The Cheers and Bramble family, who've fished in the lake for 140 years, say mullet, flathead and whiting have been washing up on the shoreline severely underweight since Christmas.

They say their concerns have fallen on deaf ears with the Marine Parks Authority and Fisheries Department unwilling to conduct any research.

"There is a serious problem in the water and we need to find out what's causing this, because it looks like they're all going to die," said Les Cheers, who began his fishing career with his father-in-law Lyle Bramble more than 50 years ago.

"I've never seen anything like this."

The family has documentary evidence of the lake's condition leading back to 1932, when Lyle Bramble's father began a diary of catches and water quality. However they've found no entries similar to conditions they're experiencing at the moment.

"We're not just talking out of our hat, we've got documentary evidence," Kath Bramble said.

A spokesperson for the Department of Environment, Climate Change and Water told the Advocate: "The department is following up on reports from residents and commercial fishers. It has taken samples of mullet, whiting and flathead, which have been handed over for testing."

The spokesperson would not say what they were testing for and declined to make further comment until results were available.

Fishing in the General Use Zone of the lake one month ago (the other half is protected by Marine Parks as a Sanctuary Zone), Les and his sons caught 200kg of fish but could only salvage 50kg for commercial sale.

"The Marine Parks are there to protect the fish and they could well be killing them. We're reporting something that's serious here," Les said.

"Smiths Lake was well known for perfect beautiful fat fish. We were catching extra large mullet that weighed up to 2.5kg. Now they don't even weight 0.5kg."

"It's a disgrace you can't eat them."

He said mullet and whiting have been found dead and dying around they lake but doesn't know how many flathead have been affected as they are bottom dwellers that sink when they die.

The family suspect Smith's Lake is overstocked and fish are competing for food, but are urging Marine Parks and Fisheries to conduct research.

"We just want to know what's wrong," said Lyle Bramble, who's in his 80s.

"One [fish] jumped up on the bank the other day - God I don't even know how he was swimming."