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Autumn diet of recolonising female
New Zealand sea lions based at Otago
Peninsula, South Island, New Zealand
AA Augé a , C Lalas b , LS Davis a & BL Chilvers c
a Department of Zoology, University of Otago, Dunedin, New
Zealand
b Department of Marine Science, University of Otago, Dunedin,
New Zealand
c Department of Conservation, Aquatic and Threats Unit,
Wellington, New Zealand
Version of record first published: 11 Oct 2011.
To cite this article: AA Augé, C Lalas, LS Davis & BL Chilvers (2012): Autumn diet of recolonising
female New Zealand sea lions based at Otago Peninsula, South Island, New Zealand, New Zealand
Journal of Marine and Freshwater Research, 46:1, 97-110
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Autumn diet of recolonising female New Zealand sea lions based at Otago
Peninsula, South Island, New Zealand
AA Auge
´
a
*, C Lalas
b
, LS Davis
a
and BL Chilvers
c
a
Department of Zoology, University of Otago, Dunedin, New Zealand;
b
Department of Marine Science,
University of Otago, Dunedin, New Zealand;
c
Aquatic and Threats Unit, Department of Conservation,
Wellington, New Zealand
(Received 26 March 2011; final version received 14 July 2011)
New Zealand (NZ) sea lions (Phocarctos hookeri) are slowly recolonising the Otago coast, South
Island, New Zealand. The increase in their numbers may lead to resource competition with other
marine predators and fisheries. We determined the diet of female NZ sea lions at Otago during
autumn. In total, 571 scats and 110 regurgitations were collected on Otago Peninsula during
2008 and 2009. Barracouta (Thyrsites atun) and jack mackerel (Trachurus sp.) were the two main
prey species and accounted for 26% and 31% of the reconstituted biomass, respectively. This
was consistent between two years. Only five other species contributed 5% of the diet by
biomass in either year. Prey species are all found on the narrow continental shelf surrounding
Otago Peninsula. The main prey species of Otago NZ sea lions may be of higher energy content
than prey in the Auckland Islands (remnant breeding area). Resource overlap with other marine
predators and fisheries appears to occur around Otago Peninsula. A marine trophic model of
the area off Otago Peninsula would help understanding potential competition between marine
predators and fisheries in this area.
Keywords: pinniped; prey species; foraging; competition; recolonisation; feeding; Otago
Peninsula; Hooker’s sea lion; New Zealand sea lion; Phocarctos hookeri
Introduction
Investigating the diet of animals allows an
understanding of their needs in term of prey
availability and quality (Capitani et al. 2004;
McKenzie & Wynne 2008; Herreman et al. 2009;
Chilvers et al. 2010). Firstly, prey species vary
geographically, have different handling times
and risks, and do not all have the same energy
content (Rosen & Trites 2000; Bowen et al. 2002;
Meynier, Morel, Mackenzie et al. 2008). Sec-
ondly, large marine predators can compete for
resources with fisheries (Bjørge et al. 2002;
Alonzo et al. 2003; Wilkinson et al. 2003;
Huckstadt & Krautz 2004). Knowing the diet
of marine mammals can consequently improve
our understanding of their habitat needs and
help manage their interactions with fisheries.
New Zealand (NZ) sea lions, Phocarctos
hookeri (Gray 1844), were extirpated from the
New Zealand mainland by the 1830s but they
have now started recolonising the Otago coast,
on the east coast of the South Island of New
Zealand, for approximately 50 years (Wilson
1979; Childerhouse & Gales 1998; McConkey,
Heinrich et al. 2002). However, consistent breed-
ing only started in 1994 on Otago Peninsula
(45850?S, 172800??E; McConkey, McConnell
et al. 2002). Up to 2010, 45 pups had been born
there (Auge
´2010). A population of large marine
predators is consequently increasing in number
and may compete with other marine predators
*Corresponding author. Email: amelie.auge@gmail.com
New Zealand Journal of Marine and Freshwater Research
Vol. 46, No. 1, March 2012, 97110
ISSN 0028-8330 print/ISSN 1175-8805 online
#2012 The Royal Society of New Zealand
http://dx.doi.org/10.1080/00288330.2011.606326
http://www.tandfonline.com
Downloaded by [JAMES COOK UNIVERSITY] at 17:34 30 September 2012
and fisheries. Resource competition can signifi-
cantly affect established populations of smaller
and less competitive species when a larger spe-
cies that was previously absent re-establishes in
an area (Herreman et al. 2009; Richard et al.
