Mosquitofish
From Wikipedia, the free encyclopedia
| Mosquitofish | ||||||||||||||
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Secure
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| Gambusia affinis (Baird & Girard, 1853) |
The mosquitofish (Gambusia affinis) is a species of freshwater fish, also commonly known simply by its generic name, gambusia, although such usage is ambiguous. It is sometimes called the western mosquitofish, to distinguish it from the eastern mosquitofish (G. holbrooki). In Hong Kong, it is also known as the live-bearing tooth-carp. It is a member of the family Poeciliidae of order Cyprinodontiformes.
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 larval and pupal 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 °C 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. The majority of these introductions were foolish; in most countries where mosquitofish have been introduced it is has been proved that the endemic fish species were already providing maximal mosquito control, and that the introduction of mosquitofish has been both unnecessary and highly damaging to endemic fish and other endemic aquatic life. In Australia G.holbrooki has caused great damage to native fish and frog species. For example it is considered responsible for the extinction of rainbowfish in sub-tropical streams around Brisbane.
The mosquitofish is 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, which belong to the same taxonomic family. Sexual dimorphism is pronounced; mature females reach a maximum overall length of 7 cm (2.5 inches), while males reach only 4 cm (1.5 inches). Sexual dimorphism is also seen in the physiological structures of the body. The anal fin on adult females resembles the dorsal fin while the anal fin of adult males is pointed. This pointed fin is referred to as a gonopodium and is used to deposit sperm inside the female.
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. Live-bearing gives their young a much higher survival rate than in most species of egg-laying fish, which typically suffer from egg predation.
Under favourable conditions, mosquitofish live two to three years. Estimates of their breeding potential have therefore demonstrated an incredible ability for this species to multiply and dominate new habitats into which they have been introduced. Their 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.
[edit] Introduction to Australia
The name 'gambusia' means 'useless' in Portuguese, a point that would have been helpful to know before introducing the mosquitofish to most of the world. Mosquitofish were first introduced to Australia in 1925, spreading from the northeast coasts farther south to New South Wales, Southern Australia, and parts of Western Australia by 1934[1]. Currently there are known populations of wild mosquitofish in every state and territory except the Northern Territory, and are found in swamps, lakes, billabongs, thermal springs, salt lakes, and ornamental ponds. Mosquitofish are considered a noxious pest, especially in New South Wales and Queensland, and it is illegal to release them into the wild or transported them live into any of the states or territories[2]. Mosquitofish were introduced by military and local councils to control mosquito populations, however there has been no evidence that gambusia has had any effect in controlling mosquito populations or mosquito borne diseases[3]. In fact, studies have shown that gambusia can suffer mortalities if fed only on mosquito larvae, and survivors show poor growth and maturation[4]. Gambusia typically eat zooplankton, beetles, mayflies, caddies flies, mites and other invertebrates; mosquito larvae make up only a small portion of their diet[5].
Many ichthyologists believe that native species are more effective in population control than mosquitofish[6]. These include species such as the western minnow and pygmy perches[7]. Unfortunately, gambusia may have exacerbated the mosquito problem in many areas by outcompeting native invertebrate predators of mosquito larvae. Because of their aggressive nature and high birthrate, mosquitofish can overtake most native species in an area, drastically harming local populations. Even if they were needed for mosquito control, studies have also shown that at least 5000 fish/ha would be needed for effective control. However, mosquitoes breeding environments are migratory and unreliable, so regular fish predators can have little effect from year to year[8].
Mosquitofish have a high tolerance for salinity, low oxygen, high temperatures (from 0.5-35 degrees Celsius), and pollutants, and are therefore able to live in many areas where other fish cannot. Certain thermal adaptations have allowed them to live in places from 55 degrees North to 44 degrees South, expanding their natural range[9]. They have a resistance to a wide variety of pollutants, including organic waste, heavy metals, insecticides, herbicides, rotenone, phenol, and radiation[10]. Ichthyologists believe that the reason for low mosquito levels in areas populated by gambusia is not because of the fish, but rather the insecticide in the water killing the larvae.
Gambusia has harmed native fish populations in many ways. By consuming algae eating zooplankton, they increase the chances of algae blooms in the water, reducing the water quality. They are very aggressive, and tend to attack other fish and nip their fins, leading to infection or death[11]. Studies have shown that the purple spotted gudgeon fish population has declined as a result of high densities of mosquitofish[12]. They have also negatively impacted populations of beetles, backswimmers, rotifers, red finned blue eye, Edgbast goby, crustaceans and mollusks, to name a few[13]. In a study done by Keane et al., 2004, one major threat to Tasmanian fauna could be to native galaxiids, as suggested by the fact that mosquitofish are known to attack and kill adult Galaxias gracilis in New Zealand[14]. If gambusia became native in certain areas by eradicating other fish populations, it could put a great deal of stress on the remaining flora and fauna. Decreasing the number of native species would also benefit mosquitoes by decreasing the competitive pressure from other fish. Because of their high reproductive rate (an average of 50 young per brood, with up to nine broods per year), fast maturation (sexual maturity is reached in two months), and aggressive behavior, mosquitofish can outcompete almost any native fish[15].
Mosquitofish also pose a threat to native frog populations, (such as Limnodynastes ornatus) in natural water bodies where these species co-occur. In one study conducted by Keane et al., gambusia consumed the tadpoles of L. ornastus, leading researchers to believe that over a period of time, they could wipe out the L. ornastus population as a whole. Gambusia are known predators of tadpoles of other closely related frog species such as L. aurea and L. dentate. The green and golden bellfrog in Tasmania is now directly threatened by gambusia, and is listed on Tasmania’s Threatened Species Protection Act of 1995[16]. Another study conducted by Keane found that tadpoles are better suited to environments without introduced fish. Responses observed in the green and golden bellfrog suggest that tadpoles are naïve to mosquitofish because they did not respond to them. Additionally, gambusia and other introduced fish may have reduced the suitability of permanent water bodies as breeding sites for pond-breeding amphibian species such as the green and golden bellfrog[17].
One species of fish, the Edgbaston hardyhead, is restricted to one spring-fed pool in the Edgbaston Spring Complex located in Edgbaston Station, central Queensland. The hardyhead competes with the mosquitofish for food and resources; if the mosquitofish outcompetes the hardyhead, they will have eradicated an entire species in one fell swoop. That is why the hardyhead is now considered a high priority for conservation recovery actions in Queensland, where steps are now being taken to exterminate the mosquitofish from the spring[18].
The mosquitofish was nominated as an environmental hazard under the NSW Threatened Species Conservation Act in 1999, making it illegal for landowners to do anything that would help spread gambusia[19]. This includes releasing mosquitofish pets into the wild, one of the biggest problems effecting local rivers throughout eastern Australia. Proposals for gambusia population control have included introducing viral, bacterial, or fungal diseases and parasites into an overpopulated area. However many diseases can jump species and would harm native fish[20]. The Tasmanian government has taken steps to eradicate mosquitofish by containing current populations and minimizing paths of dispersal[21]. In Gordon’s Lagoon at the Clarkesdale Bird Sanctuary in Linton, Victoria, which experiences major infestations of mosquitofish year-round, park managers have taken advantage of recent dry conditions by draining ponds. Residual wet areas are treated with lime solutions, changing the Ph of the water and killing off the fish[22]. Other ideas have been proposed, but attempting to isolate them to affect only the mosquitofish is unrealistic as of yet.
