Vol. 358: 273–287, 2008 MARINE ECOLOGY PROGRESS SERIES Published April 21 doi: 10.3354/meps07305 Mar Ecol Prog Ser

Colony-based foraging segregation by Antarctic fur seals at the Kerguelen Archipelago

Mary-Anne Lea1,*, Christophe Guinet2, Yves Cherel2, Mark Hindell1, Laurent Dubroca2, Sam Thalmann1

1Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, PO Box 252-05, Hobart, Tasmania 7001, Australia 2Centre d’Etudes Biologiques de Chizé-CNRS, 79360 Beauvoir-sur-Niort,

ABSTRACT: The foraging behaviour of conspecific female Antarctic fur seals (AFS) was compared simultaneously at 2 breeding colonies at Îles Kerguelen (S Indian Ocean). A remnant colony at ÎIes Nuageuses (IN) thought to have escaped sealing is hypothesized to be the source of increasing fur seal numbers at Cap Noir (CN) on the Kerguelen mainland. Inter-annual variability in foraging areas is known to occur in response to local oceanographic changes at CN. Given the distance between the 2 sites (~160 km), we hypothesize that seals from the 2 colonies may show spatial segregation in for- aging due to variability in local prey resource availability, although the transfer of foraging knowl- edge between sites via emigration may override such behaviour. The foraging zones, diving activity, diet and foraging success of seals were compared between sites using satellite telemetry, dive recorders and faecal analysis. No evidence of spatial foraging overlap was observed, with seals from IN conducting longer foraging trips, typified by a longer initial transit phase, than CN seals, which spent less time diving at night and dived more deeply. Pups nevertheless received higher absolute and daily energy delivery rates at IN. Diet was superficially similar at ~98% myctophid consumption; however, IN seals favoured the high-energy Gymnoscopelus nicholsi, indicating that local hetero- geneity in marine resources likely influences the foraging zone choice of AFS. Finally, distribution patterns of 54 female AFS tracked during summer months from 1998 to 2006 reveal the importance of both on-shelf (<500 m) and shelf-break regions as foraging habitat. The core foraging area for CN in all years (10 400 km2) was small (~10% of total foraging space); however, time spent in this region alone totaled 38%. The likelihood of spatial overlap in foraging range is higher on the east coast of Kerguelen.

KEY WORDS: Fur seal · Segregation · Myctophid · Diving ·

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INTRODUCTION such as seals and penguins, exhibit foraging plasticity both within and between sites in response to changes Intra-specific spatial and dietary segregation is in physical oceanography and associated marine pro- common between the sexes and the different age ductivity (McCafferty et al. 1998, Tremblay & Cherel classes of top level marine predators like seals (Ster- 2003, Lea et al. 2006, Trathan et al. 2006). Favourable ling & Ream 2004, Page et al. 2006), seabirds (Baird foraging and reproductive success are likely posi- 1991) and penguins (Forero et al. 2002). Such differ- tively influenced by the prevalence of proximal ences may be explained by variation in body size and, and/or predictable concentrations of prey (Mori & consequently, diving ability, or in relation to temporal Boyd 2004, Trathan et al. 2006). Temporal and spatial constraints on foraging behaviour related to breeding variability in marine productivity and availability also (Salamolard & Weimerskirch 1993, Charrassin et al. influence individual and colony-wide foraging behav- 1998), as in the case of central place foragers (Orians iour, while competition between similar niche species & Pearson 1979). High latitude species in particular, may be equally important in regulating foraging

*Email: [email protected] © Inter-Research 2008 · www.int-res.com 274 Mar Ecol Prog Ser 358: 273–287, 2008

activity (Barlow et al. 2002). At sub-Antarctic South Previous foraging studies of AFS females from the Georgia Island, where the majority of predators feed CN breeding colony indicate that an area extending on Antarctic krill Euphausia superba during summer from the northeast to the east of the colony is an impor- months, macaroni penguins Eudyptes chrysocome are tant foraging area (Lea et al. 2006). Given the potential a case in point. Penguins tracked from adjacent importance of the IN population to the recolonisation islands show considerable foraging overlap, while of the Courbet Peninsula at Îles Kerguelen and the those from more distant breeding sites displayed no proximity of the 2 islands within the foraging range overlap whatsoever. A 7 yr study of foraging habitat previously recorded for AFS at Îles Kerguelen (Bona- at one breeding site helped interpret these data. In all donna et al. 2001, Guinet et al. 2001, Lea et al. 2006), years, penguins concentrated their foraging efforts on some overlap in foraging habitat between sites may a bearing to the north of the colony, only varying the also be expected to occur. Consequently, the primary distance out to the shelf between years (Trathan et al. aim of this study was to determine whether the AFS 2006), suggesting that inter-annual variability in shelf population at IN displays evidence of foraging segre- prey availability determined the extent of foraging gation in terms of spatial foraging distribution, dive movements. behaviour, foraging success (such as pup mass gain) Few sympatric, conspecific comparisons of pinniped, and diet in comparison to seals from the derivative and particularly, otariid, foraging distribution patterns colony at CN. have been undertaken. Northern fur seals Callorhinus ursinus in northern sub-polar regions display a high degree of individual (Call et al. in press) and colony- MATERIALS AND METHODS related foraging site fidelity (Robson et al. 2004) during summer foraging trips, although the degree of overlap Study sites and population size. The Kerguelen varies temporally within a season and between years Archipelago (49° 20’ S, 70° 20’ E) in the southern Indian (Call et al. in press). Again, at South Georgia, female Ocean (Fig. 1) is comprised of approximately 300 Antarctic fur seals Arctocephalus gazella (AFS) from 2 islands with a combined coastline greater than colonies located only 90 km apart exhibited no spatial 3000 km. One of the most isolated land masses on overlap, even though their potential foraging range Earth, it is located more than 3000 km from both Africa was up to 300 km (Boyd et al. 2002). and Australia, and approximately 2200 km north of At sub-Antarctic Îles Kerguelen, AFS numbers are Antarctica. recovering after considerable sealing pressure in the The 2 study colonies were located 164 km apart 1800s (Budd & Downes 1969). Breeding individuals on at CN (49° 07’ S,70° 45’ E) on the northeast coast of the Courbet Peninsula of Kerguelen were first discov- the Courbet Peninsula (CN, the site of a 3 yr study) ered in the early 1980s (Bester 1981) and are thought to and at Île de Croy (48° 38’ S, 68° 38’ E, IN, Fig. 1) in have immigrated from a remnant population in the the IN. The colony at CN currently extends along the northwestern sector of Îles Kerguelen (Bester 1981), coast over a distance of approximately 1 km. Seals subsequently shown to be at Îles Nuageuses (IN) (Jou- breed on pebbly beaches with mothers and pups ventin et al. 1982). Studies of female AFS foraging moving onto the Cotula plumosa covered slopes and behaviour conducted on the Courbet Peninsula be- plateau as pups become more independent from tween 1998 and 2000 have established high individual their mothers. At the rarely visited IN colony, the foraging-site fidelity (Bonadonna et al. 2001) and a majority of breeding activity is centred around a strong correlation between many foraging parameters small, protected bay on the east coast of the island. and change in environmental parameters such as sea In 1984 seals were reported as breeding on the surface temperature, the location of frontal structures, headlands north and south of the bay and on an distance to the colony and the distribution of fish prey offshore islet to the southeast, with an estimated resources (Guinet et al. 2001, Lea & Dubroca 2003, Lea 1500 females on the headlands and over 5000 seals et al. 2006). To date, no studies of the foraging behav- in total (Jouventin & Stonehouse 1985). The IN iour of seals from the remnant IN population have been colony was visited from 5 to 20 January 2000, while undertaken. Given the inter-annual variability in for- an ongoing study was conducted at CN from De- aging success exhibited by AFS at Cap Noir (CN) in cember 1999 to March 2000. Analyses are restricted response to changing oceanographic conditions and to the common period of 5 to 20 January 2000 in prey availability (Lea et al. 2006), and the foraging early summer. habitat preferences predicted for central place-forag- Systematic whole colony counts of tagged and ing female AFS by Guinet et al. (2001), segregation in untagged pups at CN were made in late January foraging distribution and diet may well be expected in 1999 and 2000 as part of an ongoing 3 yr study (1998 between the 2 colonies if prey availability is sufficient. to 2000) at this site (see Table 1 and Lea et al. 2006). Lea et al.: foraging segregation 275

