Fish Movement

Weirs and regulators in rivers and lakes can impede or block the movement of native fish, impacting their ability to migrate and move to suitable feeding or breeding areas.  

While all fish need to move around to some degree, ‘flow-specialists’ like Golden perch and Silver perch need to move along rivers as part of their breeding and development, with some recorded traveling hundreds (even thousands!) of kilometres along and between river systems. Golden perch and Silver perch are particularly mobile species that often migrate and breed in response to flow events in rivers, with their eggs and larvae then drifting back downstream in the flow pulse, sometimes over long distances. Many of these young are transported into lakes and wetlands, which provide excellent ‘nursery’ habitat with warm, still water and plenty of food – a bit like a fish kindergarten.

To complete their life cycles, many of these fish will later return from lake and wetland areas back into river channels, where some take the opportunity to head upstream, and others move downstream exploring connected rivers.

Golden perch juveniles in a container, near Lake Cawndilla, 2021.

Golden perch juveniles pictured in a container near Menindee Lake, 2020.

A juvenile Golden perch near Menindee Lake.

However, with the many dams, weirs and regulators throughout the Murray Darling Basin, native fish now need our help to migrate. The NSW Department of Primary Industries-Fisheries has been working closely with fish ecologists and partner agencies to help native fish move past barriers in some rivers and wetlands in NSW, particularly through introducing fishways.

Key collaborators include:

• Water NSW, who operate and manage flow regulation and water storage structures across NSW
• Fish passage experts such as those from Charles Sturt University and Jacobs Consulting
• The Murray-Darling Basin Authority
• The Commonwealth Environmental Water Office and  
• NSW Department of Planning and Environment (Water) (Environment and Heritage), who deliver environmental water in NSW, and Water Infrastructure NSW.
• NSW National Parks and Wildlife Service.

Figure 1. Golden perch adult (left) and juvenile (right). Credit: NSW DPI Fisheries.

The Menindee Lakes

The Menindee Lakes are a series of shallow natural ephemeral lakes on the mid Darling-Baaka River in far west NSW. Through the 1950’s and 60’s, the Menindee Lakes were modified for water storage and regulation via construction of a series of weirs, regulators, levees and channels (Figure 2). The Lakes and the Lower Darling-Baaka River are the connection between the Northern and Southern Murray-Darling Basin.

Although downstream movement of fish into the Menindee Lakes is possible when inflows are filling the lakes, the regulatory structures in the Menindee Lakes unfortunately restrict ‘upstream’ movements by fish between the Lower Darling-Baaka River and the lakes. In the river channel itself, the Main Menindee Weir is a major barrier for fish trying to move upstream from the Southern Murray-Darling Basin (i.e. the Murray River and its tributaries) into the Northern Murray-Darling Basin (i.e. the Barwon- Darling River and its tributaries).  

Furthermore, under most operating conditions the high speed of inflows through Lakes Pamamaroo and Menindee inlet regulators prevent fish from exiting the lakes and dispersing ‘upstream’ to the Barwon-Darling River and the Northern Basin. It is only on rare occasions when the lakes are full and there is no drop in water level that fish can move ‘upstream’ within or between lakes in the system.  

So, while fish regularly enter the Menindee Lakes system from the Northern Basin upstream, opportunities to return to the Barwon Darling River and the Northern Basin are limited.

Figure 2. The Menindee Lakes system in far west NSW. Lakes Pamamaroo and Menindee inlet regulators are indicated in yellow.

Making Connections

In 2020 and 2021, NSW DPI Fisheries researchers identified multiple cohorts of juvenile golden perch in the Menindee Lakes and downstream in the Lower Darling-Baaka. Following severe drought from 2017-2020, these juveniles were the product of spawning in response to flow pulses upstream in the Barwon-Darling River system and its tributaries. Importantly, those flows had been protected by the NSW Government to support a ‘first flush’ along the parched Barwon-Darling River. This flush facilitated downstream drift of fish larvae spawned during the flow events, with many reaching the Menindee Lakes.

