Fish and river experts say the recent opening of the main weir at Menindee will benefit native fish populations that instinctively migrate up and downstream.
It is the first time in 10 years that the gates have risen completely from the Darling River.
In anticipation of further flooding in the river, outflows from the weir, south-east of Broken Hill, have been increased by Water NSW over the past few weeks to lower the capacity of the Menindee Lakes system.
Menindee resident and regional engagement officer for the Murray-Darling Basin Authority Richard Unsworth said the weir’s six gates were fully opened by November 27, which was a rare occurrence.
“The gates have been open for some time, but this is where the gates have actually come clear of the water, so there’s no barrier on either side of the weir,” Mr Unsworth said.
“The gates were out of the water when there was high water back in 2012 for 29 days, and the time before that was in 1998 when they were out of the water for 41 days.”
Native fish are the real winners
In addition to helping prevent the local system from overflowing, opening the gates fully also creates an environmental side benefit.
“Based on research that was done previously, even if the gates were in the water by 100 millimetres, that creates a real issue for fish being able to travel either north or south past the main weir,” Mr Unsworth said.
With this barrier now removed, fish seeking to migrate north during the warmer months can travel unimpeded to either side of the basin for the first time in a decade.
Many species of fish, including those living in the Darling River, seek to migrate to different habitats during feeding and breeding season.
NSW Department of Primary Industries senior fisheries manager Iain Ellis said this travel was likely to be happening in both directions.
“It means for at least the next couple of weeks, we’ll be having that [adult] fish movement upstream, but also smaller fish moving downstream,” Mr Ellis said.
“Usually, if [baby fish] go through a partially closed gate at that weir, they get mushed up in the turbulence, velocity and pressure changes, [so] find it hard to move from upstream to downstream.”
Native fish species, which people can expect to be migrating include silver perch, golden perch, Murray cod, bony brim and gudgeon.
Hopes for a future permanent alternative
There is currently no specified timetable for when Water NSW will close the weir gates again, although Mr Unsworth says it would likely be after Menindee’s gauge reaches the predicted peak of 9.6 metres.
However, Mr Ellis said he was hopeful regular migration in the system could eventually be restored by a state government-funded solution, such as a fish ladder.
A fish ladder is a structure built on artificial water barriers, such as a weir which uses a series of small dams and pools to make a long, sloping channel for fish to travel around them in either direction.
“Back in 2019, after those big fish deaths in Menindee, the previous government did commit to providing more permanent fish passage,” he said.
“That especially means a fish ladder at the main weir or nearby to allow fish to move from downstream to upstream.
“We’re pretty hopeful that’ll still happen in the next couple of years or so.”