Over 50,000 juvenile king salmon per day are dying as a result of disease spurred by high water conditions on the American River’s Nimbus Fish Hatchery in Rancho Cordova – and little can be done about it, according to Department of Fish and Game officials.

“We have lost 1.5 million of the 5 million fish that we are raising in the hatchery,” said Bob Burks,  assistant manager at Nimbus Fish Hatchery. “Before the disease runs its course, we could lose anywhere from 50 to 100 percent of the run.”

The fish mortality in the hatchery started when high flows from Sierra Nevada snowmelt put a high and unhealthy amount of air into the water. The problem, known as “dissolved gas supersaturation,” causes gas bubble disease in the fish, as evidenced by the presence of gas bubbles in the eyes, gills and fins of the juvenile salmon. The fish become stressed, just like when a diver contracts a case of the “bends” when he/she comes up too quickly to the surface.

Although the gas saturation would have become fatal to some fish, the real mortality started when the stressed fish contracted two diseases, cold water disease and IHN, according to Burks.

Cold water disease, a bacterial infection, can be treated with antibiotics. However, IHN (Infectious Hematopoetic Necrosis), formerly called Sacramento River chinook disease, is a virus and is not treatable.

IHN is found in the wild, but doesn’t cause the same problems in the wild that it does when it infests a hatchery. The hatchery fish are more vulnerable 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.

“IHN is mainly a threat to the hatchery system, rather than to wild salmon and trout,” said Mike Healey, DFG associate fishery biologist. “First, the wild fish are generally tougher and more resistant to the disease. Secondly, the crowded conditions found in the hatchery cause the disease to spread all at once.”

Burks, hatchery manager Terry West and other hatchery staff are working day and night to remove the dead chinooks from the runways and ponds, hoping that the disease will soon run its course soon.

The flows on the American are normally around 2,500 to 3,000 cfs this time of year, but flows have gone as high as 35,000 cfs this winter and spring. At press time, releases to the river below Nimbus Dam were 11,000 cfs.   

Fish pathologists from the DFG are currently trying to determine the cause of the IHN outbreak. While the causes are being studied, the hatchery ponds and raceways are closed to the public to prevent the public from making contact with hatchery water and spreading the disease to other facilities. The hatchery visitor’s center will remain open.

If people were to spread the IHN infestation to the DFG’s American River Trout Hatchery, located right next to Nimbus Hatchery, it would be a disaster for the DFG’s trout planting program in the Sacramento Valley region, according to Burks.

One possible source of contamination could be king salmon, kokanee salmon and rainbow trout in Lake Natoma and Folsom Lake. After a similar IHN infection hit the Feather River Fish Hatchery in Lake Oroville in the mid 1980s, the DFG stopped planting chinook salmon and brown trout in Oroville, since they were hosts for the IHN virus. The Department started planting coho salmon instead because they aren’t susceptible to the disease, according to Bill Cox, senior DFG fishery biologist.  

“At this point, we still don’t know the source of the IHN infection,” said Burks. ”If this occurs one time only and it doesn’t spread into other years, it won’t have that bad impact upon the salmon populations. However, if it continues in other years, it could become a real problem.”  

Healey noted that the salmonids put in Folsom and Natoma are free of IHN before they are planted into the lake, so he doesn’t think it’s likely that planted salmon and trout are the cause of the disease. More likely sources of the IHN are the sediments that were stirred up by high flows or the inadvertent human spread of IHN.

“I’m pretty confident that it isn’t the planted trout and salmon at Folsom that are causing the disease,’” said Healey. “The disease could possibly come from people walking around the hatchery who put their hands in the hatchery water after contact with river water.”

Bill Cox, the DFG’s senior fish pathologist, said there are two major areas of investigation in tracking down the source of the IHN virus - internal contamination at the hatchery itself and the water supply.  

Even though the adult salmon received at the hatchery are infected with IHN, the disease isn’t transmitted to eggs or young fish because the eggs are disinfected. “It’s very unlikely that the virus would be transmitted this way, since the eggs are treated, although a break in procedures is possible,” said Cox.

The second possibility is that the virus is in the water supply. “A number of fish species are susceptible to the virus, but the principal ones are chinook and kokanee and they go through a rigorous process to ensure they are free of the virus before being planted,” he stated. “But if there is a break in procedures, there is a possibility that infected fish were put in there.”

The DFG will be looking at rainbow trout, chinook salmon and kokanee salmon at Folsom to see if they detect the virus in the fish.  

This disease infection comes at a time when commercial salmon fishermen along the coast are reeling from the impacts of the most severely restricted salmon season in history, the consequence of low projected salmon returns this fall on the Klamath River caused by the Bush administration’s mismanagement of Klamath River water.  

The American River is no stranger to fish kills and disease infection. Usually low, warm water is the culprit, as during the autumns of 2001, 2002 and 2003 when pre-spawning mortality among adult fish was very high. In the worst year, 2001, a total of 87,626 adult king salmon - 67 percent of the run died before spawning.    

More recently, many juvenile steelhead in the summer and early fall became infected with “rosy anus,” a viral infection.  

In spite of the river’s water and disease problems, large numbers of chinooks have returned to the system in the last five years – and the majority of the fish found in the system are wild spawners.

In Fall 2005, 54,000 naturally spawning adult salmon and 2,842 grilse (2-year-olds) returned to the American, according to Healey. An additional 20,569 adult chinooks and 1780 grilse retuned to Nimbus Fish Hatchery in 2005.

During the previous year, 75,991 naturally spawning adult chinooks and 13,756 grilse ascended the river. The hatchery took in 12,741 adults and 13,659 grilse; an unusual year because the grilse outnumbered the adult salmon.

The impact of the high flows on juvenile king salmon naturally spawned in the river is yet to be seen. Many become stranded along the river and its floodplain during the high flows, but high flows also wash the fish downriver more quickly, away from predators in the river and Delta. However, the fish have a higher rate of survival when they migrate to salt water as smolts five inches or larger.

Nimbus Fish Hatchery and the American River are significant contributors to the Central Valley king salmon population, the most vibrant remaining chinook salmon stock on the West Coast. The four Central Valley hatcheries produce a total of 24 million salmon per year, with Nimbus contributing around 4 million salmon.

“We still don’t know what the high flows have done to the wild salmon that were in or emerging from the gravel in the river in December,” said Felix Smith, board member of the Save the American River Association and a frequent critic of U.S. Bureau of Reclamation water policy on the American and other Central Valley rivers. “Do they also suffer from over saturation of nitrogen, cold water disease or IHN? We do know that numbers of out migrating salmon found in the screw traps that the DFG operates have been significantly lower this year than in previous years.”

Smith noted that the reservoirs are as much cold sinks as they are heat sinks. On rivers below major dams, it takes longer for both the river to warm up and cool down. As a consequence, the fish are subject to diseases spurred by cold water, such as IHN and the cold water disease.