Fig. 1. Arctocephalus gazella. Time spent per 20 × 20 km grid cell for female Antarctic fur seals from Îles Nuageuses (west, n = 5) and Cap Noir (east, n = 7) in January 2000. Time spent (seal hours) is weighted by the number of seals and calculated for each colony separately. Inset map 13134 (Edition 1) courtesy of the Australian Antarctic Data Centre

A simplified mark-recapture technique using the the fur on the rump with peroxide hair-dye (Bristol- number of flipper-tagged pups resighted as a propor- Myers Squibb). During the attachment of the devices, tion of those known to be tagged was used as a cor- adult seals were held on a wooden restraint board. Fol- rection for counts of untagged pups. At IN, a single lowing the deployment, which lasted an average of 15 count of unmarked pups on the beaches and tussocks to 20 min, seals were released with their pups. The was made on 18 January 2000. A factor of 4.1 was used colonies were checked at least twice daily for the to convert pup counts to population size (Page et return of marked females overnight (0800) and during al. 2003). the day (1900). Foraging activity. Time-depth recorders (TDRs, Three types of dive recorders were deployed on Wildlife Computers) or a TDR and platform terminal seals from the 2 sites in January 2000. A single Mk7 transmitter package (ST-10 PTT, Telonics Pty, pack- TDR was attached dorsally to the fur 10 cm anterior to aged by Sirtrack) linked to the ARGOS satellite system the rump of 9 seals at IN and 10 seals at CN using 2- were deployed on adult females to study the diving part Araldite (AW 2101, Ciba Specialty Chemicals) for activity and foraging distribution of the seals (see the study of diving behaviour. TDR/PTT packages Bonadonna et al. 2000, Guinet et al. 2001, Lea et al. were also deployed on seals at both sites for the study 2002b, 2006). Seals were captured ashore using a hoop of at-sea distribution patterns. At CN, Mk7 TDR/PTT net and were subsequently weighed, a standard length packages (Lea & Dubroca 2003) were deployed on measurement was recorded (nose to tail in ventral 8 seals, while at IN, Mk5 TDR/PTT packages recumbancy) and an individual number was applied to (Bonadonna et al. 2000, Guinet et al. 2001) were deployed on 7 seals (Table 1). Rela- Table 1. Arctocephalus gazella. Pup production counts at Cap Noir (CN) and Île tively large seals with healthy pups de Croy (Îles Nuageuses) breeding colonies in 1999 and 2000 were selected for these deploy- ments. These packages were at- Date Size Pups counted Actual no. Estimated pup Est. colony tached dorsally between the scap- Tagged Untagged tagged pups production Size ulae. In total, devices were attached 22 Jan 1999 Cap Noir 130 545 140 724 2968 29 Jan 2000 Cap Noir 137 512 158 748 3067 to 33 seals (15 at IN and 18 at 18 Jan 2000 Île de Croy – 3279 – ~3600a 14 700 CN). During the course of the study aAdditional pups seen on inaccessible islet in east harbour 1 seal returned without the TDR/ PTT device (CN), 1 seal was not 276 Mar Ecol Prog Ser 358: 273–287, 2008

