Tilapia
Tilapia are farmed in Africa, Israel and throughout South-East Asia, but their introduction or escape to streams is disastrous for native fishes and for stream or lake ecology.
In countries where they have been introduced, tilapia have eliminated many native fishes, or threaten them with extinction.
Tilapia are very hardy, and can survive in conditions that kill other fish. They are prolific breeders and mouth-brooders (i.e. the female protects the eggs and fry by holding them in the mouth); this ensures high survival of the young.
They are herbivores, but grow rapidly and compete aggressively with native fishes, attacking and nipping their fins.
A number of tilapia species have been accidentally released into NSW waters, and one species is quickly becoming widespread in Queensland and Western Australia. Three species are listed as noxious in NSW and it is illegal to possess them or stock them into farm dams.
Gambusia, mosquito fish or top-minnow
(Gambusia)
holbrookii, previously Gambusia affinis) Often incorrectly called a ‘guppy’, the gambusia is a small livebearer with high survival of the young. They are aggressive, nip the fins of larger species, and out-compete gudgeons, perches, galaxias and other small native fishes. Wherever there are gambusia, few young of other species survive.
Since gambusia can destroy many fingerlings, they should be removed if possible. Gambusia are listed as noxious and are illegal in most of NSW (except the greater Sydney area).
Redfin, perch, English perch or European perch
(Perca fluviatilis)
Redfin are aggressive and prey heavily on fry and juveniles. They also carry a virus detrimental to some native fishes and trout, and a tapeworm that can infect humans. In dams, they breed readily and can overpopulate the dam, producing a large number of stunted individuals.
Carp
(Cyprinus carpio)
Carp are a large fish (to 1 m) with large scales, and two pairs of barbells near the mouth. They are not great predators, but are known for out-competing native fish and surviving in poor conditions. Carp have proven
capable of invading new waterways very quickly, and are renowned for muddying streams and eroding banks. The Koi carp is an ornamental variety of the same species.
Eels
(Anguilla australis & A. reinhardtii)
Eels are common in farm dams on the coastal side of the Great Dividing Range. Eels are real survivors, and will prey heavily upon young fingerlings. Eels spawn at sea, and the young (elvers) migrate to freshwater in spring and summer, then upstream to the headwaters. Adults and young enter farm dams at night, moving over grass wet with dew or rain. Eels are attracted to lights and are readily caught using fresh meat. A few nights of hard fishing can usually rid a dam of eels.
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