Anglers fishing lakes Wanaka and Dunstan are being asked for small pieces of skin from any salmon they catch.
The Otago Fish and Game Council and the Cawthron Institute are surveying the Chinook salmon populations of the Otago lakes.
Council officer Paul van Klink, of Cromwell, said on Wednesday he had collected enough samples from Lakes Wakatipu and Hawea but needed 20 from Lake Wanaka and also some from Lake Dunstan.
The skin samples are being DNA-tested by Ag Research in Dunedin.
Mr van Klink said most of the lake salmon were landlocked because of the Clyde and Roxburgh dams.
However, DNA testing had shown some young salmon migrated down the Clutha River and managing to survive passing through the dam turbines.
"They go out to sea they grow to big salmon and they come back as far as the Roxburgh Dam and they can't come [any further]," he said.
"Prior to the dams there was a really big salmon fishery in the Clutha.
"It was a big run of thousands of fish.
"Last year there were a few hundred but this year there was just about none."
"It's just getting worse and worse."
Mr van Klink said DNA testing was able to provide genetic evidence of which lake the returning salmon had come from.
The study began more than a year ago, but Mr van Klink said he still had not got any samples from Lake Wanaka.
"Hawea's fishing really well and so is Wakatipu but Wanaka just doesn't have many.
"If you've got a salmon, let us know and we will come and collect some skin off it."