They were dead on the sand and shoreline amid a strong odour, forcing red flags to be flown to prevent people from swimming in the area.
Officials have collected samples of the water, which was black, and specimens of dead fish have been taken for detailed analysis.
As residents voiced their concerns about a waste spill or pollutants being pumped in the sea, biologists said the deaths could be from natural causes, possibly due to a loss of oxygen in the water from the heat and winds that inhibit the flow of water and therefore fish are unable to breathe.
Comment: See also: Hundreds of methane plumes erupting along U.S. Atlantic coast
Casualties of seafloor methane gas release? Hundreds of thousands more fish found dead in Plymouth tidal pool, UK