Mosquitofish

Mosquitofish breed throughout the summer months and have a quick maturation rate of 4 to 6 weeks. Gestation only lasts 3 to 4 weeks. Up to 3 generations can be born each year. Generally, broods can have 40 to 60 young. Mosquitofish give birth to live young rather than laying eggs, and juveniles will begin consuming mosquito larvae as soon as they are born. Mosquitofish have a high rate of brood survival. They generally live to about 1 year.

Mosquitofish are extremely good invaders due to their rapid population growth, short gestational period, and high rate of brood survival.

Mosquitofish are primarily a freshwater fish but have also been observed surviving in brackish water (water that is saltier than freshwater, but less salty than marine water). They have been observed in a variety of natural and human-made waterbodies, including rice fields, floodplains, swamps, rivers, freshwater lakes, streams, lagoons, estuaries, ornamental ponds, hot springs, canals, dams, and saltwater lakes. Mosquitofish are robust and able to survive in diverse environmental conditions, but their one limitation is their preference for stagnant and slow-moving waters.

They are capable of surviving in waters ranging in temperature from 0.5°C to 39°C. However, they thrive in warmer waters from 25°C to 35°C (Pyke, 2005). They are known to survive ice-covered waters as well as hot bores, and therefore have a vast range of temperatures they can survive in.

Mosquitofish also have a high tolerance for salinity and can survive direct transfer from freshwater to saltwater environments. They are also known to tolerate low oxygen conditions and can even tolerate anoxic (oxygen-absent) conditions if they have access to the water’s surface. Mosquitofish are also more resistant than other fishes to pollutants such as herbicides, pesticides, and even radiation.

In its native range, Mosquitofish prefer shallow water with dense vegetation and warm water temperatures. The mosquitofish’s tolerance to a large range of water conditions speaks volumes about its ability to invade a variety of habitats including those with harsh environmental conditions not suited for other organisms.

Due to the mosquitofish’s ability to grow its population so quickly, the main sign of its presence is the death and disappearance of native fishes and other organisms from a waterbody. Mosquitofish are known to prey on mosquito larvae, but have diverse diets that include many invertebrates, vegetation, tadpoles, and other fishes. Due to their rapid population growth, a waterbody populated by mosquitofish will undergo rapid transformation, losing much of its native species. Their rapid population growth coupled with their diverse diets will outcompete other native fish species for space and resources, such as food. Their diet and overpopulation can cause anoxic conditions due to the decrease of plant and other native life. Waterbodies populated by mosquitofish will become depleted of oxygen and resources required to sustain the life of less robust native species.

The main mode of distribution for the mosquitofish has historically been commercial use for the predation of mosquito larvae. People have favoured the introduction of mosquitofish due to their ability to decrease mosquito populations in urban areas. Mosquitofish have been traded internationally for ornamental purposes as well. The Mosquitofish’s hardiness allows it to be able to survive and thrive when introduced to a new habitat with potentially harsh conditions.

The Western Mosquitofish (Gambusia affinis) is native to the eastern and central United States, down to parts of Mexico. These areas include southern New Jersey and the Mississippi River Basin from central Indiana and Illinois, down to the Gulf of Mexico. This species has been distributed globally, and is present in Africa, Asia, Europe, North America, Oceania, and South America. Records identify the Western Mosquitofish as being introduced to Canada as early as 1923 (Seebens et al., 2017).

The Eastern Mosquitofish (Gambusia holbrooki) has a similar native range to the Western Mosquitofish, ranging from central and eastern USA down to Mexico. This includes the Mississippi Basin and the tributaries down to the Northern Gulf of Mexico, as well as New Jersey down to the Gulf of Mexico and Northern Florida. Its invasive range is similar to the Western Mosquitofish as well, invading every continent except for Antarctica.

