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Hatchery Fish Implicated in Frog Decline

A discussion on the NANFA e-mail list, introduced by Jay DeLong (WA), reinforced the reasons why we can't release any fish back into the wild after it has been maintained in captivity: frog and toad populations can suffer. A decline in anuran (frog and toad) populations has been noted around the world. Many theories for the decline, ranging from disease to habitat degradation, have been discussed with varying degrees of blame attributed to each. A team of researchers from Oregon and Pennsylvania State Universities has thrown a new theory into the mix. The new theory involves a fungus, Saprolegnia ferax, being fatally spread to anuran populations by hatchery reared fish. A similar idea was introduced to explain the decline of Australian rain forest frogs where the most likely culprit was a virus introduced via imported aquarium fish. The fungus appears to be on the fish's scales or in their mucus layers and is introduced when the fish are released. Laboratory research showed a 15 percent increase in toad egg mortality when toad eggs were exposed to infected rainbow trout. The fungus lives on the outer membrane of the toad embryo and as it grows, it ruptures the membrane and smothers the embryonic toad. How big of a potential problem is this? Hatchery reared fish have been introduced into nearly half of the 16,000 mountain lakes in the western U.S., including a large number of lakes in national parks and wilderness areas! The popularity of fishing makes it difficult to discontinue stocking with hatchery reared fish, but this is one of those areas where biologists and economists and everyone involved must examine the potential environmental costs of their activities.

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