Ontogenetic Prey Preferences of Pallid Sturgeon

Transition from endogenous to exogenous feeding is a significant event during early life history of fishes. However early exogenous feeding larvae have functional, anatomical, physiological and behavioral limitations, that affect prey detection, capture, and ingestion. As fish grow, foraging efficiency improves which may ultimately result in diet shifts. So far, little is known about food habits and potential ontogenetic diet shifts during the early life history of pallid sturgeon (Scaphirhynchus albus), a federally endangered species, endemic to the Missouri and Mississippi river drainages. We conducted laboratory experiments to examine prey selection by several size classes of pallid sturgeon from first feeding larvae (20 mm) to juvenile fish (325 mm fork length [FL]). Zooplankton and benthic invertebrates (Chironomidae and Ephemeroptera larvae) were offered to larval pallid sturgeon, and benthic invertebrates and fathead minnows (Pimephales promelas) were offered to juveniles in varying density combinations. Larval pallid sturgeon preyed primarily upon benthic invertebrates, but selection patterns differed between prey density combinations. Thus, our results suggest that opportunistic feeding is common for larval pallid sturgeon when selection patterns are influenced by prey availability. During the early juvenile life stage (≤ 250 mm) opportunistic feeding became less pronounced and pallid sturgeon strongly selected for Chironomidae larvae. Fish larger than 250 mm FL showed a neutral selection for both, Chironomidae and fathead minnows, suggesting that benthic invertebrates are still an integral diet component, but the importance of fish prey increased.