Seasonal and interannual variability in the diet of young and subadult male Antarctic fur seals as revealed by stable C and N isotope ratios

Abstract

Detailed knowledge of marine mammal diet is critical to understand their ecological roles and for the adequate management of marine resources. Antarctic fur seals ( Arctocephalus gazella) in the Southern Atlantic Ocean rely largely on Antarctic krill ( Euphausia superba) during the summer months, but their winter diet remains largely unknown. Here, stable carbon and nitrogen isotope ratios in whiskers and blood of young (2–3 years old) and subadult (4–7 years old) Antarctic fur seal males from the South Shetland Islands were used to assess the seasonal and interannual changes in the krill contribution to their diet. The stable isotope ratios revealed that krill dominated the diet of young and subadult male Antarctic fur seals year-round, with penguins, and not fish, as the second major prey, regardless of year and season. The year-round reliance of young and subadult male Antarctic fur seals on krill should be considered for ecosystem-based fisheries management, as they represent the bulk of the fur seal population remaining off Antarctica during the winter months.

Future climate‐induced distribution shifts in a sexually dimorphic key predator of the Southern Ocean

“…More extensive sea ice cover may certainly result in a more northerly location of the winter foraging grounds, but this is unlikely to be the reason for the differences reported here between the winter foraging grounds of males and females, more than 1000 km apart. Previous research using stable isotope analysis and satellite telemetry and conducted over several years has already demonstrated that males forage closer to Antarctica than females during the winter months, both in the Atlantic and the Indian sectors of the Southern Ocean (Arthur et al., 2017; Cherel et al., 2009; Drago et al., 2023; Jones et al., 2020; Kernaléguen et al., 2012, 2015, 2016; Lowther et al., 2020; March et al., 2021; Raymond et al., 2015; Walters, 2014). Hence, interannual differences in the extent of sea ice may certainly influence the exact location of the winter foraging grounds of each sex, but there is little doubt that males remain closer to Antarctica than females during the winter months, irrespective of sea ice extent.…”

“…The overall evidence suggests that krill remaining at 100–200 m of depth during the winter months may be out of reach of females but is still accessible to males. This explains why krill remains the staple food of males in the Atlantic sector year‐round (Drago et al., 2023), whereas females move to northern foraging grounds and shift to myctophid fishes and squid (Walters, 2014). It should also be noted that segregation between male and female Antarctic fur seals at winter foraging grounds emerges before they reach sexual maturity (Jones et al., 2021; Kernaléguen et al., 2016), which can be explained by the larger size of males since weaning (Payne, 1979).…”

“…Potential changes in the Antarctic fur seal's distribution and spatial use, derived from our habitat suitability models, will largely depend on the response of their prey to increased warming. Antarctic krill is the staple diet of male Antarctic fur seals throughout the year (Drago et al., 2023) and has already experienced a southward distribution shift induced by climate change, especially in the Bellingshausen Sea (shift for larvae: 832 km SW; for adults: 1400 km SW; Atkinson et al., 2019, 2022). Overall, winter foraging grounds of male Antarctic fur seals may align with the projected shift in krill distribution by 2100, particularly during the early and mid winter months.…”

“…The overall evidence suggests that krill remaining at 100-200 m of depth during the winter months may be out of reach of females but is still accessible to males. This explains why krill remains the staple food of males in the Atlantic sector year-round (Drago et al, 2023), whereas females move to northern foraging grounds and shift to myctophid fishes and squid (Walters, 2014).…”

“…Furthermore, they rely on ice‐free areas of land for breeding, and females are highly philopatric (Hoffman & Forcada, 2012). Antarctic krill, Euphuasia superba , is the staple food of both sexes during the summer months in the Atlantic sector of the Southern Ocean (Atkinson et al., 2022; Borras‐Chavez et al., 2023; Drago et al., 2023; Osman et al., 2004; Polito & Goebel, 2010) and for males during the winter months (Drago et al., 2023). However, females rely mostly on myctophid fishes and squid in winter (Walters, 2014).…”