Rainbow Trout in a Regulated River below Glen Canyon Dam, Arizona, following Increased Minimum Flows and Reduced Discharge Variability

Abstract

We examined the effects of dam operation on the relative abundance and relative condition of rainbow trout Oncorhynchus mykiss captured by electrofishing between 1991 and 1997 in the Lee's Ferry tailwater below Glen Canyon Dam, Arizona. Higher minimum, higher mean, and more stable flow releases from the dam after 1991 provided conditions that supported greater relative abundance of rainbow trout. Though the relative abundance of most length-classes increased following the onset of a stabilized flow regime, relative condition declined, particularly in large rainbow trout (≥305 mm). Correlation analyses suggested that the small rainbow trout (<305 mm) were more strongly influenced by physical factors (flow variation, mean discharge, and water temperature) than the large trout (≥305 mm). However, the relative condition of small trout was negatively correlated with that of all trout, and the condition of trout 305–405 mm in length was negatively correlated with the relative abundance of trout within this length category, suggesting food limitation or density-dependent influences on physiological well-being. Our findings enhanced the understanding of rainbow trout ecology in the Lee's Ferry tailwater by demonstrating that a stabilized flow regime provides conditions supportive of greater relative abundance.