Foraging site fidelity of lactating New Zealand sea lions
B. L. Chilvers
Marine Conservation Unit, Department of Conservation, Wellington, New Zealand
Keywords
foraging ecology; New Zealand sea lions;
Phocarctos hookeri; satellite tracking;
individual specialization.
Correspondence:
B. Louise Chilvers, Marine Conservation
Unit, Department of Conservation, PO Box
10420, Wellington 6143, New Zealand.
Tel: +64 4 471 3073; Fax: +64 4 471 1082
Email: [email protected]
Editor: Andrew Kitchener
Received 18 February 2008; revised 22 April
2008; accepted 28 April 2008
doi:10.1111/j.1469-7998.2008.00463.x
Abstract
Ecologists often describe the foraging ecology of a species as a whole, treating
conspecific individuals as ecologically equal. However, individual specialization
has potentially important ecological, evolutionary and conservation implications.
Foraging studies are usually based on the foraging behaviours of individuals
sampled in only 1 year. In this study, the site fidelity of foraging locations of nine
female New Zealand sea lions (NZ sea lions Phocarctos hookeri) from Enderby
Island, Auckland Islands, were investigated by repeating their satellite tracking
between 1 and 4 years after they were first tracked. Females were monitored during
early lactation in the austral summers of 2001–2005. The kernel ranges of females’
foraging satellite location concentrations overlapped consistently within and
between years. As predicted for benthic foragers, NZ sea lions show low individual
variability in foraging behaviour and greater specialization. It is important to
understand the spatial and temporal limitations on an individual’s foraging-site
fidelity, because they can affect a species’ ability to cope with environmental
changes and anthropogenic impacts. This has significant implications for NZ sea
lions management and conservation.
Introduction
Ecologists often describe the foraging ecology of a species as
a whole, treating conspecific individuals as ecologically
equal. However, individual specialization has potentially
important ecological, evolutionary and conservation impli-
cations, particularly taking into account the timescale over
which specialization operates (Bolnick et al., 2003). The
foraging behaviours of air-breathing marine predators have
been used to identify their prey consumption and fisheries
interactions (Boyd et al., 1994), differences in sexual segre-
gation of foraging habitat (Clarke et al., 1998; Hedd, Gales
& Brothers, 2001; Breed et al., 2006), differences between
sympatric species (Burns & Kooyman, 2001; Bailleul et al.,
2005), oceanographic variables (Georges, Bonadonna &
Guinet, 2000; Staniland, Reid & Boyd, 2004), environmen-
tal variation (i.e. El Nino, La Nina changes; Boyd et al.,
1994; McCafferty et al., 1998; Etnoyer et al., 2006), seasonal
variability (Boyd, 1999; Beauplet et al., 2004), and to define
species distributions and areas for protection (Jouventin &
Weimerskirch, 1990; Croll et al., 1998; Hooker & Gerber,
2004; Hennen, 2006; Doyle et al., 2007). However, most
species foraging data come from individuals sampled in only
one season or year, under one set of environmental condi-
tions. Few studies have tested whether foraging behaviour is
consistent by recapturing and attaching satellite equipment
to the same individual in the following seasons/years, within
the same time period or in the same environment. In most
cases this is not possible due to logistical difficulties in
finding the same animals again (because of a lack of
marking, loss of marks or the large numbers of animals at
capture sites), animal ethics restrictions or the need to use as
many different individuals as possible to increase sample
sizes for experiments and to gain representation of the entire
population in the area. However, as impacts from climate
change, pollution and human interactions with wildlife
increase around the world (Alverson, 1992; Wickens et al.,
1992; Pascual & Adkinson, 1994), whether foraging loca-
tions obtained from wildlife satellite data from 1 year are
representative needs to be tested.
The New Zealand sea lion (NZ sea lion Phocarctos
hookeri) is one of the world’s rarest and most highly
localized pinnipeds; it is classified as ‘Vulnerable’ by the
IUCN (Reijnders et al., 1993) and ‘Threatened’ under the
New Zealand threat classification system (Hitchmough, Bull
& Cromarty, 2007). NZ sea lions breed on New Zealand’s
subantarctic islands between the latitudes of 481S and 531S
(Gales & Mattlin, 1997). Their population size is one of the
smallest reported for an otariid and pup production has
shown a 30% decline in the last 8 years (Campbell et al.,
2006; Chilvers, Wilkinson & Childerhouse, 2007). Eighty-
three per cent of all breeding is highly localized, occurring
on Dundas and Enderby Islands (only 8 km apart) in the
north of the Auckland Islands group (Fig. 1; Chilvers et al.,
2007). Over the past decade, there has been concern about
the interaction between NZ sea lions and the arrow squid
Nototodarus sloanii trawl fishery, which operates on the
Auckland Island shelf between February andMay each year
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London28
Journal of Zoology. Print ISSN 0952-8369
(Gales, 1995). The presence of the squid and the fishery
around the Auckland Islands coincides with the first
4–5months of lactation for the NZ sea lions (Gales, 1995).
