Friends of Perry Lakes

Australia, like many other countries, has a major problem with feral fish in freshwater waterways. These fish have usually been kept as ornamental fish or used in aquaculture and become established in rivers and lakes after being released into the wild.
Feral fish have inhabited Herdsman Lake for many years and their eggs and larvae have translocated to Perry Lakes following the establishment of the water diversion from Herdsman Lake to Perry Lakes.
While feral fish may be a source of food for water birds and turtles, they can have a negative impact on aquatic ecosystems by a) competing with native species in the consumption of a wide range of food items, b) uprooting of large aquatic plants (macrophytes), which adversely affects water quality, c) predation on fish and frogs, or their larvae, and invertebrates, and (d) transmitting parasites to native fish.
The most commonly detected feral fish in Herdsman Lake are Koi Carp (Cyprinus rubrofuscus), Goldfish (Carassius auratus), Eastern Gambusia (Gambusia holbrooki; also known as mosquitofish), and Pearl Cichlids (Geophagus brasiliensis).

Dr Mark Allen (Harry Butler Institute, Murdoch Uni) holds a carp caught in Herdsman Lake inJune 2023

Both Carp and Goldfish are native to central and south‐eastern Asia but are now widespread in wetlands along the Swan Coastal Plain. They are found in a range of sizes (up to 40 kg for Carp and 4.5 kg for Goldfish) and in the wild have often reverted to a bronze or olive-gold colour.
They were likely present in Herdsman Lake at least as far back as the 1960s and Carp are now very abundant. Scientists from Murdoch University demonstrated this in May 2023 when they undertook a fish survey using boat electrofishing, whereby fish are stunned by an electric shock and unwanted species euthanised humanely while native fish are left to recover. The survey, conducted over 2 days, collected 114 Carp, of which the largest was 842 mm in length and weighed 8.9 kg (see image in Background section above).
Carp have a pair of ‘whiskers’ under their chins that are used to feel for prey, thereby stirring up bottom sediments, which increases water turbidity and nutrient levels. This impairs the vision of underwater feeders and also contributes to eutrophication of lake water.
Both Carp and Goldfish can transmit parasites, such as the Asian Fish Tapeworm (Schyzocotyle acheilognathi), to native fish. However, a positive finding of the electrofishing survey in Herdman Lake was that there was no evidence of Carp infestation by this tapeworm, even though Carp and Goldfish in nearby lakes are infested.

Eastern Gambusia and Pearl Cichlids are native to southeastern USA and southeastern Brazil, respectively. Both are much smaller than Carp and Goldfish (Gambusia up to 60 mm and Pearl Cichlids up to 300mm in length) and may swim in small schools.
Gambusia were introduced into Australia in the misguided belief that they would control mosquitoes better than native fish. They often outcompete smaller native fish species when feeding and show aggressive fin-nipping behaviour. Their presence in streams and lakes around Perth usually means that native fish will be absent.
Pearl Cichlids feed on invertebrates (insects and small native crayfish), as well as vegetation when larger in size, and may also be aggressive towards native fish.

Electrofishing in West Lake arranged by the Town of Cambridge in November 2025 resulted in the removal of 801 feral fish, consisting of 7 large Carp, 794 small Carp (under 8cm) and one Pearl Cichlid. Neither Goldfish nor gambusia were detected. As expected, no native fish were detected.
Carp are likely to be detrimental to water quality and adversely affect aquatic animals and waterbirds. However, Pearl Cichlids may not create a major threat because native fish are absent (as a result of repeated drying of the lakes over many years) and they could provide a source of food for water birds, such as cormorants.
The movement of feral fish eggs/larvae into East Lake will likely occur via the balance pipe between West and East Lakes (but not via the aquifer) and this needs to be considered if the pipe is kept open to equilibrate water levels.
The Town of Cambridge plans to monitor feral fish numbers and types.
Specialised water filters that prevent translocation of feral fish eggs/larvae are available and could be considered for use at Perry Lakes.
Public fishing for carp is not permitted.

Fresh water pest ID. Department of Fisheries, Government of Western Australia. Fresh water pest ID – fish.wa.gov.au.
Beatty, S., Palermo, C., Allen, M. & Morgan, D. (2023). Removal and disease assessment of Koi Carp from Herdsman Lake, Perth, Western Australia.
Centre for Sustainable Aquatic Ecosystems, Harry Butler Institute, Murdoch University report to Regional Parks Unit, Parks and Wildlife Service, Sawn Region, Department of Biodiversity, Conservation and Attractions.

Feral Fish