2010). Otago Peninsula is a breeding area for the
endangered yellow-eyed penguin (Megadyptes
antipodes) (Seddon et al. 1989) and for the New
Zealand fur seal (Arctocephalus fosteri) (mini-
mum estimate of 20,000 fur seals [Lalas 2008]).
The recolonisation of the area by NZ sea lions
may affect these populations.
In this paper, we determined the diet of
female NZ sea lions based at Otago Peninsula,
southeast South Island, New Zealand, during
autumn using undigested prey remains in scats
and regurgitations. The identification of undi-
gested remains of prey (bones, body parts,
feathers etc.) in scats and regurgitations is
commonly used to investigate the diet of carni-
vores, including pinnipeds, as it is non-invasive,
low-cost, and samples can easily be collected on
land (McLellan & Hovey 1995; Capitani et al.
2004; Ruhe et al. 2008; Paralikidis et al. 2010).
By identifying remains in these samples, the prey
species, number of prey species and sizes of prey
can be determined, hence revealing the diet of
the animals. Typically, the identification is only
conducted on otoliths for fish, and on beaks for
cephalopods (McKenzie & Wynne 2008) but the
digestion of otoliths is species-dependant and
this overestimates the amount of larger fish
(Browne et al. 2002). Along with otoliths, the
identification of other species-specific remains
increases the accuracy of the method by detect-
ing more prey items, including from species that
were not present or under-represented in the
reconstructed diet using otoliths only (Browne
et al. 2002; Tollit et al. 2003; Tollit et al. 2007;
Trites & Calkins 2008). While some studies have
only used scat samples, there is a large bias
against larger fish and cephalopods in this case
and the combination of scats and regurgitations
appears to represent the diet of pinnipeds more
reliably (Lalas 1997). It is possible to estimate
length and mass of prey from the size of remains
whenever allometric relationships are available.
In case several different bones from the same
species are found in a sample, these prey size
estimates can be used to determine from how
many different prey they came from. Along with
direct counts of parts when only single remains
per species were found, this enables the estima-
tion of the minimum number of prey of each
species that were taken. Biomass reconstruction
calculated from the mass estimates of prey
provides a more accurate assessment of diet
than frequencies of occurrence in samples or
numerical abundances of prey when looking at
energy intake and resource overlap based on
available biomass (Tollit et al. 2007).
Potential competition between fisheries and
NZ sea lions around the main remnant breed-
ing areas of NZ sea lions in the Auckland
Islands (50840?S, 166810??E) is of concern as
fisheries may affect the resource availability of
prey species of sea lions (Childerhouse et al.
2001; Meynier et al. 2009; Meynier et al. 2010;
Robertson & Chilvers 2011). It is, therefore,
important to determine the likelihood of this
situation around Otago Peninsula as the popu-
lation grows. This will allow precautionary
management decisions to ensure the establish-
ment of new breeding colonies on the New
Zealand mainland as this is the main step to
remove the species from its ‘nationally critical’
conservation status (Department of Conserva-
tion 2009; Baker et al. 2010). The diet of male
NZ sea lions was previously investigated on
Otago Peninsula (Lalas 1997). However, sexual
segregation in diet is often found in pinniped
species (Beck, Iverson et al. 2007; Meynier,
Morel, Chilvers et al. 2008; Trites & Calkins
2008; Breed et al. 2009). Consequently, the
investigation of the diet of the female compo-
nent of the Otago population was essential.
Material and methods
Scats and regurgitations of female NZ sea lions
were collected from 23 March to 28 May 2008
and 2009 at Victory Beach and Papanui Inlet
(45850?S, 170843??E) on Otago Peninsula. This
area was the main site used by all Otago-born
98 AA Auge´ et al.
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female NZ sea lions aged two years and over in
2008 (n14) during this study period, and was
where all lactating females nursed their pups
(Auge
´2010). Consequently, the entire popula-
tion of females from Otago Peninsula was
sampled. When a female sea lion was found,
any fresh scat or regurgitation nearby was
collected in a zip-lock plastic bag (Minigrip
Redline, Kennesaw, USA). The study area was
searched at least twice daily and females’ loca-
tions and identities (each female could be
individually identified, see Auge
´[2010]) were
recorded. Occasionally, males were found in a
part of the area, in which case samples were not
collected and all scats or regurgitations found in
this part were covered with soil or vegetation so
they were not re-sampled later. Regurgitation
samples were kept chilled after collection, and
later stored in a 20 8C freezer within 15 days
of collection and until further analyses to avoid
putrefaction of flesh often present. Scat samples
were soaked in water with liquid detergent
(laundry soap) in zip-lock plastic bags for up
to a week to allow the soft part of the scats to
break down. The undigested remains in scats
were recovered by washing the contents of the
bags with water through a sieve of mesh size 0.6
mm. All remains collected were stored in fresh
water in small bottles and kept in a fridge until
further analysis. Regurgitations were placed in
large containers in water with detergent the day
before identification process, and sieved with the
same mesh size as the scats before identification.
