The fish kills were found between the Woodman Point Ammo Jetty and the Omeo Wreck, south-west of Perth, with onshore winds causing some to wash up on Coogee Beach.
Beachgoers reported seeing several hundred dead fish floating in the water, with some fish eventually washing up on shore.
The “unintended failure” of a commercial fishing net has been blamed for an event that forced the closure of a Perth beach.