Dive behaviour, foraging locations, and maternal-
attendance patterns of Australian fur seals
(Arctocephalus pusillus doriferus)
John P.Y. Arnould and Mark A. Hindell
Abstract: The dive behaviour, foraging locations, and colony-attendance patterns of female Australian fur seals
(Arctocephalus pusillus doriferus) from Kanowna Island (39 10 ′S, 146 18 ′E) in Bass Strait, southeastern Australia, were
determined throughout lactation during 1997 1999. Foraging-trip durations increased as lactation progressed, being
shortest in summer (3.71 – 0.24 days; mean – 1 SE) and longest in winter (6.77 – 0.57 days, P < 0.05), but maternal-
attendance periods did not differ in duration (1.70 – 0.10 days, P > 0.5). Individual mean attendance periods and trip
durations were positively correlated (r
2
= 0.21, P < 0.005). Diving commenced shortly after seals left the colony (2.6 –
0.4 h), was continuous for long periods (up to 36 h), occurred mostly during daylight hours, and lacked regular diel
variation in depth. The majority of dives (78%) were typically U-shaped and reached depths corresponding to the pre-
vailing depths in Bass Strait (65 85 m), indicating that these animals forage mostly on the benthos of the shallow con -
tinental shelf in this region. Such behaviour is unusual for fur seals but is reminiscent of that of some sea lion species.
Mean dive durations varied between 2.0 and 3.7 min (maximum 8.9 min) and the theoretical aerobic dive limit (3.91
4.26 min) was exceeded on 17.3% of dives. Dive frequency (8.3 – 0.6/h) and the proportion of time at sea spent div-
ing (40.7 – 2.1%) were weakly negatively related to the duration of the foraging trip (r
2
= 0.07, P < 0.004, and r
2
=
0.13, P < 0.0001, respectively). Data from at-sea locations showed that lactating females forage almost exclusively
within Bass Strait during all seasons.
RØsumØ: Nous avons ØtudiØ le comportement de plongØe, les zones d alimentation et la prØsence dans la colonie chez
des femelles de l Otarie d Australie ( Arctocephalus pusillus doriferus) dans l le de Kanowna (39 10 ′S, 146 18 ′E),
dans le dØtroit de Bass, dans le sud-est de l Australie, pendant toute la pØriode d allaitement, de 1997 1999. La durØe
des excursions de quŒte de nourriture augmente pendant toute la pØriode d alllaitement et c est en ØtØ qu elle est le
plus courte (3,71 – 0,24 jours; moyenne – 1 erreur type) et en hiver qu elle est le plus longue (6,77 – 0,57 jours,
P < 0,05), mais la durØe de la prØsence maternelle ne varie pas selon la saison (1,70 – 0,10 jour,P > 0,5). La durØe
de la prØsence individuelle et la durØe des excursions sont en corrØlation positive (r
2
= 0,21, P < 0.005). Les otaries
commencent plonger peu aprŁs leur dØpart de la colonie (2,6 – 0,4 h), plongent pendant de longues pØriodes conti-
nues (jusqu 36 h), surtout durant les heures de clartØ et la profondeur de leurs plongØes ne suit pas de pattern parti-
culier selon l heure. La majoritØ des plongØes (78 %) sont des plongØes en U jusqu aux profondeurs correspondant aux
profondeurs les plus communes du dØtroit de Bass (65 85 m), ce qui indique que les otaries cherchent leur nourriture
surtout dans le benthos de la plate-forme continentale de cette rØgion. Un tel comportement est inusitØ chez une otarie,
mais rappelle celui de certains lions de mer. La durØe moyenne des plongØes va de 2,0 3,7 min (maximum 8,9 min)
et la limite thØorique d une plongØe aØrobie (3,91 4,26 min) s est trouvØe dØpassØe dans 17,3 % des plongØes. La
frØquence des plongØes (8,3 – 0,6/h) et la proportion du temps en mer consacrØ aux plongØes (40,7 – 2,1 %) sont en
corrØlation nØgative faible avec la durØe des excursions (respectivementr
2
= 0,07, P < 0,004 et r
2
= 0,13, P < 0,0001).
