Thousands of dead fish have washed up along the foreshore at Moana Beach. Source: adelaidenow

THOUSANDS of dead fish found washed up on Adelaide's southern beaches this morning may have been dead for some time.

Southern suburbs residents have reported the small fish, about two inches in length, across long stretches of sand at Moana, Moana South, O'Sullivans Beach and Christies Beach.

Biosecurity SA is investigating the cause of the deaths but the incident is likely to have been caused by heat.

Seaford Rise resident Phyl Gava said the stretch of beach affected runs from Moana to Maslin Beach.

It's not known why scores of fish have ended up dead on Adelaide's southern beaches.

Ms Gava, who has been walking the beach for over 17 years, said she has never seen anything like this.

Biosecurity SA said the cause of the death is likely to be heat, as was the case with similar incidents on the Eyre Peninsula earlier this month.

Dead fish litter the foreshore at Moana Beach. Picture: Roger Wyman

Spokesman Ken Pearce said many of the fish, mainly leatherjackets, are already partially decomposed.

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Scores of dead fish on our beaches