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Western Mosquitofish

Western Mosquitofish

Western Mosquitofish are small, non-native live-bearing fish that have been established throughout California since their introduction in 1922 from Texas for mosquito control. These hardy fish are found in 67 different watersheds across the state.

Scientific Name: Gambusia affinis

Physical Characteristics:
  • Adults typically reach 1.5-2.5 inches in length, with females being larger than males.
  • They have dull gray or brown coloration with black-outlined scales, small black spots on dorsal and caudal fins, and distinctive teardrop markings below the eyes.

What to know

Habitat Distribution

  • Found in 67 different watersheds across the state.
  • They inhabit shallow, stagnant ponds, lake edges, slow streams, brackish sloughs, and disturbed low-elevation waters with aquatic vegetation, often staying near the surface where oxygen levels are highest.

Key Locations

  • Sacramento River system, San Francisco Bay, San Joaquin Delta, coastal lagoons including Los Penasquitos, Newport Bay, Ballona Marsh, Malibu Lagoon, and Morro Bay, and inland waters like Lake Tahoe, Crowley Lake, and the Salton Sea.
  • Coastal locations like lagoons and estuaries near freshwater inflows are productive, especially in areas like San Francisco Bay, Newport Bay, and southern California coastal waters.

Specific Spots

  • Shallow, vegetated areas along pond edges, slow-moving stream margins, and backwaters.
  • In urban areas, ornamental ponds, abandoned swimming pools, and water troughs can hold good populations.
  • Focus on shallow, sunlit portions of ponds and along vegetated shorelines where they concentrate.

Western Mosquitofish FAQ

Answers from onWater's species guide.

What flies work for Western Mosquitofish in California?

Tiny dry flies including size 18-22 midge patterns, small ant patterns, and micro terrestrials that imitate their natural diet of insects and zooplankton.

When is the best time to fish for Western Mosquitofish in California?

Most productive from October through March, avoiding their April-September breeding season. Winter fishing can be challenging as they become less active and move to deeper water, though they remain catchable in thermal areas and spring-fed waters that maintain warmer temperatures.

How do you catch Western Mosquitofish in California?

Fish slowly in shallow water. Early morning and late afternoon often provide the best action when they're actively feeding at the surface.

What do Western Mosquitofish eat?

These opportunistic surface feeders consume mosquito larvae, algae, zooplankton, terrestrial insects, and various invertebrates, choosing the largest food items they can swallow.

Where do Western Mosquitofish live in California?

Found in 67 different watersheds across the state. They inhabit shallow, stagnant ponds, lake edges, slow streams, brackish sloughs, and disturbed low-elevation waters with aquatic vegetation, often staying near the surface where oxygen levels are highest.

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