recaptured and thus PTT data were available without annelids) and other miscellaneous items in faeces was dive data (IN), and in 4 cases (2 per colony), the Mk7 noted. Percentage FO is expressed as a percentage of TDR failed to record dive data. PTTs (2 MK10-A satel- the number of faeces containing prey items. Numerical lite GPS tags, Wildlife Computers, and 3 ST-10 PTT) abundance of fish and squid was estimated by the were also deployed on 16 females at Pointe Suzanne sorting and identification to species level of otoliths on the southeastern Courbet Peninsula from 22 Janu- (sagitta, asteriscus and lapillus) when possible. See ary to 21 February 2006. The movements of these adult Lea et al. 2002a for further details. females, together with those at IN (from 2000) and CN Dietary comparisons between sites were conducted (from 1998 to 2000), constitute the total tracking using a stepwise Discriminant Function Analysis research undertaken at Îles Kerguelen and conse- (Systat 9.0, SPSS) with tolerance level set at 0.15 and quently provide the most complete island-wide esti- p values of 0.05. Measures of mass gain were com- mate of foraging habitat use. pared between sites using a 2-sample t-test. All vari- Transit and foraging phases were determined from ables were tested for normality and homogeneity of the onset and ending of bout diving during the forag- variance with Levene’s statistic. ing trip (see Lea et al. 2002b for bout analysis details). Foraging success. Inter-site comparisons of pup Inter-site comparisons of diving behaviour have been mass: A random cross-sectional sample of pups (n = restricted to those seals at each site carrying the small, 100) was weighed at each breeding site on 14 January rump-mounted Mk7 TDRs to minimize the device 2000 to test for differences in mean size of pups effects associated with carrying larger PTTs. An analy- between populations. Pups were weighed using a sis of similarity (ANOSIM) and a similarity of percent- Salter Weightronix spring balance (25 ± 0.02 kg) by ages (SIMPER) analysis with site as a factor were con- field teams moving systematically through the colony ducted using PRIMER 5.0 (Primer-E Limited). Seven to prevent the repeat weighing of individuals. Com- dive parameters were included in the analyses: mean parisons were made per sampling period by 2-way dive depth (m), dive duration (min), foraging trip ANOVA on log-transformed mass data with ‘sex’ and length (d), proportion of time spent diving, proportion ‘site’ as factors (SPSS). of dives in bouts, mean dives per bout and dive fre- Pup mass gain per foraging cycle: Mass gained by a quency (h–1, see Lea et al. 2002b for greater detail). pup during a foraging cycle (1 foraging trip and subse- Proportional data were arcsine-transformed and other quent shore bout) has been shown to represent a rela- data were log-transformed. tively easily obtainable measure of maternal energy Spatial analyses. Argos movement data of location transfer to the pup in several studies (Georges & quality 0 to 3 (see Bonadonna et al. 2001) were filtered Guinet 2000, Guinet et al. 2000). The pups of females for an upper speed of 3.0 m s–1 using the filtering algo- equipped with PTT and/or TDRs were weighed each rithm in the R-Software Trip package (Sumner 2006). day during the fasting period while females were at The resulting track was split into 60 min segments, sea, prior to the arrival of the female and again imme- which were then summed within 20 × 20 km grid cells diately after her departure, to estimate the mass to create time spent in area plots for habitat use. Time gained by the pup during the foraging cycle (FC). In spent in cells was converted to a proportion by colony instances where the body mass was not recorded on or year to standardize effects of sample size for the the arrival day of the female, it was estimated by linear various deployments. The centroid of the 20 × 20 km regression of pup mass lost during the previous fasting grid cells was spatially joined in ArcMap (ESRI) to the bout. A control group consisting of pups from nearest bathymetry value (GEBCO One Minute Grid) unequipped females was also monitored simultane- to provide a water depth measurement per cell. Bathy- ously at each colony (IN: n = 12; CN: n = 21). metric domains were defined as inner shelf (<500 m), outer shelf (500 to 1000 m), shelf break (1000 to 1500 m), and deep waters (>1500 m). RESULTS Diet. Faecal samples were collected from female haul-out sites in the colonies at CN and IN during the CN-IN comparison course of the study (6 January to 23 January 2000). Individual samples were frozen at –20°C on site and Foraging habitat use returned to the laboratory for sorting and prey identifi- cation. Samples were elutriated overnight and subse- There was no overlap in the foraging zones used by quently sieved with a 1 mm and 500 µm mesh. The fre- seals tracked simultaneously at IN (n = 5) and CN quency of occurrence (FO) of fish (otoliths, scales and (n = 7) in January 2000 (Fig.1). Seals from the CN bones), squid (beaks, radulas, pens and eye lenses), colony foraged within a ~240 km arc from the NE to the invertebrates (amphipods, isopods, nematodes and SE of the island (Table 2, Fig. 1), whilst the majority of Lea et al.: Antarctic fur seal foraging segregation 277 ; ) date (cm) (kg) sex (cm) (kg) duration (d) distance (km) Mean 114.6 ±1.2 30.1±1.0 72.9 ±1.5 6.7 ±0.7 1599.4 ± 195.9Mean 7.0 ± 0.6 117.4 ± 1.5 33.9 ± 1.1 240.1± 39.3 72.5 ± 1.2 8.5 ± 0.3 1830.2 ± 134.5 8.4 ± 0.3 247.6 ± 20 . Summary statistics for female Antarctic fur seals studied at Iles Nuageuses (IN) and Cap Noir (CN), Kerguelen. L: length PTT: platform transmitter terminal;PTT: TDR: time-depth recorder; Mk7: Mk7 TDR; y: yes; BDG: Behaviour Dive Group (Lea et al. 2002b Arctocephalus gazella Site Unit Seal BDG Deploy Length Mass Pup Pup L Pup Mass Dives PTT TDR Trip Maximum CNCNCN Mk7CN Mk7CN Mk7CN Mk7 J1CN Mk7 J2 Mk7 J3CN Mk7 J6CN 2 J7CN 2 J9 PTTCN 2 J10 PTTCN 2 7 Jan PTTCN 2 7 Jan PTT H2CN 2 6 Jan PTT 107.5 H3 2 7 Jan PTT 109.5 H4IN 8 Jan PTT H5 109.5 1IN 8 Jan 24.5 H6 113 8 Jan 3IN 24.8 120.5 H7 Mk7 1IN 119.5 H8 26.8 Mk7 15 Jan 1IN 111 F Mk7 29 Jan 26.9 2IN 32.4 F IN8 Mk7 117.5 30 Jan 2IN IN10 33.0 M Mk7 114.5 61.5 30 Jan 1 IN11 24.9 Mk7 M 69.5 31 Jan IN M 113 IN12 Mk7 34.3 1 29 Jan IN 71 110 IN13 F 2 34.8 6.0 31 Jan IN 117 IN14 4 F 65 PTT 73 3.2IN 115 IN15 29.4 2 M PTT 119.3IN 5 Jan 71 29.7 1 5.1 6 Jan PTT FIN 66.5 33.9 2 8 Jan IN1 1213 PTT 8.1IN 112.5 7.9 M 74 28.3 1 6 Jan IN2 PTT 31.5 706 128 77.5 F 5.8 8 Jan IN3 PTT 112 4.3 M 1118 7 Jan IN4 PTT 76 – 114 29.4 1 F M 8 Jan IN5 6.8 120.3 2230 73.5 2186 1 12.3 41.0 – IN6 80.5 126 1 30.7 1100 IN7 – 1299 122 3 F y 7 Jan 83 30.0 79 8.9 36.8 6.2 – 1 M 6 Jan – 1163 y 2453 9.3 . 5 Jan 35.5 F 121.3 73.5 – y 1 – 5 Jan 40.0 F 6.0 5.16 75.5 112.3 F 8.8 y 2055 8 Jan y y 1141 3.2 113 y M 68 10 Jan 36.8 2863 112.3 7.3 y 65.5 5.1 M 71.5 7 Jan y 33.7 8.3 114 915 y 8.1 2855 123 7.9 y 83 y y M 31.4 28.0 – 112.8 6.5 y 74 9.1 5.8 9.0 M 1213 4.3 – 32.7 2226 y y y 76 37.0 M 12.3 – y 6.8 8.2 32.2 F y 70 6.9 1178 – 2847 – – 1674 F 74.5 – M y y 8.9 8.7 – 73 F 6.2 – 2335 9.3 8.8 448 – 230 y – 73 1367 – 7.5 71 y 66.5 5.2 8.8 8.9 2155 – 193 y y 186 7.3 11.4 2015 – y 10.0 259 8.3 8.3 1410 – y 1718 114 251 6.5 9.1 y y 9.0 1809 y 1845 – – y 8.2 y – y 6.9 – y y – – 8.7 – y y 8.8 y – 7.5 y – – 8.9 11.4 – 8.3 281 8.8 202 201 – 301 253 Table 2. Table 278 Mar Ecol Prog Ser 358: 273–287, 2008

