GAMBUSIA; AN UNDERESTIMATED MENACE.
Mosquitofish (Gambusia affinis) are widely used by public health and mosquito control agencies throughout the world by being introduced into waterways in which the nuisance mosquitoes breed. Their adaptability and hardiness, and their ability to produce large numbers of young in a short time, can make them valuable biological control agents. However, in an effort to minimise unwanted environmental impacts, the authorised personnel introducing these fishes must still refrain from planting mosquitofish in sources that are known or thought to be habitat for endangered or threatened species. Care must be especially taken when planting mosquitofish in waterbodies where they can migrate to habitats used by endangered or threatened species. For most of these countries, it is also against their own fisheries regulations for private citizens to plant mosquitofish in the waters of that locality without a permit. Lately in New Zealand however, these fishes have been the source of worries for environmentalists concerned about their continuing introductions, both illegal and legal, into our country’s waterways.
These fish are native to the watershed of the Gulf of Mexico, where it has long been known that they feed readily on the aquatic larvae and pupae stages of mosquitoes. They are remarkably hardy, surviving in waters of very low oxygen saturations, high salinities (including twice that of seawater!), and high temperatures; they can even survive in waters up to 42 oC for short periods. For these reasons, this species may now be the most widespread freshwater fish in the world, having been introduced as a biocontrol to tropical and temperate countries in both hemispheres, and then spreading further both naturally and through even further introductions.
They are a small and stout dull grey, robust fish with a rounded tail and a terminal and upward pointing mouth adapted for feeding at the water’s surface. In these features and their small size they resemble the tropical guppies from which family they also belong (the 'live-bearers', or Poeciliidae). Mature females measure 50 - 60 mm, and males to only around 25 - 35 mm long. Females can reach sexual maturity in only six to eight weeks, and they may bear three to four broods of young in a single season. The first may number only a dozen, but later broods include 60 to 100 young. Females store sperm in their reproductive tract for up to two months and give birth to live offspring. Being a live-bearer their young encounter a much greater survivorship of young than do egg-laying fishes which typically suffer from egg predation. Under favourable conditions, Gambusia live two to three years. Estimates of their breeding potential have therefore demonstrated an incredible ability for this species to multiply and dominate their new habitats by sheer numbers in which they have been introduced. Their own success in a new environment is almost guaranteed by their rapid maturation, by breeding several times a year and producing broods of around 50 advanced live young. Individual populations have been recorded expanding from 7,000 to 120,000 in five months!
 Gravid female Gambusia (Gambusia affinis) (20 mm). |
Gambusia 'Mosquitofish', probably first established from a small population introduced into a pond in the Auckland Botanical Gardens in 1930. Very soon after, concern was expressed as to the potential repercussions of the introduction of this fish on whitebait and other native fishes (McDowall, 1990). Gambusia are now foun throughout much of the North Island; both as a combination of dispersion through connected waterbodies, but predominantly as a result of deliberate amateur introductions. This national distribution now also includes many wetland, lake, and river nature reserves for endangered freshwater species such as the ecologically significant Whangamarino Wetland (Waikato).
These introduced fish are well established as an aggressive competitor and predator of native fish species around the world, with evidence for these negative interactions being found in many other countries including Australia. Native fish species tend to decline, or are eliminated in areas where Gambusia become established. In addition to competing for resources, Gambusia are aggressive and often attack fish more than twice their size en masse. This results in damage to fins and scales, leaving fish susceptible to disease. Gambusia also prey on eggs and fry of native fish directly in addition to displaying aggression toward the adults. I quote, "near centres of human habitation, Gambusia has become the dominant fish, sometimes to the exclusion of native fishes. The introduction of one Poeciliid fish [i.e. the guppy family to which Gambusia belongs] has an appreciable effect on the small, surface inhabiting native fishes." (McKay, 1984).
