A single-celled parasite has ravaged an endangered fish species that is reared at the Willow Beach fish hatchery about 12 miles downriver from Hoover Dam. Hatchery manager Mark Olson said a massive die off of razorback sucker fish was first detected in late October.

“We’ve lost an estimated 30,000 fish,” Olson said. “We still have some that are infected and we’re trying to save them, but it’s a tough customer. It’s hard to kill.”

Olson said the parasite, Ichthyophthirius multifilliis, commonly called “ich” likely reached the hatchery raceways from the Colorado River. He said formalin is being used to kill the parasite when it is free swimming, though he noted the application does not work once the parasite attaches to fish.

Olson said the 51,000 rainbow trout fingerlings that were planted in other raceways at the hatchery are so far uninfected.