Mosquitofish (Gambusia affinis) are small freshwater fish known for their ability to thrive in various water conditions. Originally introduced to many ecosystems as a biocontrol agent for mosquitoes, they are now considered an invasive species in some regions. These hardy livebearers are omnivorous, with a diet primarily consisting of insect larvae, particularly mosquito larvae. This article delves into the dietary habits of mosquitofish, their feeding behavior, and how their diet contributes to their effectiveness in mosquito control.
Introduction to Mosquitofish
Mosquitofish are rarely found in pet stores due to their invasive nature. They are commonly kept in backyard water gardens or ponds to help control mosquito populations. These fish can tolerate cold temperatures and poor water quality, making them adaptable to various environments. However, it is crucial to avoid releasing them into the wild to prevent further ecological imbalance.
Mosquitofish Biology and Habitat
Mosquitofish are small, typically ranging from ¾ of an inch to 3 inches in length. Females are generally larger than males. They thrive in sunlit areas of water bodies and can tolerate a wide temperature spectrum, from 33 to 104 degrees Fahrenheit. They are often found in shallow waters and dense vegetation, where mosquito larvae and pupae hide.
Their hardiness, adaptability, and ease of handling make them effective in mosquito control programs. Mosquitofish are native to the southern and eastern parts of the United States and were introduced to California in the early 1920s for mosquito control.
Feeding Behavior and Diet
Mosquitofish are omnivorous but primarily feed on insect larvae. They have a voracious appetite for mosquito larvae, which is why they are called mosquitofish. Their flattened head and protruding mouth enable them to readily prey on surface-feeding mosquito larvae and pupae. A large female can consume up to 500 larvae per day. All ages, sexes, and sizes of mosquitofish eat mosquito larvae, other small aquatic invertebrate insects, and algae. They are visual predators and feed during daylight hours.
Read also: Mosquito Feeding Habits
Primary Food Sources
- Mosquito Larvae: The primary food source for mosquitofish, especially in areas where mosquito populations are high.
- Aquatic Insects: They also consume other small aquatic insects and their larvae.
- Algae: Algae provide a supplemental food source, especially for fry, and also offer shelter.
- Zooplankton: These microscopic animals are a part of the Mosquitofish diet.
- Detritus Material: Mosquitofish consume detritus material.
Supplementary Feeding
Mosquitofish are low maintenance and seldom need supplementary food in established ponds. However, during cooler months when mosquito larvae and other aquatic organisms are scarce, it is acceptable to feed them tropical fish food flakes or dry dog/cat food. Overfeeding can reduce their appetite for mosquito larvae and pollute the water.
Mosquitofish in Mosquito Control
Mosquitofish are an attractive and effective alternative to chemical insecticides, referred to as biological control. Many mosquito control districts maintain fishponds to produce mosquitofish for distribution. These fish are routinely stocked in various water sources, including irrigation ditches, industrial and ornamental ponds, unmaintained swimming pools, and wetlands.
Considerations for Stocking
- Mosquitofish are suitable for ornamental ponds, water troughs, and neglected swimming pools with a minimum depth of 18 inches.
- They typically do not thrive in heavily shaded locations.
- Abundant leaf litter can lower the water's pH level, which can be lethal to the fish.
- To avoid competition with sensitive native fish and other native aquatic organisms, mosquitofish should not be stocked in habitats where such species are present.
Reproduction and Life Cycle
Mosquitofish are live-bearing fish, meaning they give birth to live young. They breed throughout the summer months and may have 3 to 5 broods annually, with each brood containing 40-100 young fry. Optimal growth occurs when water temperatures are between 77-86 degrees Fahrenheit.
Reproduction Process
- Fertilization: The male uses his gonopodium (modified anal fin) to secrete milt into the female's genital aperture.
- Gestation: The gestation period lasts 16 to 28 days.
- Birth: Newborn mosquitofish are about 8 to 9 mm in length.
Factors Affecting Reproduction
- Temperature: Higher temperatures (20 to 30 °C) decrease the age at first reproduction and increase brood size.
- Photoperiod: A photoperiod of less than 12.5 hours triggers the development of the next clutch of oocytes.
Mosquitofish Care and Maintenance
Mosquitofish are relatively easy to care for, making them suitable for beginner aquarists. They can be kept in tanks as small as 10 gallons, though 15 gallons or larger is recommended. An aquarium tank cover is advisable.
Tank Environment
- Tank Size: Minimum 10 gallons, recommended 15 gallons or larger.
- Water Quality: Mosquitofish can tolerate a wide range of water conditions but thrive in clean, well-maintained water.
- Temperature: They can tolerate temperatures from 33 to 104 degrees Fahrenheit, with optimal growth between 77-86 degrees Fahrenheit.
- Hiding Spots: Adding plants or other hiding spots can help fry survive in the presence of adults.
