This article was downloaded by: [Port Elizabeth Museum], [Ms D Pitman]On: 01 November 2014, At: 07:00Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

African Journal of Marine SciencePublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tams20

Trophic investigations of Cape fur seals at theeasternmost extreme of their distributionM Connana, GJG Hofmeyrab, MJ Smaleab & PA Pistoriusa

a Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, SouthAfricab Port Elizabeth Museum at Bayworld, Port Elizabeth, South AfricaPublished online: 30 Oct 2014.

To cite this article: M Connan, GJG Hofmeyr, MJ Smale & PA Pistorius (2014) Trophic investigations of Cape furseals at the easternmost extreme of their distribution, African Journal of Marine Science, 36:3, 331-344, DOI:10.2989/1814232X.2014.954619

To link to this article: http://dx.doi.org/10.2989/1814232X.2014.954619

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

African Journal of Marine Science 2014, 36(3): 331–344Printed in South Africa — All rights reserved

Copyright © NISC (Pty) LtdAFRICAN JOURNAL OF

MARINE SCIENCEISSN 1814-232X EISSN 1814-2338

http://dx.doi.org/10.2989/1814232X.2014.954619

African Journal of Marine Science is co-published by NISC (Pty) Ltd and Taylor & Francis

Variations in the demography, diet, foraging behaviour and other behavioural parameters of marine top predators frequently reflect the structure, function and condition of their environment (e.g. Weimerskirch et al. 2003). Seals are no different. Although they forage almost entirely at sea, during their obligatory terrestrial phases they are more easily accessible to research. These two factors contribute to their use in assessing marine environmental changes. Studies on the foraging ecology of seals have revealed their importance in terms of assessing ecosystem functioning (e.g. Wright et al. 2007), and have also shed light on their interactions with fisheries (Szteren et al. 2004), and on aspects of fish biology (Wilhelm et al. 2013).

Initial investigations of seal diet, based on analysis of prey remains recovered from scats or stomach contents, indicated the content of the most recent meals only and hence may not have been representative of the overall diet. In addition, the analyses were associated with unavoidable biases, including retention in the stomach and subsequent regurgitation of large and hooked squid beaks (Yonezaki et al. 2003) and rapid digestion of soft-bodied prey (Jackson et al. 1987). Whereas studies of captive pinnipeds have attempted to calculate correction factors, a number of limitations remain (Bowen and Iverson 2013). To address these requires the development of indirect complemen-tary methods such as the analysis of stable isotopes (Kernaléguen et al. 2012), fatty acids (Beck et al. 2007) and DNA (Casper et al. 2007).

Stable isotopes are being used increasingly to investi-gate various aspects of pinniped foraging ecology, including diet, individual variability and ontogenic changes (Arnould et al. 2011; Hückstädt et al. 2012). Carbon (13C/12C, 13C) and nitrogen (15N/14N, 15N) stable isotope values in tissues of consumers reflect those of their prey species in a predictable manner. Most of the 13C variations in the biosphere occur at the base of food webs in primary producers, but very little along food chains (Michener and Kaufman 2007). Therefore, 13C provides essential information on sources at the base of trophic food webs. On the other hand, 15N increases by an average of 2–4‰ between prey and its consumer due to isotopic discrimination during assimilation, protein synthesis and excretion, and is therefore a good indicator of trophic positions within food webs (Hobson and Welch 1992). Finally, different tissues have different protein turnover rates. Whereas some tissues – such as plasma – are renewed continuously with turnover rates of days, others, like fur and whiskers (vibrissae), are metabolically inert once fully grown and can reflect prey sources over longer periods (Cherel et al. 2009; Newsome et al. 2010). However, whereas stable isotopes can indicate trophic level of the predator, they cannot be used to identify prey species except in very simple systems, and are therefore best used in conjunction with scat-, regurgitant- or stomach content analysis.

Cape fur seals Arctocephalus pusillus pusillus are endemic to southern Africa, with breeding colonies extending from Baia dos Tigres in Angola to Algoa Bay (Eastern Cape) in

Trophic investigations of Cape fur seals at the easternmost extreme of their distribution

M Connan1*, GJG Hofmeyr1,2, MJ Smale1,2 and PA Pistorius1

1 Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa2 Port Elizabeth Museum at Bayworld, Port Elizabeth, South Africa* Corresponding author, e-mail: maelle.connan@gmail.com

The diet of Cape fur seals Arctocephalus pusillus pusillus in the eastern portion of their distribution has received little attention previously, and was studied using traditional methods only. In 2013 we therefore assessed the diet of seals at the easternmost colony at Black Rocks, Algoa Bay, South Africa (33°50′ S, 26°16′ E) from both scats and the analysis of stable isotopes in blood, guard hairs and whiskers. Information from both sources indicated that seals at this site are generalist predators feeding on a mix of pelagic (mainly anchovy Engraulis encrasicolus and chokka squid Loligo reynaudii) and benthic species (tonguefish Cynoglossus spp., East Coast sole Austroglossus capensis and horsefish Congiopodus spp.). Stable isotope analysis revealed that the diet of individual females may differ consistently as a result of different individual foraging preferences or strategies. There were no differences in the diet between summer and winter. Furthermore, our results suggest that stable isotopes in three-month-old pups can be used as proxies for the diet of lactating females, using female-to-pup discrimination factors presented here. Future research should consider an extension of this approach to fully resolve ecological partitioning between individual Cape fur seals to better understand their role in ecosystem dynamics.

Keywords: Agulhas Current, Algoa Bay, Arctocephalus pusillus pusillus, carbon and nitrogen stable isotopes, scat analysis, seasonal diet, South Africa

Introduction

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius332

South Africa (Kirkman et al. 2013). More than 95% of the population is associated with the cold, nutrient-rich Benguela Current along the west coast of South Africa, with only three colonies occurring east of Cape Agulhas, all inshore of the warm Agulhas Current (Huisamen et al. 2012a). Pupping and mating take place from November to early January each year, followed by lactation, which typically lasts between 8 and 11 months (Shaughnessy 1985).

