A sudden outbreak of disease has claimed a tenth of Idaho's trout population and a loss of $40,000, including fish food and labor.
The outbreak of Ichthyophthirius multifilis has killed around 250,000 trout from a hatchery in southwestern Idaho- the state Department of Fish and Game hatchery, in Nampa. This is the second such outbreak in the region and official say that it probably resulted when stress from overcrowding weakened the fish, making them more susceptible to the parasite.
AdvertisementThe outbreak, which began in January, was made public only this week because the state agency is trying to manage remaining stocks of 6- to 8-inch fish at its five other hatcheries. This is in order to make certain lakes and streams still get enough fish to satisfy anglers.
According to Tom Frew, who manages the Nampa site, careful manipulation of stocks at other facilities should make up for the losses. He says scientists are assessing just what went wrong. One possible change to avoid future outbreaks, he suggests, might be to reduce the number of fish raised at the Nampa hatchery and increase it elsewhere.
Says Frew:"The parasite multiplies very rapidly. 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 them.
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.
"Normally, they're capable of sloughing off the parasite", says Frew. "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. An unfortunate fact is that due to the hatchery's design, it is not possible to empty the raceways of water to sterilize them, which leaves the parasite present year after year.
Though hatchery officials haven't changed their fish-raising regimen in a dozen years, the disease appears to have gained a more lethal toehold in 2006 and 2007, according to Frew. "For some reason, the last couple of years, we've had some problems with ich at the Nampa hatchery. There's not really a lot we could do, without a complete rebuild of the Nampa hatchery", Frew was quoted.