Other recent works have demonstrated further impacts on native fishes being found with the Australian Pacific blue-eye "they’re vermin, like rats with fins, taking over the habitat of native fish" (Anon., 1997), and even native frogs "There’s no wild habitats we can reintroduce the frogs to in NSW, because they all have Gambusia." And, "This little fish drives away native fish and eats frog eggs and tadpoles" (Carwood, 1997). Populations of the Australian green frog (Litoria aurea) and golden bell frogs (Litoria raniformis), already endangered in their own home country, are now thought to be at risk from this aggressive fish in New Zealand (Don Newman DoC, pers. comm). Evidence for competition and predation with native wildlife has also been found in South Africa and South-East Asia (Myers, 1965).
In New Zealand, even our native invertebrates are at risk with Rowe (1987) suggesting that predation by Gambusia has led to decline in distribution in New Zealand lakes and wetlands of our largest New Zealand dragonfly, the baron dragonfly (Hemianax papuensis) This threat to a beautiful feature of our still waters has been further borne with recent findings (John Early Auckland Museum, pers. comm.) of wetlands in the Auckland region failing to show this native species where Gambusia has been introduced. As Rowe, a foremost expert on New Zealand dragonflies, himself wrote, "it is unfortunate that they [Gambusia] have achieved a popular and undeserved reputation for mosquito control. In New Zealand mosquito larvae are rare in permanent waters, the sorts of habitats where Gambusia flourishes. Few Gambusia are ever likely to find their way to tree holes, old tyres, tin cans, and undrained swimming pools and boats where many mosquitoes are."
Interactions of Gambusia with our native freshwater fish species have barely been studied; but what work has been done suggests that Gambusia is likely to affect recruitment, especially when native fishes are small and vulnerable to predation as has been observed with the fry of the endangered black mudfish (Neochanna diversus) (Barrier & Hicks, 1994). A recent work has found that Gambusia are likely to be responsible for the decline of threatened dwarf inanga (Galaxias gracilis) populations in Northland dun lakes; a demise that was previously attributed to the introductions of rainbow trout (Oncorhynchus mykiss) for sports-fishing. Even larger dwarf inanga have been seen to be attacked and quickly immobilised by Gambusia only a 1/3 of their size (Rowe, 1998), with bites from the mob of attacking Gambusia being aimed at their fins and eyes.
 Female Gambusia (Gambusia affinis) (25 mm) hiding in oxygen weed. |
The use of Gambusia as a biological control in an attempt to control mosquito populations has never been proven to be more effective than our native species would be, or already are. "It is arguable whether Gambusia offer better control than some native fishes" (cited in Courtenay & Meffe, 1989). Even the natural presence of native mosquito-larvae eating insects such as backswimmer (Anisops spp.), and carnivorous water-beetles (Coleoptera spp.) etc. have been recorded as being considerably more effective than introduced Gambusia which then destroy these natural bio-controls (Hoy et al., 1972). This failure to perform as the advocates would propose is also borne with the New Zealand experience. New Zealand’s foremost expert on our country’s freshwater fishes, McDowall (1990) has written "Mosquitofish were introduced into New Zealand with the intention of using them to control mosquitoes, but their effectiveness and superiority over native fishes has never been demonstrated." Even more simply, Rupp, (1996) asks "if Gambusia is so effective as a predator, how is it there are so many mosquitoes in areas that are its native habitat?"
Furthermore, native fish have already long been known to control mosquito problems very well. "Absence of mosquitoes from streams inhabited by G. fasciatus [banded kokopu] indicated [that] this fish is an active natural enemy of mosquitoes and a promising control agent" (Graham, 1939). Indeed, the use of native fishes as a superior control without the ecological repercussions of an introduced pest is recognised by both the American Mosquito Control Association, and the World Health Organisation (WHO). A recent feature in the journal of this former organisation which specialises in the control of mosquito problems around the world, damned the use of Gambusia as biological control (Rupp, 1996) promoting the use of country’s native fish species instead. This journal also features many regular article outlining research that supports the use of native fish species as alternative methods for control, typically finding them superior as control agents to Gambusia.
