Anoxia may have killed fish

IT may well have had something to do with the weather, but the Otago Regional Council still has not worked out the cause of hundreds of fish and eels dying in the Silverstream, near Mosgiel, in January.

The fish died in the wake of the January floods.

The council is also still trying to work out how to reduce the likelihood of similar incidents in its flood drainage infrastructure in the future.

Council manager engineering Michelle Mifflin said water samples taken from the stream by council staff at the time the complaint was received did not point to a cause of the fish deaths.

While there was no obvious pollution detected in the water samples, the environmental factors of the recent flooding, followed by warm summer temperatures, were thought to have played a part, she said.

Council staff were investigating, with input from an independent expert, how its flood protection and drainage scheme could best operate to avoid exacerbating environmental risks for fish and eels.

Staff were working closely with Fish and Game to better understand what happened.

At the time, Fish and Game staff said the deaths seemed to be a result of warm, lowoxygen inflows of ponded floodwater water from surrounding farmland.

Rotting grass and vegetation beneath ponded floodwater was known to decrease water oxygen levels as the material broke down and that, in combination with heat, was a perfect recipe for an anoxic environment to form, they said.

Ms Mifflin said the council wanted to ensure it was being a good environmental steward as well as ensuring the continued effective operation of its flood schemes, which were critical to the communities they protected.