title: Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover.
abstract: Ontogenetic niche shifts are widely prevalent in nature and are important in shaping the structure and dynamics of ecosystems. Stable isotope analysis is a powerful tool to assess these shifts, with δ(15) N providing a measure of trophic level and δ(13) C a measure of energy source. Previous applications of stable isotopes to study ontogenetic niche shifts have not considered the appreciable time lag between diet and consumer tissue associated with isotopic turnover. These time lags introduce significant complexity into field studies of ontogenetic niche shifts. Juvenile Chinook salmon (Oncorhynchus tshawytscha) migrate from freshwater to marine ecosystems and shift their diet from feeding primarily on invertebrates to feeding primarily on fish. This dual ontogenetic habitat and diet shift, in addition to the long time lag associated with isotopic turnover, suggests that there is potential for a disconnect between the prey sources that juvenile salmon are consuming, and the inferred prey sources from stable isotopes. We developed a model that considered ontogenetic niche shifts and time lags associated with isotopic turnover, and compared this 'ontogeny' model to one that considered only isotopic turnover. We used a Bayesian framework to explicitly account for parameter uncertainty. Data showed overwhelming support for the ontogeny model relative to the isotopic turnover model. Estimated variables from best model fits indicate that the ontogeny model predicts a much greater reliance on fish prey than does the stomach content data. Overall, we found that this method of quantifying ontogenetic niche shifts effectively accounted for both isotopic turnover and ontogenetic diet shifts; a finding that could be widely applicable to a variety of systems.
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.
authors.forename: Eric
authors.initials: E
authors.lastname: Hertz
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.|Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC, Canada, V9T 6N7.
authors.forename: Marc
authors.initials: M
authors.lastname: Trudel
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.
authors.forename: Rana
authors.initials: R
authors.lastname: El-Sabaawi
authors.affiliation: Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC, Canada, V9T 6N7.
authors.forename: Strahan
authors.initials: S
authors.lastname: Tucker
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.
authors.forename: John F
authors.initials: JF
authors.lastname: Dower
authors.affiliation: Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC, Canada, V9T 6N7.
authors.forename: Terry D
authors.initials: TD
authors.lastname: Beacham
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.|Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo, BC, Canada, V9T 6N7.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Edwards
authors.affiliation: Department of Biology, University of Victoria, PO Box 3020, Station CSC, Victoria, BC, Canada, V8W 3N5.
authors.forename: Asit
authors.initials: A
authors.lastname: Mazumder
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D002247:Carbon Isotopes / Q000378:metabolism*; D004032:Diet; D017753:Ecosystem; D005399:Fishes; D005618:Fresh Water; D005766:Gastrointestinal Contents / Q000737:chemistry; D007448:Invertebrates; D008962:Models, Theoretical; D009587:Nitrogen Isotopes / Q000378:metabolism*; D012474:Salmon / Q000254:growth & development* / Q000378:metabolism*; D012623:Seawater; D013997:Time Factors
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes
keywords: Bayesian; Chinook salmon; niche; ontogeny; trophic level