Further to the frequent inappropriate use of this species, international evidence points to Gambusia as not being the 'silver bullet' to cure a location’s mosquito ills. Documented overseas experiences have ranged from "I believe their [Gambusia’s] effect on mosquitoes has been negligible" to the all-too-common-biocontrol-gone-mad story with "Mosquitofish can increase [my emphasis] mosquito populations by eliminating other mosquito predators" (both cited in Courtenay & Meffe, 1989).
In conclusion, the policy of the Native freshwater Fish Society of New Zealand is not to support the use of Gambusia as a mosquito biological control. We are not alone in these feelings, with a dominant proportion of ichthyological societies being unprepared to support their use.
This is further recognised in national legislation covering noxious fishes in New Zealand;
Freshwater Fisheries Regulation 1984
Section 64 (a). No unauthorised catching or keeping of certain fish (amended).
No person shall, except with the prior consent in writing of the acclimatisation society for the district within which it is proposed to catch or keep the fish and except in compliance with such conditions as the society may lay down:
(a) Obtain or keep in captivity any live sports fish or any mosquito fish (Gambusia affinis) or catfish or brown bullhead (Ictalaurus nebulosus).
Conservation Act 1987
26zm. Transfer or release of aquatic life.
(1) No person shall transfer live aquatic life or release live aquatic life into any freshwater except in accordance with this section.
(2) The prior approval of the Minister of Fisheries shall be required for the following: (a) The movement of live aquatic life between sites in which it already exists. (b) The movement of live aquatic life between the islands of New Zealand.
(3) The prior approval of the Minister of Conservation is required for the following: (a) The transfer of live aquatic life to or the release of live aquatic life in a new location where the species does not already exist (including the transfer of a new species to or the release of a new species in an existing or a new fish farm). (b) The transfer of a species of live aquatic life to any land or water managed or administered under this act or any other act specified in the First Schedule of this act.
It is not without reason that internationally this demon guppy is known as "Damnbusia!"
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REFERENCES CITED
Anon. 1997: A fish fiasco. Australian Geographic 45: 12.
Barrier, R. F. G. & Hicks, B. J. 1994: Behavioural interactions between black mudfish (Neochanna diversus) and mosquitofish (Gambusia affinis Baird & Girard 1854). Ecology of Freshwater Fish 3: 93-99.
Carwood, M. 1997: Spawn of an era. Australian Geographic 48: 35-51.
Courtenay, W. R.; Meffe, G. K. 1989: Small fishes in strange places: a review of introduced poeciliids. p. 301-331. In, Meffe, G. K.; Snelson, F. F. (eds), Ecology and evolution of livebearing fishes (Poeciliidae). Prentice-Hall. New Jersey, USA.
Graham, D. H. 1939: Mosquito life in the Auckland district. Transactions and Proceedings of the Royal Society of N. Z. 69: 210-224.
Hoy, J. B.; Kauffman, E. E.; O’Berg, A. G. 1972: A large scale field test of Gambusia affinis and Chloropyrites for mosquito control. Mosquito News 32: 162-171.
McCullough, C. D. 1998: The voracious mosquitofish: Gambusia or Damnbusia? Forest & Bird November: 20-21. (Page 1, Page 2)
McDowall, R. M. 1990: New Zealand freshwater fish: a guide and natural history. Heinemann-Reed. Auckland, New Zealand. 553p.
McKay, R. J. 1984: Introduction of exotic fishes in Australia. In, Courtenay, W. R.; Staffer, J. R. (eds), Distribution, biology, and management of exotic fishes. John Hopkins University Press. Baltimore, USA. 430p.
Myers, G. S. 1965: Gambusia: the fish destroyer. Tropical Fish Hobbyist: 31-54.
Rowe, D. 1998: Management trials to restore dwarf inanga show mosquitofish a threat to native fish. Water and Atmosphere 6: 10-12.
Rowe, R. J. 1987: The dragonflies of New Zealand. Auckland University Press. Auckland, New Zealand. 260p.
Rupp, R. 1996: Adverse assessments of Gambusia affinis - an alternate view for mosquito control practitioners. Journal of the American Mosquito Control Association 12: 155-159.