SLUGGISH and dead moon jellyfish were spotted two weeks ago along a 5km section of the Swan River between Caversham Avenue and Fish Market Reserve in Guildford.

Bassendean resident Tracy Steer said she saw a lot of dead jellyfish in Lilac Hill Reserve when she was walking her dog.

“It was a bit of a shock – what stood out was how many there were and how stagnant the water looked,” she said.

“My dog usually has a bit of a splash but not that day. I wondered whether they were being culled or there was something wrong with the river.

“I took some photos to post on Facebook to see if anybody knew what was happening.”

A Department of Biodiversity, Conservation and Attractions (DBCA) spokesperson said the department is responding to reported sightings.

“DBCA has undertaken water quality surveys in the affected area and is continuing to work with the Department of Primary Industries and Regional Development to determine the cause of the unusual mortality event,” they said.

“Since Friday, August 7, the jellyfish have dispersed and there have been no further reports of large numbers of dead jellyfish.”

DBCA warned visitors to avoid handling dead jellyfish and swimming where dead jellyfish are concentrated or where visible surface scum or discoloured water is present.

Visitors should also keep pets away from affected areas and wash exposed skin, equipment and watercraft after leaving the water.

Local councils have also been asked to report any other significant jellyfish accumulations, odour, access or amenity issues, or impacts on fish, birds or other fauna.

If visitors see dead or sluggish marine life, they can report it to FishWatch at 1800 815 507.