The tiny, silvery fish are being offered to residents and municipal authorities across the parched desert county, which has tens of thousands of swimming pools, and one of the highest foreclosure rates in the United States.

"The abandoned pools become a stagnant little swamp that breeds mosquitoes in the middle of a neighborhood," said John Townsend, Maricopa County Vector Control manager.

"It is an important public health issue to keep the mosquito population down ... and the fish are very effective at that," he added.

West Nile virus, which came to the United States from Africa in the late 1990s, is now endemic in the county. Severe cases can produce high fever, stupor, tremors and paralysis, and can prove fatal.

In the first three months of the year, 17,214 homes in Maricopa County were offered for foreclosure sales, a more than three-fold rise on the previous year, according to RealtyStore.com.

In addition, an unknown number of local homes were simply abandoned as the owners skipped out on their spiraling debts, many leaving untreated backyard pools to stagnate, local authorities say.

To meet a sharp increase in reports of green pools, Townsend said the county has begun a program to breed 40,000 mosquitofish this year using tanks at Phoenix Zoo, and is making the voracious minnows available countywide.

The fish are proving an attractive alternative to chemicals or pesticides for some local authorities, facing a sharp rise in reports of abandoned neighborhood pools they have to treat as the mortgage crisis deepens.

"Chlorine is not effective as it is burned off by the sun in a couple of days, and we would have to go back and treat it again," said Daniel Anderson, a city official in Chandler as he released a glittering shoal of the fish into one murky backyard pool of one abandoned home.

"Once these fish are in the pool, we are not concerned about mosquitoes until someone either buys the house or the pool dries up."

(Reporting by Tim Gaynor; Editing by Sandra Maler)