All scat and regurgitation samples from each
female during one year were gathered and
organised by date so that each sample could be
attributed to a foraging trip (i.e. period at sea)
based on presenceabsence of the females in the
study area. This limited the bias of repetitive
sampling of the same prey in multiple scats or
regurgitations. Foraging trip was defined as the
sampling unit in the analyses.
All remains found after sieving were chec-
ked for possible identification. Those non-
identifiable remains (either broken, too eroded
or unknown) were discarded. The identification
of the undigested remains was completed using
the reference drawings in Furlani et al. (2007)
for otoliths, in Leach (1997) for fish jaw bones,
in Lu and Ickeringill (2003) and Lalas (2009)
for beaks of cephalopods, and the private
collection held by Chris Lalas for all remains of
species not described in these references (this
collection was used in Lalas [1997], Fea et al.
[1999] and Mattern et al. [2009]). Cephalopods
were mainly identified from beaks, but we
also used undigested body parts (e.g., radula or
arms) to assess the occurrence of this group.
Otoliths and jaw bones were mainly used to
determine fish species, although other parts
contributed to more identifications than these
in some species (e.g. lateral scutes of jack
mackerel [Trachurus sp.], green bones of green-
bone [Odax pullus], caudal peduncle and teeth
of barracouta [Thyrsites atun], denticles of rough
skate [Raja nasuta] and spines of spiny dogfish
[Squalus acanthias]). The occurrence of crusta-
ceans corresponded to the recovery of exoskele-
tal remains. Species of birds were identified from
feathers and feet.
For remains with a set number in each prey
but no allometric equation available or remains
that were broken or too eroded to be measured,
but were still identifiable, it was possible to
determine the correspondent minimal number
of prey ingested during the trip by identifying
the side (left or right) of the structure whenever
possible. Owing to the large amount of feathers
often ingested during seabird predation, if a
definite occurrence was detected during a fora-
ging trip, the sole presence of a few feathers in
the samples of the following trip was attributed
to the previous occurrence. This allowed the
determination of the minimal numerical abun-
dance of each prey taken during each foraging
trip. When allometric equations were available,
the estimated fork length for fish and wet mass
for all prey were calculated based on the
remains. In this case, if all structures of the
same species gave approximately similar sizes
and there was no replication of the same
structure, they were attributed to the same
prey item. Allometric equations were from
Leach et al. (1996) for barracouta, Leach,
Diet of Otago female NZ sea lions 99
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Davidson & Horwood (1997) for blue cod
(Parapercis colias), Leach et al. (2001) for red
cod (Pseudophycis bachus), Leach, Davidson,
Samson et al. (1997) for wrasses, Labridae spp.,
Lalas (2009) for Maori octopus (Macroctopus
maorum), and equations obtained from diag-
nostic structures in Furlani et al. (2007) or the
reference collection held by Chris Lalas, for all
other species.
The percentage of sampled foraging trips
during which each prey species was identified
(percentage of occurrence) and the minimal
number of individuals of each prey species
(numerical abundance) were determined. All
numbers calculated for these two values are
given as minimal numbers. The diet was
described by calculating the percentage that a
particular species contributed to the overall
total reconstituted biomass of prey taken dur-
ing each study season (percentage of reconsti-
tuted biomass).
Results
Totals of 166 and 405 scats and 42 and 68
regurgitations were collected, respectively in
2008 and 2009. At least one scat or regurgita-
tion was collected for 104 foraging trips in 2008
and 154 foraging trips in 2009. We identified
386 prey in 2008, and 551 prey in 2009, from 38
different species including 27 fish, 2 cephalo-
pods, 4 crustaceans, 3 seabirds and 2 salps.
Twenty of the prey species were found during
both years. Pieces of bryozoans were also
detected during 21 foraging trips. Reconstituted
biomasses added up to totals of 243 kg in 2008
and of 401 kg in 2009.
