OLYMPIA - An estimated 75,000 to 90,000 yearling Twisp River spring chinook salmon died April 13-14 when debris borne by heavy rains clogged water intake screens at a remote salmon rearing pond site, according to the Washington Department of Fish and Wildlife (WDFW).
The fish were a significant portion of the annual hatchery production of Twisp River spring chinook. Chuck Johnson, WDFW area hatchery manager, said roughly 10,000- 25,000 chinook remain alive in the pond, and about 50,000 more fish are at the Department's nearby Methow Hatchery.
In addition, there are estimated to be hundreds of thousands of juvenile chinook in the Twisp River system, the progeny of naturally-spawning chinook. Both the hatchery and naturally-produced fish are listed as "endangered" under the federal Endangered Species Act.
Johnson said it's too early to gauge how much of an impact this fish loss will have on upper Columbia River spring chinook recovery efforts.
"Fortunately, chinook salmon don't all return to spawn at the same ages," Johnson said. "They can come back as three, four or five-year-olds, so we could have some Twisp River spring chinook returning to the system at the same time that these fish would have, which lessens the blow of the loss for this year's class."
Johnson said water flows in the Twisp River increased 300 percent overnight on April 13-14 as a result of heavy warm spring rain and melting snow, unleashing a flood of debris that clogged the gravity intake screen at the acclimation pond. The debris caused the river intake screens that provide river water to the pond to plug.
The alarm system to warn WDFW staff of such an event failed. The fish kill wasn't discovered until the morning of April 14 when hatchery staff arrived to feed the fish and maintain the ponds.
The fish, which are typically kept in the acclimation ponds for a month to 45 days, were scheduled to be released into the Twisp River beginning Monday, April 15.
"This loss is a blow to the entire hatchery staff," said Guy Weist, WDFW hatchery specialist at the Methow Hatchery. "We have nurtured these fish for the better part of a year and a half, and to lose such a large number of them so close to their release is devastating to all of us."
Twisp River adult chinook are collected as they return to spawn. The fish are spawned, the eggs are incubated in the nearby Methow Hatchery, and juveniles are moved to remote acclimation ponds to grow and imprint on the river's unique water "signature," ensuring that the fish will journey back to the Twisp River when they return to spawn as adults.
The exact cause of the alarm system failure has not been determined. Johnson said the type of water flow alarm system used at the remote acclimation ponds is a simple, time-tested mechanism that monitors water levels through the pond intake system and automatically phones hatchery staff if a problem is detected.
"While this type of alarm system is widely used and has been extensively tested for reliability, remote sites are by their definition vulnerable to these kinds of problems," Johnson said. "We don't have the resources to staff a caretaker up there 24 hours a day, seven days a week, so we have to rely on an alarm system to monitor conditions around the clock."