|Top: female/Bottom: male|
The eastern mosquitofish, Gambusia holbrooki, is a species of freshwater fish, closely related to the western mosquitofish, Gambusia affinis. It is a member of the family Poeciliidae of order Cyprinodontiformes. The western mosquitofish has a larger distribution throughout the United States, while the eastern mosquitofish is native to eastern and southern United States spanning from Florida to Delaware and as far interior as Alabama and Tennessee.
G. holbrooki is a small, light colored fish with semi-transparent fins. The females usually have a black stripe near their eye area and light spots can be seen on the caudal and dorsal fins of both sexes. Due to its similar size, shape and reproductive habits, it can easily be mistaken for a guppy. Generally, males reach 1.5 in (3.8 cm) and females 2.5 in (6.4 cm). These fish are a live bearer species and as such the females are larger and more rounded then the males. Pregnant females are also easily recognizable by their gravid spot; a darker area on their bellies where they hold the fry.
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.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. 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.
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. This species thrives in shallow water between 31 and 35°C, 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, 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.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. No decrease in this species due to human activities has been noted. G. holbrooki is easily maintained and has never been considered an endangered or threatened species due to its ability to thrive in its native habitat. Due to releases in new areas, G. holbrooki has actually increased its range. It tolerates chemical and temporal changes quite easily, and this might be one reason they have not shown a decrease in population due to environmental changes caused by humans.
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. 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. The variability of the average brood size is due to many variables, including temperature, age, and available nutrients. Higher temperatures have been shown to increase the fecundity of this species. The gestation period for this species is between 22 and 25 days.
The 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. Sexual selection in this species is based on the size of the male. Females tend to choose larger, 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. 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.
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 its ability to thrive in many different environmental conditions which are usually lethal to other fish species. The main recommendation for this species is to find a way to decrease their numbers in areas where they are considered an invasive species. In Australia, they have been introduced as a means to control the mosquito population. However, they have become an invasive species that causes harm to native species which have an aquatic larval stage. 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 electrifying a lake known to have been invaded. The cost and loss of native fish was so great, this method was dropped. The main reason it failed was these fish stay in the shallows, which receive the smallest charge from the electrification method used. 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. Eastern mosquitofish were introduced 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.
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. Using population density would be the best way to keep track of the population in the areas where it is considered invasive; since it is 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 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 sampling 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.
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 its eating the larvae of some top predators, which include frogs and other fish.
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