Only nine species were taken during more
than 10% of the sampled foraging trips in
any one year, and 22 species were represented
by fewer than 10 individual prey items in the
diet (Table 1). Barracouta and jack mackerel
were taken during 4060% of the foraging
trips while all other species were found in less
than 25% of the trips. Fourteen species were
identified in the samples of only one individual
(seven species during a single foraging trip).
Mean sizes of the prey species of female NZ
sea lions at Otago are presented in Table 2. The
largest prey species taken by female NZ sea
lions at Otago consisted of yellow-eyed penguin
(Megadyptes antipodes) (5.4 kg), and trumpeter
(Latris lineate) (4.1 kg). Neither was found in
the top five prey species of the diet, which made
up 74.0% and 81.4% of the reconstituted
biomass during 2008 and 2009. All other prey
were less than 2.5 kg. All of the top five prey
species in the diet were relatively large prey
(range of mean masses 0.51.6 kg). During
both study years, jack mackerel and barracouta
were the main prey species, adding up to 51.0%
and 62.5% of the total reconstituted biomass,
in 2008 and 2009 respectively. Only four other
species made up more than 5% of the female
diet during any one year (Table 1).
From Table 1, the inter-annual difference in
diet at the population level can be assessed.
Percentages of trips and of reconstituted bio-
mass can be compared between years. The
numerical abundance cannot be directly com-
pared, as the amount of sampled foraging trips
differed between years. There was no difference
in the percentages of reconstituted biomass
made up by the prey species in the diet of
female NZ sea lions between years. Conse-
quently, the diet at the population level ap-
peared similar in 2008 and 2009. This
similarity, however, relied in a large part on
the high proportions made by barracouta and
jack mackerel in the diet as there was a
difference between years for prey species of
lower importance in the diet. The third to sixth
prey species in order of importance in the diet,
constituting more than 5% of the diet of the
population in any one year, varied. Species
making up less than 5% also varied between
years, especially in numerical abundance, as
they had small sizes (Table 2).
100 AA Auge´ et al.
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Table 1 Diet of the female population of New Zealand sea lions in autumns 2008 and 2009 on Otago
Peninsula, New Zealand. Species ranked by biomass in the diet for the two years combined.
Percentage of
occurrence
Numerical
abundance
Percentage of
biomass
Common name Species 2008 2009 2008 2009 2008 2009
Barracouta Thyrsites atun 43.3 58.4 49 110 24.8 33.4
Jack mackerel Trachurus sp. 57.7 57.1 52 95 26.2 29.1
Arrow squid Nototodarus sloanii 20.2 13.6 68 27 15.2 3.7
Red cod Pseudophycis bachus 3.8 22.7 4 50 1.3 9.8
Maori octopus Macroctopus maorum 12.5 9.1 13 15 6.5 5.4
Yellow-eyed penguin Megadyptes antipodes 2.9 1.9 3 2 6.8 2.8
Wrasses Labridae, two spp. 21.2 16.2 24 38 3.5 3.4
Greenbone Odax pullus 11.5 9.7 12 16 3.6 2.9
Blue cod Parapercis colias 24.0 16.2 31 37 2.8 2.0
Little penguin Eudyptula minor 1.9 3.2 2 6 1.2 2.3
Ling Genypterus blacodes 3.8 43.3
Trumpeter
1
Latris lineata 0.6 22.1
Rough skate
1
Raja nasuta 1.9 21.9
Yellow-eyed mullet Aldrichetta forsteri 2.9 3.9 5 16 0.3 0.6
Spiny dogfish Squalus acanthias 4.8 3.9 5 5 0.4 0.3
Catfish Crapatalus sp. 1.9 1.3 4 3 0.5 0.2
Estuary stargazer Leptoscopus macropypus 4.8 11.7 8 23 0.2 0.3
Spotted shag
1
Stictocarbo punctatus 1.0 0.6 1 1 0.6 B0.1
Blue moki
1
Latridopsis ciliaris 0.6 20.3
Rock cod Lotella rhacinus 1.9 3.2 2 5 0.1 0.2
Witch Arnoglossus scapha 1.0 4.5 1 20 B0.1 0.2
Lemon sole Pelotretis flavilatus 1.9 1.3 2 2 0.2 0.1
Marblefish
1
Aplodactylus arctidens 1.0 10.3
Paddle crab Ovalipes catharus 3.8 5.2 4 10 0.1 0.1
Tarahiki
1
Nemadactylus macropterus 0.6 10.2
Opalfish Hemerocoetes sp. 1.9 40.2
Sprat Sprattus sp. 6.7 75 0.2
Pigfish
1
Congiopodus leucopaecilus 1.9 30.2
Lobster
1
Jasus edwardsii 0.6 10.1
Common roughy
1
Paratrachichthys trailli 3.9 24 0.1
Ahuru
1
Auchenoceros punctatus 1.9 27 0.1
Flounder
1
Rhombosolea sp. 1.0 1B0.1
Camouflage crab
1
Notomithrax sp. 0.6 1B0.1
Garfish
1
Hyporhamphus ihi 0.6 1B0.1
Salp Two spp. 6.7 1.3 14 5 B0.1 B0.1
Mantis shrimp
1
Squilla armata 1.0 1B0.1
Note:
1
Species taken by a single individual female sea lion.
