STAFF REPORTER

Laboratory tests of sample fish from thousand that perished in the upper Swan River a fortnight ago have been inconclusive as to the cause of death.

Swan River Trust officers removed more than 4000 dead fish between Middle Swan Bridge and the Jane Brook confluence last week, largely from recreational use areas.

An unconfirmed number of fish sank to the riverbed and could not be retrieved.

Swan River Trust spokesman Mark Cugley said a clean-up of the 10 kilometre stretch of river where the fish were found was now complete and many of the fish have been taken away to landfill.

“While the laboratory tests have been inconclusive, we believe that low oxygen levels triggered by rainfall and organic loading to the river, were a key factor in the demise of these fish,” Mr Cugley said.

Although most of the dead fish collected were initially thought to be Black Bream, they have since been confirmed as Yellowtail Grunter (Amniataba caudavittatus) and Six Line Trumpeter also known as Western Striped Grunter (Pelates octolineatus).

“Testing of oxygen levels this week shows that concentrations are now at levels suitable for fish with the two oxygenation plants at Guildford and Caversham continuing to operate further down the river, helping the situation,” Mr Cugley said.

Mr Cugley said that monitoring conducted on January 30 confirmed water quality had improved.

No potentially harmful species of algae exceeded guideline values.

“Weekly testing by the Swan River Trust will continue as normal to make sure these levels are closely monitored,” he assured.