The whirling fish kill

Rainbow Trout affected by whirling disease.">Rainbow Trout affected by whirling disease. In the early hours of a frigid March morning, a dozen men in waders and coveralls plunge into the icy raceway waters of the Bear Creek fish-rearing station in Accident, Md. Using large plastic baskets, they haul more than 50 pounds at a time of rainbow trout out of the cold water and into even colder air, where the fish are weighed and then tossed into a front-end loader, to be trucked to a plant and cooked down into animal feed. "An unusual load for you?" Bob Lunsford, director of fisheries for the Maryland Department of Natural Resources, asks the truck driver for Valley Protein, who responds, "Very. We usually only see chicken, cows, and horses." "Well, we'd rather not be giving these fish to you," says Lunsford, as foot-long rainbow trout flip around madly in the baskets and front loader's bucket. In a first for the Maryland fisheries department, more than 80,000 fish are being destroyed because they are infected with whirling disease, a parasite that infects the fish brain, disrupts their equilibrium and causes them to swim in circles. Lunsford estimates a loss of about 20% for the coming stocking season. Experts estimate that the disease first arrived in the United States in loads of European brown trout sometime in the 1950's. The disease has long been present on the East Coast, but it has been only a severe problem in the rivers and streams of western states such as Colorado and Montana. Myxobolus cerebralis, the metazoan parasite that causes the disease, has a complex life cycle. Spores of the parasite float in the water column and can survive freezing and desiccation in a stream for as long as 30 years. Eventually, the parasite's intermediate host, a common sludge worm (Tubifex tubifex) ingests the spores. While inside the worm, the parasite matures into the triactinomyxon (TAM) stage, the agent that is rereleased into the water and infects fingerling trout. "Rainbow trout are the most susceptible," says Jerri Bartholomew, a researcher at the Fish Disease Research Center at Oregon State University. "They're highly susceptible until about nine weeks old. During that time if they get a fairly heavy dose of parasite, they develop whirling signs and probably wouldn't survive in the wild. But if they encounter the parasite at a very low dose, or later than nine weeks, they can still become infected but not likely to show clinical disease signs. Those are the fish responsible for spreading it all over the place." Cases of whirling disease have been observed for at least a dozen years in Maryland, but rarely cause such a mass infection. In February, workers at the Mettiki fishery, a smaller rearing station about 15 miles from Bear Creek, started losing one or two fish a day, which was unusual since hatchery fish deaths are rare. The dead fish were sent to fisheries biologist Vicki Blazer, at the Leetown Science Center in West Virginia. Blazer conducted a pepsin/trypsin digest of the fish heads and found that they were packed with parasitic spores. A follow-up histology and PCR confirmed whirling disease. During the previous fall season, some trout had been moved from Mettiki to Bear Creek. Tests of trout at Bear Creek later showed that all but three of the 13 raceways had whirling-infected fish. Maryland fishery researchers are baffled by how a few raceways at Bear Creek staved off infection. Semiresistant lineages of Tubifex worms, natural variations in habitat conditions, and fishery stocking practice may all contribute to how severely a disease affects a given area. The dearth of rainbows in Colorado's rivers and streams has become so severe that in some locations scientists are introducing rainbow trout that have been identified as parasite-resistant, despite the risk of jeopardizing the indigenous rainbow trout population. Ron Hedrick, an aquaculturist at the University of California, Davis, suspects that the more rare instances of whirling on the East Coast may make workers less vigilant. "I'm wondering if in the East it's not as much on the radar screen," Hedrick says. "In Maryland, it looks like the infections were good infections and heavy infections. Precautions may have slipped through [the cracks]." By the end of the day at Bear Creek, a dozen more fisheries workers had arrived from all over the state and loaded 35,574 pounds (nearly 18 tons) of rainbow trout into the truck. Alan Heft, a statewide biologist for Maryland fisheries service, recorded by hand the weights of each basket of fish being moved from water to truck. "For these fishery guys, [raising fish] is what they do," he says. "So it's tough for them." Watching all their work from last year get thrown away is nothing short of "heartbreaking."

The whirling fish kill

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