Squid constitute a seasonal, temporal and individually
variable proportion of a sea lion’s diet (Childerhouse, Dix
& Gales, 2001, L. Meyneir, unpubl. data). Lactating NZ sea
lions, as central place foragers, are restricted in their time
spent at sea and therefore the distance they can travel to
foraging from breeding areas. For all NZ sea lion females
who breed at the Auckland Islands this limited foraging area
overlaps with the squid fishery operational area (Chilvers
et al., 2005; Chilvers, 2008).With some sea lions and trawlers
preying on squid, resource competition and incidental cap-
tures of sea lions in squid trawl nets occur with between 17
and 140 sea lion deaths reported each fishing season (Wilk-
inson, Burgess & Cawthorn, 2003). A previous study showed
that there is a high level of variation in foraging behaviour
between individual lactating NZ sea lions; however, there
was high site fidelity within a season (Chilvers et al., 2005). In
the present study, lactating females that were satellite
tracked between 2001 and 2004 were recaptured and satellite
tracked in 2005 to investigate across-year foraging behaviour
to determine whether there is site fidelity or foraging location
specialization between years.
Material and methods
Captures and sampling
Nine branded adult female NZ sea lions that had previously
been satellite tagged from the Sandy Bay breeding area,
Enderby Island, Auckland Islands (Fig. 1, 501300S,1661170E; Chilvers et al., 2005) were re-captured and satel-
lite tagged during January to February 2005. Females were
Figure 1 New Zealand sea lions Phocarctos
hookeri northern Auckland Island pupping
sites, Enderby and Dundas Island.
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London 29
Foraging site fidelity NZ sea lionsB. L. Chilvers
previously tagged between 2001 and 2004 (Chilvers et al.,
2005). Previously satellite-tagged female NZ sea lions ob-
served suckling a pup were captured using a specially
designed hoop net, physically restrained by two handlers
and anaesthetized using isoflurane delivered with oxygen to
a mask via a portable-field vaporizer (Gales & Mattlin,
1998). Satellite-linked platform transmitting terminals
(Telonics 300mW ST6, potted in epoxy, 130� 35� 15mm,
175 g, Telonics Mesa, AZ, USA), VHF transmitters
(70� 30� 15mm, Sirtrack, Havelock North, New Zealand)
and time–depth recorders (Mk9, 40� 30� 22 and
65� 18� 18mm, Wildlife Computers, Redmond, WA,
USA) were attached to the females back at the shoulder
(for more details see Chilvers et al., 2005, 2006). Once the
units were attached to the sea lion, the flow of anaesthetic
was stopped and the animal allowed to recover; it was then
reunited with its pup. Following restraint, each animal was
observed until it was fully conscious and had returned to the
group or location where captured. All females were recap-
tured before the end of the field season (18 February) to
retrieve instruments. Satellite tags were programmed to
work continuously, but were fitted with a saltwater wet–dry
switch to ensure that transmission only occurred when
animals were at sea and on the surface.
Foraging locations
Sea lion at sea locations were calculated by reference to three
satellites and were assigned to six classes by Argos on the
basis of their accuracy. NZ sea lions dive almost continu-
ously while at sea (Gales & Mattlin, 1997; Chilvers et al.,
2006); therefore, all trips and satellite locations were assumed
to be part of a foraging trip and to represent foraging
locations. Locations were filtered by an algorithm described
by McConnell, Chambers & Fedak (1992). This iterative
forward/backward averaging filter identifies fixes that would
require an unrealistic rate of travel. Locations were filtered
using a maximum swimming speed parameter of 2m s�1
(Crocker, Gales & Costa, 2001; Chilvers et al., 2005).
Filtered locations were used to estimate travel distances.
