A NSW Government website

Green swordtail


Male green swordtails have an elongated sword-like tail while females do not. Photo Gunther SchmidaThe green swordtail (Xiphophorus helleri) is native to Central America. It is commonly known as green swordtail owing to the olive-green natural colour that it exhibits in wild populations in Central America. There are other species in the same genus referred to as ‘swordtails’ including variants such as marbled swordtail and highland swordtail. There are no native swordtails in Australia.

The green swordtail is a small-bodied colourful species which are popular aquarium fish. Swordtails are generally less than 80 mm in length but can grow up to 120 mm (Allen et al. 2002). Swordtails found in aquariums are usually orange in colour, however feral populations often take on a bronze or olive colour in natural waterways. Green swordtails have been introduced into two waterways on the north coast of NSW.

It is extremely difficult if not impossible to eradicate pest fish once they become established in the wild. It is therefore vital to prevent non-native species such as the swordtail from entering or spreading within our waterways.

Natural distribution and biology

Green swordtails originate from Mexico and Belize, Central America. It is a member of the Poeciliidae family (livebearing toothcarps), which also includes the pest fish Gambusia holbrooki commonly referred to as eastern gambusia (also known as 'mosquito–fish'), and pest fish Xiphophorus maculatus (also known as the platy-fish). Swordtails are highly variable in colour but are usually bright orange on both body and fins with alternating silver and orange lines along the trunk. Feral populations often take on a bronze or olive colour with an orange-red mid lateral stripe in natural waterways (McDowall 1996).

Like many small native species, the exotic green swordtail is omnivorous and consumes algae, detritus and small insects. The males are easily identified by the elongation of the lower part of the tail, whilst the females do not have a sword-like tail but a very angular dorsal fin (Allen et al. 2002). They are a live-bearing species and reproduce at a young age, producing several broods per year.

Where are they in NSW?

Green swordtails have been introduced into at least two water bodies in NSW, Lake Ainsworth, near Ballina and Burringbar Creek, and near Murwillumbah in north-eastern NSW. The species is thought to be restricted to these two systems, as it has not been recorded during surveys of the surrounding waterbodies within the same sub-catchments (Bishop 1999; The Ecology Lab 2002; Knight 2001). The most comprehensive records for this species come from Burringbar Creek, where the species was captured from three sites on six occasions from 1999 to 2002 (The Ecology Lab 2002). These sites are located within a 200 m stretch of Burringbar Creek, near the township of Mooball, in the immediate vicinity of the new Pacific Highway bypass.

How did they get here?

Green swordtails are a popular aquarium fish and feral populations are most likely the result of aquarists discarding unwanted specimens into waterways or escapees from ponds reaching waterways through flooding events.

What is NSW DPI doing?

NSW DPI will continue its ongoing monitoring of inland and coastal rivers. If any new populations of green swordtails are discovered through these surveys, NSW DPI will assess the potential control or eradication options.

What are their impacts?

There have been no studies on the impacts of green swordtails on NSW aquatic ecosystems. However, swordtails are closely related to Gambusia holbrooki which can quickly establish large populations and out-compete native fish for habitat and food, so there are concerns regarding the potential impact of the green swordtails on native species and ecosystems in NSW.

In Queensland it is believed that increasing populations of green swordtails have been associated with the decline of nine native fish species (Western Australia Fisheries 2003).

What you can do

  • Be on the lookout for new species in your local waterways and report pest species immediately!
  • Give unwanted aquarium fish to friends or a pet shop, rather than letting them go in the wild (Note: it is illegal to release live fish into NSW waterways without a permit, and heavy penalties apply).
  • If you are involved in fish stocking, obtain a fish stocking permit from NSW DPI before buying fish for restocking. Also, buy fingerlings from local suppliers rather than outside your region or interstate, to minimise the chances of introducing other species not native to your area.
  • If you are a fish farmer, comply with aquaculture permit conditions designed to prevent the escape of fish (e.g. screened water outlets), and keep to your approved species list.
  • If you find any fish that you think might be a green swordtail or another non-native species, take a digital photo or freeze the fish whole and report it!

References and further reading

  • The Ecology Lab, 2002. Fish fauna study – Oxleyan pygmy perch, Yelgun to Chinderah – Contract No. F/99/01. Progress Report for Sampling Times 1-6. Report to NSW Road and Traffic Authority, Pacific Highway Office by The Ecology Lab Pty Ltd, Brookvale, NSW.
  • Bishop, K. A., 1999. Threatened Fish Species Surveying and Habitat Assessment in the Emigrant Creek Catchment, Richmond River System, New South Wales, Report prepared in relation to the proposed Pacific Highway Upgrading: The Ballina Bypass, for Connell Wagner Pty. Ltd on behalf of the NSW Roads and Traffic Authority.
  • Faragher, R. A. and Lintermans, M., 1997. Alien fish species from the New South Wales Rivers Survey. In Fish and Rivers in Stress: The NSW Rivers Survey. (Eds. J. H. Harris and P. C. Gehrke), NSW Fisheries Office of Conservation and the Cooperative Research Centre for Freshwater Ecology, NSW. pp.201-223
  • Knight, J. T., 2001. Distributional limits of the endangered Oxleyan pygmy perch Nannoperca oxleyana (Whitley 1940) in northeastern New South Wales (Research Phase 1: 31/05/2001-28/08/2001). Interim report to Environment Australia by the Office of Conservation, NSW Fisheries, Port Stephens.
  • McDowall, R. M., 1996. Livebearers. In Freshwater Fishes of south-eastern Australia, (Ed. R. M. DcDowall), Reed Books, NSW. pp. 116-122.
  • Western Australia Fisheries 2003. Introduced freshwater species in Western Australia – Swordtails.