Mass stranding in South Africa

Not sure of the species, but the entire beach at Fish Hoek, just south of Cape Town on the False Bay coast of the Cape peninsula, was covered in dead jellyfish. They were on average about 10 cm in diameter and covered a stretch of beach about 50 m wide and at least 500 m in length, spread in density as per the photograph. (My camera's batteries died, so I couldn't get more pics. Sorry!) It wasn't particularly windy the night before, but the sea was quite rough.