Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover

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 δ15N providing a measure of trophic level and δ13C 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 sou...