Description: The Antarctic fur seal ( Arctocephalus gazella ) is a key marine predator in the Southern Ocean, a region that has recently started to show changes as a result of global climate change. Here, carbon ( δ 13 C) and nitrogen ( δ 15 N) stable isotope analyses on whole blood and plasma samples were used to examine the isotopic niche of lactating female Antarctic fur seals. Using recently developed Bayesian approaches to determine changes in isotopic niche, a significant increase in δ 13 C and δ 15 N was found between 1997 and 2015; this change occurred at an average rate of 0.067‰ ( δ 13 C) and 0.072‰ ( δ 15 N) per year over this period. This suggests that a marked isotopic niche shift has occurred over this period, which very likely corresponds to a shift in diet towards prey at a higher trophic level, such as fish (replacing krill). Although our sampling design prevented us from exploring a seasonal trend in a conclusive manner, our data suggest that concurrent increases in δ 13 C and δ 15 N might occur as the breeding season progresses. At a seasonal scale, an average decrease of −0.7‰ per month (95% confidence interval=[−0.9; −0.6]) in δ 13 C might have occurred, concurrently with an average increase of 1.1‰ per month in δ 15 N. The results of this study constitute the first isotopic assessment for female Antarctic fur seals from Bouvetøya and provide a baseline for the use of this predator species as a sentinel of the marine trophic system in one of the least studied areas within this species’ distributional range. Keywords: Arctocephalus gazella ; krill; predation; Southern Ocean; stable isotopes; trophic relationships. (Published: 27 October 2016) To access the supplementary material for this article, please see the supplementary file in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 31335, http://dx.doi.org/10.3402/polar.v35.31335