Article Index

Invasive plants and animals

Invasive species are the greatest threat to Australia’s biodiversity after habitat loss. Once established, invasive species can alter an ecosystem and reduce local biodiversity. New species may be competitors, disturbers, consumers, or prey. They can cause local extinctions through predation, competitive exclusion, niche displacement and altered genetics of native species. The Swan Canning Riverpark is widely affected by affected by introduced weeds, fish and invertebrates.


A weed is an alien or introduced plant that is invasive. Weeds on the foreshore and in the water can be a serious problem for the Swan Canning Riverpark, competing with local plants for space and light. They usually don’t suffer from pests or diseases, and animals may not feed on them, so they spread easily. They can overtake the natural vegetation, reducing native species diversity and habitat value.

Foreshore weeds mostly come from nearby gardens, farmland and market gardens. Seeds are blown or washed into bushland near streams and into drains leading to streams. Birds can introduce them in droppings and people often thoughtlessly dump garden waste containing seeds, bulbs or cuttings in bushland near the river.

Aquatic weeds may clog the waterway with their own biomass or by causing sediment build up. Stagnant waters are more likely to be deprived of oxygen, causing the death of aquatic life. Habitats for birds and other animals may be lost, recreational areas ruined and irrigation pumps clogged with plant material.

Invasive fish

Invasive fish species Swan Canning Riverpark include pearl cichlids (Geophagus brasiliensis), goldfish (Carassius auratus), koi carp (Cyprinus carpio), mosquito fish (Gambusia holbrookii), silver perch (Bidyanus bidyanus), spangled perch (Leiopotherapon unicolour) and redfin perch (Perca fluviatilis). Their introduction and spread can cause problems in waterways as they flourish, breed and compete with native species.

Introduced fish can:

  • establish feral populations that compete for food such as crustaceans, insects and plants, or feed directly on native fishes, causing native species to decline
  • become aggressively territorial, reducing available habitat for native fish and other water dwelling animals, and exhibit aggressive behaviors – such as fin nipping – that can seriously injure or kill native fish
  • introduce parasites and spread disease that may affect native fish and other freshwater animal life
  • alter habitats by digging in riverbeds, uprooting plants and muddying the river: this can increase the level of nutrients in the water and may lead to excessive algae growth

The Department of Fisheries is the lead agency for managing feral fish. 

Many exotic fish common in the aquarium trade are hardy and have wide environmental tolerances and will survive and thrive in our local waterways. Pearl cichlids (Geophagus brasiliensis), a popular aquarium fish native to coastal rivers of Brazil and Uruguay, were first found in the Bennett Brook in March 2006 and are now well established in the upper Swan and Canning rivers.

Introduced invertebrates

Three invertebrate species introduced to the Swan Canning Riverpark are described here.

  • The Australian mud whelk (Batillaria australis) is a large marine snail with a persistent shell that disturbs soil and sediment and is widely distributed at a very high abundance. It is likely to be impacting the ecology and biogeochemical cycling of the river.
  • The Asian paddle crab (Carybdis japonica), also known as lady crab, is a swimming crab native to South East Asia (normally found in the waters of Japan, Korea and Malaysia) typically found in estuaries. It is not known to have established in Australian waters. Four individuals have been found in the lower Swan estuary since 2012. 
  • The white sea squirt (Didemnum perlucidem) is an invasive colonial tunicate that has established sustaining populations around a number of ports in Western Australia. It remains listed as a marine pest and suspected and confirmed detections should be reported so its distribution can be tracked. Growth of this species appears to be highly seasonal. It can grow very fast in favourable conditions and is known to foul mussel beds and artificial structures. In the Swan Canning Riverpark it has been observed fouling seagrass meadows. 

Managing invasive fauna

In Western Australia, the Department of Fisheries is the lead agency responsible for responding to and managing invasions of introduced fish and invertebrates. Actions are often a cooperative approach between the departments of Parks and Wildlife, Fisheries and Water, local government authorities and other major stakeholders.

How you can help

The community’s help is needed to curb the spread of invasive species in the Swan Canning Riverpark and the wider catchment.

The Department of Fisheries has launched a Don’t Dump that Fish campaign to promote awareness of the damage aquarium species can cause if they are released.

You can also join in the fight against aquatic pests by reporting sightings of Asian Paddle Crab, the White Sea Squirt and other unwanted species. Download the PestWatch app.

Downloads and further reading


Invasive fish

Introduced invertebrates