Bosman informed the Department of Conservation (DoC), which contacted local guardian group Ngā Kaitiaki o Ngā Wai Māori and hapū coordinator Hona Edwards.
The Ministry for Primary Industries (MPI) is investigating.
Biosecurity NZ acting team manager Rissa Williams said its primary role was to rule out the involvement of exotic diseases.
“It’s possible the mortality could have been caused by a stress event,” she said.
Williams said this could be connected to a trap and transfer programme, but all possibilities were on the table.
The programme, run by Northpower with local hapū support, moves elvers and eels past the Wairua power station to other parts of the Wairua River.
Josie Boyd, Northpower Network’s chief operating officer, said work was underway to understand the cause of the deaths, and any environmental factors at play.
She said the trap and transfer strategies were continuously reviewed and adapted, but they could only do so much as an eel’s expected survival rate through its migration cycle was “very low” at 4 to 8 per cent.
Edwards, who is part of the local guardian group that monitor the area, said the stream’s water quality had declined in the past couple of years.
Ngā Kaitiaki o Ngā Wai Māori has launched its own investigation and sent some elvers for laboratory testing. The results are still pending.
More than 3500 elvers were found dead in a stream near Poroti. Photo/ Michael Cunningham.
Edwards believes, based on information from residents and collected data, that the stress event is linked to climate change.
He said elvers thrive in a waterway with good water flow levels, water temperatures between 16 to 18C and a “good presence” of dissolved oxygen between 7-8.5 mg/L levels.
But after testing the waterway upstream and downstream the day after the deaths were reported, Edwards found none of the parameters were satisfactory.
“We noticed some algae build up which generally is due to the stream water warming up.
“Then the biggest percentage of the stream was observed to range from very limited to no water flow. When there’s no flow, the dissolved oxygen reduces, and the toxins build up in the waterway,” he said.
Elvers migrate upstream usually between late November and early March . Photo/ Amber Bosman
Cawthron Institute freshwater ecologist Dr Simon Stewart said eels faced multiple stresses before they ended up at the Pacific Islands to spawn.
“They have hurdles like being predated in the ocean, climate change, polluted waters, absence of habitat or food and so on.
“But it’s difficult to diagnose the exact reason as to why the elvers died in Northland. It’s also quite sad because they already surpassed the majority of their obstacle-filled journey.”
Stewart said an eel’s lifespan was between 11 and 52 years. In autumn, adult eels that felt confident to spawn left NZ and travelled about 5000km to tropical islands like Tonga to breed.
The male eels then fertilised the eggs released by females, then die. After months of drifting in the sea, the eggs hatch into larvae and change into glass eels, elvers and finally into adult eels.
“The entire process takes place in a year or so,” Stewart said.
Dr Simon Stewart says eels were tropical fish before they migrated south millions of years ago.
The scientist said eels historically were tropical fish.
He speculated that eels travelling to the tropics to spawn and die was connected to their “tropical heritage and origin”.
Edwards said the recent incident was a reminder for communities to stay vigilant and take care of the awa.
“It’s a signal for all of us to do what we can to improve the health of our waterways in NZ to avoid situations like these.”
Avneesh Vincent is the crime and emergency services reporter at theNorthern Advocate. He was previously at the Gisborne Herald as the arts and environment reporter and is passionate about covering stories that can make a difference. He joined NZME in July 2023.
