Effect of Water Velocity and Temperature on Energy Use, Behavior, and Mortality of Pallid Sturgeon \u3cem\u3eScaphirhynchus albus\u3c/em\u3e Larvae

Pallid Sturgeon Scaphirhynchus albus are a federally endangered species endemic to the Missouri River basin. Natural reproduction has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. Understanding factors that affect survival of Pallid Sturgeon larvae is key given their critical status and ongoing recovery efforts. In this study, I evaluated the effects of water velocity and temperature on energy use, behavior, and mortality of endogenously feeding Pallid Sturgeon larvae (\u3c 25 mm TL). In the first experiment, I examined the effect of water velocity (0 – 8.3 cm s−1) on larval growth, energy density, swimming activity, and mortality. Larval growth rate at 16.3 °C was similar across treatments (0.97 mm d−1), but energy reserves of larvae reared at 0 cm s−1 decreased significantly compared to larvae reared at velocities of 3.5 or 8.3 cm s−1. Larvae maintained at zero to low water velocities exhibited significantly greater activity and mortality than those maintained at greater velocities. In the second experiment, I examined the effect of water temperature (18.7, 20.4, or 23.3 °C) on larval growth, energy density, and rheotactic behavior at water velocity averaging 8.8 cm s−1. Larvae reared at 23.3 °C exhibited a significantly greater growth rate (0.85 mm d−1) than at 18.7 or 20.4 °C (0.55 and 0.65 mm d−1, respectively). Energy reserves of larvae maintained at 23.3 °C decreased significantly compared to those maintained at cooler temperatures. Additionally, larvae reared at 23.3 °C made a behavioral transition from negative to positive rheotaxis (drifting to settling) significantly faster than larvae in other treatments. In natural environments, areas with zero to low flow (reservoir headwaters) may pose a significant source of mortality to Pallid Sturgeon larvae, owing to their negative rheotactic behavior. I recommend that water velocities \u3e10 cm s−1 be considered during the construction of shallow water habitat (current U.S. Army Corps of Engineers’ definition states s−1). The manipulation of dam releases to increase riverine temperature may aid survival of Pallid Sturgeon larvae by decreasing the extent of spatial and temporal drift, mitigating the drift habitat deficiency caused by impoundments