Feeding Practices
- Natural Diet: Primarily mosquito larvae and other small aquatic insects.
- Supplementary Diet: Tropical fish food flakes, dry dog or cat food during periods of scarcity.
- Feeding Frequency: Avoid overfeeding to ensure they continue to prey on mosquito larvae.
Potential Problems
- Aggression: Mosquitofish can be aggressive and are known fin-nippers, so avoid keeping them with long-finned fish like Fancy Guppies.
- Organic Matter: Excessive organic matter, such as pine, oak, and eucalyptus leaves, can be harmful.
- Algae Overgrowth: While small amounts of algae are beneficial, too much algae can limit their ability to hunt mosquito larvae.
Growing Mosquitofish
Eastern mosquitofish are quite adaptable to their environment and, therefore, can be grown with many different culture techniques and containers. Growing and maintaining populations of eastern mosquitofish in ponds is less technical and more labor efficient than growing them in tanks.
Read also: Are Crane Flies Really Mosquito Hawks?
Pond Culture
When using earthen ponds for mosquitofish production, careful planning and cooperation with the appropriate regional permitting agencies is recommended. Once dug, the pond should have a water source to maintain the water level, which is reduced by evaporation and seepage. This is best achieved through access to wells containing water that has not been chemically treated. Well water often contains hydrogen sulfide, which can be toxic to fish. Thus, it is best to have the well water inlet placed above the pond's water surface to agitate incoming water, which degasses hydrogen sulfide. Once filled with new freshwater (for the first time or after subsequent cleanings), the water should be fertilized to encourage growth of phytoplankton and zooplankton. These tiny plants and animals will serve as food for the stocked fish population.
Tank Culture
Tank culture has advantages and disadvantages relative to pond culture. The eastern mosquitofish can tolerate varied water quality conditions. However, tank culture allows for more control of those water quality parameters, keeping them in the range for optimal growth. Although preference has been shown for temperatures between 31°C-35°C (88°F-95°F); they tolerate a range from 1°C-40°C (33°F-104°F). Temperatures closer to the preference range will provide for better growth, reproduction, and survival. In general, pond culture is far easier, usually cheaper (if one has access to a pond), and is more productive. Eastern mosquitofish in tank culture often show aggression and kill one another, can be more susceptible to disease outbreaks, and will eat most of their own offspring.
Harvesting
There are two ways to harvest small fish from a pond; trapping and netting. Traps are often used with livebearers because the fish reproduce continually and can be graded for size once harvested. Using a trap is sufficient for harvesting a portion of the population without disrupting the water quality and majority of fish within the pond. There are different types of traps that can be utilized for this procedure, all of which are easy to operate. Trapping is done by baiting the trap and returning after sufficient numbers of fish are caught. A dip net or hand net can also be used to harvest small numbers of fish from a pond with minimal effort. Seine nets are used to harvest most of the fish that are in a pond at one time.
Ecological Impact and Invasive Status
While mosquitofish are effective in controlling mosquito populations, their introduction outside their native range can have harmful ecological impacts. They can consume or injure other small fish or harm them through competition. In Australia, mosquitofish are considered noxious pests, posing a threat to native fish and frog populations.
Negative Impacts
- Competition: Outcompete native invertebrate predators of mosquito larvae.
- Predation: Consume or injure other small fish.
- Habitat Disruption: Alter ecosystems through their feeding habits and reproductive success.
Positive Impacts
- Mosquito Control: Reduce mosquito populations, potentially decreasing the spread of mosquito-borne diseases.
- Biological Control: Offer an alternative to chemical insecticides.
Mosquitofish vs. Eastern Mosquitofish
The western mosquitofish (Gambusia affinis) is often confused with the eastern mosquitofish (Gambusia holbrooki). Both species are used for mosquito control, but there are some differences.
Read also: Incorporating Fish into a Carnivore Meal Plan
Distinguishing Features
- Gonopodium: The gonopodium of G. affinis has a smooth third ray, while that of G. holbrooki bears minute denticles.
- Gravid Spot: Adult female mosquitofish can be identified by a gravid spot they possess on the posterior of their abdomens.
- Coloration: The eastern mosquitofish coloration is silver to forest green on top and on the sides and sometimes exhibits a dark bar below the eye.
Alternative Native Species
While mosquitofish have the namesake, other native Florida species will also consume mosquito larvae. Other native poeciliids (live bearers) found in Florida include the sailfin molly (Poecilia latipinna) and least killifish (Heterandria formosa); the latter is considered one of the better mosquito control candidates. Numerous fish species will consume mosquito larvae at some time during their life cycle, especially during the early growth periods when mosquito larvae can constitute a portion of fry or juvenile fish diets. The smaller fish will continually graze down mosquito larvae populations while the larger species will out-grow them and move onto larger prey items.