Hopes fish passageway will prevent mass kill ahead of hot, dry summer

Fishheart co-founder Mika Sohlberg at the fishway entry in the Darling-Baaka River at Menindee. (ABC Broken Hill: Em Jensen)

The fishway at Menindee has reopened for the final season of its three-year trial, with 50,000 fish expected to travel through it before late May.

The reopening coincides with concern as the lakes and river experience low levels, poor water quality and algal bloom alerts.

The Basin Plan Review Panel will review findings after consultation with water users, holders and communities along the river.

A fish passageway allowing fish to travel upstream into the Menindee Lakes has reopened for the final season of its three-year trial, as low water levels and poor water quality raise concerns about another potential fish kill in the Darling-Baaka this summer.

The passageway, operated by Finnish company Fishheart, is expected to see up to 50,000 fish travel through between now and late May or early June.

The entry to the Fishheart fish passageway, in the Darling-Baaka River near Menindee. (ABC Broken Hill: Em Jensen)

The technology was installed to help reduce the risk of mass fish kills, including the 2023 event that saw about 30 million fish die in the area.

Fishheart co-founder Mika Sohlberg said the increasing number of fish using the passageway each year had come from a better understanding of the river.

"The first season we got 7,000 fish, and then around 30,000 fish," he said.

Fishheart regularly consult with a local group of traditional owners, who are able to share advice on fish behaviour.

"Obviously the local fish are new to us," Mr Sohlberg said.

Cameras in the passageway capture photos of fish as they travel through. (Supplied)

Regular upgrades to the cameras, lighting and artificially intelligent technology has helped to improve the system's ability to identify fish.

The fishway has reopened as the Menindee Lakes sit at a combined capacity of 29 per cent, with low rainfall in the catchment and algal bloom alerts in effect in parts of the lakes.

Cameron Lay, director of Freshwater Fisheries and Threatened Species at the New South Wales Department of Primary Industries and Regional Development (DPIRD) Fisheries, said the fishway could help reduce the risk posed by fish becoming concentrated in the Menindee weir pool.

Cameron Lay says there is concern around blue-green algae. (ABC Broken Hill: Em Jensen)

"Fish now have the ability to leave the Menindee weir pool which has been the site of a lot of those significant fish kills. Some of that has been because there's been an accumulation of fish and there hasn't been sufficient resources, food, oxygen et cetera, for them to survive.

"So some of these initiatives, if they can allow fish to move out of the Menindee weir pool and up into the lake system and to migrate further up the Darling, then hopefully we've got a better change that we won't see large scale fish kills like we've seen in the past."

Mr Lay said there was concern around blue-green algae, although oxygen levels in the water were generally fine.

Blue-green algae can occur naturally in small amounts, but can rapidly multiply when weather is warm, water is still or slow-flowing, and excess nutrients enter the water.

In large amounts, blue-green algae can deoxygenate water, potentially leading to fish suffocation.

"Things can change very quickly when you get extended periods of 40-degree days," Mr Lay said.

A sign next to Lake Wetherell warns of algal bloom risk. (ABC Broken Hill: Em Jensen)

"Probably the biggest concern is the potential of a really hot, dry summer.

"With El Niño developing, we have had challenging circumstances in the past when we've had really hot, extended dry periods, and that might be as big a concern as the amount of water that's actually in the river at the moment."

It is yet to be confirmed whether the fishway will be extended after the trial's conclusion, but Mr Lay said the fish passage was only one part of the broader work needed to improve river health.

He said outcomes from the recent Basin Plan Review would begin being implemented after they are released in December.

One of the common themes the review panel heard during their tour of the river was that investment should be spent more broadly.

"There's been a lot of focus in sort of purchasing water and water recovery and that has been an essential step," Mr Lay said.

The fishheart passage sends fish upstream into Lake Wetherell. (ABC Broken Hill: Em Jensen)

"The river can't survive unless it's got sufficient flow so that was probably the obvious and first priority area to focus on. But it's not the only problem that our rivers are facing."

Along with the fish passage, he said water temperature and quality as well as habitat were vital to maintaining a healthy river system.