Global frog, toad, and salamander declines in recent decades have been linked to climate change, increased exposure to UV-B radiation, and pathogens (Environ. Sci. Technol. 1998, 32, 352A). This list of threats now includes hatchery-reared fish, which may be infecting toads in the Cascade Mountains of Oregon with a lethal fungus, according to new research from Joe Kiesecker, a biologist at Pennsylvania State University.

Mountain lakes in Oregon are annually stocked with more than 10.5 million rainbow, brook, and brown trout raised in hatcheries, Kiesecker says. Reared in unnaturally crowded conditions, the fish are susceptible to infections by the water fungus Saprolegnia ferax, which is lethal to western toad embryos.

Kiesecker and his colleagues hypothesized that western toad (Bufo boreas) embryos may pick up the fungus directly from infected fish or from lake and stream soil carrying fungus left by sick fish. They exposed toad embryos in the lab to healthy rainbow trout, infected trout, and trout raised in a hatchery. About 15% of the embryos died of fungal infections when exposed to infected and hatchery-raised fish, suggesting that seemingly healthy hatchery fish may transmit the fungus directly to toad embryos. Toad embryos exposed to experimental soil developed fungal infections only from soil that had been in contact with heavily infected fish.

While only 15% of the toad embryos in the lab developed infections, that number is expected to jump in the field, where low water levels are exposing eggs to stressful UV-B radiation, Kiesecker says. His previous research (Nature, 2001, 410, 681–683) has shown that more than 50% of toad embryos die of fungal infections in waters made shallow by climate-induced drops in rain and snowfall. (Conserv. Bio. 2001, 15, 1–8) —JANET PELLEY