Mass fish die-off at Marlborough children's fishing event

The dead fish (Newshub.)
The dead fish (Newshub.)

The Marlborough District Council is adamant a Blenheim river is safe, even though dozens of fish died minutes after they were set free in the waterway.

The salmon were released into the Opawa River for a children's fishing event organised by Fish and Game.

However staff were shocked when the fish suddenly died before the kids could get their lines in, leaving behind the rotting remains of a fishy failure.

"[They're] pretty badly decomposed and pretty smelly, so it's best we haul them out of the water and go bury them for plant fertiliser," says Nelson-Marlborough Fish and Game field officer Vaughn Lynn.

He placed 100 salmon in the river last week last week ahead of the children's event.

"A couple of salmon beached themselves on the bank, so we were placing them back in the water, and we noticed a couple turned belly-up - then a salmon by the upstream net turned belly-up," he says.

A video from a previous event shows how the healthy fish were pumped into the river from an oxygenated tank.

Marlborough District Council ecologist Pete Hamill says the salmon appear to have died from a lack of oxygen in the water.

Fish have trouble coping when the dissolved oxygen level drops below 80 percent. The Opawa River had just 15 percent.

"It's definitely something that's unusual to have happened," Mr Hamill says.

Around 60 local children had signed up for the event - one of many held by Fish and Game.

Mr Hamill says earthquake runoff entering the stream and recent flooding could be to blame.

"The area here in question is controlled by floodgates - and that would have actually stopped the movement of water going through, so it made the pulse of contaminants, or whatever was in the water, just move very slowly," he says.

Contaminant tests are due back next week, but Mr Hamill says the river is already safe.

And Fish and Game has promised to test waterways itself before future fishing events, to avoid a repeat of scenes like this.