Influence of Diet and Environmental Variation on Physiological Responses of Juvenile Pallid Sturgeon (Scaphirhynchus albus)

Shortly after the pallid sturgeon Scaphirhynchus albus was listed as federally endangered in 1990, a recovery plan was drafted to outline efforts to restore the pallid sturgeon throughout its’ native range in the Missouri and middle and lower Mississippi rivers. The plan set forth a number of recovery efforts such as habitat restoration, population assessment, a comprehensive sturgeon research program and a propagation program to mitigate the decline of pallid sturgeon. Stocking of hatchery raised pallid sturgeon has been an important short-term recovery effort for the prevention of local extirpations. Thus, producing healthy pallid sturgeon, maximizing genetic diversity, and preserving locally adapted populations are critical components of the pallid sturgeon propagation program. In this study, I compared feeding, growth and energy status of juvenile pallid sturgeon fed a commercial or invertebrate diet and explore whether pallid sturgeon exhibit physiological adaptations based on recent, genetic stock structure analysis. To compare growth, food consumption, energy status, and routine metabolic rates of pallid sturgeon fed formulated and natural diets, I conducted a laboratory study using twenty-four hatchery reared pallid sturgeon. Pallid sturgeon were fed a daily ration of either commercial pellets (1 mm, slow sinking; 45% protein, 19% fat), or chironomid larvae for 5 weeks. Pallid sturgeon fed chironomid larvae had faster average growth (0.5 g/d) than fish fed a formulated diet (0.06 g/d). Moreover, mean metabolic rate was significantly higher (82%) among fish fed a natural diet, likely owing to increased cellular metabolism associated with higher growth rates. Energy density was significantly higher among sturgeon fed chironomids (3.7 kJ/g wet weight) compared to those fed commercial diets (0.73 kJ/g wet weight), despite lower energy density (86%) of chironomid prey. In contrast, the hepatosomatic index (HSI) was significantly higher (44%) in fish fed formulated feed, indicative of a high lipid diet. Natural feeds resulted in superior growth and condition than formulated diets. Use of a natural diet prior to stocking may maximize growth, energy stores and post-stocking survival for hatchery reared pallid sturgeon. To determine whether pallid sturgeon are locally adapted to regional environmental conditions, I conducted a common-environment experiment using individuals from two genetically distinct source populations. I measured absolute growth, food consumption, feed conversion efficiency and routine metabolic rate of pallid sturgeon from the upper basin (UB; Montana, North Dakota, South Dakota) and middle basin (MB; Gavins Point Dam, SD to Kansas City, MO) of the Missouri River at 10, 14, 18, 22, 26 and 30°C. Upper basin pallid sturgeon exhibited faster growth at temperatures greater than 18°C. Feed conversion efficiency by UB sturgeon was significantly greater than MB sturgeon at 18 and 22°C. Upper basin pallid sturgeon had higher consumption rates at 22, 26 and 30°C (32.3%, 15.2%, and 15.2%, respectively) and greater metabolic rates (20.4%) at 18°C than MB sturgeon. Pallid sturgeon from the UB exhibited faster growth, greater consumption rates and greater metabolic rates than MB sturgeon, at temperatures greater than 18°C. These results indicate that UB sturgeon have developed a higher capacity for growth at warmer temperatures. Rapid growth by UB fish could potentially be a compensatory adaptation to contracted growing seasons typically experienced in northern latitudes. Previous pallid sturgeon microsatellite analysis indicated a pattern of genetic stock structure that follows a latitudinal pattern, similar to our results of increased growth and consumption by UB fish. Understanding the presence and pattern of local adaptations in broadly distributed species is important for preservation of such adaptations as a means to conserve genetic diversity