Stable isotopes suggest stomach content underestimates prey fish consumption by nonnative piscivores in a shallow western lake

Background/Question/Methods
Understanding aquatic food webs has progressed from quantified stomach contents and predator-prey experiments, to stoichiometric and stable isotope analyses. We used a combined stable isotope and diet analysis to estimate the relative contribution of prey fish to different size-classes and species of nonnative piscivores in Utah Lake, Utah. We first used carbon and nitrogen stable isotopes to construct the Utah Lake food web, including determining relative trophic positions. We then compared the isotope-derived food web with one constructed using quantified stomach contents.

Results/Conclusions
Our stable isotope-derived food web illustrated two sub-webs, 1) a more pelagic sub-web, and 2) a more benthic/littoral sub-web. Surprisingly, trophic position estimates placed some of the smallest individuals, young-of-year white bass (Morone chrysops) and fathead minnow (Pimephales promelas), near the highest trophic level. Observed and constructed isotope values were in general agreement for piscivores when prey fish contributed >%50 of the total diet biomass. For the more generalist piscivores, constructed δ¹³C (‰) values were considerably higher, and δ¹⁵N (‰) values considerably lower than observed values, which indicated the dietary contribution of the pelagic sub-web may have been underestimated. Mixing model results suggested the pelagic invertebrate prey, daphnia, was underestimated, and the benthic/littoral invertebrate prey, chironomidae, was overestimated in the diet for most of the generalist piscivores. Generally, an adjustment of the relative dietary contributions for these invertebrate prey corrected constructed δ¹³C (‰) values; however, δ¹⁵N (‰) values could not be corrected without a substantial increase in prey fish contribution to the total diet biomass. Based on stomach contents alone, our assessment of prey fish dietary contribution was misleading, as stable isotopes suggested prey fish contributed more substantially to the diet of some of the more generalist piscivore species and size-classes.