ECOLOGY AND COEXISTENCE OF SCAPHIRHYNCHUS STURGEON AT MULTIPLE SCALES

Transforming river systems and their surrounding habitats is the most widespread threat to the function of lotic ecosystems. To meet increasing demands for the limited supply of fresh water, humans have extensively altered river systems through diversions and impoundments, use for irrigation, drinking water, food fishing, generating hydro-electricity, and transporting goods and services. These modifications have resulted in an oversimplification of riverine habitats and global declines of many fluvial dependent species. Sturgeons are a group of fluvial dependent species affected by river modifications and may be the most imperiled group of species on Earth. Most sturgeons are threatened across the globe due to a combination of unregulated harvest and habitat loss. The federally endangered pallid sturgeon (Scaphirhynchus albus) and a congener, the threatened shovelnose sturgeon (S. platorynchus) are two species that depend solely on the large rivers of North America. Both species have experienced declines from habitat loss within the Mississippi River basin and overharvest due to the demand for black egg caviar, and are now protected from harvest where they overlap in range. However, pallid sturgeon populations continue to decline while shovelnose sturgeon have remained stable. Efforts to conserve pallid sturgeon have been ongoing for decades, but most studies describing pallid sturgeon life history have been conducted in altered habitats, making it difficult to determine unknown aspects of pallid sturgeon ecology and conservation needs. As a result, knowledge gaps still remain in our understanding of habitat needs during early life and adulthood for both species. Further, little information exists on how these similar sturgeons coexisted historically and why they are now on such different population trajectories. To address these issues, I investigated Scaphirhynchus sturgeon ecology and coexistence at multiple scales. I first examined the environmental life history of pallid and shovelnose sturgeon within the Mississippi River basin (Chapter 2). Using microchemistry techniques, my goal was to assess how to improve species conservation by identifying ecologically relevant management boundaries for pallid and shovelnose sturgeon. My results highlighted a mismatch between conservation boundaries and Scaphirhynchus sturgeon river use and I suggested that managers should consider expanding current protections for pallid sturgeon to include the unprotected sections of the Mississippi River. Chapters 3 and 4 of my dissertation centered on habitat use at smaller spatial scales (\u3c 1 m2) and the individual differences between pallid and shovelnose sturgeon that may occur within different microhabitats (e.g., substrate and velocity). For example, alluvial sand dunes are thought to provide energetic relief for benthic fishes in energetically costly riverine landscapes. However, use of alluvial dune habitat is not well understood, and it is unclear whether dunes provide refuge that effectively reduces energetic costs. I designed a scale-relevant experiment to examine the energetic responses (measured as oxygen consumption; MO2) associated with sand dune habitat in rivers (Chapter 3). Using respirometry, I tested whether pallid sturgeon experienced reduced energetic costs with different configurations of simulated sand dune habitat. Sturgeon displayed distinct station holding behaviors when behind a dune, in front of a dune, and with no dune present. Dune location did not affect energy expenditure, but sturgeon MO2 was on average 16−20% higher in the absence of a sand dune configuration. This was the first experiment to provide a potential mechanism for over two decades of research on why sturgeon, and other benthic fishes, exhibit selection for sand dune habitat in large rivers. In Chapter 4, I combined a separate respirometry study with morphological analysis to identify mechanisms behind differences in ecological success between pallid and shovelnose sturgeon. I proposed that identifying a complex of subtle characteristics in which these and other species differ may be more effective in elucidating both historical coexistence and divergent ecological success of similar species in contemporary habitats compared to identifying just one limiting similarity between species. Using this ‘subtle difference hypothesis’, I showed how metabolic rate associated with habitat use and internal and external morphology can lead to different predicted growth rates for pallid and shovelnose sturgeon, which correspond well to observed differences in ecological success between these species in the wild. Lastly, I reviewed multiple aspects of pallid and shovelnose sturgeon ecology within a framework for assessing surrogate species approaches to conservation in fisheries science (Chapter 5). I suggested that sufficient data regarding life history, niche overlap, genetic similarity, and population vital rates collected at multiple spatial and temporal scales need to be available for surrogates and their targets before implementing a surrogate approach. Although similar ecologically, subtle to large differences at multiple life stages between pallid and shovelnose sturgeon may hinder the use of shovelnose sturgeon as surrogates to infer conservation needs of pallid sturgeon. However, broader similarities between the species may be important for their joint conservation