Cephalopod diet of the Cape fur seal, Arctocephalus pusillus pusillus, along the Namibian coast: Variation due to location

Cephalopod diet of the Cape fur seal,Arctocephalus pusillus pusillus, along the

Namibian coast: variation due to locationP.J.N. de Bruyn

1*, M.N. Bester1, S.P. Kirkman

1, S. Mecenero

2, J.P. Roux

3& N.T.W. Klages

4

1

Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, 0002 South Africa2

Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch, 7701 South Africa3

Ministry of Fisheries and Marine Resources, Directorate of Resource Management, Lüderitz Marine Research,

P.O. Box 394, Lüderitz, Namibia4

Institute for Environmental and Coastal Management, Nelson Mandela Metropolitan University, P.O.Box 1600,

Port Elizabeth, 6000 South Africa

Received 21 October 2004. Accepted 22 February 2005

Scats of the Cape fur seal, Arctocephalus pusillus pusillus, were sampled at four mainlandcolonies, Cape Cross, Atlas Bay, Wolf Bay and Van Reenen Bay, along the Namibian coast over aperiod of eight years (1994–2001) to assess the diversity and spatial variability in thecephalopod component of the seal diet. Additional scat samples were collected from thePossession Island seal colony (1999–2000) to gain a broader perspective of spatial variation. Auniform and low diversity of cephalopods, only six species amongst all colonies, was identi-fied, indicating that independently the scat sampling method is unsatisfactory for determiningspecies diversity within the diet. Given the ease of scat collection, this method does, however,provide valuable insight into the variability of the most important species in the diet.Ommastrephids dominated the cephalopod component of the diet of seals from Atlas/WolfBay and Cape Cross, both in terms of wet mass and numbers. Sepia australis proved to benumerically the most important cephalopod in the diet of seals from Possession Island and VanReenen Bay, while Octopus magnificus dominated at these colonies in terms of wet mass.Contrary to previous findings it is suggested that seals from Van Reenen Bay and PossessionIsland forage south of the upwelling cell at Lüderitz (in the southern Benguela ecosystem),while previous evidence of Atlas/Wolf Bay and Cape Cross seals foraging north of thisupwelling cell (in the northern Benguela ecosystem) is supported. Prey specimen size differences,within species between colonies, were identified, but lack of cephalopod life history andmovement data, and scat sampling biases preclude adequate explanation of these findings,indicating the necessity for further studies.

Key words: Arctocephalus pusillus pusillus, cephalopods, spatial variation, diet, scat samples,Namibia.

INTRODUCTIONFor many of the 36 extant pinniped species (Martin& Reeves 2002), single studies addressing spatialvariability in the diet of any one species are scarce(e.g. Lipinski & David 1990; Green & Burton 1993).This is probably due to logistical difficulties ofsampling over extensive spatial scales. Morecommon are localized studies of diet, often overextended time frames (e.g. Bester & Laycock 1985;Rodhouse et al. 1992; Klages & Bester 1998; Nayaet al. 2002). Attempts at extensive coverage of theranges of wide-ranging species have often beenbased on opportunistic sampling (e.g. Green &Williams 1986), or have been unable to addressspatial variability because samples from distantsites were pooled together (e.g. Mori et al. 2001).

The Cape fur seal, Arctocephalus pusillus pusillus,occurs at island and mainland sites along the

Namibian coast and the west and south coasts ofSouth Africa (Oosthuizen & David 1988). Some65–70% of the annual pup production of Cape furseals takes place in Namibia (Wickens et al. 1991).The distribution of easily accessible colonies alongmuch of Namibia’s coast lends itself to a spatialcomparison of the diet of this species using scatscollected from widely separated study sites.

Scat or faecal analysis is a popular means forassessing seal diets, owing mainly to the non-disruptive nature of the method and the relativeease of obtaining samples (Gales et al. 1993; Daneriet al. 1999; Naya et al. 2002). Sources of bias associ-ated with the method have been well documentedand are discussed in some depth by de Bruyn et al.(2003) and others (Harvey 1989; Gales et al. 1993;Staniland 2002). Certainly, any assessment of dietcomposition of a seal population based only onscat analysis is unlikely to be accurate, in view of

African Zoology 40(2): 261–270 (October 2005)

*Author for correspondence. E-mail: [email protected]

the many potential sources of error. Nevertheless,the method may be suitable for investigatingvariation in seal diet composition between sites orover time, although such studies should considerthat the influence of certain biases may varyspatiotemporally. For example, should variationin prey distribution influence the average distancethat seals have to cover between their last mealand their haulout site, the quantity and quality ofprey hard parts in the scats could be affected.However, since the akward shape of cephalopodbeaks increases their retention time in the gut(Staniland 2002) relative to other prey hard partssuch as otoliths, estimates of cephalopod intake byseals from scat analysis may be least affected bysuch variability.

