The foraging ecology and provisioning strategies of sympatric fur seals Arctocephalus gazella and Arctocephalus tropicalis at Macquarie Island

• Main Author: Robinson, Susan Andrina
• Format: Thesis
• Language: English
• Published: 2001
• Subjects: Southern fur seals; Antarctic; Austral; Antarc*; Arctocephalus gazella; Macquarie Island; Southern Fur Seals
• Online Access: https://eprints.utas.edu.au/21424/; https://eprints.utas.edu.au/21424/1/whole_RobinsonSusanAndrina2002_thesis.pdf

Antarctic Arctocephalus gazella and subantarctic A. tropicalis fur seals (Pinnipedia: Otariidae) have very different lactation strategies (ie. duration of pup rearing periods), the former taking four months to rear its pups and the latter, ten months. At Macquarie Island these two species are sympatric, which is unusual for fur seals, and their lactation periods overlap for the first four months. The aim of this study was to examine the importance of phylogenetic constraints and environment on the lactation strategies of these seal species at Macquarie Island. This was undertaken by comparing their provisioning strategies at this location, primarily in terms of maternal energy acquisition at sea (foraging behaviour and diet) and on shore energy transfer to the pup (attendance behaviour, milk composition, milk intake, pup growth and fasting). The diet, diving behaviour and foraging location of A. gazella and A. tropicalis females were compared during the austral summer period when pup-rearing of the two species overlapped. The prey of the two fur seal species was very similar, with the myctophid Electrona subaspera being the predominant prey item (94 % numerical abundance froth identifiable fish remains). There were no major differences in the diving behaviour, with both species diving almost exclusively at night with very short and shallow dives averaging 10 - 15 m and 0.5 - 0.9 mm. Both species foraged north of the island with most activity concentrated at two sites: within 30 km, and around 60 km north. Comparing the foraging strategies of A. gazella and A. tropicalis from different locations showed their behaviour to be flexible. At Macquarie Island, under the same environmental conditions, the two species chose a similar diet, and used similar diving behaviour and foraging locations.The maximum foraging range, area prospected and foraging trip duration were investigated in lactating A. gazella in relation to central place foraging theory. There was substantial overlap in area use between years, between foraging trips of different females and between foraging trips of the same individuals. Distance to foraging areas, total distance travelled, area foraged and pup mass gain increased with increasing foraging trip duration. This agrees with the predictions of central place foraging theory: that both energy gained from, and time spent in, a foraging patch should increase with the increasing distance of that patch from the central place. The influence of phylogenetic and environmental factors in shaping pup growth strategies for A. gazella and A. tropicalis were also examined. For A. gazel la and A. tropicalis respectively, birth mass (6.0 and 5.8 kg), growth rate to 120 days (84 and 83 g/d), and weaning mass (14.8 and 14.8 kg) were similar for both species. Maximum mass (15.4 and 19.0 kg) age at maximum mass (1 08 and 200d) growth rates to maximum mass ( 89 and 70 g/d) and weaning (83 and 42 g/d), and age at weaning (122 and 298 d), for A. gazella and A. tropicalis respectively, varied significantly between species. Sex-based differences in mass and growth were evident in both species. Growth rates and weaning mass were relatively high compared to similar measurements for these species at other locations. Lactation length (weaning age) did not vary from other populations of these species. The local environment appeared to have a greater influence on birth mass, growth rates and weaning mass. Despite phylogenetic differences and differences between current and ancestral marine environments, the pup growth of the two fur seal species was very similar. Weaning age, being phylogenetically fixed, however, had the strongest influence on pup development and growth. The provisioning strategies of the two fur seal species were examined with respect to the composition of their milk, the amount of energy transferred and the attendance behaviour of the mothers. Fasting mass loss gave an indication of the energetic cost of body maintenance in the pups of the two species, and was 23 % lower in A. tropicalis pups suggesting a lower energy demand. The milk composition of the two species was similar for the period of overlap and milk lipid increased throughout lactation. Milk lipid and protein contents averaged for the four month lactation period of A. gazella were 41.8 ± 0.73 % and 10.8 ± 0.16% (n = 135) respectively, and for the longer lactation of A. tropicalis, 49.05 ± 2.03 % and 9.5 ± 0.28 % (n = 26) respectively. Milk and milk energy consumption for the two species was not significantly different. Attendance behaviour of mothers of both species was similar with respect to the duration of overnight (<24 h) trips, extended (>24 h ) trips and shore attendance, but the proportions used by each species varied significantly. Lactating A. gazella spent less time in overnight trips, more time at sea and, overall, undertook less foraging trips than A. tropicalis. I conclude that the phylogenetically controlled energy demands of the pups drives the provisioning process but the environment dictates how often and how much energy can be supplied. Fur seal mothers respond to pup energy demands and environmental variables through their attendance cycles. The greater energy demands of A. gazella pups results in their mothers spending more time at sea and undertaking longer foraging trips. Mothers of A. tropicalis pups can meet lower pup demands with mostly short foraging trips. The regular use of overnight foraging trips by both species is possible at Macquarie Island because prey resources are abundant and close to the island, and the fur seal population is small, reducing competition for nearby resources. Provisioning strategies and pup growth are governed by a combination of evolutionary and environmental factors. Pup growth strategies have evolved with physical and physiological limitations on development which they cannot change, even in a situation of rich prey resources. A. tropicalis pups have evolved with energy conserving behaviours and/or physiology and cannot change their rate of development to wean in a shorter time like A. gaze/la. Of the two maternal strategies, it is likely the strategy of A. gaze/la is more expensive during the period of lactation overlap, but overall, with a longer lactation period, it is likely to be more expensive for A. tropicalis to raise a pup.