Total distance travelled was calculated from the interpola-
tion of all filtered locations. The duration of trips and
periods ashore were calculated from satellite and automatic-
tracking VHF data. Calculations of mean distance travelled
per trip and maximum distance from the breeding area were
restricted to complete trips (those defined by final locations
at the breeding area or within 10 km from the breeding area
while the animals were travelling towards it). Locations from
incomplete trips are represented in the figures and tables but
were not used in the calculation of trip statistics. Kernel
ranges representing out to 99% of all satellite locations for
each female were calculated using smoothing factors calcu-
lated via least-square cross-validation (Seaman & Powell,
1996), which were created using the Animal Movement
Extension of ARCVIEW (Hooge, Eichenlaub & Solomon,
2000). Central foraging locations were determined based on
kernel range information for 50% of all locations per animal.
Kernel ranges representing 50% of all of the locations were
considered representative of the main concentration of fora-
ging effort (Chilvers et al., 2005).
All trip variables derived from location data were analysed
using Excel, SPSS and ARCVIEW (Microsoft Office Excel
2003, SPSS 2004, ARCVIEWr 1998 ESRI Inc., Redlands,
CA, USA). Individual variations in trip variables (i.e. time at
sea, trip distance) were analysed using one-way ANOVAs,
with significance at Po0.05. All means are presented� SE.
The arrow squid fishery operational locations data and
sea lion bycatch data were supplied by the Research Data
Management section of the Ministry of Fisheries, New
Zealand. Fisheries data represent all start/stop locations
for trawl shoots undertaken each year. Kernel ranges
representing 50 and 95% of all trawler activity were created
to show areas of highest activity (Hooge et al., 2000). These
figures were calculated in the same way as the foraging areas
for NZ sea lions, outlined above.
Results
Female NZ sea lion site fidelity to foragingareas
For the nine NZ sea lion females who were satellite tagged
for a second season during the austral summer of 2005, 2232
locations were recorded, with equipment deployed and
active for 15–34 days for each female (Table 1, Fig. 2a–e).
An average of 248 locations was recorded per female
(minimum 62, maximum 386, Table 1). The comparison of
foraging trip parameters and locations between deployment
years is shown in Table 1 and Fig. 2a–e. There were no
significant differences between the mean times at sea or
ashore, or trip distances for each female across the 2 years
she was studied. From the 2005 data, the maximum distance
away from breeding area averaged 103� 8.5 km (range
45–134 km), mean trip distance was 265� 30.5 km (range
132–373 km) and maximum trip distance was 327� 31.7 km.
For each female, there was a maximum of 25 km between
sampled years in the maximum distance recorded from the
Enderby breeding area (Table 1). Similarly, the distance
females were foraging from the breeding area to the centre
of their foraging concentrations differed by up to 16 km
between the years studied, except for female 1391, for whom
the central foraging area could not be defined due to a
diffused and widespread foraging pattern in both years (Fig.
2b, Table 1). The angle from the Enderby breeding area to
the centre of each female’s foraging location varied little
between the two study years for each female (variation
mean=2.61, SE=0.61, maximum=61). The water depths
of females’ satellite locations were consistent across years
(Fig. 2a–e). There were no significant differences in mean
water depths for each female across years (Fig. 2a–e). Six of
the nine females rarely had satellite locations recorded from
water deeper than 250m (o10% of all fixes). The exceptions
were 1439 (Fig. 2d) with 30% at a depth of 250–500m, 1470
(Fig. 2c) with 70% at a depth of 250–500m and 5% at
500–750m, and 1456 (Fig. 2e) with 14% at a depth of
250–500m and 11% at 500–750m.
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London30
Foraging site fidelity NZ sea lions B. L. Chilvers
Figure 3 shows the overlap between all filtered satellite
locations represented as a kernel range for all females tagged
from Enderby Island (Chilvers et al., 2005, present study)
and the kernel range areas of 50 and 95% of all squid fishery
trawl operation between 2001 and 2004. The 2005 fishery
locations data were not obtained, but the concentrations of
fishing areas are likely to be similar each year (Chilvers et al.,
2005) and overall catch and fishing effort are shown in
Table 2. Two fishing areas predominate: one lies south-east
of the Auckland Islands along the 250m bathymetry line
(44% of all tows undertaken 2001–2004); the second lies
north/north-west of the Auckland Islands (56% of all tows
undertaken 2001–2004). There is little overlap between the
foraging locations of females from Enderby Island and the
fishing area south-east of the Auckland Islands; however,
significant overlap occurs in the north/north-west area
(Fig. 3, Chilvers et al., 2005). Sea lion bycatch is usually
concentrated within the 50% kernel range areas of fishery
operation (Chilvers et al., 2005).
