Determinants of Growth and Survival of Larval Pallid Sturgeon: A Combined Laboratory and Field Approach

Missouri River modifications caused a loss of shallow water habitats, which was identified as potential cause for pallid sturgeon Scaphirhynchus albus recruitment failure. Consequently, recovery effort has focused on habitat restoration, however, ecological requirements of larval pallid sturgeon are largely unknown. To inform recovery efforts, we studied the transition from endogenous to exogenous feeding in pallid sturgeon and quantified prey taxa-specific growth and survival for zooplankton and Chironomidae and Ephemeroptera larvae in discrete pallid sturgeon size classes (first feeding larvae, 20 to 30 mm, 30 to 40 mm). We quantified pallid sturgeon prey selection offering zooplankton and Chironomidae and Ephemeroptera larvae to larval pallid sturgeon, Chironomidae and Ephemeroptera larvae to age-0 juvenile pallid sturgeon, and Chironomidae larvae and fish prey to age-1 and age-2 juvenile pallid sturgeon. We evaluated four shallow water habitat types in the Lewis and Clark Delta (i.e., backwaters, side channels, main channel depositional zones, and Lewis and Clark Lake headwater habitats) regarding their suitability as nurseries for pallid sturgeon and strived to identify variables that foster growth and condition (i.e., energy density) of pallid sturgeon. We did not observe mixed endogenous and exogenous feeding in pallid sturgeon and first prey (i.e. zooplankton) was consumed when the yolk sac had been absorbed. Growth in first feeding larvae was highest for Chironomidae larvae, while in larger larvae tended to be highest for Ephemeroptera larvae. Survival was high for all prey types in all size classes (i.e. 88.3 to 100 %). Larval and juvenile pallid sturgeon selected for Chironomidae larvae throughout all size classes, but consumption of other prey increased when Chironomidae larvae densities were low. Pallid sturgeon growth, energy density, and survival did not differ among habitat types in the Lewis and Clark Delta. Sites which fostered high energy densities had lower velocities, finer substrate, and higher macrophyte, zooplankton, and benthic invertebrate densities and regression analysis revealed that pallid sturgeon energy density increased with increasing Ephemeridae and Caenidae larvae densities. Overall, our work supports the further creation of shallow water habitat as a tool for pallid sturgeon recovery. However, the specific habitat type is less important and the goal of habitat creation should be the increase of primary and secondary productivity, with focus on macrophytes, zooplankton, and Chironomidae and Ephemeroptera larvae