White sturgeon (Acipenser transmontanus) are the largest, longest-lived freshwater fishes in North America. These historically anadromous fish have experienced fragmented habitats and reduced abundance following the development of hydropower facilities on the Columbia River. In particular, the population in the Lake Roosevelt reservoir, between the Grand Coulee (WA, USA) and Hugh Keenleyside (BC, Canada) Dams, has experienced chronic recruitment failure since the 1960s. One explanation is larval starvation, which seems likely due to the timing at which mortality occurs and the consequences of low feeding success, including greater susceptibility to disease and predation. This study focuses on Upper Columbia white sturgeon and their primary food source: benthic macroinvertebrates (BMIs). The primary objective is to characterize the BMI community in the reservoir when sturgeon begin exogenous feeding to determine if suitable prey density exists or if starvation is contributing to recruitment failure. Benthic samples were taken from nine sites in the riverine portion of Lake Roosevelt, where sturgeon are known to spawn, during the month of July annually from 2007-2010. Samples were collected at each site by pulling a benthic sled over the substrate, in both littoral and thalweg zones. Seasonal BMI variation will be investigated with additional sampling in 2017.
Preliminary analysis of samples from 2007-2010 showed a substantial variation between years, with 2007 demonstrating more than twice the BMI density of any other year (2007 mean density=479 BMIs/m2, 2008-2010 mean density=191, 160, and 118 BMIs/m2, respectively). We also saw a trend towards higher BMI density in the littoral zone (mean=397/m2) as opposed to the thalweg (mean=148/m2). BMIs in the littoral zone may be less available to sturgeon larvae than those in the thalweg, as larvae are drifting in the thalweg during this period. Despite differences in abundance, we found minimal differences in diversity between years or zones. The most common taxa found in benthic sled samples were copepods from the family Cyclopidae, small Cladocerans like Bosmina sp., and Hydra polyps. Similarly, cyclopoids and Hydra were also found to be the most abundant taxa of organisms in a separate analysis of drift net samples from the same sites. By characterizing the BMI community in the upper reaches of Lake Roosevelt, we will be able to identify if food availability is limiting white sturgeon recruitment, and inform management decisions regarding this population.