Diet of Otago female NZ sea lions 101
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Table 2 Mean sizes (including masses used to calculate the reconstituted masses) of prey species found in the
diet of female New Zealand sea lions on Otago Peninsula, New Zealnd in autumns 2008 and 2009.
Fork length for fish
(cm)
1
Mass (g)
2
Species Mean9se Max. Min. Mean9se Max. Min. n
Mass (g) from
references
3
Ahuru 10.093.9 18.8 5.5 8910 34 8 15
Arrow squid 29.593.3 36.8 22.5 5439218 1218 214 54
Barracouta 61.3913.3 87.3 38.1 12639728 3208 250 72
Blue cod 23.395.6 41.1 15.8 219177 506 51 16
Blue moki 600
Camouflage crab 80 1
Catfish 28.5910.0 32.0 18.7 125956 215 61 4
Common roughy 3.491.3 6.7 1.8 17912 51 8 14
Estuary stargazer 16.693.8 25.9 9.9 56939 161 25 19
Flounder 17.7 120 1
Garfish 25.1 36 1
Greenbone 35.398.2 49.0 23.3 7349604 1869 100 12
Jack mackerel 42.496.3 50.1 26.5 12259232 1605 789 29
Lemon sole 22.6915.4 38.8 8.1 2259128 638 105 4
Ling 2000
Little blue penguin 1100
Lobster 500
Mantis shrimp 10
Marblefish 32.8 778 1
Maori octopus 14.493.9 21.0 6.3 144191013 3783 133 17
Opalfish 35.5913.0 51.7 21.5 1779124 399 46 4
Paddle crab 57931 161 18 19
Pigfish 200
Red cod 38.498.9 49.3 25.5 7819583 2437 64 36
Rock cod 23.795.1 29.2 25.5 162994 285 117 7
Rough skate 69.5 2284 1
Salp B1
Spiny dogfish 39.7 217 1
Spotted shag 1200
Wrasses 20.498.1 33.2 5.8 3569250 1093 31 44
Sprat 8.691.0 12.5 7.0 59320 3 61
Tarahiki 34.0 714 1
Trumpeter 63.7 65.2 62.3 4145 4430 3860 2
Witch 17.395.1 28.0 10.7 49951 173 9 14
Yellow-eyed mullet 20.593.8 28.7 18.8 155998 313 62 5
Yellow-eyed penguin 5400
Notes:
1
For squid and octopus, this is the mantle length;
2
obtained from the undigested remains of this study;
3
obtained
from references Ayling & Cox (1982), Robertson & Heather (1999), Paul (2000) and Paulin et al. (2001).
102 AA Auge´ et al.
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Discussion
Prey species of female NZ sea lions around
Otago Peninsula
This first study of the diet of female NZ sea
lions based on Otago Peninsula showed that
barracouta and jack mackerel were the most
consumed prey and likely represent critical prey
species for female NZ sea lions during autumn
at least. Only two other species (arrow squid
and red cod) were taken in significant amounts
by the female population of NZ sea lions at
Otago (representing approximately 10% and
over of reconstituted biomass during any one
year; see Table 1). Barracouta and jack mack-
erel are pelagic but can be found near the
seafloor from 0 to 200 m depth within the
continental shelf (Ayling & Cox 1982). Both are
schooling fish during the day but barracouta
disperse at night, when they may be feeding at
the bottom (O’Driscoll & McClatchie 1998).
