Fish are best for mosquito and tadpole control in marron ponds
The first Australian colonisation of the mosquito fish (or Gambusia) from Central America occurred in drains and canals in the Sydney area in 1925. Mosquito fish were introduced to Western Australia in 1934 into irrigation ditches in the Perth suburb of Nedlands. The intention was to control mosquito larvae, in an effort to reduce the likelihood of transmission of malaria, yellow fever, dog heart-worm and dengue fever.
Before the early 1900s the mosquito fish was called a ‘top minnow'. However, the common name was changed to ‘mosquito fish' when it was thought it would be an effective, natural form of biological control for removing disease-carrying mosquito larvae from waterways. As a result of this name change, and the misconception that it would eradicate mosquito larvae, it is the most widely distributed freshwater fish in the world and is found on every continent except Antarctica.
Although there are some 35 different species of Gambusia that are native to Central America, in most cases it is Gambusia affinis that has been spread throughout the world. Since the first introduction of the mosquito fish, it has been translocated (spread) mainly due to the suggestive common name and the desire of people to eradicate mosquito wrigglers from rainwater tanks, backyard ponds, and other water bodies near populated areas, without using chemicals. In recent years they have been introduced onto commercial marron farms to reduce the number of mosquito larvae in marron ponds.
Biology and problems
The female Gambusia is slightly larger and rounder than the male and gives birth to live young (about 50 or more), which are fully developed to look after themselves from birth. Mosquito fish are very adaptable and breed more than once a year, in almost any water quality conditions. As found with the introduction of other foreign species for biological control, there are many problems associated with the introduction of mosquito fish:
- Gambusia are naturally more aggressive than the smaller south west native fish species. In aquaria they have been frequently observed to attack other species and even harass larger fish by fin nipping, causing infection and death.
- In a new locality without their natural predators, Gambusia tend to over populate and out compete the native species of small fish, frequently replacing them. Native fish are particularly vulnerable to replacement by the very hardy mosquito fish when a riverine or wetland habitat has suffered some degree of man-made degradation, eg. due to eutrophication (excessive algal growth caused by fertilisation of catchment areas).
- The breeding success of mosquito fish is independent of habitat structure, because of live bearing, allowing their invasion of many different types of microhabitats.
- Other environmental conditions in many new localities may increase the already high survival of young. In warmer localities, a longer breeding season encourages a population explosion, due to prolonged and repeated spawning by individual fish.
- Gambusia (and tadpoles) will eat any marron pellet or similar food source which is fed to ponds. Large numbers of mosquito fish can impact significantly upon marron feed, affecting growth rates and the increasing the cost of feeding marron.
- Mosquito larvae are not the natural food of mosquito fish, which if fed solely upon mosquito larvae, show signs of delayed maturity and heavy mortalities. It has been shown that native fish actually eat more mosquito larvae than mosquito fish (Lloyd, 1987).
- The name Gambusia is from the Portuguese word gambusino meaning useless which should have been noted before the fish were widely translocated from their native habitat in Central America, where they were named.
- Mosquito fish can catch mosquito larvae in open water but the success of their predation decreases in areas of high aquatic vegetation.
Native fish alternatives
While native fish will not totally eradicate mosquito larvae in marron ponds, or any other water body, they are much more environmentally acceptable than mosquito fish.
Within the south west of WA and the external distribution of marron farming, there are several types of small native fish available. There are two which appear to be suitable to place in marron ponds; the western minnow (Galaxias occidentalis) and pygmy perch (Edelia vittata). Western minnows are most effective for controlling the breeding of tadpoles in marron ponds, and will consume the tadpole spawn. Western minnows will not consume pellets, as they primarily feed upon small terrestrial insects falling onto the water surface.
Both of these types of fish may breed in marron ponds; however they will only breed once a year and providing the ponds environmental conditions are suitable for them. They will not over-breed and over populate within the ponds; thus they will not cause the problems that the Gambusia do, especially if the ponds are drained once a year, as recommended for good marron farm management.
There are several licensed, native fish farmers, who can sell you stock of western minnows and pygmy perch if you are looking for a species of fish to reduce the number of mosquito larvae in your ponds.
Please note It is illegal to catch freswater native fish from the wild, without a relevant licence from the Department of Fisheries.
References
Allen G.R. 1982. A Field Guide to Inland Fishes of Western Australia. Western Australian Museum.
Kailola P. 1981. Gambusia is a pest. SAFIC Vol 5 (5) pp 11.
Lloyd L. 1986. An Alternative to Insect Control by "Mosquitofish", Gambusia affinis.
Lloyd L. 1987. Biological Control of Insects With Fish. A paper presented at the workshop "Mosquito Vector Control in Australia: Current Status and Future Prospects". Commonwealth Department of Health, Canberra.
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