Individual specialization and temporal changes in Antarctic fur seal trophic ecology from a multi-tracer approach

Abstract

Substantial individual variation in diet and foraging strategies can exist within populations. These differences can enhance individual foraging efficiency, reduce intraspecific competition, and provide fitness advantages that increase the resilience of a population. Understanding the extent and stability of individual variation, and its contribution to population niche dynamics, is therefore important for interpreting ecological responses, particularly under changing environmental conditions. We used bulk and compound-specific stable isotope analyses together with tracking data to assess individual- and population-level variation in the foraging strategies of adult female Antarctic fur seals (Arctocephalus gazella) from Marion Island (southern Indian Ocean), during the winter migration period, over a decade (2008–2018). We aimed to (1) determine the existence and degree of individual specialization within the population, (2) identify foraging strategies and assess potential influences of body size, and (3) evaluate the temporal stability of foraging strategies, trophic position, and the population isotopic niche, considering potential changes in isotopic baselines. Isotopic data from 129 whiskers from 80 individuals revealed four main foraging strategies, with individuals ranging from extreme specialists to generalists. Corresponding tracking data highlighted differences in the use of oceanographic features niches among these groups, notably in the time spent south or north of the Polar Front. Observed foraging patterns were not explained by body size but instead likely reflect a combination of intrinsic (e.g., energetic costs, experience, genetic legacy) and extrinsic factors (e.g., intra- and interspecific competition and environmental variability). Interannual fluctuations in the population's isotopic niche were also detected, potentially linked to changes in environmental conditions, highlighting the population's adaptability to ecosystem changes. Foraging strategies appear to be temporally stable, as 61% of the 31 females sampled over multiple years showed fidelity to foraging areas, and 74% in their resource use. This suggests that while the population exhibits adaptability to environmental variability, individual seals may consistently employ specific foraging behaviors to limit the potential costs of shifting foraging area and diet. Given the accelerating impacts of climate change on the Southern Ocean, this study underscores the value of stable isotope analysis of whiskers as a powerful, minimally invasive tool for long-term ecological monitoring.