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Swordtail (Xiphophorus hellerii)

Swordtails are popular aquarium fish that have been introduced into Australian waterways, including Queensland, New South Wales, the Northern Territory and Western Australia. They can establish in the wild and may affect native fish and aquatic invertebrates, although their specific ecological impacts are not fully understood.

Key facts

Type: Pest animal, fish.

Native to: Central America, from Veracruz in Mexico to northwestern Honduras.

Established in: QLD, NSW, NT.

Other names: Green swordtail, Red swordtail.

Identification

  • The fish has an elongated, laterally compressed body and a pointed head.
  • Males develop a long, sword-like extension from the lower rays of the tail fin.
  • Males also have a modified anal fin, called a gonopodium; females lack this feature.
  • Adults can reach about 140 mm in males and 160 mm in females, but are usually much smaller, around 50 to 70 mm.
  • Colour varies: wild fish are usually light greenish with a red or brown stripe along the side, while ornamental fish may be orange, red, yellow, black or otherwise patterned.
  • Look-alike: Platies, Xiphophorus maculatus, and variable platies, Xiphophorus variatus, can look similar; swordtails typically grow larger, and males have the distinctive tail sword.
  • Look-alike: Ornamental varieties and hybrids can have colours and markings unlike wild swordtails, so colour alone is not a reliable identification feature.

Where it lives

In Australia, swordtails occur in creeks and waterways, especially coastal drainages of southern and central Queensland, and can thrive in modified, weedy habitats.

Why it matters

  • Swordtails may compete with native fish for resources and may prey on native fish eggs or juveniles.
  • They feed on aquatic invertebrates, so predation may affect invertebrate communities and food resources for native fish.
  • Impacts on aquatic ecosystems have been observed where swordtails occur in high numbers, particularly alongside other introduced poeciliid fishes.
  • The specific ecological effects are not well understood, and habitat and hydrological changes can make it difficult to separate the effects of swordtails from other causes.

How it spreads

  • Human-mediated movement is a major pathway: unwanted aquarium fish may be released into waterways, and the species is widely traded as an ornamental fish.
  • Swordtails may also have been released for mosquito control.
  • Natural spread may occur during flooding, particularly where populations are already substantial.

Control methods

  • Seek advice from the relevant state or territory fisheries or biosecurity authority about approved control methods and reporting requirements before acting.
  • The species is considered very difficult to remove once established; no information about published eradication attempts is reported.
  • Rotenone is commonly used to collect or eradicate small fishes, including poeciliids, but any chemical use must comply with local requirements and product labels.
  • Monitor treated or removal sites afterwards and arrange follow-up action with the relevant authority if swordtails remain or are detected again.

Site hygiene and welfare

  • Do not move live swordtails or aquarium water between waterways, and never release unwanted aquarium fish into the environment.
  • Avoid transferring water or aquatic material between sites when moving boats or gear, and clean equipment before using it in another waterway.
  • Use methods set out in the national model codes of practice and standard operating procedures for the species.
  • Coordinate control with neighbours and the local government biosecurity plan, as isolated effort is quickly undone by reinvasion.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: General biosecurity duty. Not prohibited matter in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Not a declared pest. Not a declared pest or prohibited matter in Tasmania. Importing animals is still controlled by Biosecurity Tasmania.
  • Northern Territory: Not a declared pest. Not on an NT declared or prohibited list. Check with the NT Government before keeping, moving or releasing it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources