As the weather has warmed, the Department of Biodiversity, Conservation and Attractions (DBCA) and Department of Health have found higher than normal levels of algae in the Swan River.
The algae, which can cause a reddish-brown water discolouration, is primarily occurring between Matagarup and Garret Road Bridges in the Swan River.
While the algae isn't known to be harmful to humans, the Departments are advising that people and pets avoid swimming in water that is discoloured, murky or smells unpleasant.
DBCA officers are monitoring the algae. If you see any sluggish fish, please report it to FishWatch on 1800 815 507.