Mystery surrounds the death of up to 700 fish that have washed up dead at a popular fishing spot in Cockburn Sound, south of Perth.According to an article in the Sydney Morning Herald, the Department of Fisheries investigated reports various fish species had been seen floating dead in the sound on Friday. On Saturday, Fisheries Officers scoured the area around Garden Island between the Causeway and Careening Bay and found approximately 600 dead fish.On Friday, officers also found another 100 dead fish, from a range of different species, at Challenger Beach, near Naval Base. Fisheries Aquatic Environment manager Rae Burrows said it was not known at this stage what killed the fish. He said samples of the fish and the water where they were found would be tested, but the results wouldn't be available until next week. "We appreciate the help from those people who have called the FishWatch reporting line in the last few days, with sightings of dead fish," he said. "The public involvement is a very important part of the department's Fish Kill response program. "The earlier we hear about fish kill events, the better chance we have of taking suitable samples that will help us establish what the cause might be." Recfishwest posted on their Facebook page the fish deaths were "extremely concerning". "Immediate action must be taken to find the source these mystery fish deaths," the post said. The recreational fishing community are distressed at seeing large dead pink snapper and numerous other fish species washing up along shoreline around the sound. "Recfishwest understand that dead pink snapper and other fish were first been observed floating on the water throughout the sound two days ago. "Recfishwest has had concerns about water quality issues in the Sound since the collapse of the crab fishery in 2010," the post said. Further information can be found on the Recfishwest Facebook page, including the following message from CEO Dr Andrew Rowland.