Discussion
Individual specialization in foraging behaviour has been
recorded in a wide range of animals (Fedriani & Kohn,
2001). Specialization can result from a number of factors
including different prey capture or harvest tactics (Bence,
1986), morphological differences (i.e. age, size, gender;
Werner & Gilliam, 1984), intra-specific competition (Mili-
nski, 1982), experience (Pernal & Currie, 2001), maternal or
social learning (Smolker et al., 1997; Sargeant et al., 2007)
and habitat restrictions (Heithaus et al., 2002). Foraging
behaviours may also change through time for an individual
due to physiological energy requirements (i.e. reproduc-
tion), prey distribution and availability, environmental con-
ditions, climate change, competition with other species or
interactions with humans (Boyd et al., 1994; McCafferty
et al., 1998; Bailleul et al., 2005). Consequently, in most
cases it is unknown whether the foraging behaviours dis-
played by an individual during 1 year are representative of
the individual’s long-term foraging behaviour. Therefore,
determining the timescale over which specialization persists
is important as it will have implications for the ecology and
conservation of individuals and therefore species as a whole
(Bolnick et al., 2003).
The foraging behaviours of lactating NZ sea lions show
high levels of individual variation in foraging location and
diving behaviour. However, individuals showed strong site
fidelity to specific foraging areas within a breeding season
(December–February each year; Chilvers et al., 2005, 2006).
The re-capture and re-satellite tagging of nine lactating NZ
Table 1 Age, deployment year, number of days and locations during deployment, foraging trip and location parameters for nine New Zealand sea
lions satellite tagged twice within a 4-year period (2001–2005) from Sandy Bay breeding area, Enderby Island, Auckland Islands
Female
ID Age Yeara
No. of
days
deployed
Total
no. of
locations
No.
of
trips
Mean
time
at sea (h)
Mean
time
ashore
(h)
Mean
trip
distance
(km)
Maximum
trip
distance
(km)
Maximum
distance
from
Enderby
(km)
Distance
from
Enderby
to centre
of foraging
area (km)b
Angle from
Enderby to
centre of
foraging
area (1)b
1474 8 2001 36 588 10 65.0� 6.6 20.6� 3.3 248�27.8 385 69 53 24
12 2005 30 330 8 66.4� 5.4 33.7� 2.5 209�14.1 278 94 58 22
1423 10 2002 31 366 8 66.1� 7.6 15.7� 3.1 316�52.8 586 86 84 28
13 2005 34 259 6 68.3� 6.9 31.4� 5.4 373�24.0 478 98 68 34
1433 12 2003 33 351 6 79.6� 5.7 29.1� 5.3 309�29.4 407 119 115 58
14 2005 31 358 6 78.6� 5.3 54� 6.8 322�19.1 366 124 104 60
1391 10 2003 31 328 10 43.8� 5.6 20.3� 1.3 188�27.8 338 86 Diffuse Diffuse
12 2005 29 221 10 45.3� 4.1 30.3� 1.9 132�21.7 236 95 Diffuse Diffuse
1397 11 2003 30 463 6 96.4� 14.8 36.8� 3.1 362�17.5 432 133 103 67
13 2005 27 202 5 100� 5.9 44.7� 8.6 353�28.9 428 121 110 68
1385 17 2004 28 91 4 83.7� 12.1 44.4� 1.6 291�18.8 342 135 117 66
18 2005 28 210 5 76.9� 5.9 39.1� 1.6 328�9.5 357 134 115 62
1470 15 2004 33 657 7 82.0� 7.3 30.1� 3.0 420�42.8 563 110 103 1
16 2005 15 217 3 87.4� 8.8 42.5� 1.3 308�24.3 351 107 100 3
1439 10 2004 33 398 8 53.8� 3.0 33.3� 3.8 280�16.6 364 91 90 10
11 2005 26 62 3 50.6� 8.8 35.5� 3.5 224�54.0 278 105 99 9
1456 13 2004 33 374 14 35.4� 3.5 23.8� 1.9 194�39.4 269 67 29 282
14 2005 31 381 13 36.4� 2.9 25.8� 1.6 138�7.9 175 45 25 285
Mean
values
2001–2004 32� 0.8 401� 54 8� 0.9 67.3� 6.7 28.2� 3.0 290�24.9 409� 34.8 100.0� 8.6
2005 28� 1.8 248� 32.9 7� 1.1 67.8� 6.9 37.4� 2.9 265�30.5 327� 31.7 103� 8.5
All means presented� SE.aAll instruments were deployed for various time periods between 12 January and 18 February within each year.bCentre of foraging area determined from cluster of satellite locations for each female calculated as 50% of all her locations (see Fig. 2 a–e).
