Queensland's Department of Environment and Science (DES) is continuing to investigate the cause of a large fish kill in the Mulgrave River, downstream from Gordonvale in far north Queensland.
While some locals believe the fish kill numbers to be in the tens of thousands, a DES spokesperson says its officers estimate hundreds of native and introduced fish species were killed earlier this month, including barramundi, catfish, eels, shrimp and tilapia.
Fish and water samples collected by DES officers are being analysed via laboratory testing.
While the cause of the fish kill is still unknown, it is suspected to be the result of low levels of dissolved oxygen in the water which can occur during periods of high temperatures and low river flows.
Aloomba cane farmer Tony Rossi agrees this is the most likely cause.
He said the recent heatwave conditions along with tidal flows potentially created 'the perfect storm'.
"The spring tides were happening at exactly the same time … [that's] when we get the big tidal influence on the rivers and they pulse, he said.
"[That] can hold water back and in combination with the thermal load on the river and other pressures, it [probably] made this low dissolved oxygen event."
While large numbers of tilapia, considered a pest in far northern waterways, were killed, Mr Rossi is particularly alarmed by how many eels died from the event.
"When you see eels taken out, they're really tough, so it must've been pretty low [dissolved oxygen]," he said.
"The one that's got me worried is the eel-tailed catfish … to see a lot of [them] amongst the dead ones is a bit worrying because we've seen iconic species leave these northern rivers before."
Environmental group calls for council water re-think
Cairns and Far North Environment Centre (CAFNEC) spokesperson Bronwyn Opie said the fish kill was concerning in light of plans to take more drinking water from the local waterway.
"Cairns Regional Council's Water Security Stage 1 proposal to extract water from the Mulgrave River may exacerbate the issue of low flows on the river, causing more incidences of the same conditions that are believed to be behind this mass death," she said.
"The climate is changing, and with that, our rainfall patterns are changing and heat waves are becoming more frequent and severe.
"To adapt, we need solutions that are resilient and address the root cause of our water supply shortage - overuse and inefficiency."
Tony Rossi, who is also Chair of Mulgrave Landcare, said the fish kill highlighted the need to let scientific data determine the balance between drinking water reservoirs and environmental flows.
"It can't just be a nice round number that fits everyone's numbers, it's really got to be sensitive to the requirements of the biodiversity in the river," he said.
"These sorts of events are just not acceptable anymore; we are realists, but we've got to find a way forward."
A spokesman for Cairns Regional Council said it would ensure environmental flows are maintained to protect the river system "for the benefit of the communities, fauna and flora that rely on it".
"Significant work and modelling has been undertaken by council to ensure there is a robust understanding of how the river system operates, which will safeguard and preserve the established ecosystem," the spokesman said.
The State Government is reviewing a council submission on water entitlements and will set environmental protections and strict operating conditions, he said.