Scats collected from breeding colonies werelargely deposited by lactating females, whichconstitute the overwhelming majority of animalspresent at the colony at any one time, other thanthe pups of the year (de Bruyn et al. 2003). More-over, the duration of foraging trips and rangingbehaviour of lactating fur seals are constrained bytheir need to regularly return to shore to feed theirpups (a mean of 2.9 days per feeding trip up to 100days postpartum; David & Rand 1986). Therefore,their diets at the different colonies are representa-tive of what is available at varying distances fromthose particular shores, and is unlikely to overlapdue to the large distances between the studycolonies (see below).

De Bruyn et al. (2003) assessed the temporalchange in the cephalopod component of the dietof Cape fur seals and found that cephalopoddiversity in the diet was low (six species) and thatthe relative contribution of these species to the dietvaried somewhat with time, particularly betweenseasons. However, that study was based only onthe Wolf Bay (WB) and Atlas Bay (AB) breedingcolonies (samples from which were combineddue to their close proximity and designated WA)situated adjacent to the Lüderitz Upwelling Cell(LUC), using the most suitable continuous data set(1994–2001) for a study of temporal variation in thediet.

Lipinski & David (1990) investigated the dietof Cape fur seals over their entire range, usingstomach samples. In that study, samples collectedfrom nearly the whole length of the Namibiancoastline, during research cruises that took placein several different years, were pooled. Thus anyvariation in diet between the different subregionswithin Namibia was not distinguished. In thepresent study, scats were also collected from three

other colonies in tandem with those at WA,although with less continuity due to fewer visits.These were the two mainland breeding colonies atVan Reenen’s Bay (VRB) situated approximately60 km to the south of the LUC, and Cape Cross(CC), some 560 km to the north of WA, as well asthe island breeding colony of Possession Island(PI), situated less than 5 km offshore, between WAand VRB (Fig. 1).

The occurrence, abundance and biomass ofcephalopods in the seal diet at these colonies, asestimated from scat analyses, are presented inthis paper, and are compared with WA, and thefindings of Lipinski & David (1990).

MATERIALS & METHODSFrom January 1994 to September 2001 (a period of93 months), field technicians of the NamibianMinistry of Fisheries and Marine Resources(NMFMR) collected scats at WA, CC and VRB.Logistical difficulties prevented sampling on amonthly basis and complete continuity in samplingeffort was precluded, and varied amongst thecolonies (Table 1). The most continuous samplingeffort was achieved at WA, while VRB had themost missing data points. Monthly samples wereobtained from PI only from September 1999 toAugust 2000, with the exception of December 1999.Together with the observation that the compositionof the cephalopod component of the seal dietvaried somewhat with time (Augustyn et al.1992; de Bruyn et al. 2003), particularly betweenseasons, comparison between the four sites wasnot restricted to any single time period, but in-cluded all years of the study (1994–2001). Sampleswere obtained for each delineated season at allof the colonies, including PI, which was onlysampled during a single year (Table 1).

Samples were collected randomly within coloniesuntil a plastic bag was filled with approximately15–40 scats, depending on their size. The scatswere subsequently washed under running waterand then passed through nested stainless steellaboratory test sieves, before being dried in anoven overnight at 50°C. Cephalopod beaks wereremoved from the dried material and stored,either dry or in 70% ethanol for further analysis.Identification of beaks followed de Bruyn et al.(2003). All beaks stored dry were placed in 70%ethanol to re-hydrate, at least 76 hours prior toidentification, to prevent biases in identification(Clarke 1986). Numbers of lower and upper beakswere counted to determine total abundance ofcephalopods. Owing to the lower beaks’ species-

