Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals

Abstract

The movement and dietary history of individuals can be studied using stable isotope records in archival keratinous tissues. Here, we present a chronology of temporally fine-scale data on the trophic niche of otariid seals by measuring the isotopic signature of serially sampled whiskers. Whiskers of male Antarctic fur seals breeding at the Crozet Islands showed synchronous and regular oscillations in both their delta(13)C and delta(15)N values that are likely to represent their annual migrations over the long term (mean 4.8 years). At the population level, male Antarctic fur seals showed substantial variation in both delta(13)C and delta(15)N values, occupying nearly all the 'isotopic space' created by the diversity of potential oceanic habitats (from high Antarctica to the subtropics) and prey (from Antarctic krill to subantarctic and subtropical mesopelagic fishes). At the individual level, whisker isotopic signatures depict a large diversity of foraging strategies. Some seals remained in either subantarctic or Antarctic waters, while the migratory cycle of most animals encompassed a wide latitudinal gradient where they fed on different prey. The isotopic signature of whiskers, therefore, revealed new multi-year foraging strategies of male Antarctic fur seals and is a powerful tool for investigating the ecological niche during cryptic stages of mammals' life.

Figure 1.

δ¹³C (filled circles) and δ¹⁵N (open diamonds) values along the length of a whisker from a male Antarctic fur seal. Dotted lines illustrate the isotopic estimation of fronts and water masses. APF: Antarctic polar front; STF, subtropical front.

Figure 2.

Whisker δ¹⁵N versus δ¹³C values of four different male Antarctic fur seals. The numbers in parentheses refer to the number of isotopic measurements per whisker. Dotted lines illustrate the isotopic estimation of fronts and water masses. Abbreviations are the same as given in figure 1.