Exploring Interactions Between Walleye and Smallmouth Bass in South Dakota Waters

Field: Agricultural and Biological Sciences (Life Sciences)

Walleye Sander vitreus are the most popular fish among South Dakota anglers, but smallmouth bass Micropterus dolomieu were introduced to provide new angling opportunities. Some walleye anglers reported subsequent reductions in the quality of walleye fisheries since the introduction of smallmouth bass in South Dakota waters and attribute this decline to consumption of young walleye by smallmouth bass and competition for shared prey resources. I used field- and experimental-based approaches to determine whether smallmouth bass negatively influence walleye in South Dakota. The objectives of this study were to: 1) determine food habits, diet overlap, and the influence of prey and temperature on walleye and smallmouth bass in one Missouri River reservoir; 2) investigate whether population demographics and dynamics are inversely related for the two species across a range of South Dakota water bodies; and 3) evaluate competition and associated feeding behaviors between walleye and bass under various conditions. For the first objective, I quantified the diets of walleye and smallmouth bass in the lower reaches of Lake Sharpe, a Missouri River reservoir, calculated diet overlap between both predators, and determined whether they partitioned the shared prey based on prey size. I also quantified walleye diets in the upper reach of the reservoir, which has a different prey base and allowed me to examine differential annual growth of walleye within Lake Sharpe. Age-0 gizzard shad Dorosoma cepedianum composed a substantial proportion of diets of both predators, regardless of location, for most of the growing season; patterns in shad growth and vulnerability likely drove observed patterns in diet overlap values. Smallmouth bass consumed a smaller size range of gizzard shad compared to walleye, which consumed a wide range of sizes of shad. Smallmouth bass consumed Sander spp. in some months but in very low quantities (< 5% of diets by weight). Walleye also consumed Sander spp. on rare occasion, and one walleye consumed a smallmouth bass in 2006. Because global climate change is expected to alter population and community dynamics in Great Plains reservoirs, I also used bioenergetics models to assess potential effects of limiting prey availability (specifically, the absence of gizzard shad and rainbow smelt Osmerus mordax) and increased water temperatures (as projected from global climate change models) on walleye and smallmouth bass growth. Scenarios indicated the absence of rainbow smelt from upper Lake Sharpe walleye diets reduced growth, but the absence of gizzard shad would have a marked negative effect on growth of both predators. Bioenergetics scenarios showed increased water temperature would have a negative influence on walleye growth; however, smallmouth bass growth was predicted to increase under increased temperature simulations. For the second objective, I examined relationships between walleye and smallmouth bass population dynamics and demographics in two Missouri River reservoirs and five glacial lakes to indirectly determine whether bass introductions have affected South Dakota walleye populations. I compared relative abundance, size structure, condition, growth, and total annual mortality for both species where available. Of the 26 correlations assessed, only six (23%) were significant (i.e., condition at lakes Francis Case and Poinsett, size structure at Pickerel Lake, relative abundance at lakes Kampeska and Poinsett, and mean length at age 3 at Lake Brant). Importantly, only half of these relationships (i.e., size structure at Pickerel Lake and relative abundance and condition at Lake Kampeska) were negative as would be expected if interspecific competition or predation exists between these two species. The other three significant correlations were positive. While this study does not provide direct observations of competition or consumption, the results of my study did not support the premise that introduced smallmouth bass interfere with walleye population dynamics in South Dakota waters. Rather, other density-dependent and –independent factors likely influence walleye recruitment, growth, and mortality in South Dakota.