09 April 2020

Algal bloom update 6 - Swan and Canning rivers

Algae in the Swan and Canning rivers lessens; ‘Do not consume fish, crabs, or shellfish’ warning remains

The Department of Health is reminding people that the warning associated with persisting toxic Alexandrium alga in the Swan and Canning Rivers remains in effect:

Fish, crabs or shellfish collected from within the following waterways should not be eaten due to the potential for unsafe levels of paralytic shellfish toxins:

  • the Swan River – from Pelican Point, Crawley to the South of Perth Yacht Club, Applecross and upstream to Middle Swan (Reid Highway) Bridge. Middle Swan includes the commonly known areas of Como Jetty, Matilda Bay, Perth Waters, Elizabeth Quay, Barrack Street Jetty, Claisebrook Cove, Maylands Yacht Club, Ascot Waters, Hind Reserve, Riverside Gardens, Garvey Park, Sandy Beach Reserve, Point Reserve, Kings Meadow, Fish Market Reserve and Woodbridge Riverside Park.
  • the Canning River – from the South of Perth Yacht Club and upstream to Kent Street Weir (this includes commonly known areas of Canning Bridge, Mt Henry Bridge, Salter Point, Shelley Bridge, Riverton Bridge, and Castledare).

The concentration of Alexandrium in both rivers has decreased significantly over the last month, and testing has confirmed that there are now only a few areas within the Swan River where this alga remains. However, testing of mussels collected from within the affected areas of both rivers has revealed that toxin concentrations remain above a comparable food safety guideline level.

The current health warning will remain in effect until Alexandrium alga are at or close to non-detectable levels, followed by an additional period of time to allow for elimination of toxins from the flesh of shellfish, crabs and fish. Consequently, this warning is likely to remain in place until at least May 2020.

Although the size of the area of both rivers covered by this warning is considerable, it includes buffer zones as an added precaution.

Testing has confirmed that this alga primarily impacts shellfish and the internal organs of crabs. Fish species tested to date, do not appear to be as significantly affected.

The ingestion of toxins produced by this alga can produce a type of poisoning known as paralytic shellfish poisoning (PSP). Cooking does not destroy these toxins.

People who consume wild shellfish, crabs or fish collected from the affected area of the Swan or Canning rivers may experience symptoms including:

  • tingling or numbness of the lips prickliness of the fingertips and toes nausea or vomiting
  • impaired balance dizziness
  • slurred speech double vision weakness
  • difficulty in swallowing or breathing loss of fluids and diarrhoea.

In severe cases, PSP may cause muscular paralysis in people who consume affected shellfish, crabs or fish. Shellfish includes oysters, mussels, clams, pipis, scallops, cockles and razor clams.

Anyone who has consumed shellfish, crabs or fish collected from the Swan or Canning rivers and experiences any of these symptoms should seek urgent medical attention, particularly if they have difficulty breathing.

They should also retain uneaten portions of mussels, crabs or fish to help determine a likely cause of any symptoms.

As a general rule people should avoid eating recreationally collected shellfish in rivers, estuaries or other waterways where there is an increased likelihood of contaminant or nutrient inputs that may result in chemical, algal toxin and/or pathogen uptake.

Farmed shellfish purchased in supermarkets and other commercial outlets in WA are not affected because there is a strict quality assurance program to ensure they are safe for human consumption.

Other recreational activities including swimming, skiing and boating are not affected by this microalgae species, but as a general rule swimming should be avoided in areas of discoloured water.

Health warning signs advising against crabbing, shellfish collection and fishing have been primarily erected and maintained by relevant riverfront local government authorities, at key locations including bridges, jetties, boat ramps and popular accessible foreshore areas within the affected region.

Alexandrium blooms occur globally with limited options for management and are likely to re­occur periodically within the Swan and Canning rivers, due in part to the alga’s ability to produce long­lasting seed­like cysts.

In the context of the COVID – 19 pandemic, the Department of Biodiversity, Conservation and Attractions (DBCA) will endeavour to maintain micro-algal sampling for the Swan and Canning Rivers, and incident management team response with Department of Health, and Department of Primary Industries and Regional Development to address the issues relevant to this current warning.

Read detailed FAQs on the Alexandrium algal bloom and paralytic shellfish poisoning (external site). A map detailing affected waterways can be viewed on the DBCA website