262 African Zoology Vol. 40, No. 2, October 2005

specific characters being less ambiguous thanthose of upper beaks, only the former were usedfor identification and estimation of dorsal mantlelength (DML) and wet mass. The Port ElizabethMuseum reference collection, as well as severalpublications (Clarke 1986; Lipinski & David 1990;Lipinski et al. 1992; Villanueva & Sánchez 1993;Smale et al. 1993; Ogden et al. 1998; Bianchi et al.1999) were used to identify lower beaks. Dependingon their identity, either the lower rostral length(LRL) or the dorsal hood and crest lengths of thelower beaks were measured to the closest 0.05 mmwith vernier callipers (large beaks) or a graticuleon a light microscope (Clarke 1986; Tollit & Thomp-son 1996). Species-specific regressions were usedto calculate DML and mass from these measure-ments (Clarke 1962, 1986).

The frequency of occurrence (FO)a and numeri-cal abundance (NA)b of cephalopods found within

scats were determined for each colony. By con-verting NA to mass using the species-specific re-gressions, the mass and percentage mass (% mass)contribution of each species was estimated. Thepercentage relative mass contributed and %NA ofspecies was compared between colonies. Mann-Whitney U non-parametric tests were used to testfor significant differences in the mean mass ofeach species between colonies. Possession Islandwas excluded from these comparisons due tosmall sample sizes.

RESULTSTable 2 summarizes the returns of cephalopodbeaks from Cape fur seal scats collected at thecolonies during the study period. The calculatedrelative contribution in terms of numerical abun-dance of distinct cephalopod species at eachcolony is compared in Fig. 2, and their relativemass contribution at each colony is compared inFig. 3. From the lower beaks, six species ofcephalopod (from five families) were identifiedamongst the different study colonies. Todarodesangolensis and Todaropsis eblanae (Ommastrephidae),Octopus magnificus (Octopodidae), Sepia australis(Sepiidae) and Argonauta argo (Argonautidae)were identified in scats from all colonies. Lycoteuthislorigera (Lycoteuthidae) was not found in any ofthe CC samples, but was present in the WA, VRBand PI samples (Table 3). The family Omma-strephidae dominated the cephalopod compo-nent of the seals’ diet in the WA and CC colonies,both in terms of mass (Fig. 3a,b) and numericalabundance (Fig. 2a,b). In the WA colonies, T.angolensis dominated both numerically (Fig. 2a)and in relative mass contribution (Fig. 3a), whilein the CC colony T. angolensis and T. eblanaecontributed evenly in terms of relative mass(Fig. 3b), but T. angolensis dominated numerically(Fig. 2b). At the VRB colony, O. magnificus contrib-uted just over half of the mass of cephalopodsconsumed by the seals (Fig. 3c), but only a smallfraction of the total number (Fig. 2c), most of whichconsisted of Sepia australis (Fig. 2c). PossessionIsland showed a similar pattern to VRB with themost important cephalopods in terms of bothmass and numerical contribution being O. magni-ficus and S. australis respectively (Figs 2d, 3d).

The largest cephalopod specimen in the diet of

de Bruyn et al.: Spatial variation of cephalopods in the diet of Cape fur seals 263

Fig. 1. Map indicating the Cape fur seal study coloniesalong the Namibian coast, the towns of Walvis Bay andLüderitz, and the mouths of the Cunene and Orangerivers. Although the exact location of the LüderitzUpwelling Cell (LUC) varies temporally, its generallocation is depicted here.

aThe FO of a cephalopod species is defined as the number of scatsamples, collected from a colony during a given time period, in whichlower beaks of that species were identified. Percentage FO of thatspecies is defined as the percentage of all scat samples collected at acolony during a given period, containing cephalopod hard parts, inwhich lower beaks of that species were identified.

bThe NA of a cephalopod species is defined as the number of lowerbeaks of that species found in a given quantity of scat samples.Percentage NA is the percentage of all lower beaks, found in a givenquantity of scat samples containing cephalopod remains, identified asbelonging to that species.

the seals at the WA colonies was a T. angolensis(DML = 265 mm; mass = 473 g), the heaviest wasan O. magnificus (DML = 127 mm; mass = 787 g),and the smallest specimen an A. argo (DML =6.2 mm; mass = 1.2 g). The largest cephalopodspecimen in the diet of CC seals was also T.angolensis (DML = 159 mm; mass = 120 g), theheaviest was S. australis (DML = 92 mm; mass =300 g), and the smallest an A. argo (DML = 1.2 mm;mass = 0.6 g). The largest and heaviest cephalopodconsumed by a VRB seal was an O. magnificus(DML = 231 mm; mass = 4570 g) and the smallestagain an A. argo (DML = 0.5 mm; mass = 0.2 g).

