| Eastern mosquitofish | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Cyprinodontiformes |
| Family: | Poeciliidae |
| Genus: | Gambusia |
| Species: | G. holbrooki |
| Binomial name | |
| Gambusia holbrooki Girard, 1859 |
|
The eastern mosquitofish, Gambusia holbrooki, is a species of freshwater fish that is closely related to the western mosquitofish, Gambusia affinis. It is a member of the family Poeciliidae of order Cyprinodontiformes. It is native to the eastern and southern United States, and grows to 3.5 cm in length.
Gambusia holbrooki is found in the southeastern United States and has become an invasive species in Australia, where they were released as a method to decrease mosquito populations.[1] G. holbrooki is considered a planktivorous species which consumes algae and detritus to enhance its dietary requirements. Feeding habits seem to change based on maturity and mating season.[2][3] This species thrives in shallow water between 31 and 35° Celsius, and seems to be able to acclimate to temperatures above and below this. G. holbrooki has been shown to survive in water with pH and chemical levels known to kill other fish species,[1] and prefers to live in areas where the water flows at a slow pace, is clear and without free floating plant life, and seeks shelter in rooted plants.[1] Temperature has been shown to change the length of time it takes them to reach reproductive maturity and body size. This species is also known to give birth to live young instead of laying a clutch of eggs.[1][4] Since G. holbrooki is not considered endangered or threatened, no true management plan is in effect for this species within the United States. In fact, this fish is considered an invasive species in Australia, due to their ability to thrive in many different environmental conditions which are usually lethal to other fish species.[5] The main recommendation for this species is to find a way to decrease their numbers in areas where they are considered an invasive species.
Gambusia holbrooki is native to the southeastern United States, and can be found in many of the lakes within that area, which includes lakes east of the Mississippi River. There has not been a noted decrease in this species due to human activities.[1] G. holbrooki is easily maintained and has never been considered an endangered or threatened species due to their ability to thrive in their native habitat.[5] In Australia, they have been introduced as a means to control the mosquito population.[1] However, they have become an invasive species that causes harm to native species which have an aquatic larval stage. Due to these releases, G. holbrooki has actually had an increase in its range. It has been shown in laboratory experiments to tolerate chemical and temporal changes quite easily, and that this might be one reason they have not shown a decrease in population due to environmental changes caused by humans.[4]
Gambusia holbrooki is a planktivorous species that will, if need arises, switch food sources to survive. With an increase of competition, this species will switch from a diet rich in plankton, algae, and detritus to one consisting of zooplankton, other invertebrates, the larvae of many species, and plant-associated animals.[2] The main source of competition for G. holbrooki seems to be an increase of its own species and other planktivorous species. The main problem with this is, as mentioned before, it will change its diet, this is common even among the juveniles and both sexes of its own species. The females tend to not specialize on one prey, and consume all evenly, whereas the males and juveniles specialize on one prey type. However males, females, and juveniles all consume detritus at the same rate.[1] Gambusia holbrooki fish live in areas that have slow-moving water and is free of large floating plant life. They use rooted plants as shelter. The water temperature in which Gambusia holbrooki thrive is between 31 and 35°C; however, they can tolerate temperatures above and below this. Also as mentioned before, they can tolerate a large change in chemical composition and a wide range of pH levels.[1][4] Little research has been done to determine all G. holbrooki predators, due to its own predatory nature in the areas where it has been introduced. In the introduced areas, it has been known to cause top down trophic effects due to their eating the larvae of some top predators, which include frogs and other fish.[2]
The breeding season for Gambusia holbrooki is between midspring and midautumn, with the peak breeding time being around summer. Females can have up to nine broods per mating season, with the average size ranging from five to 100.[1] The variability of the average brood size is due to many variable, including temperature, age, and available nutrients. Higher temperatures have been shown to increase the reproductivity of this species.[4] The gestation period for this species is between 22 and 25 days. These offspring juvenile stage lasts between 18 days and eight weeks. Once again, changes in temperature affects these numbers; colder temperatures decrease and higher temperatures can increase maturity. This species can have several generations within their breeding period due to the fast rate of growth. The usual life span is between one and two years, as determined by stress factors in their habitats.[1][4] Sexual selection in this species is based on the size of the male. Females tend to choose the larger and more aggressive males. Females tend to choose areas of shallow water with dark soil cover for brooding sites, while juveniles prefer more rooted plants in which to hide.[3] The main human-induced change that affects the growth rate and life history of G. holbrooki is the water temperature, which, as stated before, can increase or decrease both the growth rates and birth rates.
This species is not considered an endangered or threatened species in America because they are able to withstand temperature change, chemical change, and pH change fairly easily.[1][4] This mitigates many of the problems caused by humans, the biggest of which is its introduction to nonnative areas, such as Australia. Australia has set up conservation management plans to try to save native species from Gambusia holbrooki. One such management plan included releasing a chemical known to kill mosquito larvae. The chemicals used were found to have a strong effect on the G. holbrooki, but they became tolerant to most of them fairly rapidly unless amounts considered unsafe for native species were used. Another tried and failed attempt to decrease this invasive species was electrocuting a lake known to have been invaded. The cost and loss of native fish was so great that this method was dropped. The main reason it failed was these fish stay in the shallows, which receives the smallest charge from the used electrocution method. Later tests also revealed this species has a high tolerance for electrical shock, but the exact mechanism that allows this still seems to be questionable.[1]
The management recommendation for this species would be for the areas where Gambusia holbrooki is considered invasive, since there is no known need to have a conservation management plan set up to protect this species in its native area. This, of course, is due to its ability to grow and reproduce at a fast rate, and the ease of raising a population under controlled supervision.[2][4] Using population density would be the best way to keep track of the population in the areas where G. holbrooki is considered invasive; since they are not native, the population should be zero in these areas. A minnow seine would be the best type of equipment to use when measuring the population density, due to their small size. Samples for this species would have to be taken in the shallows of lakes where the water is slow, has little floating vegetation, and has rooted plants, where they thrive and where they have become an invasive species . Sampling frequency would be based on the overall growth and spread of this species. In areas known to be heavily populated, more frequent samplings would have to be taken to know how fast the population is increasing, which allows for better control by a concentrated attack during periods of lower population density. It seems the only safe route to remove this invasive species is by catching the adults with a net and using one of the poisons known to kill this species. Most of the young should be killed by the poison, with the adults being physically removed. This method could cause a decrease within the population, and if kept up, could eradicate them from the introduced areas.
Eastern mosquitofish were introduced to Australia to control mosquitoes, when in reality various small Australian native fish were already keeping mosquitoes to a minimal level.
They are aggressive, fin-nipping harassers of other fish, and pose a serious threat to native Australian fish and aquatic fauna. Negative impacts on rainbowfish species and at least one frog species have been documented. Several rainbowfish populations appear to have become extinct due to the impacts of introduced Gambusia.