Niche partitioning among fur seals

At Cape Gantheaume, Kangaroo Island (South Australia), adult male, lactating female and juvenile New Zealand (NZ) and Australian fur seals regularly return to the same colony, creating the potential for intra- and inter-specific foraging competition in nearby waters. I hypothesised that these demographic groups would exhibit distinct foraging strategies, which reduce competition and facilitate their coexistence. I analysed the diet of adult male, adult female and juvenile NZ fur seals and adult male Australian fur seals and studied the diving behaviour of adult male and lactating female NZ fur seals and the at-sea movements of juvenile, adult male and lactating female NZ fur seals. Female diet reflected that of a generalist predator, influenced by prey availability and their dependant pups' fasting abilities. In contrast, adult male NZ and Australian fur seals used larger and more energy-rich prey, most likely because they could more efficiently access and handle such prey. Juvenile fur seals primarily utilised small lantern fish, which occur south of the shelf break, in pelagic waters. Juveniles undertook the longest foraging trips and adult males conducted more lengthy trips than lactating females, which perform relatively brief trips in order to regularly nurse their pups. Unlike lactating females, some adult males appeared to rest underwater by performing dives that were characterised by a period of passive drifting through the water column. The large body sizes of adult males and lactating females facilitated the use of both benthic and pelagic habitats, but adult males dived deeper and for longer than lactating females, facilitating vertical separation of their foraging habitats. Spatial overlap in foraging habitats among the age/sex groups was minimal, because lactating females typically utilised continental shelf waters and males used deeper water over the shelf break, beyond female foraging grounds. Furthermore, juveniles used pelagic waters, up to 1000 km south of the regions used by lactating females and adult males. The age and sex groups in this study employed dramatically different strategies to maximise their survival and reproductive success. Their prey and foraging habitats are likely to be shaped by body size differences, which determine their different physiological constraints and metabolic requirements. I suggest that these physiological constraints and the lactation constraints on females are the primary factors that reduce competition, thereby facilitating niche partitioning.