seals at IN foraged within a ~250 km arc to the W and SW of the colony (224° to 278°). The foraging range of FT = 0.76 + 1.02FP females was similar between the 2 colonies (t10 = 0.149, p > 0.05), although the range in maximum distance R2= 0.96, p < 0.001 traveled from CN was highly variable (114 to 448 km, 8 Table 2). Time spent in different bathymetric regions varied considerably between sites. Females at IN spent more time on the inner shelf (39%) and in deep waters (33%) than on the outer shelf (14%) and shelf break (12%). At CN females spent more time in deeper 6 waters (36%) and on the outer shelf (26%) than on the inner shelf (17%) and shelf break (16%).

There is some evidence for device effects, with PTT fe- duration (d) Trip males recording longer foraging trips than TDR females

(F1,25 = 5.185, p = 0.038, Table 3). Consequently, we have only included TDR dive statistics in analyses for 7 fe- 4 males at each site (3 TDRs at CN and 1 TDR at IN failed, CN and 1 TDR at IN was not retrieved). Absolute and pro- IN portional transit and foraging phases of PTT females were similar between sites. Trip durations of TDR fe- 345678 males at IN were significantly longer than those at CN Foraging phase (d) (Table 3), as were transit times to the start of the first for- aging bout (0.70 d cf. 0.41 d; t = –2.370, p = 0.036). No Fig. 2. Arctocephalus gazella. Relationship between trip dura- 11.87 tion and foraging phase length for females at Îles Nuageuses significant differences in return transit phase, foraging (IN) (n = 7) and Cap Noir (CN) (n = 7). FT: foraging trip phase length (time between transit phases), or the pro- duration; FP: foraging phase portion of any phase in relation to trip duration were de- tected between sites, although the foraging phase tended to be longer at IN (Table 3). Foraging phase CN (r1,7 = 0.425, p > 0.05). The timing of diving activity length was positively related to trip duration (Fig. 2). varied significantly between sites (t = –3.780, p = 0.008) The diving behaviour of females at each site (IN: with females at CN conducting 94.5 ± 1.0 (SE) % of 12 840 dives, CN: 9852 dives) differed markedly when their diving nocturnally (Fig. 3, Table 4), compared compared by ANOSIM, at p = 0.023. Foraging trip with females from IN at 68.6 ± 6.8% (33 to 87%). duration, mean dive depth and the number of dives per hour accounted for 65.2% of the inter-site dissimilarity (Table 4). Trips were longer, dives deeper and the fre- Dietary preferences quency of diving less at IN (Table 4). If trip duration was removed from the analysis, mean dive depth and The frequency of occurrence of fish in the diet of fe- dive frequency alone accounted for 52.4% of the dis- male seals was high, with fish remains predominating similarity between sites (p = 0.044). Mean dive depth at both study sites (83% at IN and 89% at CN, Table 5). was also significantly negatively correlated to foraging Evidence of cephalopod ingestion was considerably phase duration at IN (r1,7 = –0.904, p = 0.005) but not at higher at CN than at IN, with beaks occurring in 67% of scats samples at CN (cf. 33% at IN). Table 3. Arctocephalus gazella. Mean durations and proportions of foraging Crustaceans and other invertebrates and transit phases for females Antarctic fur seals from Iles Nuageuses (IN) and were relatively uncommon by compar- Cap Noir (CN). Sample sizes in parentheses ison, occurring in only one-third of samples from both sites. Duration of trip IN (7) CN (7) tdfp In total 18 species of fish were recorded in the diet of female fur seals Phases (days) Mean SE Mean SE Transit phase 1 (T1) 0.70 0.09 0.41 0.08 –2.370 12 0.035 at IN and 15 species at CN (Table 6). % time in T1 0.93 0.52 1.05 0.42 –0.501 12 0.625 The diet at both sites was dominated Foraging phase (FP) 6.78 0.37 5.14 0.74 –1.991 12 0.070 by myctophids, which accounted for % time in FP 85.79 1.33 87.33 3.33 –0.431 12 0.674 97.8% at IN and 98.8% at CN. In total, Transit phase 2 (T2) 0.41 0.17 0.20 0.09 –1.102 12 0.292 1727 sagittal otoliths were recovered % time in T2 6.16 2.48 6.23 3.08 0.018 12 0.986 from the scats. Although the mean Total trip length 7.89 0.38 5.76 0.67 2.768 12 0.017 number of otoliths per scat was highest Lea et al.: Antarctic fur seal foraging segregation 279

Table 4. Arctocephalus gazella. Mean comparisons and analysis of dissimilarity for dive Discriminant Function Analysis parameters between female at Iles Nuageuses (IN) and Cap Noir (CN). PMG: pup mass (DFA) of log-transformed abun- gain during foraging trip (kg); PM: night dance data (Wilks’ Lambda =

