LAUSR

Natural reproduction of pallid sturgeon Scaphirhynchus albus has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. In this study, we evaluated the effects of water velocity and temperature on swimming activity, energy use, settling behavior, and mortality of endogenously feeding larvae. Swimming activity of drifting sturgeon larvae (i.e., fish exhibiting negative rheotaxis) increased at low water velocity. In subsequent experiments, we observed greater energy depletion and resultant mortality of larvae in no-flow environments (0 cm s-1 ) compared to tanks with water velocity ranging from 3.5 to 8.3 cm s-1 . Growth rate of drifting larvae was positively related to water temperature (18.7 to 23.3 °C). But reduced growth rate at low water temperature (18.7 °C) resulted in protracted development that extended average drift duration by ~4 d compared to larvae reared at 23.3 °C. This study provides evidence that cooler summer water temperatures, characteristic of present-day conditions in the upper Missouri River, can reduce larval development and extend both the drift duration and distance requirements of S. albus. Moreover, if dispersed into low velocity environments, such as in reservoir headwaters, larvae may experience increased mortality owing to a mismatch between early life stage drift requirements and habitat conditions in the river. Manipulation of water releases to increase seasonal water temperature below dams may aid survival of S. albus larvae by shortening the time and distance spent drifting.