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London 31
Foraging site fidelity NZ sea lionsB. L. Chilvers
sea lions in the present study showed that this individual
fidelity or specialization to foraging location and the related
foraging characteristics are consistent both within and
between years (Table 1, Fig. 2a–e). This occurred even
though it is likely that there were differences in the prey
distribution and environmental conditions between the
years of this research (2001–2005).
There are limited data on prey distribution and availabil-
ity for the Auckland Island shelf. As an indicator of prey
distribution and availability, the squid fishery catch varied
considerably between the 5 years of this research (lowest
catch 3254 tonnes in 2000–2001, highest catch 35,634 tonnes
in 2003–2004, Table 2), although the area of fishing activity
did not (Fig. 3, Chilvers et al., 2005). Despite the differences
25 km
N
250 m
500 m
1385
1423
2002 and 2004
(a1)
25 km
250 m
500 m
(a2)N
1385
2005
1423
25 km
(b1)
250 m 500 m
N
2003
1391
25 km
(b2)
250 m 500 m
N
1391
2005
25 km
(c1)
250 m
N
1433 1470
500 m
2003 and 2004 25 km
(c2)
250 m
N
1433
1470
500 m
2005
Figure 2 Kernel home ranges of all filtered satellite locations from female NZ sea lions Phocarctos hookeri during January and February for the
austral summer. Intensity of colour of kernel ranges represents percentage of time spent in the area [colour scales go from both black (highest
percentage) to white (lowest percentage i.e. 1470) and white (highest percentage) to black lowest percentage i.e. 1433]. Bathymetric contours
shown as thin black lines. (a1) females 1423 and 1385 in 2002 and 2004, respectively, (a2) females 1423 and 1385 in 2005 (b1) female 1391 in
2003 (b2) female 1391 in 2005 (c1) females 1470 and 1433 in 2004 and 2003, respectively, (c2) females 1470 and 1433 in 2005 (d1) females 1439
and 1397 in 2004 and 2003, respectively, (d2) females 1439 and 1397 in 2005 (e1) females 1456 and 1474 in 2004 and 2001, respectively, (e2)
females 1456 and 1474 in 2005.
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London32
Foraging site fidelity NZ sea lions B. L. Chilvers
in squid abundance, which indicate ecosystem variation
between years, each female showed site specificity to fora-
ging areas and consistency in foraging behaviours. NZ sea
lions are considered to be benthic foragers, having the
deepest and longest duration dives recorded for any otariids.
They dive almost continuously when at sea with diving
behaviour reported to be at or close to their physiological
limits (Gales & Mattlin, 1997; Chilvers et al., 2006). This
limits their ability to change foraging behaviours if required
due to changes in their environment or anthropogenic
impacts (Gales & Mattlin, 1997; Chilvers et al., 2006).
Added to this foraging constraint, it appears from this
research that they also show high site fidelity to foraging
areas despite possible environmental differences between
years.
Populations composed of long-term individual specia-
lists, such as female NZ sea lions, are thought less likely to
be able to respond to changes in their environment (Bolnick
et al., 2003). This is particularly true for small populations
with restricted breeding ranges that overlap with fisheries
like the NZ sea lion. The reason why lactating NZ sea lions
have adopted a physiologically extreme and spatially re-
stricted foraging behaviour has not yet been elucidated.
However, the fact that lactating females do operate at this
level and show such high site fidelity during early lactation
makes them highly susceptible to external impacts such as
direct and indirect fisheries impacts and other local environ-
mental changes.
