Blue Ringed Octopus and Other Critters¦Life in the Swan River

Northern Suburbs Branch¦November Meeting

Our guest speaker, Melanie (Mel) Turner is Dive Co-ordinator for the WA Underwater Explorers’ Club. She dives at least twice a week, including regularly in the Swan River at night.

Mel first experienced the wonders of night diving about 10 years ago while holidaying in Bali. The warm water of Bali was in stark contrast to her first night dive in the Swan River, at Bicton, a couple of years later. The river was extremely cold and her only reward was a solitary slow moving Southern Mantis Shrimp (below), but the experience was enough to draw her back; providing her with opportunities to witness and photograph the extremely diverse fauna of the river.

Southern Mantis Shrimp

Mel and her diving partners (apparently there’s a large number of night divers in Perth) have extended their range of sites to encompass numerous spots between the bridges at Fremantle and the Claremont Jetty, with the Mosman Restaurant jetty being one of her favourites. Many of the sites contain wrecks which make ideal habitats for the more mobile critters.

We were surprised to learn how many different critters there are in the river. I counted 124 just in the presentation and no doubt there are many more, including the river’s renowned jellyfish!

Mel broke her critters into recognisable categories and gave us numerous insights into the depicted fauna, all illustrated with her own wonderful underwater photos:

  • Edible delights: Western King Prawns (longer than a beer can), Western School Prawns, Blue Swimmer Crabs (which can be taken in the river without any restrictions).
  • Edible fish: include flathead, juvenile blue spot snapper, sand and King George whiting, bream, sole, flounder, cobbler, and mulloway.
  • Other edible delights: squid, cuttlefish, mussels and scallops (neither recommended due to pollution).
  • Anemones that range from the size of a thumb to over half a metre across.
  • Sea cucumbers, including some very spiny ones and other of vibrant colours.
  • Sea Stars & Brittle Stars, most of which burrow in the sand when resting.
  • Crabs: too many to identify, great at camouflage.
  • Decorator crabs, which attach sponges to themselves for camouflage.
  • Seahorses: plentiful in the river, best place for snorkelers to see them is at Bicton Jetty.
  • Pipefish: Tiger and Spotted Pipefish.
  • Sea moths, which lose their swim bladder as they age, so it then crawls along sea bed.
  • Gobbleguts: biggest consumer of juvenile prawns.
  • Blennys: there are two species—the Sabre Tooth and False Tasmanian Blenny.
  • Pretty fish: Cardinals, Moonlighter, Old Wife, latter always in pairs, grunt when stressed.
  • Lionfish, though local waters too cold for breeding so unlikely to become a pest.
  • Boxfish and Cowfish: males live in deeper water and come into shallower water breed.
  • Leatherjackets: five species, the smallest one is the Pygmy Leatherjacket
  • Striped Catfish: juveniles swim in large balls, taking it in turns to filter feed on seafloor.
  • Pufferfish and Blowies: often sleep in the shallows.

Some really unusual critters:

  • Anglerfish (Frogfish)(above), which is very good at camouflage.
  • Eels: the Red Eel is called an “Orange Eel Pout” and is common around the limestone at Claremont Jetty and also Rocky Bay.
  • Lined Dumpling Squid (above left), which is closely related to cuttlefish and hides under sand by day.
  • Shrimp: Southern Mantis Shrimp, known as “Prawn Killers”, slow movers; Pistol Shrimp, has one very large claw.
  • Sea hares: mate by forming a chain.
  • Nudibranchs: numerous species, largest up to 15cm.

Mel concluded her presentation with some things to watch out for:

  • Blue-ringed Octopus (above right)
  • Scorpionfish (Stonefish)
  • Eagle Ray
  • Bull Shark.

Don Poynton