Gambusia holbrooki

The Central Coast area contains a large variety of natural and constructed aquatic freshwater habitats including farm dams, stormwater ponds, canals, drains, creeks, rivers, lakes and estuaries. These habitats are important foraging areas for a diverse range of birds, mammals, reptiles and fish that depend on aquatic fauna species as part of their diet. With relatively constant water levels and water quality these aquatic habitats have high potential to be infested by the highly invasive introduced fish species Plague Minnow (Gambusia holbrooki), commonly known as the mosquito fish. The presence of this species in an aquatic habitat has been linked to the decline in tadpole and frog populations and is likely to result in long term negative changes to the fauna diversity of infested aquatic areas.

Key Threatening Process

• Predation by the Plague Minnow (Gambusia holbrooki) is listed as a Key Threatening Process on Schedule 3 of the NSW Threatened Species Conservation Act, 1995.  The Scientific Committee determined that predation by Gambusia is a serious threat to the survival of threatened species, especially frogs.

Description

Gambusia are small fish, predominantly olive green on the dorsal side to blue-grey on the belly side. Females (6cm) are larger than males (3.5cm) and have a dark spot near their vent. They are native to North America and were introduced in the 1920’s to control mosquitoes.¹

Impacts of Threat

• Gambusia is an aggressive and voracious predator of native fish species and their eggs, frogs and tadpole species.²
• Ponds infested with Gambusia are often depleted of aquatic insect larvae, which provide a food resource for other aquatic feeding species.
• With their high reproductive rate and long breeding season, they populate to large numbers in rivers, creeks, dams and waterways, and deplete food supplies for native species of fish.¹
• They have aggressive behaviour and chase away native species by fin nipping, which may cause bacterial infection on affected native fish.¹
• They are associated with the decline in abundance of native fish and frog species due to predation of eggs and young hatchlings.¹
• They infest waterways and ponds through movement in flood flows and migration of fish within streams.

The Local Problem

• The development of stormwater ponds in new subdivisions is creating additional habitats which are being occupied by Gambusia.
• The broad distribution of Gambusia and their adaptation to a variety of environments has resulted in negative impacts on aquatic species over a large range of aquatic habitats.
• Lack of Gambusia free habitat sites limits the distribution of native aquatic species, particularly frogs.
• Gambusia is widespread across the local waterway systems which creates difficulty in controlling and ameliorating their impacts.
• There are no local programs to control the spread of Gambusia in suitable habitats.

Community Actions

• Do not release Gambusia into creeks, ponds or dams.
• Participate in local rehabilitation programs, particularly in areas of aquatic habitats, and join your local Waterwatch group.
• Create backyard pond habitats that can be managed to maintain Gambusia free habitats.
• Report any infestations of Gambusia to your local Council.
• Educate children and community about problems resulting from transfer of fish from one aquatic area to another.

Useful Web links

• Department of Environment and Water Resources - www.deh.gov.au
• Department of Environment and Climate Change - www.dec.nsw.gov.au

Further Reading

• Mahony M (1993), The status of frogs in the Watagan Mountains area, the central coast of New South Wales, in “Herpetology in Australia: A Diverse Discipline”, D Lunney and D Ayers (eds), Trans. R. Soc. NSW, pp.257-264
• Pyke GH and White AW (2000), Factors influencing predation on eggs and tadpoles of the endangered Green and Golden Bell Frog Litoria aurea by the introduced plague minnow Gambusia holbrooki, Aust. Zool. 31(3): 496-504
• Tyler, M.J. 1997, ‘The Action Plan for Australian Frogs’, Wildlife Australia. Retrieved 8 September 2006 from http://www.deh.gov.au/biodiversity/threatened/action/frogs/7.html
• NSW National Parks and Wildlife Service 2003, NSW Threat Abatement Plan. Predation byGambusia holbrooki – The Plague Minnow. NPWS. Hurstville, NSW.

Bibliography

1. NSW Fisheries, 2001 (updated 13 December 2005), ‘Alien fish DF 98’, What Is An Alien Fish,. Retrieved 8 September 2006 from http://www.fisheries.nsw.gov.au/aquatic_habitats/aquatic_habitats/alien_fish
2. Department of Environment and Conservation, (1999 updated 1 September 2005), ‘Predation by the Plague Minnow (Gambusia holbrooki)’, Key Threatening Process. Retrieved 8 September 2006 from http://threatenedspecies.environment.nsw.gov.au/tsprofile/threat_profile.aspx?id=20016