Fish stocking may transmit toad disease

Trout raised in hatcheries can pass a deadly fungus to amphibians, researchers have found. The discovery bolsters suspicion that stocking lakes for sportfishing could be contributing to a global decline in amphibian populations.

A breeding pair of western toads, Bufo boreas. Kiesecker/Penn State

Since the 1980s, for example, the numbers of western toads have plummeted throughout much of their range in the western United States and Canada.

A few years ago, Joseph Kiesecker at Pennsylvania State University in State College and his colleagues got a glimpse of one cause of western toad mortality. They found that some stocked lakes had high concentrations of a fungus called Saprolegnia ferax. This microbe produces a fish disease that’s common worldwide and a huge problem in fish hatcheries.

Saprolegnia coats toad embryos with a lethal fuzz. Up to 90 percent of the toad embryos die at sites experiencing Saprolegnia outbreaks, Kiesecker says. However, whether the fungus killing the toad embryos comes from fish or another source has been unclear.

In the August Conservation Biology, Kiesecker and his colleagues report that Saprolegnia-infected fish can indeed transmit the fungus to toads in an aquarium.

To demonstrate this transmission, the researchers used an aquarium divided by mesh through which the fungus could pass. On one side, they placed Saprolegnia-infected trout; on the other, toad embryos. Fifteen percent of the embryos contracted the fungus and died.

While proving that fish can transmit the fungus to amphibian embryos, the data also raised questions. The laboratory infection rate was, in Kiesecker’s words, “nothing in comparison to what we see in the field.” He and his colleagues have suggested that the sun’s ultraviolet light may exacerbate Saprolegnia infections in mountain lakes.

“It’s very important, and it’s really well done,” says Roland A. Knapp of the new research. Knapp conducts amphibian surveys with the University of California, Santa Barbara’s Sierra Nevada Aquatic Research Laboratory.

Still, Knapp cautions, “it remains to be seen how general [the new findings] are for amphibians.” In contrast to the Oregon sites surveyed by Kiesecker, he has seen little amphibian mortality due to Saprolegnia in the Sierra Nevada.

Knapp and others have observed that fish kill amphibians in a far more direct way. “Fish eat amphibians,” says Jeff Ziller, a fish biologist with the Oregon Department of Fish and Wildlife in Springfield.

However, the western toad is toxic to fish. For that amphibian, Saprolegnia is probably the lethal factor in stocked lakes, says Kiesecker.

Many once-fishless mountain lakes now contain self-sustaining trout populations. But Kiesecker cautions that removing fish may not totally wipe out the fungus. In his study, Saprolegnia-spiked aquarium dirt transmitted the fungus to western toad embryos. What’s more, Kiesecker has found such fungus-tainted sediments in mountain lakes.

Fish stocking has declined since the 1970s, in part because of concern over declining amphibian populations, says Ziller. For example, in central Oregon, half as many mountain lakes get stocked today compared with 15 years ago. In the Sierra Nevada, Knapp has seen amphibian populations bounce back aggressively in previously stocked lakes after the fish were removed.

Ziller says that the toad study could result in new standards for raising and distributing hatchlings.

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