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Dead fish, including smelt, flounder, giant bullies, trout and inanga, littered the edges of the Kaikorai Stream and estuary at the Brighton Rd bridge.
ORC regulatory general manager Richard Saunders said yesterday it would investigate the fish deaths and whether there was any pollution from an upstream source.
He said the opening of the coastal mouth, which occurred over the weekend, could also have contributed to their deaths.
The council’s engineering team opened the coastal mouth at the Kaikorai estuary regularly when the stream reached a point at which it could cause water to flood and backfill into the Green Island landfill leachate fields, Mr Saunders said.
The council’s pollution team attended the site of the fish kill at Kaikorai Stream on Saturday, and took water samples for testing.
The stream had underlying water quality issues.
"The most likely scenario in this case is that warm temperatures and reduced dissolved oxygen in the water exacerbated underlying water quality issues in the stream, leading to the fish deaths," Mr Saunders said.
“Opening the coastal mouth is something [the council] does regularly as a flood protection measure, and it’s essential to avoiding leachate from the Green Island landfill entering the stream."
Opening the coastal mouth had not been associated with fish kills in the past, but it was possible that it was an aggravating factor in this fish kill event, he said.
The fish kill highlighted that water quality was everyone’s responsibility, in urban contexts as well as rural, he said.
Urban waterways had some of the most degraded water quality in New Zealand, he said.
"It’s crucial that people are conscientious about what’s going into the streams and rivers in their backyards, and avoid draining anything that might negatively impact the environment," he said.