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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). It is a member of the family Poeciliidae of order Cyprinodontiformes. The genus name ‘gambusia’ is derived from the Cuban Spanish term ‘gambusino’, meaning useless.
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.
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. 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 transporting them live into any of the states or territories. 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. In fact, studies have shown that gambusia can suffer mortalities if fed only on mosquito larvae, and survivors show poor growth and maturation. Gambusia typically eat zooplankton, beetles, mayflies, caddies flies, mites and other invertebrates; mosquito larvae make up only a small portion of their diet.
Many ichthyologists believe that native species are more effective in population control than mosquitofish. These include species such as the western minnow and pygmy perches. 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 5,000 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.