About 250,000 rainbow trout died in a sudden disease outbreak at a southwestern Idaho fish hatchery, a loss of about 8 percent of Idaho's annual output of catchable-sized trout.

It was the second such outbreak of ichthyophthirius multifilis in as many years at the state Department of Fish and Game hatchery in Nampa. Officials say it likely resulted when stress from overcrowding weakened the fish, making them more susceptible to the parasite.

The outbreak happened in January, but became public this week because the state agency is trying to manage remaining stocks of 6- to 8-inch fish at its five other hatcheries to make certain lakes and streams still get enough fish to satisfy anglers.

Tom Frew, who manages the Nampa site, said careful manipulation of stocks at other facilities should make up for the losses. He said scientists are assessing just what went wrong.

One possible change to avoid future outbreaks, he said, might be to reduce the number of fish raised at the Nampa hatchery and increase it elsewhere.

"The parasite multiplies very rapidly," said Frew, who estimated the cost of the die-off at $40,000, including fish food and labor. "By the time we see symptoms, the disease has a pretty strong hold on the animal."

In all, the state produces about 3 million catchable-sized trout every year, among some 26 million total fish produced.

The parasites, commonly referred to as "ich," are visible as white spots on a fish's gills and skin. As their attack intensifies, fish "flash," or turn on their sides as they try to scrape off the bugs.

The fish become lethargic and eventually die. In the end, the parasites become so numerous on an infected fish's gills that it simply smothers.

In addition to the outbreaks in Nampa, a sudden thunderstorm last year washed debris-laden runoff into Idaho's Sawtooth hatchery near Stanley, weakening chinook salmon and making them more susceptible to the parasite, Frew said.

"Normally, they're capable of sloughing off the parasite," Frew said. "Anytime fish are in captivity, in the aquarium industry, or where the fish are in a closed system" there's a danger of an outbreak.

Nampa's hatchery has 10 raceways, all fed by artesian wells. The disease was found in all the raceways.

Due to the hatchery's design, it's not possible to empty the raceways of water to sterilize them, leaving the parasite present year after year.

Though hatchery officials haven't changed their fish-raising regimen in a dozen years, Frew said, the disease appears to have gained a more lethal toehold in 2006 and 2007.

"For some reason, the last couple of years, we've had some problems with ich at the Nampa hatchery," Frew said. "There's not really a lot we could do, without a complete rebuild of the Nampa hatchery."