Arrow squid diurnally migrate in the water
column from the bottom to surface waters over
the continental shelf (Paul 2000). Red cod are
demersal and abundant in any type of habitat
from sand to rock at depths up to 150 m
(Beentjes et al. 2002). The pieces of bryozoans
found in several samples indicated that several
females foraged in the bryozoan thickets found
off Otago Peninsula (50100 m deep, around
815 km offshore [Batson & Probert 2000]), at
least during some foraging trips. No species
predominantly living at more than 200 m depth
(i.e. on the continental slope or beyond) were
recorded in the prey remains from female NZ
sea lions at Otago in autumn. This corroborates
the results of a satellite tracking study on some
of the Otago female NZ sea lions during
autumns 2008 and 2009 as foraging mostly
occurred in coastal waters and on the con-
tinental shelf in the area of bryozoan thickets
(Auge
´et al. 2011).
Diets of any animal populations vary to
accommodate usual seasonal variations in food
resources (McLellan & Hovey 1995; Fea et al.
1999; Womble & Sigler 2006; McKenzie &
Wynne 2008). Female NZ sea lions at Otago
may also change prey species or the percentages
that each species makes in their diet during the
year. Lalas (1997) found significant differences
in the diet of male NZ sea lions at Otago
between seasons, with some prey species being
taken only during specific months. Jack mack-
erel, one of the two main prey species in the diet
of female NZ sea lions in autumn, is likely a
migratory species and unusual off Otago
outside summer and autumn (Lalas 1997;
O’Driscoll & McClatchie 1998). Jack mackerel
was absent from scats and regurgitations col-
lected in winter at the beginning of July 2009
(H. Anderson, unpubl. data). This reinforces
that this food resource is likely only available
during summer and autumn and a seasonal
variation in diet probably occurs.
It will be necessary to conduct a year-round
diet study of female NZ sea lions at Otago over
several years to determine the extent of dietary
variations. However, in autumn 2010, the
constant presence of several highly mobile
juvenile and sub-adult males in the area where
females were found prevented sample collection
to investigate female diet from remains in scats
and regurgitations. The increase in number of
the breeding group of NZ sea lions at Otago
Peninsula may make it harder to obtain sam-
ples from females only. Outside summer, fe-
male NZ sea lions in established breeding
colonies typically disperse and become less
gregarious (Auge
´et al. 2009) and this may still
allow female diet studies using undigested
remains. Investigation of the diet of female
NZ sea lions could also be conducted by using
more invasive methods such as fatty acid
analyses of blubber or milk samples (Iverson
et al. 1997; Iverson et al. 2004; Beck, Rea et al.
2007; Meynier, Morel, Chilvers et al. 2008;
Cooper et al. 2009).
Female NZ sea lions based on Otago
Peninsula were previously reported as feeding
on yellow-eyed penguins and this raised con-
cerns for the viability of particular local popu-
lations of this endangered seabird species (Lalas
et al. 2007). We found that only two Otago-
born New Zealand female sea lions preyed on
Diet of Otago female NZ sea lions 103
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seabirds, including yellow-eyed penguins, dur-
ing each year. These two females were not
mother and daughter and their respective
daughters never fed on seabirds. The predation
by NZ sea lions on yellow-eyed penguins
consequently appears potentially to be an
individually learnt behaviour that is not trans-
mitted through generations.
Comparison with sub-Antarctic data
At Enderby Island, in the Auckland Islands, the
diet of NZ sea lions by mass (based on the
digested stomach content of by-caught adult
females and juveniles of both sexes with no
significant differences between all) was princi-
pally composed of octopus (27.8%), arrow squid
(20.8%), hoki (Macruronus novaezelandiae) and
hake (Merluccius australis) (together 19.2%),
opalfish (Hemerocoetes sp.) (4.7%) and red
cod (4.3%) (Meynier et al. 2009) during a simi-
lar season as our study (end of summer and
autumn). This was consistent with the spe-
cies found in faecal samples analysed by
Childerhouse et al. (2001). The diet of NZ sea
lions consequently differed between the Auck-
land Islands populations and Otago Peninsula
females on the mainland of New Zealand. Jack
mackerel and barracouta were taken only in
negligible numbers at the Auckland Islands
(Childerhouse et al. 2001; Meynier et al. 2009).
While the two main prey species of the
Otago females (barracouta and jack mackerel,
on average 57% of the diet) have energy con-
tent of 6.1 kJ.g
1
and 7.6 kJ.g
1
, respectively
(Pickston et al. 1982; Vlieg 1984), the two main
prey species (octopus and squid, 49% of the
diet) at the Auckland Islands have lower energy
content on average9sd (3.890.4 kJ.g
1
and
6.390.6 kJ.g
1
, respectively) (Meynier, Morel,
Mackenzie et al. 2008; Meynier et al. 2009).
