A critical assessment of marine predator isoscapes within the southern Indian Ocean

BACKGROUND: Precise and accurate retrospective geolocation of marine predators via their tissues’ isotopic composition relies on quality reference maps of relevant isotopic gradients (“isoscapes”). Additionally, a good working knowledge of any discrimination factors that may offset a marine predator’s isotopic composition from baseline isotopic values, as well as tissue specific retention rates, are imperative. We provide a critical assessment of inter-specific differences among marine predator-level isoscapes within the Indian Sector of the Southern Ocean. METHODS: We combined fine-scale GPS tracking data and concurrent blood plasma δ¹³C and δ¹⁵N values of eight seabird species (three albatross, two giant petrel and three penguin species) breeding at Marion Island to produce species- and guild-specific isoscapes. RESULTS: Overall, our study revealed latitudinal spatial gradients in both δ¹³C and δ¹⁵N for far-ranging seabirds (albatrosses and giant petrels) as well as inshore-offshore gradients for near-ranging seabirds (penguins). However, at the species level, latitudinal spatial gradients were not reflected in the δ¹³C and δ¹⁵N isoscapes of two and three, respectively, of the five far-ranging species studied. It is therefore important when possible to estimate and apply species-specific isoscapes or have a good understanding of any factors and pathways affecting marine predators’ isotopic composition when estimating the foraging distribution of marine predators via their tissues’ stable isotope compositions. CONCLUSIONS: Using a multi-species approach, we provide evidence of large and regional scale systematic spatial variability of δ¹³C and δ¹⁵N at the base of the marine food web that propagates through trophic levels and is reflected in the isotopic composition of top predators’ tissues.