Les donnØes sur les sites en mer dØmontrent que les femelles qui allaitent se nourrissent presque exclusivement dans le
dØtroit de Bass, en toute saison.
[Traduit par la RØdaction] Arnould and Hindell 48
Introduction
Throughout most of the lactation period, otariid (fur seals
and sea lions) pups are completely dependent on their mother
for nutrition (Bonner 1984). For periods ranging from 4 to
more than 24 months (depending on species), lactating fe-
males alternate between foraging trips to sea and regular
nursing periods ashore at the natal colony (Gentry and
Kooyman 1986). As all the nutrients required for milk pro-
duction are acquired by the females during these trips, their
Can.J.Zool.79:35 4 8(2001 ' 2001NRCCanada
35
DOI:10.1139/cjz-79-1-35
Received April 26, 2000. Accepted September 29, 2000.
Published on the NRC Research Press website on December 21, 2000.
J.P.Y. Arnould.
1
Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, Sydney 2109,
New South Wales, Australia.
M.A. Hindell. Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, P.O. Box 252-05, Hobart 7001,
Tasmania, Australia.
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foraging behaviour and how they apportion time spent at sea
and nursing ashore directly influence their maternal-investment
patterns (Trillmich 1996).
Since the development in the mid-1970s of electronic time
depth recorders (TDRs) for measuring diving activity and
radiotelemetry methods for monitoring the movements of
animals to and from the colony, there have been many stud-
ies in which at-sea behaviour and foraging effort of lactating
otariids were investigated (e.g., Gentry and Kooyman 1986;
Feldkamp et al. 1989; Boyd and Croxall 1992; Francis et al.
1998; Mattlin et al. 1998; Thompson et al. 1998). These
studies have revealed two broadly divergent patterns for fur
seals and sea lions. Lactating sea lions generally undertake
short trips (1 2 days) during which they exhibit a continuous
diving pattern with no diel variation, foraging mostly on the
benthos (e.g., Kooyman and Trillmich 1986b; Gales and Costa
1997; Gales and Mattlin 1997; Thompson et al. 1998). In
contrast, lactating fur seals generally undertake longer trips
(3 23 d) during which diving is mostly nocturnal and occurs in
bouts to the deep scattering layer, with pronounced diel varia-
tion in depth that reflects the vertical migration of their prey
(e.g., Boyd et al. 1991; Harcourt et al. 1995; Georges et al.
2000a). The exception is the northern fur seal (Callorhinus
ursinus), in which both foraging strategies have been ob-
served (Gentry et al. 1986b; Goebel et al. 1991).
Detailed data are still lacking for numerous species, how-
ever, making it impossible to determine whether these trends
are due to phylogenetic or environmental influences. Indeed,
recent studies have shown marked geographical differences
in foraging-trip durations and diving behaviour within both
Antarctic fur seals (Arctocephalus gazella) (Boyd et al. 1994;
Goldsworthy et al. 1997; Green 1997) and subantarctic fur
seals (Arctocephalus tropicalis) (Goldsworthy et al. 1997;
Georges et al. 2000b) from widely dispersed colonies in the
Southern and Indian oceans, suggesting that local marine
ecosystem characteristics play an important role in determin-
ing a species foraging behaviour (Gentry et al. 1986a and
Francis et al. 1998).
The Australian fur seal (Arctocephalus pusillus doriferus)
is the largest of the fur seal species, with adult females and
males weighing, on average, 76 and 279 kg, respectively
(Warneke and Shaughnessy 1985). It is a temperate-latitude
otariid with a lactation period lasting 10 11 months, but
some females may nurse a pup for a second or even third
year (Warneke 1982). The population is still recovering from
the severe exploitation of the commercial sealing era (1798
1825), with annual pup production currently estimated at
approximately 14 000, well below the estimated presealing
level of up to 50 000 (Warneke 1982; Pemberton and Kirk-
wood 1994). The breeding distribution of the species is re-
stricted to Bass Strait, a shallow body of water (average
depth <85 m) between the southeastern tip of the Australian
mainland and Tasmania, which is recognised as being of low
oceanic productivity (Murray and Parslow 1999). Natal colo-
nies are currently located on just nine islands, but there is
historical evidence that several other islands in Bass Strait
also once hosted breeding colonies (Warneke 1982; Pember-
ton and Kirkwood 1994).