Cephalopods were part of the prey range of the
Otago females but constituted only 20.2% and
13.6% of their diet each autumn (2008 and 2009,
respectively). Consequently, the food resources
at the Auckland Islands seem to be of lower
quality than the food resources available to
female NZ sea lions at Otago at least during
autumn, potentially supporting the hypothesis
that the Auckland Islands are marginal marine
habitat for this species (Chilvers et al. 2006;
Meynier et al. 2009). However, more studies on
energy content of prey throughout the year and
specifically off Otago Peninsula will be needed
to confirm this.
Resource overlap with other marine predators
Male NZ sea lions are found at Otago most of
the year, except during the breeding season
(December and January) when many migrate to
the breeding colonies in the Aucklands and
Campbell Islands (McConkey, Heinrich et al.
2002; Robertson et al. 2006). The best estimate
of the number of male NZ sea lions using
Otago Peninsula is currently 75 (S. McConkey,
New Zealand Sea Lion Trust, pers. comm.).
Male NZ sea lions at Otago had a similar prey
range as females during autumn, at least for the
main prey species in the reconstituted biomass
(Table 3), although they foraged on some
deeper water species than females. Neverthe-
less, because male diet was investigated more
than 10 years prior to this study, a concurrent
comparative investigation of Otago male and
female NZ sea lion diets during the same period
is needed to determine the overlap. In many
pinnipeds, males and females forage in different
areas and/or have segregated diets (Koen et al.
2000; Beck, Iverson et al. 2007; Trites &
Calkins 2008), including potentially NZ sea
lions at the Auckland Islands (Meynier, Morel,
Chilvers et al. 2008; Geschke & Chilvers 2009).
Southern elephant seals (Mirounga leonine),
leopardseals(Hydrurga leptonyx), orcas (Orcinus
orca), and several species of dolphins are only
occasionally sighted at Otago (J. Fyfe, DOC
Coastal Otago ranger, pers. comm.). Conse-
quently, the only other large marine mammal
that potentially competes for food resources with
female NZ sea lions at Otago is the New Zealand
fur seal. Fur seals breed in large numbers on
Otago Peninsula (between 20,000 and 30,000
fur seals [Lalas 2008]). Their main prey varies
104 AA Auge´ et al.
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between arrow squid in summer and autumn,
and barracouta, jack mackerel and octopus in
winter and spring (Carey 1992; Fea et al. 1999).
Red cod is taken during the entire year (Fea
et al. 1999). Although resource overlap between
fur seals and sea lions appeared limited during
autumn, both predators may target similar spe-
cies during half the year, and red cod all year.
All diet studies on New Zealand fur seals were,
however, conducted more than 10 years prior
to this study. To assess possible competition
between these two species, concurrent diet studies
could be conducted.
The other marine predators inhabiting the
Otago Pensinula are seabirds that all tend to
forage near Otago Peninsula (McClatchie et al.
1989; Moore 1999). Yellow-eyed penguins nest-
ing on Otago Peninsula feed mainly on blue cod
(18.3% of reconstituted biomass), opalfish
(28.6%), arrow squid (14.4%), and red cod
(7.4% [Moore & Wakelin 1997]). Three of these
species constituted a significant part of the diet
of female NZ sea lions, indicating some reso-
urce overlap but penguins feed on smaller prey
than sea lions (Moore & Wakelin 1997). Jack
mackerel was not part of the diet of yellow-eyed
penguins, and barracouta was rarely consumed
(Moore & Wakelin 1997). Most other seabird
species feed predominantly on small fish, krill
and invertebrate larvae (McClatchie et al. 1989).
Resource overlap with recreational fisheries
Both recreational and commercial fisheries
occur along the Otago coast. While commercial
fishing is well documented by the Ministry of
Fisheries, data on species and quantities taken
by recreational fishermen are sparse. Only one
survey conducted in 1991 enabled the evalua-
tion of this fishery (Teirney & Kilner 2002). The
main species taken by recreational fishermen at
Otago that overlapped with the prey range of
female NZ sea lions were blue cod (48% of
the total number of prey caught by fisher-
men), barracouta (10%), spiny dogfish (7%),
wrasse (3%), red cod (2%) and greenbone (2%).
