dead fish
© The Advertiser One of the fish washed up dead on the shores of Lake Alexandrina. Picture: Michael Milnes.

With the health of the River Murray and Lower Lake system at its best in years, the mass "fish kill" is a mystery.

Point Sturt resident Dot Ratcliffe said she was alarmed to find the problem when she went kayaking on the lake yesterday morning. "I saw them (extending) about 400m out in to the lake," she said.

"It's terribly upsetting, very distressing. There are thousands of fish washed up, something you do not want to see. I have been living here for 10 years and never seen anything like this."

Bio-security aquatic pest manager Vic Neverauskas said Primary Industries would investigate the circumstances of the fish deaths as soon as possible. Most of the fish appear to be bony bream.

"There was a "fish kill" in Milang two weeks ago associated with cold nights," he said.

"It is a natural phenomenon for bony bream. It's normal for this to happen in July and August. Until we can get someone on the ground to verify the species, which will be as soon as practical, we are unable to make further comment."

Ms Ratcliffe said friends told her they had walked from their yacht to the shoreline and were "literally walking on dead fish on the sand".

"The smell is terrible. It's a tragedy," she said.

Lower Lakes fisherman Henry Jones said it might be a natural occurrence that kills mainly bony bream after a long flood. "There is a fungus that grows in their gills and around the body," he said.