Dietary niches of three resident phocid seal species in Kongsfjorden, Svalbard, Norway

Abstract Arctic marine ecosystems are undergoing borealization due to global warming. In the Svalbard Archipelago (Norway), this has resulted in the harbor seal ( Phoca vitulina vitulina ) expanding its distribution along the west coast, such that this species now coexists sympatrically with Arctic endemic ringed seals ( Pusa hispida hispida ) and bearded seals ( Erignathus barbatus barbatus ) in several fjords. This study investigated feeding strategies and niche overlaps among the three seal species resident in Kongsfjorden using stable isotope (SI) ratios in skin and fatty acid (FA) profiles in blubber, along with both tracers in potential prey, sampled 2021–2023. Ringed seals exhibited significantly lower δ¹³C values (-25.5 ± 1.7‰) compared to harbor (-22.1 ± 3‰) and bearded seals (-20.8 ± 3.2‰), reflecting their pelagic feeding habits, while harbor and bearded seals consumed more benthic prey. Large variation in δ 15 N and δ 34 S in ringed seals suggests that some individuals get nutrients from freshwater/terrestrial sources, which is likely related to their feeding near tidewater glacier fronts. Principal component analyses of FA profiles revealed that ringed seals consumed FAs from haptophytes/dinoflagellates (18:4n3), whereas harbor and bearded seals had FAs from protozoan/macroalgae (20:4n6) and copepod (20:1n11) sources. Bearded seals had the highest FA-niche overlap with harbor seals (92.1%), suggesting foraging similarities in shallow, near-shore habitats. Ringed seals occupied a distinct FA-niche but still had considerable overlap with harbor seals (64.6%), indicating some shared prey, likely polar cod ( Boreogadus saida ). These findings highlight the vulnerability of Arctic-endemic seals facing potential competition from temperate species concomitantly with reliance on declining Arctic habitats.