Western Mosquito Fish
Gambusia affinis
Type: freshwater, brackish
Physical Characteristics
Small, robust-bodied fish with a rounded tail. They have a flattened head and an upturned mouth. Females are larger than males and often feature a dark gravid spot near the anal fin. Coloration is typically olive-tan to gray with translucent fins and small black spots.
Habitat & Distribution
Native to the Mississippi River drainage but introduced worldwide. They prefer shallow, stagnant, or slow-moving warm waters like ponds, ditches, and marshes.
Size & Weight
Females reach up to 7 cm (2.8 in) while males reach about 4 cm (1.6 in). They are very lightweight, usually under a few grams.
Similar Looking Species
Eastern Mosquitofish (Gambusia holbrooki), which looks nearly identical but has different dorsal fin ray counts; Guppies, which are more colorful in domestic varieties.
Ecological Role
Generalist feeder that controls insect populations but can display aggressive behavior toward native fish and amphibians.
Diet
Omnivorous surface feeder. Primarily consumes mosquito larvae, small insects, zooplankton, and detritus.
Behavior
Highly active and often found in small schools near the water surface. They are hardy and can tolerate low oxygen and high temperatures.
Reproduction
Livebearers (ovoviviparous). Females give birth to live young after a gestation period of about 24 days, producing multiple broods per season.
Conservation Status
Least Concern (IUCN). Population is stable and expanding globally as an invasive species.
Identification Tips
Look for the superior mouth, large eye, and rounded caudal fin. Introduced populations often lack the vibrant colors of tropical fish.
Interesting Facts
These fish were widely introduced globally in the early 20th century as a biological control method for malaria by eating mosquito larvae.
Identified on: 5/20/2026
Mode: Standard