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Productivity and Trophic Interactions in the Missouri River Impoundments
Standardized monitoring is a vital component of fisheries assessment in Missouri River impoundments. In South Dakota, annual variation in fish growth and abundance is used to monitor changes in fish populations and develop strategies (i.e. regulations) for managing recreational fishes. Although variation in fish abundance provides important insight into the status of fish populations, it can be difficult to link these changes to environmental conditions (i.e. hydrology) without concurrent information about reservoir productivity. Measures of nutrient concentration, algal biomass, and zooplankton composition/abundance provide important insights into reservoir productivity, but standardized approaches for collecting these measures have not been developed for Missouri River impoundments in South Dakota. Furthermore, development of protocols that account for spatial and temporal variation in these parameters would enhance our ability to understand factors affecting fish populations. Inter- and intra- reservoir variation in prey fish composition and abundance has an important effect on sport fish populations in the Missouri River. Coldwater habitat in Lake Oahe, for example, provides refuge for rainbow smelt Osmorus mordax --- a forage species that contributes significantly to walleye Sander vitreus production. In contrast, prey fish populations in downstream impoundments are dominated by gizzard shad Dorosoma cepedianum, cyprinids, and age-0 recreational fishes. In the late 1990s, the decline of rainbow smelt in Lake Oahe had significant impacts on walleye production. Although the smelt population has been recovering since the early 2000s, low water levels combined with the recent expansion of gizzard shad in Lake Oahe, have resulted in a unique food web never before observed in this system. As a result factors affecting gizzard shad abundance, distribution, and their contribution to the growth dynamics of walleyes in Lake Oahe are poorly understood. In addition to the trophic interactions of walleye and gizzard shad, similarities/differences between foraging patterns of walleye and sauger Sander canadenses, two similar species found throughout the Missouri River impoundments, were examined. Sauger is of particular concern in the Missouri River reservoirs due to population decline throughout the Midwest over the past 50 years. One hypothesis that could explain this downward trend in abundance is competition with walleye, a highly sought sport fish, which is frequently stocked in waters containing sauger. Examining isotopic overlap and variability in sympatric walleye and sauger populations could provide quantitative insight into energy transfer and diet breadth of each species. Linkages between Sander spp. diet and hybridization have yet to be addressed though likely important. I also wanted to develop protocols that can be used to monitor trophic linkages and energy flow using stable isotope analysis. More directly, I determined the feasibility of using nonlethal tissues for isotope analysis of walleye, and if whole fish can be substituted for muscle plugs for isotope determinations of common Missouri River prey species. To evaluate the need to document productivity throughout the Missouri River impoundments, to reveal gizzard shad population characteristics and their contribution to walleye growth dynamics, to examine isotopic characteristics between walleye and sauger, and to better standardize tissue use in future stable isotope analysis studies, I developed six primary objectives. These include: (1) develop protocols for indexing productivity in Missouri River impoundments, (2) determine gizzard shad population characteristics (age, growth, recruitment, larval growth rates, time of spawning, spawning duration) in Lake Oahe and compare these characteristics to those of gizzard shad populations in the lower Missouri River impoundments, (3) quantify the energetic contribution of gizzard shad and rainbow smelt to walleye growth, (4) compare isotopic overlap and variability between walleye and sauger in three Missouri River impoundments, (5) enhance the protocols used in stable isotope analysis and determine the usefulness of non-lethal tissues in isotope determination of South Dakota walleye and (6) compare/contrast prey fish isotope signatures using difference tissue analysis.