The man, who does not wish to be named, said he was about 40 kilometres off-shore when he spotted the light shortly before dawn.
Grabbing his camera, the man managed to capture images of the light which he said moved and changed colours before disappearing with the sunrise.
“After nine years of fishing I've never seen anything like it,” he said.
The strange sight preceded the release of a Bureau of Meteorology (BoM) radar picture showing a bizarre S-shaped formation just west of Rottnest Island on Wednesday.
Retired astronomer Ralph Martin said it was “hard to say” what caused the strange light or radar picture.
“The vast majority of these reports [of strange lights] have reasonable explanations,” he said.
“They’re usually either astronomical objects or aircrafts.
“But we can’t explain them all.”