Feral Fish

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• 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 contributes to eutrophication. 
• 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. While the findings of the electrofishing survey in Herdsman Lake suggested that Pearl Cichlids are present in low abundance, they have probably been introduced recently and their numbers might increase in the future. 

• It is currently unclear how and to what extent Carp, Goldfish, Gambusia and Pearl Cichlids will affect the aquatic ecosystem of Perry Lakes. Their presence in the Lakes has not been established yet but it is highly likely that they are present or will be in the foreseeable future. 
• Carp, and possibly Goldfish, are likely to be detrimental to water quality and adversely affect native aquatic animals and waterbirds. However, Gambusia and Pearl Cichlids might not create a major threat because native fish are likely to be absent as a result of repeated drying of the lakes over many years. Furthermore, both Gambusia and Pearl Cichlids could provide a source of food for water birds, and Gambusia might help control mosquitoes by eating their larvae. 
• The movement of feral fish eggs into East Lake will likely occur via the balance pipe but not via the aquifer. Discussions have therefore started to consider if the balance pipe between West and East Lakes should be blocked. 
• Information about fish species inhabiting Perry Lakes would be very valuable. People walking around the lakes should be observant for unusual fish activity, which is likely to be most obvious for Carp or Goldfish. Controlling and even eradication of these species from the lakes is possible through boat electrofishing. 

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.