Figure 4 compares published mean mass withthe calculated mean mass of the six cephalopodspecies identified in the present study in themainland study colonies. Possession Island wasnot included in this comparison due to smallsample sizes for all species. Overall, with theexception of S. australis in the CC colony, allcephalopod species consumed by Cape fur seals in

this study showed a markedly lower mean masscompared with the published mean mass of thesespecies (Fig. 4). The only species whose mean massin this study approached the published valueswere A. argo in the WA colonies, O. magnificus inthe VRB colony, and T. eblanae and S. australis (thelatter surpassed the published value, but wasbased on only seven specimens) in the CC colony(Fig. 4).

Argonauta argo at WA were heavier than those atVRB (Z = –5.447, P < 0.05); those at CC were largerthan at VRB (Z = –1.759, P = 0.008); and those atWA were larger than those at CC (Z = –3.56018,P < 0.05). Octopus magnificus mass at WA was sig-nificantly lower than at VRB (Z = –3.728, P < 0.05),whereas the mass of S. australis was significantlyhigher at WA when compared with VRB (Z =–1.975, P = 0.046). Todarodes angolensis mass at VRBwas significantly higher than at CC (Z = –4.067,P < 0.05), and its mass at CC was significantlylower than at WA (Z = –3.084, P = 0.002), but no


Table 2. Total numbers of cephalopod beaks (upper, lower and unidentifiable fragments) found in Cape fur seal scatsas well as the mean number of lower beaks per sample bag, the mean number of species identified per sample bag,and the total number of species identified over the entire study period, at each colony.Ranges are included in bracketsfollowing each mean value. Standard deviations are given in italics in brackets. (Unidentified beaks refer to thosefragments of both upper and lower beaks which could not be identified, nor reassembled to determine how manybeaks were represented.)

Colony Total no. unidentified Total no. Total no. Mean lower beaks Mean species No. of speciesfragments of upper upper beaks lower beaks per bag (range) per bag (range) identified

and lower beaks (S.D.) (S.D.)

Atlas & Wolf Bay 224 1817 1253 15.66 (1–158) 2.06 (1–5) 6(n = 80) (±24.01) (±1.10)Cape Cross 66 716 502 10.04 (1–76) 1.92 5(n = 50) (±13.25) (1–4) (±0.88)Van Reenen Bay 28 885 790 25.48 (1–138) 2.77 6(n = 31) (±32.18) (1–6) (±1.28)Possession Island 9 143 87 3.78 (1–19) 2.09 (1–4) 6(n = 11) (±4.87) (±1.04)

Table 1. The percentage of months during the study period (n = 93) in which Cape fur seal scat samples werecollected at each of the study colonies, and the number of months within each season* during which scats werecollected at each colony. In brackets are the number of years in which samples were collected during that season, ateach colony.

Colony % of months (n = 93) No. months sampled as divided into seasons* giving a seasonalsampled representation of samples

Summer Autumn Winter Spring

Atlas/Wolf Bay 86 22 (8) 20 (8) 22 (8) 16 (7)Cape Cross 53 8 (6) 12 (7) 12 (6) 18 (8)Van Reenen Bay 33 10 (7) 8 (5) 8 (5) 5 (4)Possession Island 12 2 (1) 3 (1) 3 (1) 3 (1)

*As in de Bruyn et al. (2003), seasons were categorized into four 3-month periods within the year; 1 December to 28 February (summer), 1March to 31 May (autumn), 1 June to 31 August (winter) and 1 September to 30 November (spring).


Fig. 2. Percentage numerical contribution, and inbrackets the range of numbers of lower beaks, of the sixcephalopod species consumed by Cape fur seals ateach of the study colonies from 1994 to 2001. a, Atlasand Wolf Bay colonies pooled; b, Cape Cross; c, VanReenen Bay;d, Possession Island (sampled only in 1999and 2000).

