Gambusia: Really Bad or Just Misunderstood?
Centre for Biodiversity and Ecology Research, The University of Waikato, Private Bag 3105, Hamilton, New Zealand.
Email: n.ling@waikato.ac.nz
Gambusia affinis and G. holbrooki are small aggressive livebearing fishes from the southeastern USA. A reputation for voracious predation on mosquito larvae earned them the name mosquitofish, and they were stocked indiscriminately throughout all tropical and temperate areas of the world.
Mosquitofish gained an equally fierce reputation for destroying native invertebrates, fish and amphibians wherever they were introduced, being implicated in the elimination of numerous species in the USA, Northern Mediterranean, Hong Kong and Australia. Species most at risk are those that occupy the same littoral surface habitat of Gambusia.
Introduced to New Zealand in 1930, Gambusia quickly became widespread. They have gained considerable notoriety despite little direct evidence of impacts on native fish and are now classed as an unwanted organism. Gambusia are implicated in the decline of dwarf inanga in some northern lakes; however, a thriving population of inanga survives at the site of the first wild release in 1933 at Lake Ngatu. Gambusia eat mudfish fry and reduce the growth of mudfish adults, although only in the absence of an alternative food source. A nine year study in the Whangamarino wetland indicates that Gambusia reduce mudfish recruitment but only in marginal habitat. Gambusia may pose only a limited threat to New Zealand fishes since no species competes for the same littoral niche. Of more concern is Gambusia’s role in habitat degradation by removing invertebrate grazers, decreasing water clarity and encouraging eutrophication. We still do not know what threat this “ecological weed” poses to New Zealand’s freshwater ecosystems.
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