Western Mosquitofish observations in Canada have been isolated to British Columbia. There have not been sightings of Eastern Mosquitofish in Canada but have the potential to spread to the Eastern provinces due to their invasive distribution in the nearby states of New York, Pennsylvania, and Ohio.

Ecological Impacts:

Mosquitofish are ravenous predators and mainly prey on invertebrates, although their diets are quite diverse. Juvenile mosquitofish have been noted as being even more voracious than their adult counterparts, devouring almost anything in sight and feeding at incredibly high amounts compared to their small body size.

Given their insatiable appetites and diverse diets, mosquitofish pose a serious threat to indigenous plants and animals, having dangerous effects on the ecosystem as a whole. Not only will native species decrease due to predation, but mosquitofish often outcompete native animals for resources, such as food, and will therefore cause a disruption in the whole food chain. This disruption can have severe effects on the ecosystem, causing massive losses to biodiversity and ultimately changing the function of the habitat.

Losses to aquatic biodiversity usually have effects to the waterbody’s chemistry, sometimes causing anoxic environments and changes to the pH. Given the mosquitofish’s hardiness and ability to adapt, they will be able to survive in these harsh conditions, while native species less adaptable will be threatened to regional extinctions.

Restoring an ecosystem that has undergone massive changes to its biodiversity, chemistry, and function can be timely and sometimes near impossible. Restoration would require removing and managing the invasive species and reintroducing the native species that have gone regionally extinct. This process is also costly, and it would be much more cost effective to avoid the introduction of mosquitofish through proper educational resources and early management tactics. 

Social and Economic Impacts:

When mosquitofish take over an ecosystem, they decrease native biodiversity, change the chemical make-up of the ecosystem, and can change the ecosystem’s overall function. In man-made habitats, such as dams, these changes can have catastrophic effects on the function of infrastructure. The mosquitofish outcompeting native species also harms the regional fishing market, both recreationally and commercially. The importance of conserving native biodiversity also has deep-rooted social impacts, such as Indigenous communities relying on healthy ecosystems to practice their own cultural customs. Managing mosquitofish come in costly forms of chemical, biological, or physical treatments, oftentimes harming native species in the process.

The most cost-effective option in managing mosquitofish is to avoid their initial introduction by increasing prevention efforts.

Physical, biological, and chemical management tactics have been used to try to mitigate mosquitofish populations globally. Physical management methods have included draining ponds, such as ornamental ponds, and blocking or cutting off pathways from colonized waterbodies to non-colonized waterbodies. These methods do not always work, as mosquitofish can travel in waters only half the depth of their body, as little as 3mm in depth. Galaxias maculatus is a known predator of mosquitofish in the southern hemisphere and has been introduced as a biological control agent for the invasive mosquitofish. However, this causes an imbalance in the food web, and can cause more long-term harm than good. Chemical control has been used in the form of the poison rotenone. However, due to the mosquitofish’s hardiness, they can tolerate a high concentration of this poison within their ecosystem before dying off, which in turn causes more harm to the native species than to the invasives.

Public awareness, education, and monitoring activities are the least costly and most effective option for avoiding the introduction and spread of mosquitofish.

Mosquitofish are now a prohibited invasive species under the Invasive Species Act of Ontario. Prohibited invasive species “cannot be brought into, deposited, released, possessed or transported in Ontario and cannot be propagated, bought, sold or traded in Ontario.”

Western mosquitofish is prohibited under the Government of British Columbia. This province has excellent monitoring and reporting resources, and even offers an invasive species training module that allows the public to learn about pathways and prevention.

The Government of Alberta has also deemed Western mosquitofish to be illegal to possess, sell, or transport under the Alberta Fisheries Act.

Mosquitofish are not prohibited under the Government of Saskatchewan but are recognized as a potential threat.

No other provinces or territories have official prohibition of mosquitofish, but under the Government of Canada, mosquitofish are recognized as “Species Subject to Prohibitions and Controls,” meaning the country recognizes the current laws in place to manage their spread.