The salmon die-off that began in May at Nimbus Fish Hatchery on the American River will not have as big of an impact upon hatchery production as first feared, largely due to the taking of measures by hatchery staff to curtail the spread of the IHN disease.

Department of Fish and Game staff originally estimated that the outbreak of IHN would kill 50 to 100 percent of the fish. However, the disease outbreak actually ended up killing 42 percent of the fish in the ponds and raceways.

Since the hatchery produces more chinooks than it expects to release, allowing for a problem like a disease outbreak, the hatchery was able to actually meet 75 percent of its mitigation goal of 4 million salmon. DFG staff released 3,002,600 “advanced fingerlings” from May through the third week in June, according to Bob Burks, assistant manager at the Nimbus Fish Hatchery.

“The fish pathologist observed the fish before they were released and said they were ready to go,” he said. “We did what we could to reduce stress among the fish, including spreading them out through the hatchery’s ponds and raceways. The die off could have been significantly worse than it actually was.”

During previous years, the hatchery has released anywhere from 4.1 to 4.6 million chinooks as they begin to smolt. Chinook salmon usually are planted at about six months of age when they are four to six inches long. The DFG puts them into trucks and plants them in Carquinez Strait between Crockett and Rodeo.

“The biologists say that they get better returns when the salmon are trucked down to the estuary and released into deep water in the strait,” said Burks. Although salmon are normally placed in the pen-rearing project of the Fishery Foundation of California to acclimate them to salt water and away from. predators, the pens weren’t ready in time for the smolt planting this year.

Outbreaks of disease are usually associated with low, warm water conditions – like the massive adult fish kill on the lower Klamath River where over 70,000 adult chinooks perished in the September 2002 when columnaris and ich, two bacterial diseases, spread rapidly among the fish in the high and warm water.

By contrast, the salmon in Nimbus Fish hatchery became stressed this spring when cold, raging flows from Sierra Nevada snowmelt put a high and unhealthy amount of air into the water. This resulted in an outbreak of “gas bubble disease” in the fish, as evidenced by the presence of gas bubbles in the eyes, gills and fins of the juvenile chinooks. The fish become stressed, just like when a diver contracts a case of the “bends” when they surface too quickly.

The gas saturation would have become fatal to some fish, but the massive fish mortality really started when the stressed fish contracted two diseases, cold water disease and IHN (Infectious Hematopoetic Necrosis). Cold water disease, a bacterial infection, can be treated with antibiotics. However, IHN, formerly called Sacramento River chinook disease, is a virus and is not treatable.

“The gas super saturation played a factor in stressing the fish,” emphasized Bill Cox, DFG pathologist. “By stressing the fish, it makes the pathogens such as IHN more virulent. However, it was definitely the IHN that resulted in the massive fish mortality. Virtually none of the mortality was caused by gas bubble disease.”

The resident rainbow trout raised in the American River Fish Hatchery, located adjacent to the Nimbus Hatchery, also suffered from the gas bubble disease. However, there was virtually no mortality there because IHN wasn’t present, according to Cox.

Hatchery fish are more vulnerable to the disease because they are crowded in a facility’s raceways and ponds. When one fish gets the disease, that fish spreads it rapidly to the rest of the population.

The fish pathologists have not determined the cause of the IHN outbreak and are still studying potential causes. It is believed that disease originated from (1) adult chinooks handled at the hatchery (2) chinooks or rainbow trout in Folsom or Natoma Lake.

In an effort to find out the cause of the infestation, Fish and Game pathologists Teresa Veek and a team of biologists went to Folsom Lake’s Granite Bay launch ramp to take samples on Saturday, June 15. They were hoping to get samples from at least 25 to 30 trout or salmon, but they were able to get only 17 fish, 16 rainbows and one chinook, according to Cox.

They were hoping for more chinooks to sample, because the chinooks are the best fish to do the tests on. However, a following trip to Folsom by DFG staff didn’t turn up any chinooks.

“We are still running tests on the fish samples,” said Cox, “but the tests so far have come back negative for IHN and we don’t expect to see it.”

In August or early September, the fish pathologists will meet with hatchery staff to develop bio-security controls for the hatchery to prevent any future outbreak of IHN. “Our goal is not have a recurrence next year – we don’t want to see over a million salmon killed again,” said Cox.

“We’re trying to be proactive as we can to stop future outbreaks of IHN,” added Burks. “We’re going to tighten up our procedures and probably restrict some access to the hatchery.”

Fortunately, steelhead haven’t been affected by the IHN outbreak. The DFG have already put this year’s fish into the ponds and raceways of the facility.

A total of 54,000 naturally spawning adult salmon and 2,842 grilse (2-year-olds) returned to the American River in fall 2005, according to Mike Healey, DFG fishery biologist, while 20,569 adult chinooks and 1780 grilse returned to Nimbus Fish Hatchery in 2005.

The good news is that there is plenty of cold water pool available for the chinooks when they move into the river to spawn in the fall. Even with sizzling hot air temperatures, the water in the hatchery, which comes from the bottom of Folsom Dam, has been 58 to 62 degrees.