LOW oxygen levels and a recent heatwave are to blame for mass fish deaths in the Serpentine River, according to the Department of Water.
The department were alerted to reports of dead fish found in the Serpentine River, near the Lakes Road Bridge north of Lake Goegrup, in February.
There were about 250 dead fish including trumpeter, herring, black bream and mulloway.
While small amounts of phytoplankton were found the department concluded that algae blooms did not cause the fish deaths.
A report was made by Department of Fisheries officers who were taking routine water samples on February 17 and also by a member of the public, via the FishWatch hotline.
Department officers took water quality and phytoplankton samples from three sites along the river near the bridge on February 17.
They were unable to collect fish samples for pathology tests as the fish were too decomposed.
Department Kwinana-Peel regional manager Leon Brouwer said there was the presence of some harmful phytoplankton species in the water samples, but the numbers were below the guideline levels.
This indicates that this was not the cause of the fish deaths.
“The likely cause is low dissolved oxygen levels as a result of the decaying organic matter and the high water temperature during the recent heat wave that included four days of more than 40 degrees from February 7 to 10,” he said.
“When the Department of Water officers visited the site there was a strong odour indicative of decaying organic matter.
“The event is not uncommon for the area and for this time of year.
The department takes part in fortnightly water quality sampling at several sites along the Serpentine River and the Peel Inlet.
Report fish deaths to the Department of Fisheries FishWatch on 1800 815 507 and algae blooms to ALGALWATCH on 08 6250 8064.