Previous diet studies of Cape fur seals have assessed the stomach contents of stranded animals only and/or animals killed at sea, or scats. On the West Coast, fur seals appear to be generalist feeders that exhibit spatial and temporal variations (e.g. David 1987; Mecenero et al. 2006a, 2006b). Although there are fewer studies of the diet of populations from the south coast of South Africa (east of Cape Point), they show a similar pattern (Castley et al. 1991; Stewardson 2001; Huisamen et al. 2012b).

The aim of this study was to investigate variations in the diet of Cape fur seals at various temporal scales at the easternmost extreme of their distribution, using a combina-tion of scat and stable isotope analyses. Specifically, we attempted to identify prey species from hard remains in scats, which reflect the diet in the days immediately prior to sampling. Concurrently, blood was collected from adult females and pups and carbon and nitrogen stable isotope values were measured to assess diet over the month prior to sampling (Kurle 2002). Guard hairs and whiskers were collected and stable isotopes determined to assess trophic level and foraging areas at the time these tissues were growing (Hobson et al. 1996; Cherel et al. 2009). The use of pups as proxies to determine the foraging habits of the adult females was also investigated.

Material and methods

SamplingFieldwork was conducted on Black Rocks (33°50′ S, 26°16′ E; Figure 1) in the Bird Island group during late summer (17–20 March) and in winter (2–3 August) of 2013. Black Rocks consists of five small, exposed rocks, of which only the two largest are used by fur seals for breeding (GJGH pers. obs.). Counts of pups from aerial photographic censuses are typically between 391 and 804 (lower and upper quartiles respectively; Kirkman et al. 2007a), but, with no trend apparent between 1979 and 2012, the population appears stable (Kirkman et al. 2013).

Sampling was conducted on the largest rock (surface area 1 ha, highest altitude 6 m). During the two sampling sessions, all whole scats were collected, stored individu-ally in plastic bags and kept frozen until processing and analysis. Scats collected were assumed to be fresh (<1 wk old) because of frequent wave exposure and high seal traffic.

Samples for stable isotope analyses were collected from live animals caught on the rock. Pups were caught by hand, whereas a hoopnet was used to capture and restrain adult females (David et al. 1990). Blood samples (maximum 0.5 ml) were collected from the hind flipper, a fur sample was trimmed mid-dorsally, and a whisker was clipped at the base from the rear right of the muzzle. Blood samples were kept on ice in the field and then air-dried at the end

of each day. Air-dried blood, fur and whisker samples were then stored at 20 °C until further processing. All individuals handled were weighed and double-tagged with uniquely numbered plastic tags (Dalton Tags Ltd, Henley-on-Thames, UK), and pups were sexed. It was not possible to capture mother–pup pairs because the rough terrain and the small area of the rock enabled animals to escape to sea easily, soon after disturbance.

Scat analysisEach scat was processed individually. They were thawed overnight and prey remains (e.g. otoliths, fish lenses, cephalopod beaks and lenses, crustacean exoskeletons, feathers, shells, etc.) were separated from other organic material by rinsing the samples – with water – through a sieve with an aperture diameter of 0.5 mm. All water that had been through the sieve was checked visually to ensure that all prey items had been retained.

Prey items were identified to the lowest possible taxonomic group using Smale et al. (1995) and Clarke (1986), as well as the otolith reference collection housed in the Port Elizabeth Museum. A digital photograph of each sagittal otolith was taken with a Leica EC3 camera mounted on a dissecting microscope, and images were processed using the freeware image-processing tool ImageJ 1.47 (Abramoff et al. 2004). Identifiable otoliths were first assigned to one of the three following groups according to their degree of erosion (adapted from Reid 1995; and Huisamen et al. 2012b): (A) little or no sign of erosion, (B) medial relief and margins of the otolith smoothed by erosion, and (C) heavily eroded with little or no medial relief and margins generally rounded. Each photograph was scaled using a microruler and otoliths were measured

Port Elizabeth

20 km

ALGOA BAY Bird IslandBlack Rocks

20° E

30° S

NAMIBIA

Free State

Mpumalanga

Limpopo

GautengNorth-West

BOTSWANA

INDIAN OCEAN

KwaZulu

-Nata

l

EASTERN CAPE

Western Cape

Northern Cape

AFRICASOUTH AFRICA

SOUTH AFRICA

Figure 1: Location of the Black Rocks Cape fur seal colony on the South African coast

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 333

across their greatest diameters (Smale et al. 1995). Lower rostral length of squid beaks was measured to 0.1 mm with a vernier calliper. The minimum number of individual prey species per scat was determined as the highest number of either left or right otoliths and either upper or lower cephalopod beaks.

Prey remains were quantified using percentage frequency of occurrence (FO), corrected percentage numerical abundance (% NAcorr), and corrected reconstituted mass (% Wcorr) (Kirkman et al. 2007b). The FO represented the proportion of scats containing otoliths or beaks of a particular prey species. The % NAcorr represented the number of retained prey items of each species expressed as a percentage of the total number of retained prey items (i.e. otoliths and squid beaks), with a correction for otolith numbers lost through digestion (Mecenero et al. 2006a; Kirkman et al. 2007b). When calculating % Wcorr, erosion correction factors were applied to otoliths in groups B and C (Mecenero et al. 2006a; Kirkman et al. 2007b), but not to identified cephalopod beaks because they showed no evidence of erosion. Only fish otoliths and cephalopod beaks were used to estimate total lengths and reconsti-tuted mass of prey, by applying allometric equations (Clarke 1986; Smale et al. 1995; Mecenero et al. 2006a).

Because data did not meet assumptions of normality and homoscedasticity (Shapiro–Wilk and Levene tests), Mann–Whitney U-tests were used to test for significant differences in fish lengths between the two seasons.

