Winter distribution of juvenile and sub-adult male Antarctic fur seals (Arctocephalus gazella) along the western Antarctic Peninsula

Abstract

Detailed knowledge of habitat use by marine megafauna is critical to understand their ecological roles and for the adequate management of marine resources. Antarctic fur seals (Arctocephalus gazella) inhabiting the Atlantic sector of the Southern Ocean prey largely on Antarctic krill (Euphausia superba) and play a central role in managing the krill fishery. Here, we assessed the demographic structure of three post-mating, early moult male haul-outs in the South Shetland Islands in early March and calculated the relative contribution of juveniles (1–4 years old) and sub-adult males (5–6 years) to the population remaining in maritime Antarctica after the breeding season. We also satellite tagged 11 juvenile males and four sub-adult males to analyze their movements and develop a species distribution model including both age classes. Our results highlighted the dominance of young individuals in the male population, revealed that they do not behave as central place foragers and identified key environmental drivers that affected their distribution at-sea throughout winter. Predicted potential foraging habitat overlapped highly with the known distribution of Antarctic krill, and identified the waters off the western Antarctic Peninsula and the Scotia Sea as the core of the distribution area of juvenile and sub-adult male Antarctic fur seals in winter. This pattern is similar to that of adult males but totally different from that of adult females, as the latter overwinter in areas at latitude 45–55° S. This segregation has implications for the ecology and management of the krill fishery.

Evaluating threats to South Shetland Antarctic fur seals amidst population collapse

“…While adult females from Cape Shirreff migrate hundreds of miles north to forage off southern Chile and Argentina in the austral winter (Arthur et al 2017, Hinke et al 2017), males and newly weaned juveniles do not. Adult and juvenile males generally moved into, and south of, the krill fishing grounds (March et al 2021). Young of the year from Cape Shirreff remained predominantly over the continental shelf and slope north of the pack ice within the northern Antarctic Peninsula region, overlapping with current krill fishing grounds (Krause & Watters 2022).…”

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

“…In the case of ties, we prioritized models with larger learning rates, smaller tree complexities, and fewer trees to reduce overfitting. To account for the repeated‐measures structure derived from telemetry data, we incorporated a block factor in the cross‐validation process (March et al., 2021; Reisinger et al., 2018; Roberts et al., 2017). We used individual Antarctic fur seals as folds in a leave‐one‐out cross‐validation, meaning that all data from a given animal (both observed and simulated locations) were excluded from the training dataset and used to validate the model.…”

“…However, simulated tracks can generate replication at the same locations of the real track, hence leading to contradictory information in binomial models (i.e., same location and date defined as either presence or absence), and potentially reducing model performance. To reduce the amount of pseudo‐replication and prevent overlap between real and simulated tracks, we gridded all presence and pseudo‐absence locations per individual at 1° degree for each day and filtered out pseudo‐absences that were adjacent to any presence grid cell (i.e., all individuals considered) within a temporal window of 2 days (March et al., 2021). This reduces the probability that presences and pseudo‐absences at adjacent grid cells have similar environmental conditions on similar dates, to maximize model performance, although other temporal windows could also be useful.…”

“…Under this assumption, the modeling consisted of fitting binomial models with a smooth, monotonic decreasing constraint using the “scam” R package (Pya & Wood, 2015), as accessibility should decrease with the distance to the ice edge and/or distance to the colony. Antarctic fur seals may occur in areas where sea ice concentration is >15% (Reiss et al., 2017; Ribic et al., 1991), but satellite telemetry data indicates that they usually avoid such areas (Arthur et al., 2017; March et al., 2021) and hence those were considered not to be accessible in our models. As in the habitat model, we used a bootstrap approach to account for model uncertainty and fitted the model 50 times by sampling half the data (with replacement).…”

“…Tracking data was collected from individuals captured in two islands 40 km apart in the South Shetland Islands, off the Western Antarctic Peninsula (i.e., Deception Island: males; and Cape Shirreff, Livingston Island: females) between the years 2006 and 2019. Both sexes spend the non-breeding season at sea, although males remain close to Antarctica and females move to the northern limit of the Southern Ocean (Arthur et al, 2015(Arthur et al, , 2017Lowther et al, 2020;March et al, 2021). They also differ widely in their behavior during the breeding season.…”