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 δC and δN 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 δC and δN 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 δC and δN 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 δC and δN at the base of the marine food web that propagates through trophic levels and is reflected in the isotopic composition of top predators' tissues.