an area after it has been through a disaster (such as fire).
  • It allows the fish to go through all the critical life cycle stages from spawning, maturing to breeding.

Watch the video below to see how the WGCMA went about this fishway project.

Construction of the fishway began in February 2019, after almost 2 decades of planning. It involved excavation of Horseshoe Bend’s bedrock and widening of its channel. Revegetation and re-creation of the natural riparian conditions were also ensured. 60% of the water still flows through the tunnel to ensure its heritage status is maintained. The other 40% of the flow importantly now flows through the fishway, allowing a protected easy passage for native fish.

The Thomson River from Poverty Point Bridge. Photo Credit: Lyndsey V https://www.weekendnotes.com/profile/924864/

Fish surveys conducted by the Arthur Rylah Institute and Austral Research have seen a significant increase in native fish numbers, such as the Tupong and the Australian Grayling, in the Thomson River following the construction of the fishway. The results are already promising and it is hoped these native fishes continue to migrate upstream into new habitats. The following video was filmed when the fishway was opened.

Further information

 

Feature image: Rehabilitation and revegetation works have commenced around the newly commissioned fishway which has been designed to be a visual low-impact waterway. Photo credit: Victorian Department of Environment, Land, Water and Planning.

Kate McKenna
Author