Foraging behaviour of sympatric Antarctic and subantarctic fur seals: does their contrasting duration of lactation make a diVerence?
Résumé
The duration of periods spent ashore versus foraging at sea, diving behaviour, and diet of lactating female Antarctic (Arctocephalus gazella, AFS) and subantarctic (A. tropicalis, SFS) fur seals were compared at Iles Crozet, where both species coexist. The large disparity in lactation duration (SFS: 10 months, AFS: 4 months), even under local sympatry, has led to the expectation that AFS should exhibit higher foraging eVort or eYciency per unit time than SFS to allow them to wean their pups in a shorter period of time. Previous evidence, however, has not supported these expectations. In this study, the distribution of foraging trip durations revealed two types of trips: overnight (OFT, <1 day) and long (LFT, >1 day), in common with other results from Macquarie Island. However, diving behaviour diVered signiWcantly between foraging trip types, with greater diving eVort in OFTs than in LFTs, and diving behaviour diVered between fur seal species. OFTs were more frequent in SFS (48%) than in AFS (28%). SFS performed longer LFTs and maternal attendances than AFS, but spent a smaller proportion of their foraging cycle at sea (66.2 vs. 77.5%, respectively). SFS dove deeper and for longer periods than AFS, in both OFTs and LFTs, although indices of diving eVort were similar between species. Diel variation in diving behaviour was lower among SFS, which foraged at greater depths during most of the night time available than AFS. The diving behaviour of AFS suggests they followed the nychthemeral migration of their prey more closely. Concomitant with the diVerences in diving behaviour, AFS and SFS fed on the same prey species, but in diVerent proportions of three myctophid Wsh (Gymnoscopelus fraseri, G. piabilis, and G. nicholsi) that represented most of their diet. The estimated size of the most important Wsh consumed did not vary signiWcantly between fur seal species, suggesting that the diVerence in dive depth was mostly a result of changes in the relative abundance of these myctophids. The energy content of these Wsh at Iles Crozet may thus inXuence the amount and quality of milk delivered to pups of each fur seal species. These results contrast with those found at other sites where both species coexist, and revealed a scale of variation in foraging behaviour which did not aVect their eVort while at sea, but that may be a major determinant of foraging eYciency and, consequently, maternal investment.