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.

The southern king crab, Lithodes santolla, is a well-known predator/scavenger species during its adult phase but its feeding strategy in early stages is less studied. This information is important to understand their role in ecosystems and to improve fishery management (i.e., stock enhancement). Based on stomach contents and stable isotope analysis, we determined variation in the composition of diet and niche overlap in vagile and cryptic phase collected within and outside a kelp forest, Macrocystis pyrifera, of Aguila Bay at the Magellan Strait in Patagonia, Chile. Results of juvenile stomach content analysis showed 60% dissimilarity between cryptic and vagile juvenile phases. Algae dominated the volumetric contribution in cryptic juveniles while crustacean dominated the diet in vagile phase. Exoskeleton of other king crabs occurred in 43% of juveniles with crustaceans in their stomach. This fact confirms cannibalistic behavior in the wild in this species, which is consistent with findings in massive laboratory cultures. There was no evidence of isotopic niche shift between cryptic and vagile juvenile phases. Overlapping isotopic niches of different-sized juveniles suggest that they exploit similar food resources. However, vagile individuals occupy a higher trophic position than cryptic individuals, which could suggest a switch in dietary preference, from detritivorous/herbivory within kelp forests to omnivory outside of kelp forests, and an increase in the level of cannibalism in vagile juveniles.

To study how Odontaster validus can influence the spatial structure of Antarctic benthic communities and how they respond to disturbance, it is necessary to assess potential dietary shifts in different habitats. We investigated the diets of O. validus from Maxwell Bay and South Bay in the West Antarctic Peninsula. A multifaceted approach was applied including in situ observations of cardiac stomach everted contents, isotopic niche, and trophic diversity metrics. Results confirm the flexible foraging strategy of this species under markedly different environmental conditions, suggesting plasticity in resource use. The data also showed evidence of isotopic niche expansion, high δ15N values, and Nacella concinna as a common food item for individuals inhabiting a site with low seasonal sea ice (Ardley Cove), which could have significant ecological implications such as new trophic linkages within the Antarctic benthic community. These results highlight the importance of considering trophic changes of key species to their environment as multiple ecological factors can vary as a function of climatic conditions.