With its limited distribution, small population size, and
slow rate of recovery, the Australian fur seal is in sharp con-
trast to the Cape fur seal (Arctocephalus pusillus pusillus),
which has a population size of 1.7 million, has experienced
rapid population growth since conservation measures were
introduced in 1893, and occupies a wide distribution along
the coastline of Namibia and South Africa (Butterworth et
al. 1995). The disparities between the subspecies may be
due to the substantial difference in marine productivity be-
tween the Benguela Current, which runs along the southwest
coast of Africa, and the nutrient-poor waters of Bass Strait
(Warneke and Shaughnessy 1985). However, relatively little
is known about the Australian fur seal, particularly its forag-
ing behaviour (Hindell and Pemberton 1997; Hindell et al.
1998), so it is not possible to ascertain the influence of its
nutrient-poor marine habitat on maternal-investment patterns
or population demography.
The aims of this study, therefore, were to determine, in
lactating females, (i) foraging-trip durations and maternal-
attendance patterns; (ii) dive behaviour; (iii) foraging loca-
tions; and (iv) how these vary according to season and stage
of lactation.
Materials and methods
Study site, animals, and capture
The study was conducted between December 1997 and July
1999 on Kanowna Island (39 10 ′S, 146 18 ′E), the site of an Aus-
tralian fur seal breeding colony with an annual production of ap-
proximately 1600 pups (Arnould and Littnan 2000). The island is
situated in central northern Bass Strait, where the distance to the
nearest continental shelf edge (200 m depth contour) is 150 and
280 km to the east and west, respectively (Fig. 1).
Fieldwork was conducted in 2- to 3-week periods every 2
3 months during which randomly selected nursing females were
captured using a modified hoop net (Fuhrman Diversified, Sea-
brook, Tex., U.S.A.). Upon capture, each animal was given an in-
tramuscular injection (ca. 0.15 mg•kg
1
) of the sedative Midazolam
(Hypnovel
fi
, Roche Products Pty Ltd., Dee Why, NSW, Australia)
to facilitate handling and reduce capture stress. Once the sedative
had taken effect, the seal was transferred to a restraint board (Gen-
try and Holt 1982). A subset of animals was weighed using a
spring scale (200 – 0.5 kg; Salter, Peterborough, U.K.), and mea-
sured (straight-line length) to the nearest centimetre with a tape
measure. Individual numbered plastic tags (Super Tags
fi
, Dalton,
Woolgoolga, Australia) were placed in the trailing edge of both
foreflippers and the seal was instrumented with electronic record-
ing devices (see below) before it was released. The device was re-
moved by cutting the fur beneath it upon recapture of the animal.
All capture and handling of seals were conducted in accordance
with the principles and guidelines of the Canadian Council on
Animal Care.
Instrumentation, data collection, and statistical analyses
To monitor the attendance of individuals at the colony, a small
VHF transmitter (Sirtrack Ltd., Havelock North, New Zealand)
was glued to the dorsal fur using quick-setting epoxy (RS Compo-
nents, Corby, U.K.). The presence or absence of individuals was
recorded by an automatic scanning receiver and data logger (RX-
900E; Televilt Int. AB, Lindesberg, Sweden) located on a slope
above the colony. The receiver was programmed to continually
scan each frequency for 30 s every 10 12 min (depending on the
number of frequencies monitored). As Australian fur seals often
enter the water to thermoregulate without leaving the vicinity of
the colony, only absences of greater than6hwereconsidered to be
foraging trips. For logistic reasons, the deployment of transmitters
' 2001 NRC Canada
36 Can. J. Zool. Vol. 79, 2001
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