Concurrent Sessions C: A Cooperative Approach to Solving Fish Habitat Recovery - Malad River Fish Passage-Restoration of a Fluvial Life History for Resident Rainbow Trout

The Malad River supports one of the few self-sustaining, tributary populations of fluvial rainbow trout with connectivity to the middle Snake River, Idaho. Construction of two diversion dams divided the river into three reaches each approximately 1.6 km in length and created upstream migration barriers. It is believed that the Malad River likely provided spawning and juvenile rearing habitat for fluvial redband trout that would migrate from the Snake River to the Malad Riverseasonally to spawn. Recent research by Idaho Power Company (IPC) revealed that large(>35 cm total length [TL]) non-native rainbow trout introduced to the Snake River through past hatchery plantings used the lower reach of the Malad River for spawning and displayed a fluvial life history. Rainbow trout above the first diversion were smaller in size with few trout >25 cm TL. Restoring a fluvial life history strategy to areas above the diversion dams was the management goal of the Idaho Department of Fish and Game. As mitigation for a new federal license by the FERC for the Malad River Hydroelectric Project, IPC proposed to construct fishways at both diversion dams on the Malad River to restore connectivity of the lower 4.8 km of the Malad River to the Snake River. The lower diversion fishway, completed in March 2008, is a vertical-slot weir ladder with a hydraulic length of 70.1 m and has 17 pools (plus the entrance pool) rising to a height of approximately 4 m. The vertical slot width is 22.9 cm with a 22.9 cm hydraulic drop per pool, resulting in a slope of 10 cm/m. The fishway was designed to withdraw about 1.98 m3/s (70 cfs) from the river at its upstream end; all but 0.34 m3/s (12 cfs) would be screened out of the ladder flow and provided as attraction water at the downstream end of the fishway. The high amount of water entering the upstream end of the fishway was intended to attract downstream migrating trout. Since becoming operational, a total of 32,391 wild rainbow trout have passed either upstream or downstream through the fishway. More than 22,000 of these trout have used the fishwayto migrate downstream towards the Snake River. Upstream passage through the fishway has totaled more than 10,200 since 2008 and during the last three years has averaged over 2,600 rainbow trout annually. On average about 200 of these trout are spawning-sized adults (> 30 cm TL) including an average of 119 that are larger, fluvial-sized trout (> 35 cm TL). Overall, most downstream migrating rainbow trout are juvenile-sized at 15-22 cm TL.Population surveys in the Malad River above and below the fishway during November 2011 showed high densities of rainbow trout in the reach above the fishway. These densities were similar to previous years of population sampling (without the fishway in place) even though more than 20,500 trout had migrated downstream out ofthe reach during the previous four years. In contrast, densities in the reach downstream of the fishway were lower than previous surveys, suggesting that downstream migrating fish are continuing their migration through the Malad River to the Snake River. Large, fluvial sized rainbow trout (>35 cm) continue to migrate upstream through the fishway each year during time periods consistent with observed spawning in the reaches above and below the fishway, suggesting a spawning migration. These movements, coupled with the high numbers of juvenile-sized fish migrating downstream through the fishway, indicate an increasing fluvial population of rainbow trout in the Malad/Snake rivers since the fishway became operational in 2008.
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