Photo courtesy of UC Davis, Cooperative Extension

The Contra Costa Mosquito & Vector Control District raises 1 million mosquito-eating fish every year to distribute for free to members of the public for use in backyard ponds and water sources where mosquitoes can lay their eggs. The District also uses these fish as the main tool to prevent mosquitoes in abandoned and neglected swimming pools. Since the housing crisis began in late 2007, Contra Costa County has been home to thousands of neglected swimming pools that can produce millions of disease-carrying mosquitoes. The mosquitofish (Gambusia affinis) is highly effective at eating the young mosquito larvae, preventing them from developing into adults that can transmit diseases including West Nile Virus.

Each tiny fish can eat up to 500 mosquito larvae per day, proving an extremely valuable weapon in the fight against mosquito-borne illnesses! But the fish the District has used for decades is not native to California. Biologist Chris Miller oversees the District's mosquitofish program. For more than 20 years, he has raised the fish for distribution and District usage and conducts extensive research on other types of fish, namely native-California fish, in an effort to find a home grown version that might be just as effective at preventing mosquitoes.

Miller has recently accomplished a first of its kind event in the search to find that new fish. Let's take a closer look as we go Under the Microscope in the District's Fish Hatchery.

What is the big news on the current fish you are studying?

I'm taking a look at the California Roach (Lavinia symmetricus). It's a fish usually found in California streams. There are seven subspecies of California Roach. The District is studying the Sacramento/San Joaquin subspecies which is native to Contra Costa County and the surrounding area.

During eight weeks from April - June, 41 adult fish spawned in the District aquarium.  They produced 4817 larvae during 10 spawning events. It's very exciting because, to the best of my knowledge, this is the first time this species has ever spawned in captivity!

With such a rare occurrence, what special precautions did you take to increase the odds of success?

I used an egg trap and moved the eggs to a separate aquarium to prevent the adults from mistaking them for food. California Roach emerge from the eggs as larvae that swim to the surface of the water when they are about 8mm in length.  After they begin to swim it takes 61 days for the fish to become strong enough to transport. In their natural habitat they become fully mature and able to reproduce at two to three years old. However, I believe they might mature more rapidly in captivity. The California Roach train on a commercial diet quickly, and unlike the current mosquitofish, the Roach are not cannibalistic.

Once the Roach are about two to three months old, they average 36-41mm in length. At that time, we will stock them in neglected swimming pools to evaluate their ability to control mosquitoes.

If the current mosquitofish is so effective, why is the District at the forefront of research to find a replacement that is native to California?

The Gambusia affinis is effective at mosquito control, but they are not native to California so the District is careful to restrict their use to closed water sources. Because the Roach are native to California, if they turn out to be effective in mosquito control, we could potentially use them in many more locations, increasing our biological control of mosquitoes in Contra Costa County. Ultimately, the more you can use fish, it's possible you can reduce the amount of pesticides used against adult mosquitoes because the fish eat the larvae, preventing them from developing into adults.