Spatial and temporal entrainment of fish from Hauser Reservoir, Montana
Spinelli, Justin Philip
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Management of fish populations in Hauser Reservoir, Montana, is hindered by undesirable and unpredictable downstream entrainment of fish through Hauser Dam. My objectives were to estimate spatial and temporal entrainment of fish larger than 100 mm total length through Hauser Dam and identify environmental and operational conditions influencing entrainment. I quantified entrainment using hydroacoustics at the turbine intakes from July 2007 to November 2008 and the spillway from May to July 2008. Species composition was characterized using multiple netting gears. I investigated the relationships between entrainment and conditions in the reservoir and at the dam using multiple linear regression. Total entrainment was 145,470 (95% CI = 138,144 - 152,796). About 60% of entrained fish were smaller than 220 mm. Annual entrainment from summer to autumn was higher in 2007 (N = 79,031; 95% CI = 73,861 - 84,201) than in 2008 (N = 52,513; 95% CI = 47,830 - 57,196). I applied species composition by size to the hydroacoustic data to identify entrained fish species, but many fish (N = 55,529; 95% CI = 50,337 - 60,721) could not be reliably assigned to species because concurrent net catches did not include individuals of similar size. Total entrainment was mostly made up of rainbow trout (33.3%) and walleye (30.2%). Spillway entrainment was 16% of total entrainment and was correlated with spillway discharge; turbine entrainment was not. Turbine entrainment increased from summer to autumn in both years, probably in response to autumn turnover and releases of hatchery rainbow trout. On average, 9.0% (SD = 1.2%) of hatchery fish were entrained soon after being stocked in the reservoir. Most regression models were equally ranked using Akaike Information Criterion corrected for small sample size indicating that a combination of conditions were influencing entrainment. Spatial and temporal patterns of entrainment at Hauser Dam were typical of other facilities in that entrainment varied in response to a changing combination of operational and environmental conditions. Identification of these patterns of entrainment allows managers to evaluate effects to the reservoir population and make more informed decisions.