0.2474, approx. F11,16 = 4.4249, Parameter IN (n = 7) CN (n = 7) T p Cumulative % p <0.01). A backwards-stepwise dissimilarity DFA conducted on the same 1. Trip length (d) 7.89 ± 0.38 5.76 ± 0.67 2.768 0.017 26.9 data indicated that 3 species, 2. Mean depth (m) 59.7 ± 6.92 43.1 ± 1.44 2.343 0.054 48.3 Gymnoscopelus nicholsi, Elec- 3. Dives per hour 9.5 ± 0.9 10.1 ± 0.7 –0.470 0.646 65.2 4. Mean dive duration (min) 1.59 ± 0.13 1.29 ± 0.06 8.482 0.069 81.8 trona carlsbergi and Gobiono- 5. Dives per bout 10.1 ± 0.3 11.6 ± 0.7 –1.971 0.083 93.9 tothen acuta accounted for the 6. Time spent diving (%) 21.4 ± 0.01 24.2 ± 0.01 1.927 0.078 distinction in diet between sites 7. Proportion dives in bouts 97.6 ± 0.003 97.1 ± 0.004 0.794 0.443 (Wilks’ Lambda = 0.2526, ap- 8. Proportion dives PM 68.6 ± 0.07 94.5 ± 0.01 –3.780 0.008 prox. F = 23.6758, p < 0.001). 9. Time spent diving PM 41.7 ± 0.04 58.0 ± 0.02 –3.390 0.005 3,24 10. Prop Vertical depth PM 50.0 ± 6.5 87.1 ± 3.3 –5.103 0.000 Using these 3 species as dis- 11. Dives per hour PM 20.7 ± 3.6 29.3 ± 2.0 –2.127 0.055 criminators, only 2 of 28 sam- PMG (TDR) 3.1 ± 0.3 1.7 ± 0.3 –3.049 0.011 ples were incorrectly assigned PMG_FT (TDR) 0.394 ± 0.04 0.278 ± 0.02 –2.144 0.055 (Jackknife matrix 93% success, PMG_FC (TDR) 0.312 ± 0.04 0.223 ± 0.02 –2.548 0.027 IN: 100%; CN: 86%). at IN (71.6 ± 22.4 [SE], range 1 to 238), it did not differ Foraging success significantly to that at CN (51.8 ± 11.6). Diet samples from IN were dominated by Gymnoscopelus nicholsi Body mass of pups was highly variable between indi- (31.2%) and G. fraseri (29.1%), with Krefftichthys viduals, ranging from 3.9 to 13.6 kg and 3.8 to 13.4 kg anderssoni (12.0%) also important to several individu- at CN and IN, respectively. Male pups were heavier als. At CN the prevalence of G. fraseri in the diet than females pups (F1,99 = 26.50, p < 0.001) and IN pups (40.8%) outweighed that of all other species, while were heavier than CN pups (F1,99 = 7.22, p < 0.01, Electrona subaspera was also more common than in Table 7). samples from IN (Table 6, Fig. 4). Mass gained by pups (PMG) after a maternal foraging A significant difference in the dietary composition trip was measured for pups of control (IN: 2.7 ± between sites was detected by a complete linear 0.2 [SE] kg, n = 8; CN: 2.2 ± 0.2 kg, n = 20), PTT females

Fig. 3. Arctocephalus gazella. Diurnal variability in maximum dive depth at (a) Cap Noir (n = 7) and (b) Îles Nuageuses (n = 7). Shaded regions denote night 280 Mar Ecol Prog Ser 358: 273–287, 2008

Table 5. Arctocephalus gazella. Frequency of occurrence by 50 number and percentage of prey items recorded in Antarctic fur seal scats in January 2000 at Île de Croy (Iles Nuageuses) and CN Cap Noir, Kerguelen Archipelago. Number of scats sampled and the number containing prey items are in parentheses 40 IN

Île de Croy Cap Noir (37/18) (26/18) 30 n% n%

Fish 15 83.3 16 88.9

Otoliths 13 72.2 14 77.8 % Abundance 20 Eye lenses 9 50.0 13 72.2 Bones 14 77.8 12 66.7 Vertebrae 10 55.6 11 61.1 Myctophids 9 50.0 15 83.3 10 (scales or jaw bones) Channichthyids 1 5.6 0 0.0 (jaw bones or opercles) Nototheniids (jaw bones) 1 5.6 1 5.6 0 Paradiplospinus gracilis 1 5.6 0 0.0 Ea Ec Es Gb Gf Gn Gp Ka Pb Pc Pt (teeth or vertebrae) Harpagifer spinosus 0 0.0 1 5.6 Species (opercles) Cephalopods 6 33.3 13 72.2 Fig. 4. Arctocephalus gazella. Percentage abundance of 11 Beaks 4 22.2 12 66.7 myctophid species observed in the diet of Antarctic fur seals Eye lenses 3 16.7 10 55.6 at Cap Noir (CN) and Îles Nuageuses (IN). Species abbre- Gladii 2 11.1 6 33.3 viations listed in Table 6 Crustaceans 6 33.3 6 33.3 Themisto gaudichaudii 5 27.8 4 22.2 Other amphipods 3 16.7 0 0.0 (January to March) tracking studies for 33 females Isopods 1 5.6 1 5.6 from CN. Black cells demarcate the concentrated core Copepods 0 0.0 1 5.6 Others foraging region common to seals in all 3 yr (1998 to Nematodes 12 66.7 10 55.6 2000, Table 8), representing 9.7% of the total foraging Pebbles 5 27.8 15 83.3 region. Mollusk shells 2 11.1 1 5.6 Feathers (penguin) 1 5.6 0 0.0 Finally, the foraging habitat use of seals from all 3 sites is presented in Fig. 6c. Foraging effort represents a combined 403 seal days for 54 individuals encom- (IN: 2.9 ± 0.2 kg, n = 4; CN: 2.8 ± 0.2 kg, n = 6) and TDR passing 281 20 × 20 km grid cells (112 400 km2). females (Table 4). PMG varied significantly in relation to Although not sampled simultaneously, the potential unit type only at CN, where pups of TDR females gained spatial overlap between the combined foraging range less mass than PTT pups (F32,2 = 4.672, p = 0.016) while of CN seals and Pointe Suzanne females was high at control pups were intermediate (Fig. 5a). Overall, pups 31.4%; however, the overlap with the core CN region of TDR females at IN gained more absolute mass and was low at 3%. Bathymetric domains favoured by seals daily mass per trip and foraging cycle than pups at CN in order of importance were: outer shelf (34.3%), deep (Table 3). A multi-linear regression indicated trip dura- water (30%), inner shelf (19.9%) and shelf break tion (FT) and maternal length (ML) contributed equally waters (15.9%). to absolute PMG (PMG = 0.318FT + 0.098ML – 10.534, p < 0.001, r = 0.939, Fig. 5b). DISCUSSION