Stable isotope analysisCarbon and nitrogen stable isotope analyses were conducted on blood, guard hairs and whiskers. Cape fur seals moult annually in late summer with gradual replacement of guard hairs being complete within seven weeks (Rand 1956). Fur sampled from adult females in summer may have reflected isotopes incorporated during both the 2012 and 2013 moults because the 2013 moult may not have been completed, whereas fur sampled in winter could only have reflected isotopes incorporated in the 2013 moult. Pups undergo a postnatal moult four months after birth, changing their black coat for the olive–grey yearling coats (Rand 1956). The fur of summer pups thus incorporates the isotopes acquired in utero whereas the winter coat incorporates isotopes acquired after four months of age. Growth rates of vibrissae have not been determined for Cape fur seal adults or pups. Growth rates in wild individuals of the closely related Antarctic fur seal Arctocephalus gazella have been estimated to be between 0.11 and 0.16 mm d–1 (Cherel et al. 2009). An average of 0.13 mm d–1 was used in our study, and a 2 mm section of whisker was assumed to be representative of diet over a period of two weeks.

Samples of dried blood were finely ground to a homogen-eous powder. Each whisker was checked for any tissue or dirt, which was removed using a scalpel. Fur and whiskers were cleaned in a 2:1 chloroform:methanol solution in an ultrasonic bath for 5 min, rinsed in successive baths of methanol and deionised water, and then dried at 50 °C for 24 h. Guard hairs from the fur were then homogenised by cutting them finely with stainless steel scissors. A 2 mm section of the proximal end of the whiskers was clipped and analysed whole.

Results were expressed in the usual notation relative to Vienna PeeDee Belemnite for 13C and atmospheric nitrogen N2. The relative isotopic abundances of carbon and nitrogen were determined in 0.5–0.6 mg subsamples of materials (dried blood, guard hairs and whiskers) with a Thermo Finnigan Delta XP Plus mass spectrometer interfaced via a Conflo III device to a Thermo Flash EA 1112 elemental analyser at the Stable Light Isotope Unit, Univ ersity of Cape Town, South Africa. Replicate measurements of internal laboratory standards indicated measurement errors <0.15‰ and <0.08‰, respectively, for stable carbon and nitrogen isotope measurements.

The correlation between the weight of adult females and pups and their 13C and 15N values was tested using the Spearman rank correlation coefficient (rS). The correlation between 13C and the C/N ratio was tested in blood and guard hairs (both C/N ≥ 3), and between 13C and 15N in all three tissues. Comparisons of carbon and nitrogen stable isotope signatures between the different groups – defined by stages, seasons and tissues – were made using generalised linear mixed models (GLMM) with three fixed factors (stage: adult/pup; season: summer/winter; tissue: blood/guard hairs/whisker) and one random factor (individuals). For significant model results, Tukey HSD post hoc tests were then conducted. The effect of pup gender was not tested because of a low sample size. Interactions and main effects were considered significant at p < 0.05 for all tests.

Mean values are given, together with SD. All statistical analyses were performed with either R statistical software (R Development Core Team 2009) or PAST version 2.17 (Hammer et al. 2001).

Results

Scat analysisA total of 1 097 remains were identified from the summer samples and 1 114 from the winter samples, from 36 and 30 scats respectively (four scats did not contain hard remains). This corresponded to a minimum of 1 221 individual fish and cephalopods (Table 1). The average number of prey remains per scat was 35.4 (range 1–344) and the average number of prey species per scat was 2.8 (range 1–8). A total of 29 prey taxa were identified, comprising at least 18 fish families and two cephalopod families (Table 1).

After the application of correction factors for otoliths lost during digestion and erosion, anchovy Engraulis encrasi-colus was the most numerous (% NAcorr) and dominant (% Wcorr) prey species identified in the scats, followed by redspotted tonguefish Cynoglossus zanzibarensis and East Coast sole Austroglossus pectoralis (Table 1). These three species made up 86% of the NAcorr of prey remains in summer and 97% in winter, and 59% of reconstituted mass (% Wcorr) in summer and 78% in winter. Sardine Sardinops sagax were identified in only three scats, all collected in summer. Nine prey taxa were represented by a single individual in all the scats examined.

Cape fur seals foraged on a wide range of prey sizes, from the small Cheilodactylidae and Engraulidae (average length 112 and 132 mm, respectively) to the larger kingklip Genypterus capensis (two specimens of 517 and 695 mm;

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius334

Species Common nameSummer (n 36) Winter (n 30)

FO NA % NA

% NAcorr

% Wcorr

FO NA % NA

%NAcorr

% Wcorr

FishBothidae Arnoglossus capensis Cape flounder 5.6 5 0.8 0.5 0.5 Carangidae Trachurus capensis Cape horse mackerel 25.0 15 2.4 1.6 3.4 Trachurus delagoa African maasbanker 2.8 1 0.2 0.1 0.2 Cheilodactylidae Unident. Cheilodactylidae 5.6 3 0.5 0.3 0.2 Clupeidae Etrumeus whiteheadi Redeye round herring 22.2 24 3.9 2.4 6.8 Sardinops sagax Sardine 8.3 8 1.3 1.5 3.9 Congiopodidae Congiopodus spinifer Spinenose horsefish 5.6 4 0.6 0.4 1.0 6.7 2 0.3 0.2 1.1 Congiopodus torvus Smooth horsefish 5.6 6 1.0 0.7 8.9 Congridae Gnathophis capensis Southern conger 5.6 2 0.3 0.2 0.3 Cynoglossidae Cynoglossus capensis Sand tonguefish 33.3 23 3.7 2.0 2.5 10.0 3 0.5 0.4 0.6 Cynoglossus zanzibarensis Redspotted tonguefish 36.1 52 8.4 5.7 11.0 40.0 76 12.7 9.5 27.7Engraulidae Engraulis encrasicolus Anchovy 63.9 406 65.4 77.6 33.6 76.7 487 81.2 87.0 44.8Gonorynchidae Gonorynchus gonorynchus Beaked sandfish 11.1 5 0.8 0.5 1.4 Merlucciidae Merluccius capensis Shallow-water hake 8.3 3 0.5 0.3 1.1 Ophidiidae Genypterus capensis Kingklip 6.7 2 0.3 0.2 8.4Percichthyidae Synagrops japonicus Japanese splitfin 2.8 1 0.2 0.1 0.1 Pomatomidae Pomatomus saltatrix Elf 3.3 1 0.2 0.1 0.9Sciaenidae Atractoscion aequidens Geelbeck 3.3 1 0.2 0.1 2.2Scorpaenidae Helicolenus dactylopterus Jacopever 8.3 16 2.6 1.5 5.7 3.3 1 0.2 0.1 1.2Soleidae Austroglossus pectoralis East Coast sole 16.7 26 4.2 2.8 13.9 6.7 7 1.2 0.9 5.2 Unident. Soleidae 2.8 1 0.2 0.1 <0.1 Sparidae Cheimerius nufar Santer 3.3 1 0.2 0.1 5.4 Pachymetopon aeneum Blue hottentot 8.3 3 0.5 0.3 2.2 Pagellus bellottii natalensis Red tjor-tjor 3.3 1 0.2 0.1 0.5Triglidae Chelidonichthys capensis Cape gurnard 8.3 5 0.8 0.5 0.9 3.3 1 0.2 0.1 0.1 Chelidonichthys sp. 2.8 1 0.2 0.1 0.2 Unidentified 1 2.8 2 0.3 0.1 – 3.3 1 0.2 0.1 –Unidentified 2 2.8 1 0.2 0.1 – Unidentified 3 2.8 1 0.2 0.1 – Unidentified 4 2.8 1 0.2 0.1 – Total fish 615 584

