CBH confirm Cockburn Sound canola spill

CBH Group has confirmed half a tonne of canola grain was spilt into Cockburn Sound last month, as Fisheries experts continue to probe why more than 1500 dead fish have washed up in Perth's south.

An estimated 500kg - or two bathtubs full - of non-GM canola was accidentally dumped into Cockburn Sound off the Kwinana grain jetty in November, as workers were commissioning a newly renovated ship loader.

A CBH spokeswoman said the grain growers' cooperative was working with the Department of Environmental Regulation in its investigation into the cause of fish deaths in the sound.

A Department of Fisheries spokesman last week told AAP an examination of gills had shown the dead fish may have suffered respiratory stress, but there was no evidence of canola in their stomachs.