Fig. 3. Relative mass contribution (%) of six cephalopodspecies found in scat samples of Cape fur seals at eachof the study colonies from 1994 to 2001. a, Atlas and WolfBay colonies pooled;b, Cape Cross;c, Van Reenen Bay;d, Possession Island (sampled only in 1999 and 2000). Inbrackets are the range of masses (g) of the individuals ofeach species found in the scat samples at each of thecolonies.

significant difference existed between WA andVRB. At VRB Todaropsis eblanae masses were signif-icantly lower than those at CC (Z = –4.339, P <0.05), and CC significantly higher than those atWA (Z = –3.853, P < 0.05).

DISCUSSION

Diversity of cephalopod prey in the diet

The six cephalopod species identified in the scatsof the WA colony in the previous study (de Bruynet al. 2003) were also found in samples collectedfrom the other colonies. The only exception wasthat L. lorigera was absent from CC samples. It wasthought that the spatial restrictedness of samplingfor the de Bruyn et al. (2003) study may haveresulted in the low return of species relative to the65 known to inhabit the waters off Namibia(Villanueva & Sánchez 1993). However, despitesampling four colonies instead of one, of whichone occurs far south of the LUC and, therefore,theoretically in a different marine ecosystem, andthe fact that the two most distant colonies were600 km apart (Fig. 1), no additional cephalopodspecies were found.

Lipinski & David (1990) reported a total of 20species of cephalopods being fed upon over theentire foraging range of the Cape fur seal. In their‘region 3’, which coincides with the present studyarea, most of these species were identified in Capefur seal stomach samples (Lipinski & David 1990).Save for the unlikely possibility that cephalopoddiversity in the region has reduced greatly sincethe previous study, we can only conclude that thereduced diversity of cephalopod beaks found inthis study is a direct consequence of the scatsampling method employed. A further contribut-

ing factor to the discrepancy in cephalopod diver-sity may be that the samples analysed by Lipinski& David (1990) included seals of both sexes and allages above one year, whereas the present study’ssamples comprised scats primarily of lactatingfemales. This could perhaps translate to deliberatechoice of smaller prey by lactating females; how-ever this is unlikely, since seals are opportunisticpredators capable of taking large prey (David1987). Rationally, it is unlikely that a seal wouldactively avoid large prey if the opportunity forcatching such prey presents itself.

All the species of cephalopods sampled in thisstudy, except A. argo, occur in the neritic zone(Bianchi et al. 1999). This suggests that Cape furseals feed principally over the continental shelf allalong the Namibian coast, as was deduced forAtlas/Wolf Bay (de Bruyn et al. 2003) and supportsearlier findings (David 1987; Lipinski & David1990).

Spatial variation in composition and size

of cephalopods in the diet

The demersal Ommastrephidae dominated thecephalopod component of the Cape fur seal diet,as determined from scat analysis, at WA (de Bruynet al. 2003, this study) and at CC. These cephalopodsare encountered over the continental shelfwhence they ascend through the water column tothe surface at night (Bianchi et al. 1999), wherethey are exposed to seal predation. The occurrenceof some S. australis in the scats of seals at CC isnoteworthy in that the distribution of this speciesoff southern Africa is thought to extend only as farnorth as about 27°S (Lipinski et al. 1991; Sánchez &Villanueva 1991; Augustyn et al. 1995), well to the


Table 3. Percentage frequency of occurrence (%FO) of cephalopod species in all the samples that containedcephalopod beaks (n), collected from each Cape fur seal study colony over the study period (Possession Islandsamples were from 1999 and 2000 only).

Family and species Atlas/Wolf Bay Cape Cross Van Reenen Bay Possession Island(n = 80) (n = 50) (n = 31) (n = 11)

OmmastrephidaeTodarodes angolensis 79 84 68 55Todaropsis eblanae 44 60 61 27

OctopodidaeOctopus magnificus 5 4 29 18

SepiidaeSepia australis 35 6 81 73

ArgonautidaeArgonauta argo 34 38 23 27

LycoteuthidaeLycoteuthis lorigera 10 0 16 9

south of CC. The absence of L. lorigera at CC maybe a result of this species being locally uncommonin these waters, due to seals’ preference for otherspecies, or their epibenthic deep-water distributionand vertical migration in the water column(Roeleveld et al. 1992) rendering them mostlyunavailable to foraging seals. De Bruyn et al. (2003)found that L. lorigera, along with A. argo, were

mostly taken as prey in winter by WA seals,possibly due to the seals foraging farther afield atthis time of year and/or a seasonal reduction in theabundance of preferred cephalopod species.Should CC seals feed farther afield and lessselectively during winter, as is postulated for WAseals (de Bruyn et al. 2003), it may be that, althoughL. lorigera is present in the region (Villanueva &


