Predation vulnerability and trophic interactions of pallid sturgeon Scaphirhynchus albus

Recruitment of the federally endangered pallid sturgeon Scaphirhynchus albus in the Missouri River has been absent or at levels too low to sustain the population. Mortality of juvenile fishes is often greatest during their first year of life, and may have large implications for recruitment dynamics of a species. Predation and starvation are the two primary mechanisms of mortality among age-0 fishes, and can play an important role in the recruitment dynamics of fishes with intensity regulated by behavioral (i.e., prey selectivity) and/or environmental conditions. In the case of a threatened or endangered fish, such as the pallid sturgeon, understanding trophic interactions will assist in implementing a successful recovery plan.;Predation can play an important role in the recruitment dynamics of fishes with intensity regulated by behavioral (i.e., prey selectivity) and/or environmental conditions. Recruitment of pallid sturgeon Scaphirhynchus albus, a federally endangered species, in the Missouri River has been absent or at levels too low to sustain the population. We conducted laboratory experiments to quantify predation vulnerability in juvenile pallid sturgeon, and examine the efficiency of walleye Sander vitreus, flathead catfish Pylodictis olivaris and smallmouth bass Micropterus dolomieu feeding on large (40-50 mm FL) and small (80-100 mm FL) juvenile pallid sturgeon. Experiments for each predator were conducted at high and low densities of pallid sturgeon with contrasting densities of an alternative prey fathead minnows Pimephales promelas. Prey density treatments were replicated 4- 5 times under high (>70 NTU) and low (<5 NTU) turbidity conditions. During additional experiments predator behaviors (strikes, captures, and consumed prey) were quantified for each prey type. Walleye and smallmouth bass negatively selected pallid sturgeon across all treatments, indicating low vulnerability to predation. Vulnerability to predation by flathead catfish was moderate for small pallid sturgeon (neutral selection), but low for large pallid sturgeon (negative selection). Turbidity (up to 100 NTU) did not affect pallid sturgeon vulnerability, even at low density of alternative prey. Pallid sturgeon were easily captured by all predators, but were rarely consumed, suggesting mechanisms other than predator foraging behavior govern sturgeon predation vulnerability. In the presence of alternative prey, our results suggest vulnerability of juvenile pallid sturgeon > 40 mm to predation was low for the predators and conditions tested in this study.;We used stable isotope analysis (SIA) to explore size-dependent feeding patterns for pallid sturgeon and shovelnose sturgeon Scaphirhynchus platorynchus in the Missouri River. Pallid sturgeon delta 15N values (12.8 to 17.0‰) were positively related to fork length (325 to 1187 mm FL), and indicated a trophic level of separation between <500 mm and >800 mm fish. Moreover, the relationship between delta 15N values and body size for pallid sturgeon was consistent throughout the Missouri River. Shovelnose sturgeon delta15N values (14.6 to 17.2‰) were also positively related with FL (range = 341 to 719 mm). Shovelnose sturgeon delta15N values were similar to those of pallid sturgeon in the same size range (350 to 700 mm). Pallid sturgeon delta13C values (-23.4 to -18.0‰) were also positively correlated with FL, suggesting that large and small sturgeon derive energy from distinct carbon sources with a transitional group at intermediate sizes. Shovelnose sturgeon delta13C values (-23.7 to 20.1‰) were weakly correlated with FL suggesting that all sizes of shovelnose sturgeon derive energy from a single carbon source. Further analysis of delta 13C suggest shovelnose sturgeon and small pallid sturgeon (<500 mm FL) use similar energy pathways, whereas large pallid sturgeon (>500 mm FL) are able to use different energy pathways. Stable isotope analysis was an effective method for comparing feeding patterns of large river sturgeon species, and offers several advantages over gut content analysis. Patterns in isotopic composition corroborate diet studies that show pallid sturgeon shift from invertebrates to fish prey at larger sizes (>500 mm FL).