CephalopodsLoliginidae Loligo reynaudii Chokka 8.3 3 0.5 0.2 0.7 16.7 13 2.2 0.9 1.9Ommastrophidae Unident. Ommastrophidae 2.8 2 0.3 0.1 1.6 Octopus sp. 1 2.8 1 0.2 0.1 – 3.3 1 0.2 0.1 –Unidentified sp.1 6.7 2 0.3 0.1 –Total cephalopods 6 16

Table 1: Percentage frequency of occurrence (FO), numerical abundance (NA; % NA and % NAcorr; see text for details) and reconstituted mass (% Wcorr) of fish and cephalopod species identified in scats of Cape fur seals at Black Rocks, Algoa Bay, South Africa, in summer and winter 2013. Bold numbers indicate prey taxa with % NAcorr or % Wcorr >5%

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 335

SpeciesMean (SD; range)

Summer Wintern Mass (g) Total length (mm) n Mass (g) Total length (mm)

FishBothidae

Arnoglossus capensis 5 34.6 (43.9; 9.6–112.8)

144.3 (51.0; 106.5–233.6)

Carangidae

Trachurus capensis 21 74.3 (42.5; 27.8–206.9)

198.1 (32.2; 146.6–285.8)

Trachurus delagoa 1 58.0 173.2 Cheilodactylidae

Unidentified Cheilodactylidae 3 18.7 (3.9; 12.8–21.0)

111.7 (8.3; 99.3–116.6)

Clupeidae

Etrumeus whiteheadi 40 51.6 (15.8; 14.5–81.8)

189.0 (21.2; 128.4–221.2)

Sardinops sagax 7 87.8 (39.4; 21.3–141.6)

216.7 (38.6; 142.5–258.7)

Congiopodidae

Congiopodus spinifer 6 81.6 (48.8; 17.6–155.6)

185.2 (43.2; 117.2–238.3) 2 64.6–184.9 179.0–252.1

Congiopodus torvus 10 468.1 (227.6; 118.7–768.7)

348.8 (46.5; 262.9–399.5)

Congridae Gnathophis capensis 2 49.9–66.2 334.2–364.8 Cynoglossidae

Cynoglossus capensis 43 34.1 (23.0; 3.1–98.9)

169.1 (38.4; 82.1–250.0) 3 47.6

(18.2; 27.0–61.4)195.2

(26.7; 164.8–214.5)

Cynoglossus zanzibarensis 85 66.8 (45.2; 12.1–208.0)

218.5 (51.5; 127.4–339.8) 137 84.3

(47.1; 10.3–287.3)239.7

(49.3; 120.3–380.8)Engraulidae

Engraulis encrasicolus 755 15.0 (4.3; 1.7–28.7)

132.4 (13.1; 67.1–164.6) 870 12.2

(4.3; 0.7–25.1)123.1

(16.0; 51.0–157.8)Gonorynchidae

Gonorynchus gonorynchus 7 88.3 (28.8; 58.3–132.1)

269.4 (28.5; 240.2–307.6)

Merlucciidae

Merluccius capensis 3 138.2 (129.5; 26.1–280.0)

244.9 (91.2; 155.8– 333.2)

Ophidiidae Genypterus capensis 2 624.8–1671.3 517.4–695.2Percichthyidae Synagrops japonicus 2 23.5–29.4 128.5–138.2 Pomatomidae Pomatomus saltatrix 2 206.4–262.0 283.8–307.3Sciaenidae Atractoscion aequidens 1 598.4 422.3Scorpaenidae

Helicolenus dactylopterus 23 134.4 (74.8; 28.1–323.1)

197.6 (38.0; 123.8–271.2) 2 317.9–320.3 269.9–270.5

Soleidae

Austroglossus pectoralis 39 169.5 (82.7; 31.6–337.0)

301.2 (50.3; 183.9–383.5) 9 172.8

(107.9; 24.8–362.2)297.9

(67.2; 170.7–392.2) Unidentifi ed Soleidae 2 6.9–8.5 114.9–122.3 Sparidae Cheimerius nufar 1 1 480.9 468.1

Pachymetopon aeneum 3 277.0 (107.4; 180.3–392.5)

258.1 (35.8; 224.1–295.4)

Pagellus bellottii natalensis 2 126.9–147.8 211.2–221.8Triglidae

Chelidonichthys capensis 8 54.2 (30.5; 19.5–110.3)

174.9 (33.3; 129.4–227.4) 1 21.5 133.6

Table 2: Reconstituted mass and total length of prey species identified in scats of Cape fur seals collected on Black Rocks, Algoa Bay, South Africa, in summer and winter 2013

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius336

Table 2). Possible seasonal variation in the total length of fish was tested for E. encrasicolus, C. zanzibarensis and A. pectoralis. Engraulis encrasicolus consumed were signifi-cantly larger in summer than in winter (U 2.1 105, p < 0.001), whereas the reverse was the case for C. zanziba-rensis (U 4 401, p 0.002; Table 2, Figure 2). No signifi-cant seasonal variation was found in A. pectoralis (U 173, p 0.958; Table 2, Figure 2).