Fig. 4. Mean mass (g) and ranges of the six cephalopod species identified in Cape fur seal scats collected from CapeCross (CC), Atlas/Wolf Bay (WA) and Van Reenen Bay (VRB) colonies, compared to previously published meanmasses of these species (Clarke 1986; Lipinski & David 1990; Lipinski et al. 1992; Arkhipkin & Laptikhovsky 2000).Sample sizes for these means are denoted by n.

Sánchez 1993) it is unavailable to foraging sealsdue to its distribution in the water column(Roeleveld et al. 1992). These findings emphasizethe importance of top predator dietary studies, notonly to gain a clearer understanding of the fluctua-tions in the predators’ diet, but also to enhanceknowledge regarding the life histories, move-ments and distribution of the animals that arepreyed on. The limited knowledge of the biologyof most cephalopods in Namibian waters can inthis way be augmented.

The cephalopod prey in the diet of seals from theVRB and PI colonies, as determined from scatanalysis, differs markedly from the more northerncolonies, as it is dominated by S. australis and largeO. magnificus, rather than T. eblanae and T.angolensis. The abundance of S. australis hard partsin the scats at the two southern colonies supportsthe view that this is one of the most abundantcephalopods in these waters (Lipinski et al. 1991;Sánchez & Villanueva 1991). However, secondaryprey ingestion may be implicated as S. australisforms an important part of Cape hake (Merlucciuscapensis) diet, which in turn is taken by Cape furseals (David 1987; Lipinski et al. 1992; Balmelli &Wickens 1994). Whether the high incidence ofS. australis in these foraging areas at certain timescoincides with high incidence of Cape hakeremains unknown. Octopus magnificus dominatedthe cephalopods consumed in the VRB and PI sealdiets in terms of relative mass contributed, but fewspecimens were consumed, which suggests thatlarge O. magnificus were preferred above othercephalopods, if available to the seals. Octopusmagnificus was unimportant in the cephalopodcomponent of WA and CC seal diets. The similarfeeding pattern by seals on cephalopods at the PIand VRB colonies suggests that their foragingareas probably overlap, or at least that the cephalo-pod diversity and abundance encountered byseals from these colonies, are similar. In fact, PI islocated closer to the WA colonies than to VRB, butthe composition of cephalopod prey in the diet ofseals at the two southern colonies are distinct fromthat at WA.

Based on past recaptures of marked individuals atsea, Oosthuizen (1991) suggested that seals fromthe area 26.5–28°S (including WA, PI, VRB) foragemainly north of Lüderitz Bay, and therefore to thenorth of the permanent upwelling cell situated offLüderitz Bay (Gründlingh 1999; Boyer et al. 2000;Hagen et al. 2001) which divides the Benguelainto two (Shannon 1989) (Fig. 1). Seals from thearea between 21.5 and 22°S (including CC) are

suggested by Oosthuizen (1991) to forage north ofCC (Fig. 1). Thus according to the findings ofOosthuizen (1991), all seals in the present studyshould be feeding in the northern Benguela.However, the differences in the composition ofcephalopod prey in the diets of seals from PIand VRB, relative to WA and CC, and the preva-lence of S. australis and O. magnificus in the scatsof seals from the former two colonies are similarto the findings of Lipinski & David (1990) fortheir ‘region 2’ (south of Oranjemund; Fig. 1),suggesting that PI and VRB seals in fact feed southof the LUC. The results suggest that although theWA colonies are situated adjacent to/marginallysouth of the LUC, the seals from these coloniesfeed in the northern Benguela, as is the case forseals from the CC colony situated more than500 km north of the LUC. These findings lendsome support to the hypothesis that the LUCforms a barrier to the movement of certain marinespecies (Shannon 1989).

