|
Native trout can be taking a bad turn
BY CHARLES GARRATT • STAFF WRITER
WARM SPRINGS - An invasive parasite that attacks young trout and has seriously impacted trout streams and rivers in other parts of the country has been found in a Virginia hatchery, but at this point, it's not considered a high risk for Virginia waters by Department of Game and Inland Fisheries officials.
Whirling disease is caused by a microscopic parasite, Myxobolus cerebralis. The parasite infects the very young fish of the salmonid family. So far, the disease has affected mostly rainbow trout, according the Whirling Disease Foundation. The Eurasian organism was introduced into North American waters in the 1950s and has spread around the country mostly by the movement of infected fish.
"M. cerebralis is virtually indestructible - the spore can withstand freezing and desiccation, and can survive in a stream for 20 to 30 years," according to the foundation. When infected fish die, the spores are released into the water.
Fortunately for Virginia streams, the parasite has a complex lifecycle that requires an alternate host. After the spores hatch, the parasite must infect a specific species of worm, Tubifex tubifex. From the tubifex worm, the parasite is released into the water in a form that attacks fingerling trout. It feeds on the cartilage of the young fish, reproducing rapidly and affecting the fish's equilibrium.
Infected young fish tend to swim in circles and have difficulty feeding, thus the name "whirling disease." Older infected fish will also have a black tail and distorted head. This symptom is seen mostly in infected hatchery-raised trout, according to foundation executive directory Dave Kumien.
"In the wild, the parasite kills juvenile fish," Kumien said. A whirling fish in the wild is a dead fish within a month, he added. In a hatchery, infected fish are able to survive longer and exhibit the other symptoms.
Whirling disease was found in the DGIF fish hatchery at Marion in the 1970s, says cold-water hatcheries manager George Duckwall. Duckwall and other DGIF officials say the state is "managing around the parasite."
The disease has "not been a big deal in Virginia," assistant director of fisheries division Ron Southwick said. "Once we learned the lifecycle of the parasite, it was easy to manage around," he added.
The parasite only infects fish under six inches. Marion has two water sources. An uninfected spring is used for trout under six inches and the infected creek is used for larger trout. Duckwall said even though trout are being exposed to the pathogen at Marion, none show any clinical symptoms of the pathogen.
Duckwall and Southwick say whirling disease has not been found in the wild even in waters stocked with fish from Marion. Southwick said the tubifex worm required for a stream to support the parasite is found in slow moving waters. The microscopic worms are also called "sewage worms," and can't survive in clean streams, he said.
That may not be the whole story, according to Kumien. In Montana, where he is located, and other western states, the disease has had a major impact. Tubifex worms do exist in some pretty pristine streams, he said.
High flows, seasonal scouring by flooding and clean water, are "not an absolute protection," said Kumien. Clear Creek in Yellowstone Park is a highimpact. gradient stream but the worms quickly reestablish after floods, allowing the parasite to flourish.
The population of cutthroat trout has been reduced 80-90 percent in Yellowstone streams and rivers, said Kumien. And the parasite continues to spread. The biggest thing for citizens to know, said Kumien, is "you can't see anything." Since the parasite kills very small and young fish, there aren't large dead fish along the banks. Only a skilled fisheries biologist can find the infected young and then only at certain times of the year, he said.
A local fisherman who returned two weeks ago from six weeks of fishing in Montana said the infected rivers are recovering from the infection. This is a common misconception, said Kumien. The Missouri and Madison rivers are both infected, said Kumien.
"The impacts are largely invisible to many fishermen," Kumien said. He added 10 percent of the anglers catch 90 percent of the fish, and good anglers are going to catch fish in infected waters like the Missouri and Madison and Gunnison rivers.
What anglers don't see is the overall In the Madison, he said, "Growth rates in adult trout are off by 30 percent." Mortality of fish up to 2-3 inches is as much as 70 percent. The biomass of trout in the river is half what it was before whirling disease hit. The biomass is a measure of the weight of all fish of all sizes within the river and is calculated by shocking and netting fish in sections of the river.
The Madison and other rivers are still wonderful fisheries, he added, but nowhere near what they were. "None of the infected rivers have recovered," he said.
DGIF officials insist stocking from Marion does not put Virginia waters at risk, nor do they think the alternate host tubifex worm can survive in large populations in the largely clean, rapid waters of Virginia.
The native eastern brook trout is highly susceptible to the parasite, said Kumien. Rainbow trout, though not native to eastern streams, are established in many places, including the Jackson River. Brown trout, from the same part of Eurasia as the parasite, are somewhat resistant.
Whirling disease is also a threat in fish hatcheries. Facilities like Coursey Springs with natural raceways stay infected once they become infected, said Kumien. Since the spore of the parasite lives so long, there is no way to clean up the facility.
"Lots and lots of hatchery closings and reconstructions" have happened around the country, said Kumien. Maryland is closing some hatcheries now, he added. Like Virginia, Maryland had been managing around the disease. The problems in the west were made much worse by poor hatchery practice, said Kumien. So far, none of those problems have appeared in Virginia, according to DGIF officials.
The hatchery at Coursey Springs, the state's largest producer of catchable trout, will be closed next spring and completely rebuilt. Coursey Springs is currently free of whirling disease and the new facility will be largely immune to infection.
Fishermen can help prevent the spread of whirling disease by not moving fish or fish parts from one body of water to another. This includes cleaning fish caught in one stream near another stream. Equipment, including boats and waders, should be cleaned before being used in another body of water.
The parasite, once in a stream, may take years to reach the level where it begins to kill fish in substantial numbers, warned Kumien. The only cure presently known is to never spread the spores.
Check, clean, dry: Stop the spread In order to prevent the spread of didymo, whirling disease, or another water-borne pest, folks swimming or fishing in bodies of water should do the following: Check: Before leaving the river, remove all obvious clumps of algae and look for hidden clumps. Leave them at the affected site. If you find any later, do not wash them down drains. Treat them with the approved cleaning methods below, dry them and put them in a rubbish bin. Clean: Soak and scrub all items for at least one minute in either, hot (60°C) water, a 2 percent solution of household bleach or a 5 percent solution of salt, antiseptic hand cleaner or dishwashing detergent. Dry: If the above cleaning is not practical, after the item is completely dry to touch, wait an additional 48 hours before contact or use in any other waterway.
| 
