Transfer of a pathogen from fish to amphibians

Ecological studies of exotic species focus primarily on hole invaders directly affect particular resident species. In contrast, little is known about the indirect effects of introduced species on native communities, including how pathogens may be spread by introduced species. We provide evidence suggesting that introduced fish may serve as a vector for a pathogenic oomycete, Saprolegnia fer-ax, that has been associated with embryonic mortality of amphibians in the Cascade Mountains of Oregon, U.S.A. In laboratory, experiments, mortality induced by S. ferax was greater in western toad (Bufo boreas) embryos exposed directly to hatchery-reared rainbow trout (Oncorhynchus mykiss) experimentally infected with S. ferax and hatchery-reared trout not experimentally infected than in control embryos. Embryos also developed significant S. ferax infections when raised on soil that was exposed to trout experimentally infected with S. ferax. Furthermore, toad embryos exposed to S. ferax isolated from sites where Saprolegnia outbreaks are common experienced higher mortality than embryos exposed to S. ferax isolated from sites where Saprolegnia outbreaks have not occurred. Given the widespread practice of introducing hatchery-reared fishes, we suggest that fish used in stocking programs could be an important vector for diseases responsible for amphibian losses.