Effect of nutritional condition on variation in δ13C and δ15N stable isotope values in Pumpkinseed sunfish (Lepomis gibbosus) fed different diets
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Stable isotope analysis is frequently used to infer resource use in natural populations of fishes. Studies have examined factors, other than diet, that influence δ15N and δ13C including tissue-specific rates of equilibration and starvation. Most such studies completed under laboratory conditions tightly control food quantity and its isotopic composition, but it is also necessary to evaluate the influence of these factors under more natural conditions. Using pumpkinseed sunfish (Lepomis gibbosus) we evaluated whether restricted rations below minimum daily requirements affects tissue equilibration to a change in diet by holding fish on two treatments that often reflect divergent resource use in natural populations (pelagic zooplankton or littoral macroinvertebrates). Over 42 days, δ15N values increased while δ13C values did not change, additionally neither were related to diet treatment. Increased δ15N values were negatively related to body condition while δ13C values were not, indicating that stable isotope values were more affected by decreasing body condition than by diet. Additionally, δ15N values changed more in the blood and liver tissues than in white muscle tissue, indicating that restricting food availability had greater effects on tissues with greater metabolic activity. We hypothesize that stable isotope values of consumers are subject to a tissue-specific trade-off between sensitivity to changes in resource use and resistance to the effects of low resource availability. This trade-off may require consideration in stable isotope studies of wild populations facing periodic limitations of food availability.
KeywordsStable isotopes Sunfish Fish Starvation Body condition Isotope turnover
We wish to thank J. Berman for assistance in the field. R. Norris, T. Nudds, R. McLaughlin, and the M.N.R. group for discussion about this research and comments on the manuscript. We also thank two anonymous reviewers for their comments which we think significantly improved the quality of this publication. We also thank N. Bernier, B. Husband, J. Brazalot, m. Rush, P. Wright, and D. Larson for generously sharing their equipment, lab space, and time. Finally, our thanks are also extended to R. & C. Gauthier and the Ashby Lake community for their cooperation during the project. This work was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant and a Premier’s Research Excellence Award from the government of Ontario to B.W. Robinson.
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