Freshwater Ferals
Queensland has the highest diversity of freshwater fish in Australia. However this diversity is threatened by a variety of 'freshwater ferals' or exotic fish species which are fish that do not occur naturally in a particular region (i.e. they are from another catchment, state or country). Some exotic fish such as gambusia (Gambusia Holbrooki), tilapia (Oreochromis mossambicus and Tilapia mariae), carp (Cyprinus carpio), and aquarium species have established in Queensland waterways. They may adversely affect native fish through competition for food and shelter and the introduction of disease and parasites.
'Freshwater ferals' - What are the risks ?
© DERM, image by George Aldridge
The introduction and establishment of exotic fish in natural waterways can cause declines in the abundance of native fish and adversely affect aquatic habitats. Exotic fish have mostly been introduced through accidental or intentional release by humans. Ways in which these exotic pests have entered Queensland waterways include: dumping of aquarium collections into waterways (e.g. goldfish, guppies, platies, swordtails); introductions as sportfish (e.g. carp, salmon, perch, trout); the use of non-native fish as live bait; and the use of exotic species in dams and ornamental ponds which overflow into other waterways.
Noxious fish1 are of greatest concern as these harm native fish, fisheries resources and aquatic habitats. Of these, species such as gambusia, carp and tilapia have established in waterways and are now major pests in Queensland. These species threaten native fish through competition (food, space), predation (native fish eggs, hatchlings), habitat disturbance (carp feeding habits increases water turbidity and reduces water quality) or by spreading disease and parasites (e.g. exotic goldfish carry a bacterial ulcer disease which is believed to have spread to wild goldfish stocks through the aquarium trade and the disposal of aquarium fish into waterways).
The problem for native fish is that exotic fish are good invaders of foreign waters because they are often:
- Generalists - carp and tilapia for example, can tolerate varied environmental conditions and disturbance, while gambusia have varied diets and as a result, have been ineffective mosquito control agents.
- Competitive and aggressive - gambusia will compete with native species for food and other resources such as space. Where they have a size advantage, aggressive interactions with native species such as red-finned blue-eyes have been observed (e.g. biting tail fins which can lead to infections and death). The introduction of exotic aquarium fish such as guppies and swordtails often has serious consequences for small surface-dwelling native fish populations which become rare or disappear.
- Predators - exotic species such as gambusia feed on the eggs and hatchlings of native species. This can deplete native fish populations and that of other aquatic invertebrate species which form part of their diets.
- Successful breeders - frequent breeding cycles, rapid growth and an absence of natural predators are some of the factors contributing to the success of exotic species becoming established and dominant in many of the aquatic habitats they were introduced to. For example, carp may produce up to 1.5 million eggs per season, and early maturation and frequent breeding in gambusia has enabled this species to dominate some fish communities in eastern Queensland and exclude native species.
Who suffers from 'freshwater ferals'?
Edgbaston goby. Photo © Gunther Schmida
Edgbaston goby: The Endangered2 Edgbaston goby Chlamydogobius squamigenus only occurs in springs in central Queensland, and requires shallow, clear water bodies free from larger fish, to survive. Although interactions between gambusia and Edgbaston gobies are poorly known, high densities of this aggressive pest in wetland springs poses risks as Edgbaston gobies may be displaced when suitable spring habitat is limited.
Honey blue-eye: At just 3cm long, the Vulnerable3 honey blue-eye Pseudomugil mellis is one of the smallest threatened species in Queensland. It is known only from a small number of localities in lakes and streams in coastal heath areas of south-east Queensland. It is likely that gambusia could out-compete honey blue-eyes where the two co-exist.
Murray cod: The Murray cod Maccullochella peelii peelii is the largest freshwater fish found in Australia, and among the most long-lived. It occurs throughout the Murray-Darling Basin (MDB) where it is a top-order predator. Substantial declines in the abundance of this species are attributable to several factors. The establishment of exotic fish such as carp, gambusia and redfin perch Perca fluviatilis in the MDB river system may have contributed to such declines. Relationships between high numbers of carp and redfin perch and low numbers of native fish including the Murray cod, have been detected.
Ornate rainbowfish: The Ornate rainbowfish Rhadinocentrus ornatus inhabits coastal fresh water streams and is patchily distributed along a narrow strip of the south-east Queensland and northern New South Wales coast, and Fraser Island. Ornate rainbowfish share much of their habitat with gambusia and as both feed mostly on insects at the waters surface, it is likely that this and other native fish species are displaced by this pest.
Red-finned blue-eye: The Endangered4 red-finned blue-eye Scaturiginichthys vermeilipinnis occurs only in shallow artesian springs on Edgbaston, central Queensland. Gambusia are abundant in many springs at Edgbaston, and it is likely that competition between this exotic species and red-finned blue-eyes is intense when suitable habitat is reduced (i.e. in area and by extremes in water temperature).
Oxleyan pygmy perch: The Vulnerable5 Oxleyan pygmy perch Nannoperca oxleyana inhabits coastal wallum swamps, streams and lakes in south-east Queensland and northern New South Wales. Along with habitat loss, the presence of exotic fish, particularly gambusia in disturbed habitats, places further pressure on this threatened species to survive in the wild.
What can you do?
- If noxious or exotic fish are caught, never release these back into waterways or use as live or dead bait. These should be euthanased humanely (e.g. methods include placing fish into an ice slurry and then freezing) and disposed of immediately (e.g. into a rubbish bin) 6.
- Use native fish rather than exotic species for stocking purposes (e.g. native fish can be used as an alternative to gambusia for controlling mosquitos) 7.
- To help you correctly identify exotic species, obtain a copy of the noxious and exotic pest fish identification brochures and posters produced by the Department of Employment, Economic Development and Innovation (Ph: 13 25 23).
- You can help control 'freshwater ferals' in Queensland waterways by reporting exotic pest fish sightings to the Department of Employment, Economic Development and Innovation (Call 13 25 23).
Related information
Freshwater ferals factsheet (PDF, 415K)*
Department of Employment, Economic Development and Innovation, 2010. Exotic pest fish
Department of Primary Industries, Queensland. 2001. Control of exotic pest fishes - an operational strategy for Queensland freshwaters 2000-2005. Department of Employment, Economic Development and Innovation, Brisbane.
Department of Primary Industries. 1997. Fish Guide - Saltwater, Freshwater and Noxious Species. The Great Outdoors Publications. Australia (pp 136).
Department of Environment and Resource Management. A-Z of Animals - Freshwater fish
Wager. R. and Jackson. P. 1993. The Action Plan for Australian Freshwater Fishes. Australian Nature Conservation Agency, Canberra.
1. Noxious fish are those that have been declared as harmful by Australian statute law because they are or may become a pest to native aquatic communities. Department of Employment, Economic Development and Innovation has a list of species, genera and families that have been declared noxious fish in Queensland.
2. The Edgbaston goby is listed as Endangered in Queensland under the Queensland Nature Conservation Act 1992.
3. The honey blue eye is listed as Vulnerable in Queensland under the Queensland Nature Conservation Act 1992.
4. The red-finned blue-eye is listed as Endangered in Queensland under the Queensland Nature Conservation Act 1992.
5. Oxleyan pygmy perch is listed as Vulnerable in Queensland under the Queensland Nature Conservation Act 1992.
6. Noxious fish cannot be kept, hatched, reared or sold. Penalities of up to $200,000 are enforced if noxious fish are released or kept (live or dead) without a permit. Department of Employment, Economic Development and Innovation, 2009.
7. Avoid depleting native fish populations in the wild by obtaining native fish from aquarium shops and registered fish hatcheries.
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Last updated 14 June 2011