Regional habitat use Between site comparisons in the same year

Females from the previously unstudied colony at Spatial distribution and diving activity Pointe Suzanne on the Courbet Peninsula dispersed to the SE over the Kerguelen Plateau towards Heard Given the likely emigration of seals from IN to CN, Island (Fig. 6a). Effort in terms of seal hours is rela- some degree of similarity in foraging locations might be tively evenly distributed, although more time was anticipated for seals from both sites. However, spatial spent by seals at the outer reaches of the foraging zone overlap analyses of the foraging patterns of adult on the inner/outer shelf boundary (SE) and the shelf females from IN and CN in 2000 indicate complete for- break (E and S). Fig. 6b summarizes previous summer aging segregation at the 20 × 20 km scale. Sample sizes Lea et al.: Antarctic fur seal foraging segregation 281 r seals at Île de with prey items) (Cherel et al. 1997, Lea et al. 2002a) Brachioteuthis ?riisei LRL (mm) LRL (mm) Previously described as c 97.81 98.76 1002 100 725 100 FO No. OL (mm) FO No. OL (mm) – – – – 1 7.14 2 2.85 (1) 55 35.71 35.71 48 33 4.79 3.29 3.75 ± 0.07 (11) 6.07 ± 0.07 (3) 2 6 14.29 42.86 22 23 3.03 3.17 3.62 ± 0.06 5.24 (1) 95 64.29 35.717 1451 120 50.00 14.47 11.982 7.141 14.29 122 1.20 ± 0.03 (36)1 7.14 5 – 1.20 14.29 7.14 3 0.504 22 2 0.30 28.571 1.73 ± 0.03 (5) 1 0.20 14.29 – 4 0.201 3.72 ± 7.14 0.25 (3) 0.10 28.571 121 7.14 3 2.09 (1) 100 1.20 7.14 2 15 7.14 0.30 – 71.43 0.00 3 1 0.20 – 2.07 1 12 21.43 110 1 0.10 – 10 85.71 0 0.10 15.17 1.26 ± 0.04 – 71.43 0.10 4 0.00 7 –3 3.21 (1) 691 50.00 69 0.55 21.4 9.52 – 5 – 7.14 9.52 – 0 1.77 ± 0.01 (2) 35.71 – 25 3.33 ± 0.15 (8) 4 1 0.00 3.45 1 0 13 7.14 – 0 1 1.90 ± 0.03 (4) 1.79 0.00 0 0.00 7.14 2 0.00 0 1.38, 1.88 (2) 1 0 1 0.28 0.00 0 – 2 7.14 – 0.00 7.14 0.00 0.28 – 0 2 – 9 1 0.00 – 0.28 64.3 – 0.14 – – 38 – – – – 2.19 ± 0.08 (20) – – n % n % (n) n % n % (n) 1210 85.71 71.43 313 292 31.24 29.14 4.45 ± 0.04 (97) 3.58 ± 0.03 (86) 9 10 64.29 71.43 58 296 40.83 8.00 3.49 ± 0.06 (33) 4.74 ± 0.13 (8) Not included in discriminant function analyses; c b b . Frequency of occurrence (FO), numbers, otolith (OL) or lower rostral beak length (LRL) of fish and squid prey of Antarctic fu (Gn) a b (Ec) a a sp. (eroded) (range) 71.6 (1–238) 51.8 (1–138) Croy are (Îles Nuageuses) and Cap Noir in January mean ± 2000. Values SE. Numbers in parentheses: (no. of samples/no. samples Arctocephalus gazella Gymnoscopelus piabilis (Gp) Myctophidae (eroded) Gymnoscopelus nicholsi Gymnoscopelus fraseri (Gf) Gymnoscopelus Moroteuthis knipovitchi Slosarczykovia circumantarctica Krefftichthys anderssoni (Ka) Electrona carlsbergi Protomyctophum bolini (Pb) Electrona Antarctica (Ea)Electrona subaspera (Es) Protomyctophum choriodon (Pc) Protomyctophum tension (Pt) Gymnoscopelus bolini (Gb) 4Icichthys australis acuta 28.57Scopelosaurus hamiltoni Channichthyidae sp. (eroded)Paradiplospinus gracilis 4Benthalbella elongata ?Nansenia Antarctica 0.40Magnisudis prionosa 1Osteichthyes sp. (eroded) 7.14 – 2 0 0.20Kondakovia longimana 0.00Undetermined squids 0 – 7 0.00 50.00 0 12 0.00 – 1.66 0 1.51 ± 0.11 (2) 0.00 0 2 0.00 14.29 2 – 0.28 – 3 21.43 3 Significant contribution in DFA; Significant contribution in DFA; Myctophidae Mean per scat Squid Other fish Total Fish Île de Croy (37/23)a Cap Noir (26/12) Table 6. Table 282 Mar Ecol Prog Ser 358: 273–287, 2008

Table 7. Arctocephalus gazella. Mean pup body mass at Cap et al. 2001, Bost et al. 2002, Charrassin et al. 2004, Lea Noir (CN) and Iles Nuageuses (IN) in January 2000 et al. 2006) and fisheries (Cherel et al. 1996). The only previous records of fur seals to the west of Kerguelen Colony Pup sex Body mass (±SE, kg) n include large groups sighted at sea (Ensor & Shaugh- nessy 1990), and more recently sightings and interac- CN M 7.64 ± 0.25 43 tions of fur seals with fishing vessels (Capdeville 1997). CN F 6.48 ± 0.19 57 The western slope of the Kerguelen shelf and the Skiff IN M 8.21 ± 0.25 52 Bank region have been the focus of longline fisheries IN F 7.08 ± 0.21 48 since the early 1990s (Duhamel 1992, Duhamel et al. 2005). Clearly, local marine productivity is sufficiently high for IN were small and consequently should be inter- on both the east and west coasts of the island to enable preted with some degree of caution. However, concur- distinct foraging segregation between the 2 colonies rent differences in diet, diving behaviour and indirect irrespective of their close proximity (~160 km). Such a foraging success in terms of mass gain also support the distance would take ~18 hr to cover at average surface idea of segregated foraging. Seals from IN concen- swim speeds of 2.5 m s–1 (Watson 2007). Transit times trated their foraging effort westwards and to the SW to and from feeding areas and maximum distance trav- along the shelf edge towards the Skiff Bank region in eled did not vary significantly for tracked seals (Fig. 1); association with inner shelf and deep waters (Fig. 6). however, there does appear to be some effect of carry- CN females, by comparison, dispersed widely from the ing the PTT/TDR devices as foraging trips were longer colony to the east and south, focusing effort in deep wa- for these females than those carrying only a tail- ter to the NE and along the outer shelf and southwards mounted TDR. Transit time to feeding areas and forag- along the Kerguelen Plateau. The area to the NE of CN ing trip durations were, however, significantly longer is highly productive, supporting one of the largest sea- for TDR females from IN than CN, implying that the sonal phytoplankton blooms in the Southern Ocean and onset of foraging occurs earlier at CN. In addition to high fish abundances associated with the flow of the increased foraging times, IN females spent more time Antarctic Polar Front (Duhamel et al. 2000, Blain et al. diving during the day, to deeper depths (~60 m) than 2007). Consequently, it is regionally important for their CN counterparts (see Lea et al. 2002b). It is likely Antarctic fur seals and other marine predators (Guinet that females are foraging over the extended shelf

