Effects of flow fluctuations on habitat use and survival of age-0 rainbow trout (Oncorhynchus mykiss) in a large, regulated river1This article is a companion to Korman et al. 2011, published this issue.

We evaluated effects of reduced hourly variation in flow from Glen Canyon Dam on survival of age-0 rainbow trout ( Oncorhynchus mykiss ) in the Colorado River, Arizona, USA, based on monthly abundance estimates. The proportion of the age-0 population in low-angle shorelines, which are potentially more sensitive to flow variability, declined from 70% in June to 20% in November as fish grew and made an ontogenetic habitat shift to deeper habitat. Average daily instantaneous mortality between August and September was 0.008 units lower in years when there was no change in the minimum flow compared with years when there was a sudden 50% reduction in the minimum flow. However, mortality was 0.006 units higher during the fall when there was no hourly variation in flow compared with years when flows fluctuated. As a result of these opposing patterns, 3-month age-0 survival across steady (0.31) and unsteady (0.28) flow regimes were very similar. While additional replication is required to strengthen inferences about effects of steadier flows, we demonstrate the utility of early life history monitoring for evaluating effects of flow management on fish populations in regulated rivers.