Small Prey and Temperature Jointly Limit Maximum Size of Rainbow Trout in the Glen Canyon Dam Tailwater: Insights from a Drift-Foraging Bioenergetics Model

The cold, clear water conditions present below many large dams support economically important salmonid fisheries.  Many of these tailwater fisheries have experienced declines in the abundance and condition of large trout, yet the causes of these declines remain uncertain.  Field, laboratory, and modeling studies suggest that water temperature, prey availability, and prey size distribution often interact to influence growth in drift-feeding fish.  Here, we develop a drift-foraging and bioenergetics model to identify which of these factors limit Rainbow Trout, Oncorhynchus mykiss, growth downstream of Glen Canyon Dam.  We compare predicted growth with empirical estimates and demonstrate that our model approximates patterns observed in the tailwater.  We then explore the importance of temperature, prey quantity, and prey size by comparing predicted growth with scenarios representing altered conditions.  Our results demonstrate that rainbow trout growth in this tailwater is limited by the scarcity of large prey items and overall prey availability.  Additionally, warmer temperatures result in lower growth compared to cooler temperatures.  This suggests that Rainbow Trout growth in the Glen Canyon tailwater is limited by both prey availability and size distribution, and that occasional periods of warm water releases exacerbate the effects of prey limitation, further limiting growth.