Overwintering Use of Spring-Flow Tributaries by Age-0 Rainbow Trout

The Henrys Fork of the Snake River in Idaho contains world renowned rainbow trout fisheries. The number of rainbow trout in the Henrys Fork for a year-class is primarily determined by how many age-0 rainbow trout survive their first winter. Heretofore, most research and restoration of overwintering habitat for age-0 rainbow trout has focused on the mainstem river. We evaluated whether four spring-flow tributaries are used for overwintering habitat by age-0 (< 175 mm) rainbow trout using a combination of fish traps, seasonal electrofishing, and fish marking/tagging. In the largest tributary (Buffalo River; 5.66 m³/s base water flow), an average of 2,200 age-0 rainbow trout have immigrated annually (2006 to 2010) through a fish ladder near the mouth, 84% during September to December. Fish marked during this immigration were subsequently recaptured in an outmigrant trap the following spring, along with other unmarked fish. In the smaller tributaries (creeks #1 to #3; 0.03 m³/s - 0.28 m³/s base water flows), mean densities of age-0 rainbow trout increased from autumn to spring (prior to snowmelt runoff) and then decreased during the summer. Mean age-0 rainbow trout densities (number of fish/100 m) in creek #1 were: 39 in autumn (2008); 170 in spring (2009); and 30 in summer (2009). Mean densities in creek #2 were: 595 in autumn (2009); 876 in spring (2010); and 45 in summer (2010). Mean densities in creek #3 were: 2 in the summer (2010), 72 in autumn (2010), and 151 in spring (2011). To assess immigration and timing of age-0 rainbow trout in creek #2, we installed a weir and fish trap in November 2010. We captured 1,381 age-0 rainbow trout immigrating into this creek through January 2011. Our study results demonstrate that age-0 rainbow trout migrate from the Henrys Fork to spring-flow tributaries and overwinter there. Fish passage improvement and habitat restoration within these tributaries may increase age-0 rainbow trout overwintering use and survival, resulting in greater year-class strengths and increased angling opportunities in the Henrys Fork.