The Department of Primary Industries and Regions (PIRSA) has found a low tide and hot conditions killed thousands of fish found along a beach at Semaphore.
The school of small Australian anchovy was found dead near Port Malcolm on Friday.
Beachgoer Kate McAuliffe said she was shocked to see so many dead fish and tried to save the ones that were still swimming.
"I haven't seen anything like that before," she said.
"It looked shiny like shells, and then you got closer and realised it was actually thousands of tiny fish, I couldn't believe it.
"Out of thousands there were maybe 10 or so that will still kind of half-swimming around ... I was able to grab a couple of them and put them in the ocean, but I don't know how they fared once they got in."
PIRSA's general manager of aquaculture Shane Roberts said staff collected water samples on Friday which showed the water was 28 degrees Celsius.
"It seems that water quality was actually quite poor, murky water, low oxygen levels, about 30 per cent," he said.
He said the anchovies, which often come into the shallows, became caught in some of the gutters on very low tide, and being a hot day, the oxygen depleted quickly.
"The temperature was higher than their physiological tolerance unfortunately," Dr Roberts said.
Dr Roberts said PIRSA investigates on average 15 to 20 fish kills each year across the state.
"The large majority are natural events caused by extreme weather, harmful algae blooms, dodge or low tides and other environmental stresses," he said.
PIRSA urged the public to contact FISHWATCH on 1800 065 522, if they noticed unusual numbers of dead fish in an area.