Consequently, there is a resource overlap in
targeted species between female NZ sea lions
and recreational fisheries and potential compe-
tition around Otago Peninsula. However, there
is no estimate of biomass taken by recrea-
tional fishermen. This only survey of recrea-
tional fishing activities at Otago was also
conducted almost 20 years prior to this study
(Teirney & Kilner 2002). Recreational fishing
could be monitored in order to detect the
Table 3 Comparison of the top 10 prey species by percentage of biomass in the diet of male and female New
Zealand sea lions at Otago Peninsula, New Zealand.
Females during autumn
1
Males during autumn
2
Males year-round
2
Prey species % biomass Prey species % biomass Prey species % biomass
Barracouta 30 Jack mackerel 30 Barracouta 25
Jack mackerel 28 Barracouta 23 Maori octopus 14
Arrow squid 8 Maori octopus 16 Jack mackerel 13
Red cod 7 Ling 9 Skate 10
Maori octopus 6 Rough skate 7 Flounder 9
Yellow-eyed penguin 4 Red cod 3 Paddle crab 4
Wrasse 3 Greenbone 2 Ling 4
Greenbone 3 Flounder 1 Greenbone 4
Blue cod 2 Wrasse 1 Red cod 4
Little blue penguin 2 Paddle crab B0.1 Brill 2
Total 93 Total 92 Total 90
Notes:
1
Present study (mean across the two study years);
2
Lalas (1997).
Diet of Otago female NZ sea lions 105
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trend in increase or decrease in catch of parti-
cular species, and potential consequences for
the marine life foraging in the coastal area
off Otago Peninsula and adjacent coastlines,
including sea lions.
Resource overlap with commercial fisheries
Barracouta and jack mackerel have been in the
top five New Zealand commercially targeted
fish species by weight managed in the Quota
Management System (QMS, see Clark [1993])
at least since 2005 (Ministry of Fisheries 2010).
Except for wrasse and octopus (not taken in
commercial fishing activities) and seabirds, all
top 15 species in the diet of female NZ sea lions
at Otago in autumn are commercially targeted
around Otago Peninsula and managed in the
QMS (Ministry of Fisheries 2010). There is
consequently a resource overlap between NZ
sea lions and commercial fisheries, including
with three of the main New Zealand fisheries
(arrow squid, barracouta and jack mackerel, all
occurring in the marine areas around Otago
Peninsula [Ministry of Fisheries 2010]).
Marine food resources at Otago may have
been altered while the NZ sea lion was absent
from the mainland owing to fishing activities by
early Ma
¯ori and predominantly by large-scale
commercial fishing since the 1970s. Fur seals
have reached large numbers at Otago following
recolonisation, but the populations now seems
to have stabilised (Lalas 2008). Whether this is
because of the carrying capacity of the marine
habitat or of the terrestrial habitat is not
understood (Bradshaw et al. 2002). In order
to determine potential competition between NZ
sea lions and fishing activities around Otago
Peninsula, it will be necessary to gain detailed
data including primary production, available
biomass of prey species and their distribution
and migration, and masses taken out by sea
lions, other marine predators and fisheries to
integrate them into a complex marine trophic
model.
Conclusion
In autumn, the female NZ sea lions foraging
around Otago Peninsula fed predominantly on
barracouta and jack mackerel, but also to a
lesser extent on a wide range of cephalopods,
other bony fish, seabirds, cartilaginous fish,
crustaceans and salps. All prey species were
found in coastal waters or on the continental
shelf, hence highlighting that they foraged
exclusively in this area. There appears to be a
large resource overlap between female NZ sea
lions and other resident marine predators of
Otago Peninsula and recreational and commer-
cial fisheries that could lead to resource com-
petition as the population of sea lions grows.
More research is needed into marine produc-
tion around Otago Peninsula to evaluate this
possibility. Comparison between the diet of
female NZ sea lions at Otago and the diet of
NZ sea lions foraging around the Auckland
Islands indicated that prey quality may be
higher at Otago. Year-round diet studies of
male and female NZ sea lions at both sites are
needed to confirm this.
Acknowledgements
We would like to thank Nathan McNally for his help
in the field, Vivienne McNaughton and Kim Garrett
(Zoology Department, Otago University) for finding
the required gear and lab space, Jim Fyfe (DOC,
Coastal Otago) for his safety contact role, and two
anonymous reviewers for helpful comments on an
earlier draft of this manuscript.
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