Low oxygen and high temperatures appear to have caused the deaths of hundreds of eels and trout in Dunedin's Silverstream.
Mosgiel angler Bevan Campbell, who alerted Fish & Game Otago and the Otago Regional Council, said he had never seen anything like it.
Both agencies were on site last night.
Trout used the lower Silverstream as a refuge when the Taieri River was in flood and dirty, Campbell said.