a j b j 4 j m j d d j j j

m m d m m 3 d d d d m j d j d d d d d m j dj j mj j j d m d d d m d j j 2 d d d j d jj j d d Control d d Absolute mass gain (kg) j Mk7 m PTT j j d j j 1 d PMG = 0.66 + 0.27 x FT PMG = –14.44 + 0.15 x ML 2 2 j (r = 0.47, p<0.001) j (r = 0.69, p<0.001)

4681012 110 115 120 125 Trip length (d) Maternal length (cm) Fig. 5. Arctocephalus gazella. (a) Regression of absolute mass gained by pups (PMG) against maternal trip duration for control (n = 28), TDR-equipped (Mk7; n = 14) and PTT-equipped females (n = 10); and (b) regression of PMG against maternal length for TDR equipped females; CN: Cap Noir; IN: Iles Nuageuses; black: CN; grey: IN Lea et al.: Antarctic fur seal foraging segregation 283

Fig. 6. Arctocephalus gazella. Time spent in seal hours per 20 × 20 km grid by adult female Antarctic fur seals at: (a) Pointe Suzanne (2006, n = 16); (b) Cap Noir (1998–2000, n = 33); and (c) all 3 colonies at Îles Kerguelen in 1998, 1999, 2000 and 2006 (n = 54). Black circles denote core summer foraging region for female seals from Cap Noir. Grey circles denote overlap between females from Cap Noir and Pointe Suzanne 284 Mar Ecol Prog Ser 358: 273–287, 2008

Table 8. Arctocephalus gazella. Habitat use and spatial foraging overlap for adult female Antarctic fur seals from Cap Noir (CN), Iles Nuageuses (IN) and Pointe Suzanne (PS)

Colony comparisons n Seal foraging Number Foraging area Mean area Mean water hours 20 × 20 km cells (km2) per seal (km2) depth

CN: Jan 00 7 1414 95 38 000 5429 1611± 102 IN: Jan 00 5 1090 55 22 000 4400 1041 ± 87 CN: summer 98–00 33 5694 184 73 600 2230 1696 ± 79 PS: summer 06 16 2892 113 45 200 2825 839 ± 51 All colonies 54 9676 281 112 400 2082 1443 ± 58 Overlap % overlap Sum time spent (%) Core CN: Summer 98–00 – 9.7 26 10 400 38.0 854 ± 114 Core CN & PS – 3.6 14 5600 11.8 886 ± 120 CN Summer & PS – 31.4 71 28 400 37.7 876 ± 69

region to the west of IN during daytime transits to pro- 1996, Lea et al. 2002a, Lea et al. 2006) and Heard island ductive shelf-break regions and access to preferred (Green et al. 1989). At IN in 1994, G. nicholsi was the pelagic prey resources. Local oceanographic features, most numerous prey item taken by male, female and such as current movements, may contribute to the juvenile fur seals (Cherel et al. 1996). In 2000, the diet outward direction (Park et al. in press) taken by adult of AFS at both sites was dominated by myctophids. We females in addition to an individual’s prior knowledge confirm the importance of G. nicholsi in the diet of adult of seasonal food resources (Bonadonna et al. 2001); females at IN and highlight the common occurrence of the persistence of oceanic prey ‘hot spots’ may also G. fraseri at CN and IN. G. nicholsi is mesopelagic/ enable predators to reduce foraging effort (Gende & epibenthic, generally distributed along the shelf Sigler 2006). breaks and over the shelf at depth (Duhamel et al. 2005), migrating vertically to the top 100 m at night to feed (Pusch et al. 2004). Given the high occurrence of Dietary segregation G. nicholsi in the diet of fur seals at IN, and its general absence at CN, it is reasonable to assume that shelf wa- Identifiable squid remains occurred in more than ters W and SW of IN are prime habitat for G. nicholsi. 70% of scats at CN and were relatively rare by com- Rare research surveys to this productive region corrob- parison at IN (33%). The primary species taken by CN orate the existence of G. nicholsi (Koubbi et al. 2000) females, Slosarczykovia circumantarctica, is a rela- here and to the NE of Kerguelen along the shelf break tively small, abundant, pelagic migratory species from (Duhamel et al. 2005). G. nicholsi is comparable in Antarctica (Cherel & Hobson 2005). Commonly con- calorific content to G. fraseri per gram at ~10 kJ (Lea et sumed by predators — including fish, seabirds, seals al. 2002c); however, the average size of fish consumed and penguins — at both Crozet and Kerguelen (Cherel varies considerably. A 7.5 cm G. fraseri yields approxi- et al. 2004), it was also the primary squid species taken mately 45 kJ while an 11 cm G. nicholsi yields 3 times by fur seals at CN during previous diet studies (Lea et more energy, around 145 kJ (see Lea et al. 2002c). This al. 2002a). Although squid consumption is likely discrepancy may offset the longer transit times, forag- underestimated by the faecal analysis technique ing trips and deeper dives recorded for IN seals. Milk employed (as many squid beaks are regurgitated, fat content also increases concomitantly with increas- Kirkman et al. 2000), fur seal diet at Îles Kerguelen is ing myctophid content in the diet, potentially further dominated by myctophid fish. offsetting increased foraging effort by IN females by The fish assemblage in the Polar Frontal zone waters increasing the calorific content of their milk available to near Kerguelen is comprised primarily of Myctophidae their pups (Lea & Dubroca 2003). (Duhamel et al. 2000) which constitute the largest bio- Recently Reid et al. (2006) have shown that local mass in the Southern Ocean after krill (Loots et al. marine topography and oceanographic variability 2007). The most numerous of the circum-Antarctic spe- likely explain the temporal and spatial variability cies are Gymnoscopelus nicholsi and Electrona antarc- recorded in AFS diet in the Atlantic sector of the South- tica (Sabourenkov 1990). Myctophids are an important ern Ocean. Zeppelin & Ream (2006) have confirmed dietary component for many sub-Antarctic marine such influences, showing that colony-specific dietary predators, notably the (Bost et al. 2002) preferences of northern fur seals persist over many and fur seals — in particular at Kerguelen (Cherel et al. years and are indicative of foraging in distinct hydro- Lea et al.: Antarctic fur seal foraging segregation 285

