Interactions among Three Top‐Level Predators in a Polymictic Great Plains Reservoir

Abstract

AbstractAfter the introduction of hybrid striped bass (white bass Morone chrysops × 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.