Lipinski & David (1990) identified clear spatialdifferences in the composition and importance ofcephalopods as prey for Cape fur seals, at a coarseresolution across the entire foraging range of theseseals. This study supports the notion of spatialvariability in the composition of the cephalopodcomponent of the diet, and provides additionalfindings showing further variation at an evenfiner resolution for the Namibian region. Thisstudy provides some evidence that proximity inhaul-out site, particularly of lactating females,may not necessarily imply overlapping foragingranges in Cape fur seals. This factor could be ofimportance when assessing possible seal-fisheriesinteractions.

The occurrence of a wide range of sizes for eachcephalopod species in the scats at each colonyindicates that cephalopods in various stages oftheir life cycles are taken. With the exception ofS. australis at CC, the estimated mean mass of eachcephalopod prey species was lower at all coloniesthan previously published mean mass estimatesfor the same species (Clarke 1986; Lipinski &David 1990; Lipinski et al. 1992; Arkhipkin &Laptikhovsky 2000). In the case of S. australis at CC,the estimated mean mass of this species in the sealdiet is possibly unreliable, due to the small samplesize (n = 7) of beaks retrieved there over the entirestudy period. Similarly, the greater estimatedmean mass of O. magnificus at VRB (Fig. 4) relativeto the other colonies may also be an artefact of lowsample sizes for this species at all colonies.

The differences recorded between this study


(scat sampling) and published information (liveanimal and stomach content sampling) are difficultto disentangle from the biases related to bothsampling methods. However, seal preference forcephalopods at certain life stages, or variation inthe movements of different-aged cephalopods,cannot be ruled out as contributing factors forthese results. This highlights the need for furtherstudies. The most rational explanation for themean mass of cephalopod species in this studybeing generally lower than previously published,is that large cephalopod beaks (e.g. T. angolensis,T. eblanae, O. magnificus) do not readily appear inscats, perhaps being fragmented in the gut(Staniland 2002) or lost through vomiting (Rand1959). Fortuitous collection of some large beaksin the scats (from all species of cephalopodsidentified) shows that Cape fur seals do prey onlarger squid, but that the scat sampling methodskews results towards small cephalopods in thediet. The mean size of A. argo retrieved fromsamples at Atlas/Wolf Bay is similar to thepublished value (Fig. 4), probably due to the smallsize of this species, thus reducing the bias broughtabout by loss of larger beaks from samples.

CONCLUSIONCape fur seals show a uniform and species-poorconsumption of cephalopods along the Namibiancoast when scat sampling is used as a method ofanalysing the diet. The six species identified fromscat samples among the four study colonies, ismarkedly lower than the number of speciesidentified from Cape fur seal stomach samplescollected within the same region (Lipinski &David 1990). This suggests that scat sampling is anunreliable method of obtaining the completespecies spectrum of prey items. However, scatsampling does provide valuable information onvariation in the contribution of the most importantspecies in the diet. This study shows importantspatial differences in the relative consumption ofthe most important cephalopods by Cape furseals.

Ommastrephids dominated the cephalopodprey items in the diets of WA and CC seals. Theseals from the southernmost study colonies,namely VRB and PI, show S. australis as numeri-cally their most important cephalopod prey, theabundance of which decreased dramatically northof WA (Sánchez & Villanueva 1991; this study). Wesuggest that VRB and PI seals feed in the southernBenguela, contrary to what was previouslythought (Oosthuizen 1991), and WA and CC seals

feed in the northern Benguela ecosystem.Argonauta argo and L. lorigera remain unimportantcomponents of the cephalopod portion of sealdiets. Between-colony differences in specimenmass of cephalopod prey species was observed,but due to biases related to the scat samplingprocedure and limited data on life stages andmovement of the relevant cephalopods, these aredifficult to explain. It is hoped that mention ofsuch differences may initiate studies specificallyaimed at answering such questions.

ACKNOWLEDGEMENTS

Special thanks to Y.J. Chesselet, N.A. Mukapuli,K.T. Kleophas, J. Gamatham and H. Plarre(NMFMR field technicians) for the collection andprocessing of scat samples. The NMFMR providedlogistical assistance throughout. We thank K.R.Peard for assisting with the accurate completion ofthe map. The staff of the Port Elizabeth Museum atBayworld are thanked for permission to use theextensive cephalopod beak collection for identifi-cation. Lastly we thank the two anonymousreviewers for their valuable comments.

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Cephalopod diet of the Cape fur seal, Arctocephalus pusillus pusillus, along the Namibian coast: Variation due to location