Nelson Marlborough Fish and Game field officer Vaughan Lynn holds up one of the salmon that died in the Opawa River.

A children's fishing day has gone belly up after a run of salmon died just minutes after being released into a Blenheim river.

The sudden death of 100 salmon in the Opawa River on Friday has scientists testing the waterway for possible contaminants.

The Marlborough Kids Fish Out, organised by Fish and Game, was supposed to take place on Sunday.

Field officer Vaughan Lynn said he was shocked to find the salmon dead within 15 minutes of putting them in the river on Friday.

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"We put the fish in, but we noticed they looked a bit stressed, well, more stressed than they usually do when you're releasing them," Lynn said.

"Then we glanced upstream, and we saw these dead fish bodies floating around, and there was an eel looking a bit sick, and I thought, 'oh, we've got a problem'."

Lynn called the Marlborough District Council and the council's freshwater ecologist Pete Hamill ran tests on the water that morning.

Hamill said a dissolved oxygen test showed the water had 15 per cent dissolved oxygen.

"Anything below 80 per cent would cause stress to the fish, so this is well down at 15 per cent. We did measurements further upstream, and the further up you go, the levels do rise, so it appears to be something in that particular area."

Other inhabitants of the river, near the Snowden Crescent entrance, would also be affected, such as eel, whitebait and bullies.

Hamill found some dead bullies and one eel while at the site on Friday, he said.

He also took samples of the water to test for possible contaminants, but it could take until next week to receive the results, he said.

It could have been caused by a runoff, discharge or leak, probably due to earthquake damage followed by flooding, Hamill said.

"Something has gone in there and sucked the oxygen out of the water. But what and when and how, we don't know.

"It could be a whole lot of different things. We were initially concerned about the sewerage infrastructure, but we spoke to the asset services team but they don't think there's been any breakages. Who knows what it was, we'll have to wait and see what the tests say."

About 60 Marlborough children had signed up for the event, the second of its kind and in the same spot as April's fishing day, where rainbow trout were released.

Lynn heard many children were disappointed they were not able to reel in some salmon for dinner.

"We sent out emails on the Friday to let people know we'd cancelled it, but we sent someone down on the Sunday just in case someone turned up. Well one father and his two young boys came down, they didn't have email at the house.

"They'd brought their rods and everything. They were pretty disappointed."

The salmon were bred by Ormond Aquaculture in Wairau Valley at a cost to Fish and Game, and were brought to the Opawa River in a truck with an oxygenated tank.

Lynn, his son, Fish and Game staff and volunteers from the Marlborough Freshwater Anglers Club spent Monday scooping the dead salmon out of the river from a boat with a long-handled net, and burying them as fertiliser for some plantings.

"It's quite distressing really. You get the kids all excited, and then the fish all die, and then you've got to clean them up, it's a shame. But what can you do?"

Lynn was looking at new ways to hold the event.

He liked the idea of digging a man-made pond by using an excavator to reveal groundwater, which had worked well in Tasman, he said.

"We just need to find the right site, and find someone to sponsor the work. We would like to be able to do a couple of events a year. There's definitely the demand for it. If we had a custom-built pond, we could hold half a dozen events a year. But it needs good flow, good depth, a bit of shade. Close to town, maybe within a half-hour drive.

"There could be the right pond out there, we just need someone to point it out."