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: lchilvers@doc.govt.nz

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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