An estimated 800 abalone, along with other shellfish species, have been found dead along a stretch of beach north of Yallingup.
The find has sparked warnings for residents not to swim or fish in the area, and to not allow their pets to come into contact with the decomposing fish.
Investigations are underway into the marine fish kill, which included dead crabs and various molluscs across more than one kilometre of coastline.
Department of Biodiversity, Conservation and Attractions – Parks and Wildlife Service officers inspected the site today and reported a red tinge to the high water mark, which could indicate that a cyanobacterium known as Trichodesmium sp. may have been present in the ocean at the time of the fish kill.
There have also recently been low tides and hot temperatures in the area.
Samples of sea water, dead abalone and the other marine species will undergo laboratory testing in order to determine what caused the fish kill.
Department of Primary Industries and Regional Development officers are monitoring the situation.
The Department of Health has advised residents to;
• not to swim in areas of water with large numbers of dead and decomposing fish because they may contain high levels of bacteria and have an objectionable odour;
• not to fish in water with large numbers of dead fish;
• temporarily not to collect or consume fish from the waters near a fish kill;
• not to collect and use dead fish for bait or consumption because of the risk of high levels of bacteria; and
• not to allow pets and other animals to come into contact with dead or decomposing fish either in the water or on shore.
Fish kills can be reported to the FishWatch 24-hour hotline on 1800 815 507.