Origin of fish disease a mystery for Alaska biologists

Posted: Sunday, June 10, 2007

ANCHORAGE - How the dangerous pathogen Myxobolus cerebralis got into Alaska's fisheries is something scientists may never know.

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Equally unknown is the significance of the appearance here of the metazoan parasite that afflicts trout and salmon with what is commonly known as "whirling disease."

Clearly, though, M. cerebralis - usually called "Mc" for short - has been in the 49th state for some time, said chief state fisheries pathologist Theodore Meyers.

Only when DNA detection techniques entered the picture did the Alaska Department of Fish and Game discover, to the surprise of Meyers and others, that rainbow trout at the Elmendorf State Fish Hatchery were carrying M. cerebralis spores.

"After years of negative results," the agency noted in a press release earlier this year, "a new, much more sensitive molecular test based on DNA has detected the parasite."

None of the infected fish showed any outward signs of whirling disease, which causes fish to swim erratically, or whirl. Whirling makes them difficult to feed and vulnerable to other predatory fish, birds and mammals.

Whirling disease has decimated some rainbow and cutthroat trout populations Outside.

In Yellowstone National Park, for example, whirling disease has been blamed for the decline in populations of native trout that spawn in Yellowstone Lake tributaries. One of those tributaries, Pelican Creek, saw more than 10,000 Yellowstone cutthroats killed by a severe whirling disease infection.

Other Western populations of trout have been equally hard hit. Colorado was so concerned it spent more than $10 million to clean up its hatcheries after 11 of them showed signs of contamination.

To date, Mc has been reported in 24 states including Alaska and caused severe declines in native trout populations in many of the West's best trout streams.

"The heads of diseased fish may contain up to 2-3 million spores of Mc," Fish and Game noted. "The Elmendorf Hatchery rainbow trout samples contained an estimated 100 to 1,000 spores, a level too small to be detected with standard microscopic tests and too small for the fish to show any signs of the disease."

Meyers isn't sure why the Mc has failed to flourish in Alaska, but he has some theories.

"It depends a lot on the age of this fish and water temperature," he said. "We're on the very edge, possibly, of the range of the parasite."

Cold water may have protected Alaska fish.

There are a number of ways the parasite could have arrived in Alaska:

• Carried north along with fish by federal officials active in stocking when Alaska was still a territory;

• Imported to the Elmendorf hatchery with the last of the rainbow stocks brought from the Lower 48 about 30 years ago.

• Stuck to the waders of a tourist angler from Outside. The spores are extremely hardy.

• Arrived from elsewhere in the state hatchery system.

"The Fort Richardson Hatchery is negative," Meyers said, "and Fort Rich is on well water."

Elmendorf Hatchery, on the other hand, uses water from Ship Creek. All sorts of fish have been dumped into Ship Creek over the years, including the king salmon that now support the state's most visible king salmon sport fishery. Tens of thousands of anglers have gone wading there.

A parasite that enters the heads and spinal cartilage of fingerling trout, Mc is a relatively new import to North America. A native of the Eurasian continent, it is thought to have been introduced to American waters in the 1950s.

Though the disease can infect salmon, it has done the most damage to native trout populations. The parasite rapidly multiplies in a trout's brain and spine, putting pressure on the fish's organ of equilibrium. As a result, the fish starts to whirl in a piscatorial version of the way people stagger when drunk.

Fish that survive are usually left with skeletal deformities but can successfully reproduce without passing on the disease.

Brown trout, another European import, have a resistance to Mc. They become infected but seldom suffer system breakdowns. As a result, the browns - common to many Lower 48 waters but absent from Alaska - serve as ideal carriers for the disease.

When they finally die, thousands to millions of spores are released into the water. These spores are amazingly resilient. They have been known to survive in some streams 20 to 30 years.

But to grow, Mc spores need an intermediate host - one of a number of tiny aquatic worms known as Tubifex tubifex. The worms must ingest the spores before they can again infect trout fry.

As part of a national monitoring effort for whirling disease, biologists went looking for Tubifex tubifex in Alaska years ago.

The worm was absent from Southeast Alaska but found in Southcentral.

"In the Anchorage area," Meyers said, "it was found in Ship Creek. It was found in Campbell Creek. It was found in the Lower Kenai.

"These worms were tested," he added. "They were taken back to the lab, and we found no trace of the parasite.

"They (also) tried infecting the worms with a laboratory strain of the parasite," he added, and only one strain of Tubifex tubifex found in Alaska appeared capable of harboring the spores and then passing them along.

"We're lucky in many respects," Meyers said. "It may be here only at a very low level."

Still, state officials aren't taking any chances. Commissioner of Fish and Game Denby Lloyd has banned any further stockings of Elmendorf hatchery fish in creeks, streams or rivers. With the exception of already infected Ship Creek, Elmendorf fish will go only into landlocked lakes lacking natural spawning populations of trout or salmon.

That's bad news for stream anglers but good news for lake fishers.

"As a result (of Mc), over 94,000 hatchery fish that otherwise would have been destined for open watersheds will be transplanted into closed system in 2007," according to the Fish and Game press release, which added that "king salmon will continue to be stocked in Anchorage's Ship Creek, the most likely source of the parasite."

The stream has already received potentially contaminated discharge water from Elmendorf Hatchery.

"However, no Elmendorf Hatchery fish will be transferred to the Fort Richardson Hatchery," Fish and Game said. "The stocking of open systems will resume when the state builds a new, well-water-only hatchery near the existing Elmendorf facility in the next four to seven years, which will eliminate the potential of introducing Mc from Ship Creek water."

The hatchery was planned long before the Mc outbreak.

Meanwhile, Meyers said, the state agency is expanding the hunt for possible Mc contamination in waters across the state.

Neither the disease nor Mc-diseased fish pose any threats to humans, but because Mc is so hardy, the state is reminding anglers that live fish may not be transported or released into state waters without a special Fish and Game permit.

"If fish are cleaned in the field," the agency says, "clean them only in the waters from which they were caught."

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