Walleye Trophic Position Before and After a Gizzard Shad Extirpation

Authors

Bethany J. Bethke, Department of Natural Resources, FinlandFollow
Justin A. Vandehey, South Dakota State University & Wisconsin Cooperative Fishery Research Unit
Mark J. Fincel, South Dakota Game, Fish and Parks, Fort Pierre Regional Office
Brian D.S. Graeb, South Dakota State University
Mark T. Porath, Nebraska Game and Parks Commission, Fisheries Division

Date of this Version

12-2012

Document Type

Article

Citation

The Prairie Naturalist 44: 72–78; 2012

Comments

Published by the Great Plains Natural Science Society, 2012. Used by permission.

Abstract

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.