This research concentrated on the distribution of adult
lactating females because of their importance for the recov-
ery of this threatened species and because there has been a
significant decline in pup production in the last 8 years
thought to be driven by a decline in the number of breeding
adults (Wilkinson et al., 2006; Chilvers et al., 2007). Lactat-
ing pinnipeds are restricted in their use of foraging locations
due to the temporal restriction of having to return to
dependent offspring. Sub-optimal foraging conditions for a
mother attempting to optimize the return time to her pup
will increase foraging costs, hinder pup provisioning and
affect species viability (Boyd et al., 1994, 1998; Boyd,
McCafferty & Walker, 1997). For some marine predators,
changes in foraging behaviour such as foraging trip dis-
tances, locations and duration, due to environmental varia-
tion are well documented (i.e. El Nino, La Nina changes;
Boyd et al., 1994; McCafferty et al., 1998; Boyd, 1999;
Etnoyer et al., 2006). Yet it is indicated here that despite
environmental variation between years, this species shows
fidelity to foraging areas, trip lengths and duration. Benthic
resources are more predicable/stable than mid column
resources (Costa et al., 2004; Chilvers et al., 2006).
25 km
(d1)
250 m
N
1397
1439
500 m
2003 and 2004
(d2)
250 m 500 m
1397
1439
2005
N
(e1)
250 m 500 m
1474
1456
2001 and 2004
NN
25 km
(e2)
250 m 500 m
1474
1456
2005
Figure 2 continued
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London 33
Foraging site fidelity NZ sea lionsB. L. Chilvers
Therefore, species that forage benthically may be expected
to show less variability in foraging behaviours (Costa &
Gales, 2003; Mattern et al., 2007). However, benthic fora-
ging may also require greater specialization, which may
influence the ability of the species to respond to environ-
mental perturbations, which has significant implications for
their management and conservation.
Recognizing individual foraging specialization and un-
derstanding the spatial and temporal limitations over which
specialization operates are important for species conserva-
tion. Knowing that individual female NZ sea lion foraging
behaviour is consistent within and between years is impor-
tant as it indicates long-term site-specific foraging speciali-
zation despite probable environmental and resource
competition variability. The temporally and spatially re-
stricted benthic foraging behaviour of lactating NZ sea lions
indicates that they may be unable to respond to external
changes such as environmental variability or anthropogenic
impacts. This appears to be displayed by the Auckland
Island population of NZ sea lions by their low population
250 m 500 m
25 km
N
50°50′S
166°28′E
Key
Kernel range of 35 female NZSL satellite locations from Enderby Island 2001 − 2005
50 and 95% kernel ranges of fishing effort 2001 − 2004
Figure 3 The kernel range distribution of 35 lactating female sea lions’ foraging locations (nine repeated, colour intensity shows highest
percentage use – i.e. white area represents highest use area which is Enderby Island the breeding area) and overlap with squid trawl fisheries
effort (2001–2004, 50 and 95% kernel ranges represented as thick grey lines) from the austral summers of 2001–2005. Auckland Islands
represented in black. Bathymetric contours shown as thin black lines. Auckland Island Shelf represented by 500 m bathymetric boundary.
Table 2 Subantarctic arrow squid trawl fishery (SQU6 T) total catch, number of tows and catch/tow data for the fishing years 2000–2001 to
2004–2005
Fishing year SQU6 T allowed catch (t) SQU6 T actual catch (t) No. of tows Catch/tow (t) Date fishery closeda
2000–2001 32 369 3254 580 5.61
2001–2002 32 369 11 502 1653 6.96 13 April
2002–2003 32 369 6847 1383 4.95 b
2003–2004 32 369 34 634 2555 13.6
2004–2005 32 369 27 314 2646 10.3 20 April
aFisheries usually operate 1 February to 31 May each year.bFisheries closed but then reopened within days – therefore no closure.
Journal of Zoology 276 (2008) 28–36 c� 2008 The Author. Journal compilation c� 2008 The Zoological Society of London34
Foraging site fidelity NZ sea lions B. L. Chilvers
numbers and a current decline in pup production as fisheries
activities and bycatch in the area increases (Chilvers, 2008).
Acknowledgements
My thanks are due to Dr Ian Wilkinson and Dr Padraig
Duignan for initiating and making possible the first series of
satellite tracking we undertook, enabling this comparative
study. I also thank J. Amey, M. Brown, A. Castinel, W.
Hockley, F. Jonker, R. Hood, A. Maloney, L. Meyneir, P.
McClelland, F. Riet Sapriza and M. Wylie for assistance with
captures in the field. DOC Southland are thanked for their
logistical assistance. The work was conducted under permit
from the New Zealand Department of Conservation (DOC),
and was funded by DOC, Science and Research Unit (In-
vestigation no. 1638). Approval for the work was obtained
from DOC Animal Ethics Committee – Approval AEC86 (1
July 1999). Ian West, Amanda Todd and two anonymous
reviewers all provided helpful, critical reviews of the paper.
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