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

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Robinson, Susan Andrina (2001) The foraging ecology and provisioning strategies of sympatric fur seals Arctocephalus gazella and Arctocephalus tropicalis at Macquarie Island. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Southern fur seals, Southern fur seals
Copyright Holders: The Author
Copyright Information:

Copyright 2011 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s)

Additional Information:

No copying until 9/11/2003. Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:17
Last Modified: 11 Mar 2016 05:54
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