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 both Fish & Game Otago and the Otago Regional Council, said he had never seen anything like it.
Both agencies were on site on Thursday night.
Trou used the lower Silverstream as a refuge when the Taieri River was in flood and dirty, Mr Campbell said.
Otago Fish & Game chief executive Ian Hadland said the likely cause of death was warm, low oxygen inflows of ponded floodwater from surrounding farmland.
"We have taken oxygen levels in the lower Silverstream today and they are still very low — borderline lethal for fish — and it’s down to near zero in one of the drains leading into the stream.
"There are dead and decomposing fish everywhere and dead worms all over the stream bottom. It’s a tragic sight."
Rotting grass and vegetation beneath ponded floodwater was known to decrease water oxygen levels as the material broke down.
That, in combination with heat, was a perfect recipe for an anoxic environment to form, he said.
"The Otago Regional Council are taking water samples to rule out other sources but the cause of the fish kill looks to be simply a result of a natural flood event at the warmest time of the year," Mr Hadland said.
Council regulatory general manager Richard Saunders said the council received reports about 9pm on Thursday of dead fish and eels in the water and on the banks of the Silverstream, and two environmental officers were on site to investigate by 9.30pm.
Officers found one dead fish, and the member of the public who got in touch had pictures of other dead fish that had gone past. The dead fish was on the bank of Silverstream, well above the current water level, indicating it may have died when the water level was higher.
An oily sheen was observed on the riverbanks, which was determined to be natural oils from decomposing vegetation, Mr Saunders said.
The council collected water samples for lab analysis, and would continue to investigate before determining if any further action was required.
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Although the deaths might be "natural" I suspect that the cause is man-made with the draining of floodplain wetlands for agriculture to blame.
Although wetlands have often been referred to as natural sponges that soak up water, they actually function more like natural tubs, storing either flood waters that overflow riverbanks or surface water that collects in isolated depressions. As flood waters recede, the water is released slowly from the wetland soils. By holding back some of the flood waters and slowing the rate that water re-enters the stream channel, wetlands can maintain the geomorphic stability of important habitat for aquatic life by attenuating peak flows.
It also avoids agriculture runoff from from effluent, fertilizers and chemicals which are all detrimental to aquatic life.