Inter- and intraspecific variation in foraging ecology of sympatric fur seals Arctocephalus gazella and Arctocephalus tropicalis from Marion Island

Abstract

Marine mammals breeding in sympatry use resources differently depending on their time-activity budgets and prey distribution. We measured isotopic values and patterns of δ15N and δ13C in keratinous whiskers of satellite-tagged adult female Antarctic (Arctocephalus gazella) - and Subantarctic (Arctocephalus tropicalis) fur seals from three colonies at Marion Island in the Southern Indian Ocean. The three sympatric colonies were Mixed Pickle (HD_MP), Rockhopper Bay (LD_RhB), and Watertunnel (HD_WT). A comparison between two colonies of differing A. tropicalis densities will also be investigated. We investigated resource use in breeding mature females over varying time and spatial scales during summer and winter. The aim was to understand the variation in resource and habitat use at population and individual levels. The whiskers of A. gazella and A. tropicalis grow constantly at a rate of 0.16 ± 0.05 day-1 and 0.12 ± 0.05 day-1, respectively. To compare and relate behaviours, the stable isotope data and satellite tracks were grouped into winter and summer (pooled for 2010 - 2014). In summer, A. gazella from HD_WT had a larger isotopic niche width (0.85 ± 0.67‰2) compared to A. tropicalis from both HD_MP (0.18 ± 0.19‰2) and LD_RhB (0.21 ± 0.15‰2). However, the isotopic niche width of A. gazella reduced in size by more than half (0.40 ± 0.40‰2) during winter when the mature females were no longer constrained by the lactation mandate. The mean δ15N values in whiskers of both A. tropicalis and A. gazella in summer were similar (11.3 ± 0.4‰ and 11.0 ± 0.9‰, respectively). The A. gazella broadens its’ isotopic niche feeding at different trophic levels in summer (lactation period). We used a Repeatability index (R) to assess levels of intra- and inter-individual consistency. Repeatability indicated that the bigger isotopic niche width for A. gazella is attributed to high inter- and intra-individual variability in their resource use. The A. gazella individuals ranging closer to the colony consistently consumed myctophid fish. Other individuals foraged on low 15N content euphausiids (i.e., Euphausia spp. and Thysanoessa spp.) south of Marion Island in both winter and summer. The A. tropicalis from LD_RhB were repeatable in their δ15N values (trophic level) (Rglobal = 0.63; [95% CI: 0.29 – 0.80]) and δ13C values (habitat) (Rglobal = 0.69; [95% CI: 0.35 – 0.84]). At HD_MP, the A. tropicalis colony, diet was moderately repeatable (Rglobal = 0.33; [95% CI: 0.11 – 0.51]) while habitat was repeatable (Rglobal = 0.58; [95% CI: 0.33 – 0.73]). Consistency in diet (Rglobal = 0.27; [95% CI: 0.08 – 0.44] and foraging habitat (Rglobal = 0.45; [95% CI: 0.21 – 0.63] was moderate at HD_WT, the A. gazella colony. The diet varied with habitat at the individual level, across seasons. Segregation in diet and foraging habitat minimises resource-use overlap between sympatric fur seals. The behaviour of A. gazella from HD_WT (a high-density colony) might have a direct or indirect impact on the A. tropicalis from LD_RhB (a low-density colony) (i.e., competitive exclusion). Seasonality, at-sea movement, and intrinsic factors shaped the foraging behaviour and diet of adult female fur seals breeding at Marion Island. Some breeding mature females changed diets between seasons and depending on foraging habitat.