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The comparative foraging ecology of Royal Eudyptes schlegeli and Rockhopper E. chrysocome Penguins

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Hull, CL (1997) The comparative foraging ecology of Royal Eudyptes schlegeli and Rockhopper E. chrysocome Penguins. PhD thesis, University of Tasmania.

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Abstract

Penguins are well adapted to the marine environment, spending the majority of their
time at sea. Whilst their ecology is intrinsically linked to this environment, details of
how they interact with biotic and abiotic aspects of it are not well known for most
species. The majority of penguins have a limited breeding season, and commitments
at the nest necessitate that their foraging ranges are restricted, presumably placing
pressure on prey resources around nesting colonies. Sympatrically breeding species are
thought to compete for these resources, and their co-existence is thought possible by the
segregation of aspects of their ecologies, in particular foraging zones, diet or the
asynchrony in breeding timetables. Royal and Rockhopper Penguins both belong to the
Eudyptes genus, are ecologically very similar, and breed sympatrically on Macquarie
Island. This similarity provides the opportunity to explore the issue of ecological
segregation in these two species. The purpose of this study was to describe the foraging
ecology of Royal and Rockhopper Penguins and to determine the degree of overlap in
resource use. It was undertaken over three years (1993/4, 1994/5 and 1995/6) to
examine inter-annual variability. The thesis is divided into two parts, the first dealing with methodological aspects.
Morphometric indices were determined for externally sexing birds in the field. Bill
length and depth were found to be reliable measures for sexing individuals of both
species. Experiments assessing the impact of investigators on breeding success found
no significant effects, provided care was taken when working in the colony. The deployment of external devices (transmitters and Time Depth Recorders, TDRs) was an
integral part of data collection in the study, and the impact of these on Royal Penguins
was examined: No effects were found in birds carrying the small, streamlined VHF
transmitters, but the attachment of the larger, unstreamlined TDRs decreased the
likelihood that penguins would return from a foraging trip, increased foraging trip
duration, increased water influx rates, and decreased accumulated fat levels. The
different impacts of the devices was related to their size and streamlining most likely
affecting drag Some aspects of the foraging ecology of penguins carrying TDRs were
therefore not entirely representative of unencumbered birds.
The second part of the thesis examined the foraging ecology and degree of overlap in
resource use in Royal and Rockhopper Penguins. Aspects examined were: foraging
zones (using satellite telemetry, 1DRs which estimated positions using geolocation, sea
surface temperature, and foraging trip durations); diving behaviour; diet; and breeding
biology. Both species foraged offshore, to the southeast ofMacquarie Island in the polar frontal
zone, further than had previously been estimated (Royal Penguins 600 km and
Rockhopper Penguins 480 km). Foraging zones changed with stage in the breeding
season, with their extent being related to foraging trip durations, determined by
commitments at the nest. The sea surface temperatures in which both species travelled
were the same (6.8- 10.8° C), and constant between years and stages in the breeding
season. The position of the polar frontal zone changed during this period, suggesting that the species targeted a specific part of the zone.
Royal and Rockhopper Penguins were predominantly diurnal foragers, with most diving
between the hours of 04:00 and 2 1:00. They spent 38.9% and 36.6% of a 24 hour
period respectively, diving. Both species were capable of diving to over 100m, but
spent the majority of their time at depths less than 60 m in dives of less than 2 minutes
duration. This emphasis on shallow, short dives probably maximised foraging
efficiency by reducing the degree of anaerobic metabolism, with its associated cost of
removing respiratory by products, and reduced time spent descending and ascending in
the water column, which is presumably less profitable foraging time.
The diet of both species was dominated by small, gregarious pelagic prey, particularly
euphausiids ( dominated by Euphausia vallentini), and myctophid fish (dominated by
Krefftichthys anderssoni). Diet varied between years, but w a s constant across the
breeding season, although f ewer taxa were consumed before, compared to after, the
hatching of chicks. The breeding biology of both species was similar and synchronous between individuals
and years of the s!.Udy, which is most likely related to the limited temporal window
these species have in which to breed. The investment in clutches was low (6.3% in
Royal Penguins and 7.0% in Rockhopper Penguins), and breeding success was constant
between species and years (on average 53.3% in Royal Penguins and 47.3% in
Rockhopper Penguins). Most breeding failures occurred during incubation, with failures in Royal Penguins due to the late return of mates from foraging trips, and in
Rockhopper Penguins, predation by skuas. It was speculated that the two species
differed in the degree of being "capital" versus "income11 breeders.
Inter-annual differences were only found in diet, and Rockhopper Penguin fledging
masses, but foraging behaviour of both species was constant, suggesting that prey
resources were variable and the species opportunistically consumed those which are
encountered. The consistently high breeding success during the study suggests that
thf'.se years were probably all "good" years in terms of the abundance and accessibility
of prey. Although Royal and Rockhopper Penguins exhibited many similarities in their foraging
ecology, the overlap in resource use was not high. The mechanisms (particularly in
combination with each other) minimising overlap were differences in: (1) Foraging
zones (taking into account the three week asynchrony in the breeding timetables of the
two species); (2) Diet, with Royal Penguins consuming larger and more myctophid fish,
and fewer euphausiids than Rockhopper Penguins. Further, differences in the degree
of digestion of prey suggested that the species foraged on different prey cohorts; (3)
Asynchrony in the breeding season, reducing the overlap in peak food demands and the
duration of foraging trips (which determined the extent of foraging zones).
This study determined that the foraging ecology of Royal and Rockhopper Penguins was
intrinsically linked to the polar frontal zone and regulated by commitments at the nest. Although these species were similar in aspects of their ecology, the overlap in resource
use was less than has been suggested previously.

Item Type: Thesis (PhD)
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Date Deposited: 16 Jan 2013 00:44
Last Modified: 15 Sep 2017 01:06
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