Individual‐Based Energetic Coupling by a Top Predator in Lakes

Top predators are thought to couple distinct energetic pathways in aquatic ecosystems. Although clear evidence in energetic coupling exists at the population or species levels, intraspecific diversity in foraging decisions can create scope for conspecific individuals differing in their roles in energetic coupling. In particular, ontogenetic trophic niche shifts and interindividual niche variation can drive heterogeneity in resource reliance both within and among individuals that may be accounted for to predict how top predator individuals collectively couple energetic pathways among different habitats.

We measured the stable isotope composition of eye lenses (17 ± 3 isotope measures per individual) for a set of 17 large‐sized pike ( Esox lucius , body size = 102 ± 6 cm) to quantify their lifetime energetic coupling between the two distinct habitats (littoral and pelagic habitats) in the largest French lake (Lake Bourget). The combination of both trajectory and movement‐related analyses enabled us to characterize individual‐level lifelong niche trajectories and to quantify ontogenetic variation in their resource reliance related to specific habitats.

Pike exhibited a remarkable diversity of lifelong isotope trajectories dominated by right‐upward variations in the isotope space that decreased with body size based on angle and segment length analyses. Two distinct trajectory types emerged, ranging from quasi‐linear to zig‐zag‐like isotope patterns. The median lifetime littoral reliance of individual pikes was 52.5%, supporting the hypothesis of energetic coupling among lake habitats. Yet, major ontogenetic littoral‐to‐pelagic diet shifts were identified for all pikes, differing in their extent and timing among individuals, leading to unimodal or bimodal individual lifetime habitat‐related resource reliance.

Our results suggest that pike couple lake energetic pathways primarily via ontogenetic niche shift following a continuum of littoral to pelagic reliance while including among‐individual distinct trophic strategies. Consequently, both inter and intra‐individual trophic variations should be considered when quantifying energetic coupling at a population scale.