Adaptation of Traditional Habitat Improvements in Reservoir Drawdown Zones to Improve Survival of Endangered Species Act (ESA) Listed Fish During Their Migration in Reclamation Reservoirs
* Can habitat improvements in the drawdown zone of a reservoir improve survival of migratory fishes? (Drawdown zones in this proposal refer to any area of the reservoir that is seasonally dewatered due to reservoir operations.)
Many fish found in "lake" environments rely on an annual migration from reservoirs into rivers or streams to search out suitable spawning habitat. Often, fish migrating out of or back into a reservoir encounter degraded conditions during their migration due to a lack of cover in the drawn down portion of the reservoir. A river channel flowing through the drawdown zone is typically void of structure that provides security, resting, and thermal cover. Fish returning to a reservoir after spawning, like the Endangered Species Act (ESA) listed bull trout, may be physically weakened due to the physiological demands of spawning. Without deeper pools, in-stream structures and habitat diversity these fish are less likely to find prey and are themselves more susceptible to predation.
Need and Benefit
An initial literature search found no previous work that has been done to improve habitat conditions in rivers that flow through the drawdown zone of reservoirs. Past work in the Pacific Northwest Region has documented effects to ESA-listed fish due to the lack of fish cover in these areas. Furthermore, the U.S. Fish and Wildlife Service (FWS) has required Reclamation to address habitat loss due to reservoir drawdowns (FWS 2005 Biological Opinion). We would like to find solutions to this issue within the range of current operations, test these solutions on Reclamation reservoirs in the PN region and if solutions are found offer recommendations to other Reclamation facilities for implementation.
Minimum habitat availability in drawdown zones of reservoirs has been documented in virtually all reservoirs. Specifically, in the Boise and Payette river basins, radio telemetry studies conducted on ESA-listed bull trout have shown increased mortality in the river flowing through the drawdown zone of reservoirs. In Arrowrock Reservoir Hosteteller (2005) documented that 17 percent of the radio tagged bull trout were killed by predatory birds and at least two other bull trout were suspected to have suffocated when the sandy shore they used for cover collapsed on top of them. In Deadwood Reservoir, 40 percent of the radio tagged bull trout were recovered as mortalities in the dewatered section of the reservoir during 2007 (Prisciandaro 2007). Predators were thought to have killed the majority of these recovered mortalities.
Habitat improvements have been used under a wide range of conditions to improve habitat complexity and provide features necessary for fish to survive. However, habitat improvements are primarily made to "lake" or "river" environments; the drawdown zone in a reservoir is a "lake" habitat during part of the year and a "river" habitat during part of the year. Furthermore, the drawdown zone is also an area where silt and sand accumulates from the entering river or streams, so application of traditional habitat improvements may not be applicable without changes. The goal of habitat enhancements in the drawdown zone would not be to provide conditions that would attract fish year round rather to provide cover for fish as they migrate through.
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