Ontogenetic patterns in isotopic composition of northern fishes

Background/Question/Methods

Understanding the magnitude and nature of intraspecific variation in isotopic composition of wild consumers is essential to the interpretation of food web structure and dynamics at community and ecosystem levels.  Trophic ecologies and habitat preferences of aquatic consumers tend to shift as they grow, and thus, species may occupy not one niche but rather a series of ontogenetic niches during their lives.  Ontogenetic patterns in feeding ecology identified through dietary studies should also be reflected in stable isotope composition.  Variability in stable 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 stable isotope compositions (δ¹³C and δ¹⁵N) of 12 species of northern freshwater fishes (planktivores, benthivores, and piscivores) from 140 boreal lakes representing over 500 populations (with varying growth rates) to determine if ontogenetic patterns:  i) reflect the biologies and feeding ecologies of species as determined from earlier research, and ii) vary according to individual growth rates or conditions for growth in the environment. 

Results/Conclusions

Ontogenetic variation in isotopic composition is widespread in northern fishes.  The pattern observed depended to some extent on the range of body sizes examined, but significant size-based relationships were evident in both small-bodied (adult size 1 – 100 g) and large-bodied (adult size 100 – 10 000 g) species in all trophic guilds examined.  Changes in δ¹³C and δ¹⁵N with increasing body size were not always readily predictable from previously reported dietary preferences.  Across populations, ontogenetic patterns in both δ¹³C and δ¹⁵N were much more variable in benthivorous than piscivorous fishes, and ontogenetic patterns were stronger and more consistently positive for δ¹⁵N than for δ¹³C.  On average, body size accounted for 10-25% of δ¹⁵N variation for benthivores and most forage fish species (e.g., spottail shiner, yellow perch) but 50-65% of δ¹⁵N variation in some piscivores (e.g., walleye, northern pike, burbot).  There was only weak evidence of interspecific concordance in isotope vs body size slopes among lakes. The predicted negative relationship between δ¹⁵N and individual growth rate was weak and not significant in most species examined, except for walleye.  Studies comparing isotopic compositions between populations, species, or ecosystems need to account for body size covariation.