Interactions among Three Top-Level Predators in a Polymictic Great Plains Reservoir NATHAN W. OLSON* 1 AND CHRISTOPHER S. GUY U.S. Geological Survey, Montana Cooperative Fishery Research Unit, Montana State University, Department of Ecology, Bozeman, Montana 59717, USA KEITH D. KOUPAL Nebraska Game and Parks Commission, District VI Office, 1617 1st Avenue, Kearney, Nebraska 68847, USA Abstract.—After the introduction of hybrid striped bass (white bass Morone chrysops 3 striped bass M. saxatilis) into Harlan County Reservoir, Nebraska, gill-net catch per unit effort (CPUE) of walleyes Sander vitreus appeared to decline while that of white bass remained stable. This result prompted the question of whether these three species can be managed collectively in reservoir ecosystems. However, despite the frequency with which these three popular sport fishes coexist in Great Plains reservoirs, we are unaware of any studies that evaluate resource overlap among them. Therefore, we compared their diets, diet overlap, isotopic composition, vertical distribution, and vertical overlap in Harlan County Reservoir from June to September 2002 and 2003. All three species consumed similar prey (i.e., gizzard shad Dorosoma cepedianum and Chironomidae), and diet overlap was high (i.e., Pianka’s index . 40) during all months. On no occasion did all three predators consume the same sizes of gizzard shad. Hybrid striped bass consumed larger gizzard shad than white bass did in September 2002 and 2003, whereas white bass consumed smaller gizzard shad than walleyes and hybrid striped bass did in August 2002 and 2003. Stable isotope analysis corroborated the diet analysis and indicated that all three species occupied the same trophic level and that each predator derived carbon from a similar prey source. White bass were consistently located within the upper 3 m of water, whereas the vertical distribution of hybrid striped bass and walleyes varied from the surface to 10 m deep. Spatial overlap was therefore not as high as dietary overlap and was variable among species and months. Although diet overlap was high, resource partitioning (i.e., different feeding locations and different sizes of gizzard shad eaten) reduced the negative interactions among the three predators. Therefore, we conclude that concurrent management of these three sport fishes is feasible in highly productive reservoirs similar to the one in this study. Hybrid striped bass (white bass Morone chrysops 3 striped bass M. saxatilis) were first produced in 1965 and proved superior to striped bass in growth and survival in reservoirs (Bishop 1968; Logan 1968; Williams 1971), which led to large-scale introductions of hybrid striped bass into these systems. By 1978, 26 hybrid striped bass fisheries had been established in reservoirs in the southern United States (Axon and Whitehurst 1985). As has occurred in many other states, the Nebraska Game and Parks Commission (NGPC) stocked hybrid striped bass in many reservoirs to increase angling opportunities and to reduce prey fish density (e.g., gizzard shad Dorosoma cepedianum; D. Bauer, NGPC, personal communication). Many of these reservoirs already contained established fisheries for white bass and walleyes Sander vitreus. One such example is Harlan County Reservoir, where the introduction of hybrid striped bass in 1988 resulted in an angler catch rate of up to 0.56 hybrid striped bass/h (NGPC 2002b). Despite the success of hybrid striped bass stocking, walleye catch per unit effort (CPUE) in gill nets decreased fivefold over 10 years (NGPC 2002c). Such a decline was not observed in another top-level predator, the white bass. Although several abiotic mechanisms may be associated with the decline in the walleye population (e.g., spring storage ratios, temperature, and spring water levels; Quist et al. 2003), competition with other species is also possible. Gizzard shad have exhibited the ability to reduce zooplankton abundance, thus reducing the resources available for larval walleyes (Dettmers and Stein 1992). Quist et al. (2003) found walleye recruitment in Kansas reservoirs to be negatively related to abundance of white crappies Pomoxis annularis, primarily because of their ability to prey on larval walleyes. Several studies have found that hybrid striped bass posed no threat to existing * Corresponding author: nathan.olson@dnr.state.mn.us 1 Present address: Minnesota Department of Natural Resources, 811 Pine Street, Ortonville, Minnesota 56278, USA. Received October 24, 2005; accepted May 23, 2006 Published online February 19, 2007 268 North American Journal of Fisheries Management 27:268–278, 2007 Ó Copyright by the American Fisheries Society 2007 DOI: 10.1577/M05-180.1 [Article]