Habitat selection, seasonal population changes, and behaviour of the Amsterdam Island fur seal Arctocephalus tropicalis on Gough Island

Abstract

The population size and seasonal cycle of the Amsterdam Island fur seal ( A. tropicalis) at Gough Island ( 40°20'S, 9°54'W) were investigated in the summers of 1974/75 and 1975/76. Population size was determined by direct census of part of the coast, and by comparison with a reliable total census made in the summer of 1955/56. Correction factors for undercounting and seasonal cycle were applied. Population size is approximately 100 000 and is the largest known population of this species. Seasonal cycle for various age and sex groups was determined by regular counting at four study beaches. A definite summer breeding peak and an autumn moulting peak exist in A. tropicalis at Gough Island. The peak in numbers ashore during the moult is an hitherto undescribed feature of the seasonal cycle of the Arctocephalinae. Mainly nonbreeding seal colonies occur on the east coast while the breeding colonies are situated on the west coast. Their location is determined by topographical, oceanographic and climatological factors. The diurnal haulout and activity patterns of A. tropicalis are described and environmental factors that modify it are discussed. High local ambient temperatures and solar radiation during the summer have a definite influence on the fur seals. Behavioural and postural adjustments are made use of to supplement physiological means of thennoregulation. Postures and vocalizations as a means of communication in inter- and intrasexual encounters were investigated, and territorial male behaviour studied during the breeding season. Adult males show a seasonal change in territoriality and five territory types are distinguished, based on their location on the beach, time of formation, duration of maintenance, size, topographical definition, and their success in attracting females. Interactions between territorial males involve little physical contact, with boundary displays to affirm territory boundaries and advertise territorial status, being more common. Fighting technique and boundary displays are described, as well as adult male involvement with adult females. The latter is mainly directed at the containment of females on a territory, assessment of their reproductive state and possibly also has a communicative value. Females' reactions towards the attention of adult males usually have elements of submission. Mother-pup relations were investigated and an account is given of general intra- and interspecific behaviour.