Ontogenetic Patterns in Isotopic Composition of Northern Fishes

Both the trophic ecology and habitat preferences of aquatic consumers may shift as they grow, and thus, they may occupy not one niche but rather a series of ontogenetic niches during their lives.   It would be expected that ontogenetic patterns in feeding ecology identified through dietary studies should also be reflected in isotopic composition.  Variability in isotope signatures among individuals within fish populations is often high, and some of this variability is undoubtedly due to size-based differences in trophic ecology.  However, recent laboratory studies have demonstrated that the isotopic composition of aquatic organisms may also be influenced by variation in fractionation that is linked to growth rates.  We examined isotopic compositions (δ¹³C and δ¹⁵N) in relation to body size in a variety of species (both piscivores and benthivores) and populations (both slow- and fast-growing) of northern freshwater fishes to determine if ontogenetic patterns:  i) reflect the biologies and feeding ecologies of species as determined from earlier research, and ii) vary according to conditions for growth in the environment.  Understanding both the magnitude and nature of ontogenetic patterns in isotopic composition of wild fishes is essential to the interpretation of food web dynamics at community and ecosystem levels.