The rust-coloured bloom believed to be the naturally-occurring bacteria Trichodesmium, last Wednesday turned the water in Cowaramup Bay the colour of tea.

Since Sunday last week, fishermen have been reporting a band of discoloured water stretching from Cape Leeuwin to Cape Naturaliste and between 50 metres and two kilometres off shore.

Ant Bostock at Down South Camping Fishing and Outdoors, Margaret River, said: “Everyone has been talking it (discoloration) since last Sunday (February 20). There’s a band of it from Cape to Cape, but it’s thickest between Ellensbrook and Cowaramup Bay, there’s been no wind and no swell to break it up. It hasn’t been affecting the fishing, everyone is still catching plenty, but for divers it makes the water murky,” he said.

Vic Fitzsimmons of Gracetown said he first thought the red tide was an oil spill.

The Swan River Management Authority’s chief response officer for 25 years, Mr Fitzsimmons said the bloom appeared as streaks in the water running north from North Point and about 50 metres off shore. “It had a surface sheen to it and looked for all the world like a hydrocarbon spill,” he said.

A spokeswoman for the Department of Environment and Conservation confirmed that the discolouration was likely to be Trichodesmium and that the department was assisting the Health Department in monitoring the red tide which began to break up last Thursday.