Cephalopod beaks occurred in 8% of summer scats and 17% of winter scats (Table 1), with only chokka squid Loligo reynaudii being identified. The other remains were too eroded for species identification. Other invertebrates, or parts of invertebrates, were found in 28% of summer scats and 27% of winter scats. Of these, crustaceans and gastro-pods were the most common taxa recovered, and generally were well digested.

Bird feathers were found in six of the 66 scats (five in summer and one in winter). A single feather was found in each of four scats while seven and three feathers were recovered in the remaining two scats. All feathers recovered in summer were from penguins whereas the one found in winter was unidentified.

Stable isotope analysisStable isotope samples were collected from nine breeding females and 15 pups. Values of 13C ranged from 15.6‰ to 12.4‰ and those of 15N ranged from 14.8‰ to 18.7‰ (Figure 3, Appendix 1). Neither 13C nor 15N was correlated with the weight of adult females or pups (13C: adult females rS 0.083, p 0.810; pups rS 0.073, p 0.831; 15N: adult females rS 0.109, p 0.784; pups rS 0.442, p 0.174). Values of 13C in blood and guard hairs were not correlated with the C/N ratio (blood rS 0.279, p 0.356; guard hairs rS 0.429, p 0.111). However, 13C and 15N values were positively correlated in the three tissues (blood rS 0.768, p 0.002; guard hairs rS 0.531, p 0.042; whiskers rS 0.823, p < 0.001).

In general, season (summer/winter) had no effect on 13C or 15N signatures (GLMM: 13C p 0.502; 15N p 0.388). Tissue affected carbon signatures, with blood exhibiting significantly lower 13C than whiskers and guard hairs (both p < 0.001). Nitrogen values were also affected by tissue type, with blood having a significantly lower 15N than whiskers but not than guard hairs (p 0.026 and p 0.330 respectively). Stage (adult/pup) had a significant effect on 15N but not on 13C values (15N p < 0.001; 13C p 0.324). There were no significant differences in 15N values between tissues of adult females in either summer

or winter (Tukey HSD post hoc tests: all p > 0.375). In the case of 13C, however, blood values were significantly depleted compared to those in guard hairs and whiskers, in both seasons (all p < 0.001). Similarly, pup blood 13C values were significantly lower than those from guard hairs and whiskers in both seasons (all p < 0.001). In summer, the three tissues collected from pups showed signifi-cant variations in 15N values, with whiskers exhibiting the highest 15N, followed by guard hairs and blood (all p < 0.05). Pups had significantly higher 15N signatures for the three tissues in both seasons compared to adult females (all p < 0.001; blood +1.3‰, guard hairs +1.5‰, whiskers +2‰). There were no significant differences between adult female and pup 13C values (all p > 0.146), except in summer when the pup guard hairs were significantly more depleted than those of adult females (p < 0.01, 0.6‰). Results of GLMM and post hoc Tukey tests are detailed in Appendix 2.

Discussion

This study is the first to combine traditional and biochem-ical analyses to investigate the diet of Cape fur seals, and to provide stable isotope values for multiple tissues of this predator. Both techniques highlighted the importance of pelagic and benthic species in the seals’ diet in summer and winter. In addition, stable isotope signatures revealed a consistency in individual diet of adult females, probably resulting from individual foraging preferences or strategies.

Foraging ecology of Cape fur seals in Algoa Bay Traditional scat analysis highlighted the importance of fish in the diet of fur seals (with no distinction between age and sex classes) at Black Rocks, where they represented more than 97% of reconstituted mass in both seasons. Identified fish remains suggested that they targeted one species of shoaling pelagic fish (E. encrasicolus) together with non-shoaling benthic fish from diverse habitats such as sandy (e.g. C. zanzibarensis) or rocky areas (e.g. horsefish Congiopodus spp.). Fish such as horse mackerel Trachurus capensis, A. capensis and C. zanzibarensis found in the scats are also targeted by the fishing industry in the Eastern Cape. It is difficult, however, to assess whether these species were preyed upon naturally by the seals or whether they were scavenged as bycatch or discards from trawlers (Attwood et al. 2011). The length frequency distribution of E. encrasicolus in the diet suggests that the individuals targeted by the seals were mostly mature (>120 mm; Baird

SpeciesMean (SD; range)

Summer Wintern Mass (g) Total length (mm) n Mass (g) Total length (mm)

CephalopodsLoliginidae

Loligo reynaudii 3 134.6 (161.0; 22.2–319.1)

151.6 (42.9; 75.7–252.7) 13 72.6

(29.1; 22.2–150.0)132.2

(24.4; 75.8–185.3)Ommastrophidae Unidentifi ed Ommastrophidae 2 99.0–813.8 –

Table 2: (cont.)

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 337

and Geldenhuys 1973) and the increased size of anchovy in summer may be a consequence of the growth of the fish over time (Figure 2; Barange et al. 1999). Most of the C.

zanzibarensis preyed upon in winter were almost mature (>275 mm), whereas those preyed upon in summer were mainly one-year-olds (Booth and Walmsley-Hart 2000).

