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Spring fishing season arrives... and with it, amphibian diseases

Bait-shop trade likely source of pathogen spread in salamanders

Waterdogs, they're called, these larvae of tiger salamanders used as live bait for freshwater fishing.

With tiger salamander larvae, anglers hope to catch largemouth bass, channel catfish and other freshwater fishes.

They may be in for more than they bargained for: salamanders in bait shops in Arizona, Colorado and New Mexico are infected with ranaviruses, and those in Arizona, with a chytrid fungus called Batrachochytrium dendrobatidis (Bd).

"These diseases have spread with the global trade in amphibians,'" says James Collins, assistant director for biological sciences at the National Science Foundation (NSF). Collins is currently on leave from Arizona State University. "The commercial amphibian bait trade may be a source of 'pathogen pollution.'" Pathogens are disease-causing agents such as some viruses and bacteria.

Along with biologist Angela Picco of the U.S. Fish and Wildlife Service in Sacramento, Calif., Collins screened tiger salamanders in the western U.S. bait trade for both ranaviruses and Bd, and conducted surveys of anglers to determine how often tiger salamanders are used as bait, and how frequently the salamanders are let go in fishing waters.

The scientists also organized bait-shop surveys to determine whether tiger salamanders are released back into the wild after being housed in shops.

"We found that all tiger salamanders that ended up in the bait trade were originally collected from the wild," says Picco. "In general, they were moved from east to west and north to south--bringing with them multiple ranavirus strains."

Results of the research show that 26 to 73 percent of fishers used tiger salamanders as bait; 26 to 67 percent of anglers released tiger salamanders bought as bait into fishing waters; and four percent of bait shops put salamanders back in the wild after the waterdogs were housed with infected animals.

"The tiger salamander bait trade in the western U.S. is a good model for understanding the consequences of unregulated movement of amphibians and their pathogens," says Collins.

Examples of pathogen pollution are many and dramatic.

Europeans grazed cattle in African savannas, thereby introducing rinderpest, which resulted in massive losses of native African animals and changes to an entire ecosystem.

The import of Japanese chestnuts to the U.S. led to the introduction of chestnut blight, which nearly eradicated American chestnuts.

An international trade in and transport of infected timber spread Dutch elm disease throughout North America, Europe and Southwest Asia.

In the case of amphibians and reptiles, millions of kilograms of the animals may be shipped across the U.S. border each year.

"Many of them are not coming alone," says Picco. "They've got company: ranaviruses and Bd."

Waterdogs have been used as bait for at least 40 years. In 1968, 2.5 million tiger salamander larvae were sold as bait in the lower Colorado River area alone. Waterdogs in that one year were worth $500,000, equivalent to $2,766,489 in 2005 after adjusting for inflation.

"Since the tiger salamander bait trade isn't regulated or controlled in most areas of the western United States," says Picco, "there's no information about the number of individuals collected or traded annually."

She and Collins used the Web site to find a listing of bait shops in Arizona. Fourteen shops sold waterdogs, all of which were sampled in their study. The scientists collected 30 waterdogs per bait shop each month, or as often as the salamanders were available.

From March to October of 2005, 85 percent of Arizona bait shops sampled sold at least one ranavirus-infected tiger salamander.

In 2006, ranaviruses were detected in the tiger salamander bait trade between May and October in Arizona, New Mexico and Colorado, but were not found in the few bait shops sampled in Nebraska and Texas.

Three of nine shops tested in Arizona in 2007 had animals with Bd.

"If the presence of a pathogen in bait-trade salamanders is narrowed to several distributors, the movement of animals from these dealers could be stopped," says Collins.

"A quarantine program would help prevent the introduction of non-native pathogens into threatened, susceptible populations," he says. "Random monitoring of pathogen movement through the bait trade may help limit the spread of amphibian diseases."

Collins and Picco published a paper reporting the findings in Volume 22, Number 6,of the journal Conservation Biology.

Cheryl Dybas | EurekAlert!
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