Date of this Version
The Prairie Naturalist 44: 72–78; 2012
Walleye (Sander vitreus) are an ecologically and recreationally important sport fish species. Reduced growth and condition in walleye can occur when prey availability is limited. In two Nebraska reservoirs, walleye consumed gizzard shad (Dorosoma cepedianum) as their primary prey until a winterkill extirpated the gizzard shad in 2001. Because of the winterkill, walleye in the two reservoirs had to change to alternative prey items. Our objective was to determine if stable isotope analysis on archived walleye scales can be used to detect a known food web shift in two reservoir food webs. We quantified the changes in walleye trophic position following the loss of gizzard shad using stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) from archived scales. Walleye δ15N decreased and δ13C increased in both reservoirs after the extirpation of gizzard shad, indicating walleye likely fed at a lower trophic level on more benthic or littoral prey resources post winterkill. A replacement of gizzard shad by white perch (Morone americana) in Pawnee Reservoir may have ameliorated the loss of gizzard shad; in the other system, walleye appeared to feed on a wider variety of prey items as indexed by increased δ13C variability. Our results indicated that walleye were robust to gizzard shad extirpation.