Summer (n = 755)

FRE

QU

EN

CY

(%)

50–5

5

16.0

60–6

570

–75

80–8

590

–95

100–

105

110–

115

120–

125

130–

135

140–

145

150–

155

160–

165

Winter (n = 870)

Summer (n = 85)

Winter (n = 137)

12.0

8.0

4.0

20.0

16.0

12.0

8.0

4.0

120–

130

140–

150

160–

170

180–

190

200–

210

220–

230

240–

250

260–

270

280–

290

300–

310

320–

330

340–

350

360–

370

(a)

(b)

Summer (n = 39)

Winter (n = 9)

(c)

20.0

16.0

12.0

8.0

4.0

120–

130

140–

150

160–

170

180–

190

200–

210

220–

230

240–

250

260–

270

280–

290

300–

310

320–

330

340–

350

TOTAL LENGTH (mm)

Figure 2: Size frequency of (a) Engraulis encrasicolus, (b) Cynoglossus zanzibarensis and (c) Austroglossus pectoralis eaten by Cape fur seals in summer and winter

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius338

Of the other prey remains recovered, L. reynaudii was the most common cephalopod. Fur seals have been observed at squid spawning aggregations where they successfully hunt during mating and egg laying (Smale et al. 2001). Crustaceans and gastropods found in the scats were highly digested and were probably secondary prey. This notion is supported by the fact that some gastropod remains were recovered in the same scats as the otoliths of Congiopodus spp., suggesting that gastropods were present because they had been consumed by this fish (Meyer and Smale 1991). Few feathers were found in the scats under study, and all but one were identified as being from penguins. The African penguin Spheniscus demersus is the only penguin species found in Algoa Bay (Crawford et al. 2011). Individual Cape fur seals are known to prey on seabirds (David et al. 2003), but quantifying the extent of predation is difficult using scat analysis alone and would require extensive observational data.

Investigating the diet of Cape fur seals from their stable isotope signatures requires species-specific and tissue-specific discrimination factors for both carbon and nitrogen. None of these factors has been established for any tissues

in Cape fur seals, and to date, only one study has been conducted on a captive otariid, the northern fur seal Callorhinus ursinus (Kurle 2002). The author calculated discrimination factors in a lactating female, for red cells and plasma, respectively, to be +3.5‰ and +3.7‰ for 15N, and +1.2‰ and +0.5‰ for 13C. When applied to the blood of Cape fur seals, these values would equate to a mean prey value of ~+12.1‰ for 15N and ~14.1‰ for 13C. The pelagic E. encrasicolus and S. sagax exhibit lower 15N values than the adult females, at about 10–11‰, on the south-east coast of South Africa (Moseley et al. 2012). Loligo reynaudii 15N values average ~13.2‰ in the study area (MC unpublished data). Demersal fish, such as Cape hake Merluccius capensis, exhibit higher 15N values than pelagic fish (~13.5‰; Moseley et al. 2012). Unfortunately, no stable isotope signatures of benthic fish such as C. zanzibarensis or seabreams (Sparidae) are currently available. The blood 15N signatures suggested that adult female seals had consumed a mix of pelagic and benthic species over the month prior to sampling. Furthermore, 15N signatures for females were higher than those of Cape gannets Morus capensis from Algoa Bay, which are known to rely primarily on pelagic fish (Moseley et al. 2012). Regarding 13C signatures, benthic carbon sources are known to be enriched in 13C compared to pelagic carbon sources (France 1995). The higher 13C signatures in seals compared to those of the pelagic prey species (Moseley et al. 2012; MC unpublished data) showed the contribu-tion of benthic species to seal diet. The occurrence of the same 13C signature in summer as in winter suggested that females foraged in similar habitats in both these seasons.

Dietary studies of Cape fur seals in the Eastern CapeCovering the period 1992–1995, Stewardson (2001) found that C. zanzibarensis and A. capensis were more numerous than pelagic fish in the diet. The author recorded T. capensis and S. sagax, but very few E. encrasicolus. The changes observed between Stewardson’s study and this study (sample collection in 2013) are likely to reflect changes in the marine environment, with an increase in the biomass of anchovy on the South Coast since 1996 (Barange et al. 1999; Roy et al. 2007). At Robberg, a colony about 300 km south-west of Black Rocks, Huisamen et al. (2012b) showed that a decline in S. sagax remains in seal scats was strongly correlated with a decline in the size of the prey population in the east of their range (based on annual acoustic biomass surveys). This led those authors to suggest that scats could provide useful indicators of S. sagax abundance. The low prevalence of S. sagax in seal diet observed in our study may suggest a low relative abundance in Algoa Bay in 2013. This is consistent with fisheries survey results that highlighted the clear dominance of E. encrasicolus over S. sagax east of Cape Agulhas in 2013, with very few S. sagax (<1 g m–2) in the Algoa Bay region in November of that year (Mhlongo et al. 2013). The hypothesis of long-term changes in the marine ecosystem along the Eastern Cape coast since the mid-90s is supported by: (i) a similar decrease of S. sagax with a parallel increase in E. encrasicolus in the scats of Cape fur seals at the Robberg colony between 2003 and 2008 (Huisamen et al. 2012b); and (ii) the prevalence of E.

15 N

(‰)

(a)

18.0

17.0

16.0

15.0

18.0

17.0

16.0

15.0

15.0 14.0 13.013C (‰)

(b)

Figure 3: Carbon (13C) and nitrogen (15N) stable isotope signatures of blood (black), guard hairs (grey) and whiskers (clear) of adult female (circle) and pup (triangle) Cape fur seals in (a) summer and (b) winter. Mean values are represented by the symbols that are intersected by error bars (SD)

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 339

encrasicolus over S. sagax in the diet of Cape gannets at their breeding colony in Algoa Bay (Green 2014).

Fewer cephalopod remains were found in our study compared to the previous study at the same site (Stewardson 2001). This may reflect changes in the avail -ability of squids, which are known to have wide interan-nual ranges in fisheries catch rates and biomass estimates (Sauer et al. 2013).

Although scat analysis is an easy, cheap and non-intrusive method for investigating the diet of fur seals, it has several limitations, which include a limited assessment period, prey species biases, and the necessity of collecting a sufficiently large and representative number of scats (Trites and Joy 2005). Kirkman et al. (2007b) recommended a minimum number of 40 scats per sampling event when sampling Cape fur seals, which unfortunately was not possible in our study. The study site is low-lying and waves frequently wash over it, and the high density of seals (despite being a small colony) and associated traffic destroy the integrity of scats. On account of logistic constraints and the need to keep disturbance to an acceptable level, the number of days spent on Black Rocks was kept to a minimum. However, cumula-tive prey curves without rare species (i.e. species with only one individual found in all scats) approached an asymptote (data not shown), indicating that all of the main prey species were likely to have been detected.

