Fishes were sampled throughout Yellowstone Lake (Fig 2 ) during the ice-free season in 2018 and 2019 using gillnetting methods established by the NPS (see [31 (link)] for specifics on gillnetting placement and design). Diet samples of cutthroat trout and lake trout were collected by season: pre-stratification (before 1 August), stratification (1 August– 20 September), and post-stratification (after 20 September)—identical to Syslo et al. [53 (link)]. We sampled multiple individuals of each species in 50-mm total length classes starting at 100 mm during each season to account for ontogenetic diet shifts. Stomachs from cutthroat trout and lake trout that inadvertently died from gillnetting events were extracted and preserved in 70% ethanol. We pooled diet data among stratification seasons for subsequent analyses to more accurately complement stable isotope analysis from Syslo et al. [53 (link)], where stable isotope samples were not collected based on stratification season.
Fish tissue samples (~10 g of dorsal muscle tissue) were collected for stable isotope analysis. Methods for tissue collection, storage, and preparation were consistent with Syslo et al. [53 (link)]. Samples were analyzed at the University of Wyoming Stable Isotope Facility using an elemental analyzer (Thermo Finnigan Delta Plus XP, Costech 4010 and Carlo Erba 1110 Elemental Analyzer, Costech Zero Blank Autosampler, and Finnigan Conflo III Interface). Liver was used as the quality assurance material. The quality assurance of the isotope analysis is based on the standard uncertainty of the known value of the quality control reference materials analyzed during the analytical run. The standard uncertainty (1-sigma) is calculated from multiple analyses of the quality control reference materials. Stable isotope ratios were calculated using standard procedures outlined in Vander Zanden et al. [65 (link)] and Hershey et al. [66 ].
Stomach contents were analyzed for proportion of diet by wet mass [67 ], prey items were identified and separated by taxon, and the blotted wet weights were measured using the same methods as Ruzycki et al. [54 (link)] and Syslo et al. [53 (link)], thus studies were directly comparable. Invertebrates were identified to order or family and fishes were identified to species. Taxonomic identification categories were selected to match methodology used by Jones et al. [46 ], Ruzycki et al. [54 (link)], and Syslo et al. [53 (link)] and were defined as: cladocerans, copepods, amphipods, leeches, chironomids, insects (which included Ephemeroptera, Trichoptera, Plecoptera, and non-chironomid Diptera), mollusks, cutthroat trout, and unidentified fish. All field and laboratory sampling was conducted under Yellowstone National Park permit 8048. This study was performed under the auspices of Institutional Animal Care and Use Protocol 2018–72 at Montana State University.
Fish tissue samples (~10 g of dorsal muscle tissue) were collected for stable isotope analysis. Methods for tissue collection, storage, and preparation were consistent with Syslo et al. [53 (link)]. Samples were analyzed at the University of Wyoming Stable Isotope Facility using an elemental analyzer (Thermo Finnigan Delta Plus XP, Costech 4010 and Carlo Erba 1110 Elemental Analyzer, Costech Zero Blank Autosampler, and Finnigan Conflo III Interface). Liver was used as the quality assurance material. The quality assurance of the isotope analysis is based on the standard uncertainty of the known value of the quality control reference materials analyzed during the analytical run. The standard uncertainty (1-sigma) is calculated from multiple analyses of the quality control reference materials. Stable isotope ratios were calculated using standard procedures outlined in Vander Zanden et al. [65 (link)] and Hershey et al. [66 ].
Stomach contents were analyzed for proportion of diet by wet mass [67 ], prey items were identified and separated by taxon, and the blotted wet weights were measured using the same methods as Ruzycki et al. [54 (link)] and Syslo et al. [53 (link)], thus studies were directly comparable. Invertebrates were identified to order or family and fishes were identified to species. Taxonomic identification categories were selected to match methodology used by Jones et al. [46 ], Ruzycki et al. [54 (link)], and Syslo et al. [53 (link)] and were defined as: cladocerans, copepods, amphipods, leeches, chironomids, insects (which included Ephemeroptera, Trichoptera, Plecoptera, and non-chironomid Diptera), mollusks, cutthroat trout, and unidentified fish. All field and laboratory sampling was conducted under Yellowstone National Park permit 8048. This study was performed under the auspices of Institutional Animal Care and Use Protocol 2018–72 at Montana State University.
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