graphic domains. The differences in habitat use and foraging effort was focused to the SE along the Ker- prey selection observed between CN and IN would be guelen Plateau (Lea & Dubroca 2003, Lea et al. 2006). further clarified by simultaneous prey surveys in east This region is also an important foraging habitat for and west coast bathymetric domains utilized by fur king penguins Aptenodytes patagonicus breeding on seals. the east coast of Kerguelen (Bost et al. 2002). Potential competition between these 2 myctophid specialists is minimized by the diel feeding habits of both predators, Provisioning success with king penguins feeding primarily during the day at greater depths than the nocturnal fur seals. King pen- Pup body mass and mass gained per foraging trip guins have been shown to access the cold thermocline were both significantly higher at IN than at CN, per- waters below 100 m along the eastern edge of the Ker- haps indicating higher prey availability at IN. guelen Plateau (Charrassin et al. 2004), while fur seals Although median birth dates have not been measured tend to dive in the upper 50 m of the water column, at either site, they are similar at both South Georgia only descending to greater mean depths in years of and Marion Island (see Hofmeyr et al. 2007), which are anomalously warm sea surface temperatures and much farther apart, indicating that pup body mass data reduced prey availability (Lea et al. 2006). may constitute a valid index of relative prey availabil- The potential overlap between Pointe Suzanne seals ity. Daily PMG per foraging trip was also significantly and all CN individuals occurs primarily in this region higher at IN, with longer, potentially older, females over outer shelf waters. Animals from Pointe Suzanne transferring more mass to pups than smaller females. (2006) and CN (1998 to 2000) were not tracked simul- This finding implies that experienced females may be taneously and, consequently, interpretation of overlap more efficient foragers, and that body size also confers between sites is somewhat limited. Although it is advantages in terms of energy transfer rates (Beauplet unclear whether overlap is persistent or more likely in & Guinet 2007). Females at CN dive more frequently years of high and dispersed prey availability, seals during restricted bouts of night diving (29 dives h–1), from the Courbet Peninsula clearly utilize common while IN females tend to spend less time diving over inner and outer shelf domains. Our findings concur the course of foraging trips, due to longer transit times. with those on other central place foraging marine Foraging trips at CN averaged 5.7 d, while IN females predators. Trathan et al. (2006) showed that overlap in averaged 7.9 d. So although mass gain rates were foraging zones was high for closely adjacent macaroni higher at IN, overall, pup growth rates at both sites penguin colonies and minimal for more distant may be similar due to the discrepancies in mean forag- colonies. Boyd et al. (2002) reported no overlap for AFS ing trip length. Pups at CN would have 20.7 feeding females from colonies 300 km apart, which is well events over the 4 mo lactation period compared with within the foraging range of the species, while Robson 15.2 feeding events at IN. At this stage, without the et al. (2004) noted greater overlap between northern benefit of multi-year studies, it is difficult to state fur seals from colonies on the same island than whether one breeding site promotes more favourable between islands which were only 72 km apart. pup rearing conditions than the other. However, the Population-level foraging distribution patterns and rapid population growth of colonies on both coasts sug- habitat use by AFS at Îles Kerguelen in 1998, 1999, 2000 gests that fur seals have not yet attained carrying and 2006 confirm the importance of inner and shelf re- capacity at Îles Kerguelen. gions, and deep water habitats within a 250 km range of breeding colonies. Guinet et al. (2001) predicted the likely habitat use of central place foraging AFS at Îles Multi-year and site comparisons of Antarctic fur seal Kerguelen based on bathymetry, oceanographic features habitat use and fish distribution for 1998. The actual foraging distri- bution of Kerguelen seals (this study) confirms these pre- Niche overlap theory predicts that overlap should dictions when distance to the colony is excluded, indicat- decrease with increasing environmental variability ing a preference for shelf and shelf break areas to the SW and with increasing numbers of species (Pianka 1974). (IN) and E and NE (CN). The waters surrounding Kerguelen are highly dynamic and variable in terms of local productivity. This is reflected in the inter-annual variability of forag- CONCLUSIONS ing zones utilized by female AFS at CN (Lea et al. 2006). Although seals concentrated foraging in a core Habitat use by the increasing fur seal population at area along the shelf break to the NE of the colony in all Îles Kerguelen is variable between colonies and be- years, in 2000, a year of high pup growth at CN, much tween years. Competition between conspecifics from 286 Mar Ecol Prog Ser 358: 273–287, 2008

west and east coast colonies appears low, with no spa- Bonadonna F, Lea MA, Guinet C (2000) Foraging routes of tial overlap and distinct differences in the species com- Antarctic fur seals (Arctocephalus gazella) investigated by position observed within the diet. Despite the close the concurrent use of satellite tracking and time-depth recorders. Polar Biol 23:149–159 proximity of these colonies, indications are that high Bonadonna F, Lea MA, Dehorter O, Guinet C (2001) Foraging relative prey densities adjacent to the breeding ground fidelity and route-choice tactics of a marine preda- colonies and observed differences within the diving tor: the Antarctic fur seal (Arctocephalus gazella). Mar behaviour are sufficient to prevent inter-colony Ecol Prog Ser 223:287–297 Bost CA, Zorn T, Le Maho Y, Duhamel G (2002) Feeding of resource competition. Thus, it also appears that high diving predators and diel vertical migration of prey: King local prey densities also over-ride the potential transfer penguins’ diet versus trawl sampling at . of knowledge of favourable