Contribution of carbon and nitrogen stable isotopes to Cape fur seal diet studiesScat analysis provides a snapshot of the last few meals of an individual prior to defecation. Assessing longer-term diet by this means would require repeated sampling of scats of the same individual over longer periods. Not only is this unrealistic in wild, crowded and relatively inacces-sible rookeries, it would remain limited to providing informa-tion only on prey consumed in the proximity of the rookery. Furthermore, the age and sex class of seals for which diet is estimated would remain generally unknown when seals of various categories are present during sampling. Sex could be determined using DNA analyses (Reed et al. 1997), but such tests would increase considerably both the costs and the duration of analysis. In contrast, stable isotope signatures of various tissues of a known individual can indicate its trophic role over varying time scales (Newsome et al. 2010).

The differences in 13C and 15N observed between the three tissues (blood, guard hairs and whiskers) may reflect a change not only in diet and/or habitat use over time (because these tissues have different protein turnover rates), but also in their respective biochemical composi-tions. Individual amino acids exhibit a wide range of 13C and 15N values (Germain et al. 2013), and differences in amino acid compositions have been found between blood and hair in pigs Sus scrofa (Mahan and Shields 1998). Higher fractionation factors in fur compared to blood in seals that eat a consistent diet have been found in northern fur seals (Kurle 2002; Kurle and Worthy 2002) and in several species of true seals (Hobson et al. 1996). A higher discrimination factor in guard hairs may further explain the higher 15N signatures that we detected in guard hairs and whiskers compared to blood.

In several species of pinnipeds, females change their foraging strategy after parturition, switching from offshore, energy-rich pelagic prey to inshore, benthic prey of lower nutritional value (Balmelli and Wickens 1994), in order to decrease foraging-trip duration and hence to increase the proportion of time spent ashore with their pups (Drago et al. 2010). In Cape fur seals at Black Rocks, however, the similarity between summer and winter stable isotope data seems to indicate that females do not change their foraging strategy, feeding on a mix of pelagic and benthic coastal species. However, repeated sampling later in the breeding season, coupled with tracking of lactating females, would be necessary to confirm this.

Adult females and pups exhibited high individual variability in 13C and 15N signatures (Figure 3). Stable isotope signatures in predators represent a complex function of diet, together with physiological processes such as fasting and whether or not they are lactating (Kurle 2002; Arnould et al. 2011). One of the four adult females captured in summer exhibited substantially higher 13C and 15N values than the other three. Whereas fasting would cause either a lower 13C (mobilisation of lipid stores) or a higher 15N (catabolisation of body proteins; Newsome et al. 2010), this female was the heaviest of the four captured females, which suggests that she was not fasting. An alterna-tive hypothesis is that this individual was not lactating, possibly because she had not fallen pregnant the previous season (the pregnancy rate in Cape fur seals is 71%; Wickens and York 1997) or because she had lost her pup (early pup mortality is 20–35%; De Villiers and Roux 1992). Kurle (2002) estimated that, in northern fur seals, lactation decreased the prey-to-predator discrimination factor of 15N by ~1‰, and showed that lactation does not have any effect on the discrimination factor for 13C. It is noteworthy that the female in question in our study exhibited a low 13C compared to the three other females. Therefore, the different signature shown by this female may be a result of variability between individuals, which would be a consequence of an individual’s age, size-specific abilities, foraging skills and/or prey preferences (Arnould et al. 2011). This particular female may have had a prefer-ence for benthic prey, which would cause it to exhibit higher carbon and nitrogen signatures than the other females. Consistent with this, the stable isotope signatures of her whiskers and guard hairs were also the highest of the four females caught in summer, which suggests behavioural consistency. Cape fur seals are opportunistic, generalist feeders (e.g. Rand 1959; Stewardson 2001; Mecenero et al. 2006a). All previous studies of diet have used scat analysis at a population level, whereas our study of individuals, albeit based on a small sample, suggests consistency in the foraging behaviour of particular females.

Pup stable isotope signatures as a proxy of adult female foraging ecologyOwing to the considerable difficulty in sampling adult female pinnipeds, because of their size and mobility and the greater disturbance inherent in their capture, several studies have used pup stable isotope signatures as proxies to investigate the trophic ecology of females (Ducatez et al. 2008; Drago et al. 2010). During lactation, pups feed

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius340

on a fluid supplied by maternal tissues and would thus be expected to exhibit a higher trophic level than their mother until weaning. The correction factor associated with mother-to-offspring transfer of nutrients is, however, species- and tissue specific (Jenkins et al. 2001).

In summer (when samples were collected about three months after birth), Cape fur seal pups exhibited a signifi-cantly higher 15N than the adult females in the three tissues analysed, and average female-to-pup discrimination factors were +1.3‰, +1.5‰ and +2‰ for blood, guard hairs and whiskers, respectively. In winter (when samples were collected about eight months after birth), the high 15N in the two pups compared to the adult females indicate that those pups were still largely dependent on their mother’s milk rather than on a mixed diet of milk and fish/cephalo-pods. A larger number of samples is required, however, to confirm this feeding behaviour. Similar differences in 15N values between mothers and pups have been found in many species of pinnipeds (e.g. Ducatez et al. 2008; Habran et al. 2010). In contrast, fur seal milk is rich in lipids (Rand 1956), which are 13C-depleted, and hence a lower 13C was expected in pup tissues (Hobson and Sease 1998, but see Habran et al. 2010). No differences in blood and whisker 13C signatures were found between pups and adult females, but guard hairs of adult females exhibited a lower 13C signature than those of pups. A different biochemical composition of guard hairs for adults and pups and for pups of various ages may explain the difference observed. Up to four months of age, the fur of pups is black, whereas it is mainly brown in older age classes. Melanin has been found to influence the 13C of bird feathers (Michalik et al. 2010), and this may prove to be the case in fur. These results suggest that, although it is not possible to use guard hairs of Cape fur seal pups as a proxy, the blood and whiskers of pups can be used to investigate female habitat use with regard to carbon sources. Female-to-pup discrimination factors determined in this study would need to be applied to 15N in order to use pup signatures to assess the trophic level of females.

