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Rainbow Trout Suppression Plans on the South Fork Snake River

The South Fork Snake River (SFSR) supports the strongest remaining fluvial population of native Yellowstone Cutthroat Trout Ocorhynchus clarkii bouvieri (YCT) within their historical range in Idaho. The SFSR is one of only a handful of large rivers in the species’ range that supports a robust population of YCT. The primary goal for the SFSR, as directed by the public and the Idaho Department of Fish and Game (IDFG) Commission, is the preservation of the genetic integrity and population viability of YCT. In the late 1990s and early 2000s, Rainbow Trout O. mykiss (RBT) abundance increased in the main stem SFSR. Rainbow Trout and RBT x Cutthroat Trout hybrids (collectively RBT hereafter) are the biggest threat to the continued persistence of YCT in the SFSR because of risks through competition and hybridization. Abundance of RBT has increased significantly in recent years to the extent that RBT were twice as abundant as YCT in 2018 increasing the potential for hybridization and competition.

Since 2004, the IDFG and collaborators have implemented several YCT conservation management strategies in the SFSR drainage to support the viability and genetic integrity of these populations. In 2017, a statewide survey of anglers identified that management for native YCT was a top priority. This priority is a driver for the current Fisheries Management Plan which outlines the objective of limiting RBT prevalence to less than 10% of the trout species composition of the catch at the Conant monitoring reach measured during annual fall abundance surveys. The 10% threshold would return species compositions similar to those documented during the early 1990s.

Beginning in 2018, IDFG investigated the feasibility of using boat electrofishing equipment to suppress RBT abundance in the main stem of the SFSR, by electrofishing several isolated locations multiple times and observed the changes in the number of RBT caught each outing. Rainbow trout re-populated spawning areas, suggesting that a substantial number of RBT could be removed. In 2019, IDFG further investigated available equipment and staffing to estimate the number of RBT that a moderate level of suppression could achieve and the logistics it required. This effort resulted in the removal of approximately 6,000 RBT from the SFSR, which was encouraging and we assumed with additional efforts might provide a significant benefit to the YCT population.

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