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Scientists On the Trail of a Potential Salmon Killer

Nov. 9, 2011 | KUOW
CONTRIBUTED BY:
Ashley Ahearn

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  • At the Muckleshoot Hatchery coho salmon are returning to spawn after two years in the open ocean. That's a good time to test for viruses like iSA. credit: Ashley Ahearn
  • Eggs are harvested from a female and put in a bucket to be combined with salmon semen, or milt. credit: Ashley Ahearn
  • Milt! credit: Ashley Ahearn
  • Where the magic happens: the milt and eggs are set aside to incubate. credit: Ashley Ahearn
  • Male cohos turn bright red when they're ready to spawn. credit: Ashley Ahearn
  • Bruce Stewart of the Northwest Indian Fisheries Council takes an ovarian fluid sample. credit: Ashley Ahearn
  • Tissue samples from the spleen and kidney are also collected for analysis back at Stewart's lab in Olympia. credit: Ashley Ahearn
At the Muckleshoot Hatchery coho salmon are returning to spawn after two years in the open ocean. That's a good time to test for viruses like iSA. | credit: Ashley Ahearn | rollover image for more

SEATTLE — Canadian officials declared Tuesday that previous findings of the Infectious Salmon Anemia Virus in wild salmon in British Columbia were false. But many in the scientific community are calling for further testing because the virus has killed millions of farmed salmon in Europe, Chile and the east coast of North America.

Infectious Salmon Anemia is a multi-million dollar potential problem. But right now, it’s just that: a potential problem.

Scientists are scrambling to determine how real a threat it is to Pacific Northwest salmon. Bruce Stewart, a biologist with the Northwest Indian Fisheries Council, says that’s no easy task.

“The tests are so sensitive that you can pick up small copies of this virus and the test is prone to false positives,” he says. “You think you have a positive but it’s not or something else is triggering it.”

The type of testing that was done on the samples taken from wild salmon in British Columbia is called a “molecular-based assay.” This test looks at the genome or snippets of the genome of the virus and then tries to match that with existing samples of the virus from past outbreaks – like the ones that happened in Chile and Norway in recent years.

Canadian officials questioned the quality of the wild salmon samples and said that they’ve done extensive testing for ISA in their farmed salmon and all tests have come back negative.

So no one knows for sure if the virus is in Northwest waters but that doesn’t mean scientists and governmental officials are going to stop looking.

The key to confirming the ISA findings is getting a live sample of the virus in the lab, so scientists can study its intact genome.

That’s part of why Bruce Stewart is here at the Muckleshoot tribal hatchery.

The creek behind the hatchery churns with coho salmon, just returning from two years in the open ocean. While at sea, these hatchery fish have been exposed to the same viruses as wild salmon so they’re a good representation of what might be out there.

Hatchery workers net the salmon from the creek, their dappled red bodies flopping around madly.

A quick thwack on the head with a wooden baton and the salmon lies still. Then hatchery workers cut the eggs out of it’s belly.

Semen from the males is mixed with the eggs taken from the females and then set aside to incubate.

During this process 60 of these fish are set aside for Bruce Stewart. He’s hunched over a long table sticking a syringe into the abdomen of the female cohos and sucking out a clear liquid.

“So what I’m doing is taking ovarian fluid from each of these females,” he explains. “We know that ovarian fluid is a highly sensitive fluid for viruses.”

Stewart has been collecting samples like this, along with bits of kidney and spleen, at hatcheries around Puget Sound since the late ‘80s. Tribal, federal and state departments test between 35 and 40,000 fish around the region each year looking for all kinds of viruses and bacteria.

With the recent findings of ISA in British Columbia, Stewart says the testing will need to be ramped up because the standard tests he and his colleagues do aren’t designed to pick up ISA.

That means that the virus could already be here. It just might not have been detected.

“Could it be a non-virulent form that has been here for years, not causing any problems? That’s definitely a possibility,” Stewart says.

The infected salmon that were found in British Columbia were still alive – surviving with the virus. Wild salmon have been shown to be less vulnerable to the ISA virus, but they can carry it. Basically, viruses aren’t stupid. They know not to kill their host if there’s not another host nearby. So in the wild, scientists believe ISA tends to stay non-lethal.

But if that virus makes its way into a pen of farmed Atlantic salmon - Well, that’d be like Thanksgiving dinner.

With all those potential hosts, the virus can mutate out of survival mode and into kill mode, as it did in some north Atlantic salmon farms.

Researcher Jim Winton says this groups of virus is capable of adapting to other species and stocks and becoming more dangerous.

“So one thing we really need to know is if this virus is present in the West how dangerous is it currently and how dangerous could it become for wild stocks,” says Winton, chief of fish health research at the U.S. Geological Survey in Seattle.

The only way to find that out is to get that holy grail sample: the intact genome of the virus. Winton could then expose pacific salmon in his lab and see how they might react.

That would give him some idea about what this virus might be doing to wild fish in the open ocean. It would also tell him where this virus came from.

“We don’t know all of the methods by which viruses can move around the globe but as global trade increases, as aquaculture increases, there are going to be more cases of this sort of finding. Our detection technologies are getting better and we’re looking in places we didn’t used to look,” Winton says.

The Washington Fish Growers Association has expressed concern, but said that no ISA virus has been detected in their sampling.

If ISA shows up in samples like the ones Bruce Stewart collected at the Muckleshoot Hatchery, then fish farmers will most likely heighten their existing ISA testing and try to develop a vaccine.

But there’s not much that can be done to protect wild fish from the virus or prevent it from transferring to penned fish.

The lab results won’t be ready for over a month. Until then officials are gearing up for more surveillance of farmed, hatchery and wild salmon in these waters.

© 2011 KUOW
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