Transfer of a Pathogen from Fish to Amphibians

Ecological studies of exotic species focus primarily on how 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 ferax, that has been associated with embryonic mortality of amphibians in the Cascade Mountains of Oregon, US.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 hatcheryreared trout not experimentally infectd 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 introducin g hatchery-reared fishes, we suggest that fish used in stocking programs could be an important vector for diseases responsible for amphibian losses. Transferencia de un Patogeno de Peces a Anfibios Resumen: Los estudios ecologicos de especies exoticas estan enfocados principalmente en el efecto de los invasores sobre ciertas especies residentes. En contraste, se sabe poco de los efectos de especies introducidas sobre comunidades nativas, incluyendo la dispersion de patogenos por las especies introducidas. Proporcionamos evidencia que sugiere quepeces introducidospueden ser el vector de un oomicetopatogeno, Saprolegnia ferax, que se ha asociado con la mortalidad embrionaria de anfibios en las montanas Cascade de Oregon, E. U.A. En experimentos de laboratorio, la mortalidad inducida por S. feraxfue mayor en embriones de sapo (Bufo boreas) expuestos directamente a truchas arco iris fOncorhynchus mykiss) criadas en granjas y experimentalmente infectadas con S. ferax y con truchas no infectadas experimentalmente que en los embriones control. Los embriones tambien desarrollaron infecciones significativas con S. ferax cuando fueron criados en sitios expuestos a truchas infectadas experimentalmente con S. ferax Mas aun, los embriones expuestos a S. ferax aislado de sitios con brotes comunes de Saprolegnia, presentaron mayor mortalidad en comparacion con embriones expuestos a S. ferax aislado de sitios en los que no han ocurrido brotes de Saprolegnia. Dada la introducci6n generalizada de peces criados en granjas, sugerimos que lospeces utilizados en esosprogramas pudieran ser vectores importantes de enfermedades responsables de la p6rdida de anfibios. Introduction exotic species have been linked to the displacement of native species (e.g., Zaret & Paine 1973; Petren et al. The introduction and spread of exotic species poses a 1993; Gamradt et al. 1997; Kupferberg 1997, Kiesecker threat to ecological communities and global biodiversity & Blaustein 1997a), modification of trophic structure (Elton 1958; Drake et al. 1989; Lodge 1993). Introduced (Wormington & Leach 1992; Holland 1993; Nicholls & Hopkins 1993), and alteration of ecosystem function (ViPaper submitted April 12, 1998; revised manuscript accepted Notousek 1989). The introduction of exotic species into vember 8, 2000. freshwater systems may be of particular interest because