She said HBRC teams collected water samples from several locations in the Waiatai and Ikanui streams.
“These samples were tested in accredited laboratories for a wide range of contaminants.
“The findings indicate the eel deaths were caused by a natural blackwater event, made worse by the combination of extreme heat and heavy rainfall in early January, which put the entire aquatic system under severe stress.
“With conditions changing so quickly and the stream under immense stress, even resilient species like tuna were unable to survive.”
A blackwater event occurs when heavy rain washes large amounts of natural material, such as leaves, grasses, and wetland debris, into a stream. As it breaks down, it rapidly consumes oxygen in the water.
“In this case, oxygen levels dropped to just 1.9%, which is effectively no oxygen at all. In healthy freshwater streams, oxygen levels typically sit above 80%,” Siers said.
She said over time the landscape around the stream had changed, with less shade, altered wetlands, and warmer water, leaving it more vulnerable to sudden weather shifts.
“There are steps that can be taken collectively to help build the resilience of the Waiatāi Stream, including native planting, restoring wetlands and natural stream flow paths and keeping fish passages open.
Hill’s daughter Kate Eaglesome, who lives on Kairangi Station further downstream, said at the time there was nothing left alive in the stream.
“It’s good now to finally have an answer,” Eaglesome said.
“But I want to see some action. Let’s start planning now to give the stream life a fighting chance.
“The water still looks quite dark. We did see a couple of baby eels on Saturday, which was nice to see, but also sad. Usually, the stream would be teeming with them.”
Eaglesome said she was proud of her dad for all the work he did behind the scenes.
“He got the ball rolling and has been so supportive. As have Michelle McIlroy and Katarina Kawana from Wairoa Awa Mauri Restoration.”
She said autumn and winter were the perfect times to start planting.
“So let’s do it. No more talking, it’s time to get moving.”
Siers acknowledged how upsetting the eel deaths had been for so many.
“For mana whenua, landowners, and everyone with a strong connection to the area, this has been distressing to witness. I appreciate the deep care and kaitiakitanga (guardianship) shown by the community over the past month.
“Strengthening the health and mauri of the Waiatāi Stream is something we all care about, and we’re committed to working alongside mana whenua, landowners, and the wider community to protect it moving forward.”
Post-mortem couldn’t be completed
One of the dead eels was sent to Massey University for a post-mortem examination on January 15, arriving on January 16.
The university’s Associate Professor in Wildlife Pathology, Stuart Hunter, said he examined the eel the morning it arrived, but by then it was too decomposed to determine the cause of death or run any other tests.
“When it comes to issues with water quality (such as low levels of oxygen, pollutants) causing animal mortalities, post-mortem findings are not very specific, that is, there are often not characteristic changes to the internal organs that point specifically to a particular toxin.
“Water testing is much more useful,” Hunter said.
LDR is local body journalism co-funded by RNZ and NZ On Air.