Science Confirms Successful Strategy to Protect Threatened Steelhead from Virus

SEATTLE — A new and highly effective approach to control a viral pathogen that affects threatened steelhead trout in an Idaho hatchery is documented in a new paper. Researchers confirmed that the water supply from a reservoir could be used in such a way that juvenile steelhead were not exposed to river water at a time when infected adult steelhead fish contaminated the river water, drastically reducing mortality in juvenile fish.

The virus, infectious hematopoietic necrosis virus, infects both adult and juvenile fish but causes disease and sometimes death in juvenile fish. There is a well known form of this virus that is particularly deadly for steelhead trout. There are no treatments for fish infected with IHNV, and it can be a difficult pathogen to manage and a devastating one if juvenile fish start to suffer epidemic disease.

After experiencing severe losses to IHNV in the steelhead trout program at Dworshak National Fish Hatchery in Idaho for years, managers and fish health specialists from the U.S. Fish and Wildlife Service worked with researchers from the U.S. Geological Survey to identify the source of the problem and developed a strategy to control it.

“This virus has been devastating to our hatchery steelhead,” said Steve Rodgers, Manager for the Dworshak Fisheries Complex. “These losses have meant far less adult steelhead for harvest by sport and tribal anglers, which is the very reason the hatchery is here. This study will help us reach our goals.”

Analysis of virus samples taken from juvenile and adult fish in and around the hatchery provided evidence that the source of infection was from IHNV-infected adult trout in the river where the hatchery gets water for rearing juvenile fish. Based on this evidence, hatchery personnel worked with the Army Corps of Engineers, who own the facility, to modify the water delivery system. This modification allowed for extended rearing of juvenile steelhead trout on Dworshak Dam reservoir water, which does not contain adult fish infected with the virus. This strategy was implemented in 2010 and has resulted in an immediate and dramatic reduction in virus-caused death among Dworshak juvenile steelhead trout ever since.

“Our analysis provided evidence for how the new strategy worked. Since the virus and fish can adapt to their environment, the strategy may not always work. Now, the managers are better prepared if that ever happens,” said Rachel Breyta, virus ecologist with the USGS and lead author on the paper.

By conducting a genetic analysis of virus samples, USGS researchers confirmed that the reason this delayed exposure to river water strategy had worked was that adult steelhead infected with the highly virulent virus were no longer in the river acting as a transmission source by the time the juveniles were exposed to the river water. Since IHNV infected Chinook salmon were in the river at the later time and were able to transmit a less deadly form of the virus to hatchery juvenile steelhead trout, managers and fish health staff will continue to monitor IHNV in the Clearwater River.

“This long-term study, which confirmed returning adults shedding virus into the water source as the cause for all those lost juveniles in the hatchery, is critically important information,” said Rodgers. “I’m not aware of this being confirmed over a long period anywhere else as it has been here. We can apply this knowledge across other programs in the Columbia Basin, to maximize the value of our hatcheries in helping recover anadromous fish and maintain important fisheries.”

The paper “Successful mitigation of viral disease based on a delayed exposure rearing strategy at a large-scale steelhead trout conservation hatchery” was published in the journal of Aquaculture.