Thousands of tuna (eels) were found dead in Low Burn stream near Brydone, south of Gore.
Hokonui Rūnanga kaiārihi taiao (environmental lead) Riki Parata said he was saddened by the event.
"It’s devastating. We work daily with the tuna in this area, so to see this is very confronting."
Mr Parata said his team found 400 dead eels while cleaning a 500m stretch of the Low Burn stream.
He said he was not sure what killed the fish, but Environment Southland was investigating.
Mr Parata said some of the volunteers cleaning the stream were from the Hokonui Rūnaka wai Māori team.
The team monitored eels for research and had been trapping and transferring juveniles from the Mataura Falls in the Mataura River Mātaitai.
The falls were set up to protect taonga species, including tuna, he said.
"I want to appeal to people to treat our waterways with more respect.
"We need to look after our awa [river] — it’s precious."
He thanked the Te Ao Mārama and Environment Southland volunteers for their efforts in cleaning and providing updates on the stream.
For Ngāi Tahu and Hokonui Rūnaka, eels are an important taonga species for a number of reasons.
They are essential to the continuation of mahika kai, the customary take right for Māori events.
The fish are used by some iwi as an ecological health indicator to assess water and habitat quality.