With the lakes filling to capacity in 2021, Water NSW and DPI Fisheries staff saw an opportunity to temporarily assist fish in migrating back out of the lakes back into the river channel and into the Northern Basin. In late October and early December 2021, a temporary fishway was trialled in the Lake Pamamaroo inlet regulator to facilitate movement of native fish from Lake Pamamaroo ‘upstream’ into Lake Wetherell (Figure 2).

The team conceived a novel temporary fishway, which involved fitting large baffles, each with a vertical slot, either side of one of the Lake Pamamaroo inlet regulator gates. When water levels flowing from Lake Wetherell across the inlet regulator into Lake Pamamaroo dropped to less than 60cm, flow velocities through the vertical slots in the baffles were reduced. This enabled fish that were attracted to the inflows (like developing golden perch) to move through the slots and pass from Lake Pamamaroo into Lake Wetherell. From Lake Wetherell, fish could potentially then travel up the Barwon-Darling River to contribute to populations in the Northern Murray Darling Basin.

In late 2021, DPI Fisheries conducted trapping trials which confirmed golden perch did pass from Lake Pamamaroo to Lake Wetherell via the temporary fishway when the difference in water level across the regulator was less than 60cm. Dozens of golden perch from 100mm juveniles to 400mm long adults were detected passing through the fishway in just a few hours of monitoring.

One year on, and with the Menindee Lakes spilling, inlet regulators were opened by WaterNSW to allow floodwater to pass through the system. This created another opportunity to utilise the temporary fishway at Lake Menindee inlet regulator in the same way, allowing fish to move from Lake Menindee (and the connected Lake Cawndilla) into Copi Hollow. From Copi Hollow, movement into Lake Pamamaroo is typically unrestricted (see Figure 2).

Figure 3. WaterNSW staff installed a temporary fish ladder (vertical slot baffles) in one gate of the Lake Menindee inlet regulator in December 2022 to facilitate passage upstream through the regulator from Lake Menindee to Copi Hollow and Lake Pamamaroo. These simple fishways enable flow specialist species like golden perch to complete their life-history, which is typified by movement over large spatial scales (Credit: NSW DPI Fisheries, WaterNSW).

Following the installation of the temporary vertical slot fishway panels at the Lake Menindee inlet regulator, trapping at the site in December 2022 detected 15 golden perch exiting Lake Menindee through the temporary fishway towards Copi Hollow in just 2 hours.

At the same time, the Lake Pamamaroo inlet Regular gates were fully open meaning there was no difference in water level between Lake Pamamaroo and Lake Wetherell. Fish were therefore able to continue to travel into Lake Wetherell, connecting with the Northern Basin.

This was the first time since the Lakes were regulated in the 1960’s that fish could move from the bottom Lakes (Menindee and Cawndilla) through to the top lakes (Pamamaroo and Wetherell), and then onwards up the Barwon-Darling River to the Northern Basin!

To put the cherry atop the fish passage cake, WaterNSW opened the Menindee Main Weir gates fully from 22 November to 19 December of 2022 to manage inflows from the north. When fully open, these gates essentially pass river flows transparently at natural speeds. This means fish were able to pass directly from the Lower Darling-Baaka River channel at Menindee to the Barwon-Darling River upstream of the Main Weir while the gates remained open for about a month. This was the first opportunity for unrestricted fish passage through Menindee Main Weir since 2012.

Figure 4. Menindee Main Weir gates opened in December 2022, allowing fish passage from the Lower Darling-Baaka River (and the Murray River) upstream to the Barwon-Darling and Northern Murray-Darling Basin rivers for the first time since 2012. This is one of very few opportunities for connections between Northern and Southern Basin, which is crucial for supporting populations of species like golden perch. (Credit: R. Unsworth)

Management of the Menindee Lakes is extremely complex, involving multiple and sometimes competing priorities relating to water storage and river operations. The temporary fish passage in the Menindee Lakes has been a fantastic achievement by WaterNSW and DPI Fisheries staff, which highlights the potential for positive outcomes to be achieved through collaboration and innovation. On the back of this success, future opportunities to facilitate fish passage at the Menindee Lakes will continue to be explored.

The activities presented in this article were supported from the MDBA as part of the Lower Darling-Baaka Recovery Reach project under the Native Fish Recovery Strategy.

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