Spring 2002 -- Vol. 19, No. 1

Fish Stocking May Spread Amphibian Disease

Above and left: Western toads, Bufo boreas. (photos: Joseph Kiesecker, Penn State)

New research shows that hatchery-reared fish can spread a fungus implicated in the mass deaths of amphibian embryos in the Pacific Northwest. This is the first evidence that fish stocking can spread amphibian diseases.

"Fish used in stocking programs could be important vectors for diseases responsible for amphibian losses," say Joseph Kiesecker of Penn State University in University Park and his colleagues in the August 2001 issue of the scientific journal Conservation Biology.

Historically, hatchery-reared fish were introduced to nearly half of the 16,000 mountain lakes in the western contiguous United States. Today, fish are still stocked in a number of national parks and wilderness areas. Fish stocking is common at Pacific Northwest sites with mass amphibian deaths, and the associated fungus (Saprolegnia ferax) is a common disease of hatchery-reared fish.

To determine whether fish stocking could spread the fungus to amphibians, Kiesecker and his colleagues collected rainbow trout from a fish hatchery and freshly-laid western toad eggs from Lost Lake, Oregon. Western toads have declined severely since the late 1980s, and up to 90% of the toad embryos have died at sites with Saprolegnia outbreaks.

Laboratory experiments confirmed that trout can spread the fungus to toad embryos: exposing the embryos to infected trout increased their death rate by about 15%. The researchers also found that trout can spread the fungus to soil, which can then infect toad embryos. This treatment also increased the embryo death rate by about 15%.

While the death rate of fungus-exposed embryos is much smaller in the laboratory than in the wild, this is due to the fact that being exposed to the fungus is not enough to cause an outbreak. Kiesecker and his colleagues had previously shown that UV-B radiation also plays a role in Saprolegnia outbreaks.

The researchers caution that discontinuing fish-stocking may not be enough to control diseases spread by introduced fish. "If introduced pathogens become established, effects could persist even after fish stocking has been discontinued," say Kiesecker and his colleagues.

Close-up photo of healthy western toad embryos (left) and embryos infected with Saprolegnia ferax (right). (photos: Joseph Kiesecker, Penn State)

~~ Robin Meadows, Society for Conservation Biology

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