Discharge downstream from Island Park Dam on the Henrys Fork of the Snake River in Idaho is reduced each winter to facilitate storage of irrigation water. The effect this has on survival and movement of adult rainbow trout (Oncorhynchus mykiss) in this area is unknown. Additionally, fish movement during the spring has not been evaluated but may affect population estimates conducted in the tailwater monitoring area downstream from Island Park Dam prior to opening of fishing season. Therefore, we used radio telemetry to evaluate winter survival and movement of 61 adult rainbow trout in the Henrys Fork downstream from Island Park Dam under low and extremely low early winter flow conditions. Spring movement was also evaluated to asses whether the population estimates conducted in the monitoring area each spring represent fish from downstream adjacent reaches of the river, and how emigration between mark and recapture periods may affect the population estimate. Survival of radio-tagged trout was nearly 100 percent during early winter under both low and extremely low flow conditions and winter movement did not differ between the two years. Few radio-tagged rainbow trout from downriver were present in the monitoring reach during the time when the population estimate is normally conducted, indicating that large fluctuations in fish numbers in downstream reaches would likely be undetected based on population estimates conducted in the monitoring area. To remedy this, establishing a separate, regular population monitoring area in downstream reaches is recommended. We determined emigration from the monitoring reach between mark and recapture to have a minimal effect on the population estimate. However, we noted that all radio-tagged trout moving out of the monitoring reach during May moved into a short section of river between the monitoring reach and Island Park Dam, presumably to spawn. Therefore, emigration could be largely eliminated by extending the monitoring reach upstream to the dam