Conclusion

Both scat and stable isotope analyses indicate that Cape fur seals are generalist predators at their easternmost colony, although individual females seem to exhibit behavioural consistency in their diet preferences. Our study showed the value of using a relatively low-cost method (stable isotope analysis) to investigate the variability in foraging strategies between individual seals. Our conclusions need to be substantiated, however, using a larger dataset and over a longer time period than was possible during the current study. Furthermore, future studies should consider an extension of the approach used here to resolve ecological partitioning within individuals (for example, through the analysis of whole whiskers), and between individuals, sexes, age classes and populations. This information will lead to a better understanding of: (i) the role of Cape fur seals as predators in the marine ecosystem; and (ii) how they might adapt to natural or anthropogenic perturbations in resource availability.

Acknowledgements — We thank Chris de Villiers and Akhona Mlondi for help during the fieldwork, and the marine section of the Addo Elephant National Park (South African National Parks – SANParks) for logistical support and accommoda-tion on Bird Island. George Kant and Steve Kirkman are thanked for their valuable advice at different stages of this study. Isotope samples were analysed by Ian Newton at the Stable Light Isotope Laboratory of the University of Cape Town, under the supervi-sion of John Lanham. All samples are lodged in the marine mammal collection of the Port Elizabeth Museum. MC acknow-ledges funding from the Nelson Mandela Metropolitan University through a post-doctoral fellowship. In addition to the animal ethics clearance granted by the Nelson Mandela Metropolitan University (A13-SCI-ZOO-008), this study was undertaken under a permit granted by the Department of Environmental Affairs (RES2013/26) to the Port Elizabeth Museum, and a research agreement between the Port Elizabeth Museum and SANParks. Two anonymous reviewers are thanked for their beneficial comments.

References

Abramoff MD, Magelhaes PJ, Ram SJ. 2004. Image processing with Image. Journal of Biophotonics International 11: 36–42.

Arnould JPY, Cherel Y, Gibbens J, White JG, Littnan CL. 2011. Stable isotopes reveal inter-annual and inter-individual variation in the diet of female Australian fur seals. Marine Ecology Progress Series 422: 291–302.

Attwood CG, Petersen SL, Kerwath SE. 2011. Bycatch in South Africa’s inshore trawl fishery as determined from observer records. ICES Journal of Marine Science 68: 2163–2174.

Baird D, Geldenhuys ND. 1973. Biology and fishery of the anchovy in South Africa. South African Shipping News and Fishing Industry Review 28: 43–49.

Balmelli M, Wickens PA. 1994. Estimates of daily ration for the South African (Cape) fur seal. South African Journal of Marine Science 14: 151–157.

Barange M, Hampton I, Roel BA. 1999. Trends in the abundance and distribution of anchovy and sardine on the South African continental shelf in the 1990s, deduced from acoustic surveys. South African Journal of Marine Science 21: 367–391.

Beck CA, Rea LD, Iverson SJ, Kennish JM, Pitcher KW, Fadely BS. 2007. Blubber fatty acid profiles reveal regional, seasonal, age-class and sex differences in the diet of young Steller sea lions in Alaska. Marine Ecology Progress Series 338: 269–280.

Booth AJ, Walmsley-Hart SA. 2000. Biology of the redspotted tonguesole Cynoglossus zanzibarensis (Pleuronectiformes: Cynoglossidae) on the Agulhas Bank, South Africa. South African Journal of Marine Science 22: 185–198.

Bowen WD, Iverson SJ. 2013. Methods of estimating marine mammal diets: a review of validation experiments and sources of bias and uncertainty. Marine Mammal Science 29: 719–754.

Casper RM, Jarman SN, Deagle BE, Gales NJ, Hindell MA. 2007. Detecting prey from DNA in predator scats: a comparison with morphological analysis, using Arctocephalus seals fed a known diet. Journal of Experimental Marine Biology and Ecology 347: 144–154.

Castley JG, Cockcroft VG, Kerley GIH. 1991. A note on the stomach contents of fur seals Arctocephalus pusillus pusillus beached on the south-east coast of South Africa. South African Journal of Marine Science 11: 573–577.

Cherel Y, Kernaléguen L, Richard P, Guinet C. 2009. Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals. Biology Letters 5: 830–832.

Clarke MR. 1986. A handbook for the identification of cephalopod beaks. Oxford: Clarendon Press.

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 341

Crawford RJM, Altwegg R, Barham BJ, Barham PJ, Durant JM, Dyer BM, Geldenhuys D, Makhado AB, Pichegru L, Ryan PG, Underhill LG, Upfold L, Visagie J, Waller LJ, Whittington PA. 2011. Collapse of South Africa’s penguins in the early 21st century. African Journal of Marine Science 33: 139–156.

David JHM. 1987. Diet of the South African fur seal (1974–1985) and an assessment of competition with fisheries in southern Africa. In: Payne AIL, Gulland JA, Brink KH (eds), The Benguela and comparable ecosystems. South African Journal of Marine Science 5: 693–713.

David JHM, Cury P, Crawford RJM, Randall RM, Underhill LG, Meÿer MA. 2003. Assessing conservation priorities in the Benguela ecosystem, South Africa: analysing predation by seals on threatened seabirds. Biological Conservation 114: 289–292.

David JHM, Meÿer MA, Best PB. 1990. The capture, handling and marking of free-ranging adult South African (Cape) fur seals. South African Journal of Wildlife Research 20: 5–8.

De Villiers DJ, Roux J-P. 1992. Mortality of newborn pups of the South African fur seal Arctocephalus pusillus pusillus in Namibia. In: Payne AIL, Brink KH, Mann KH, Hilborn R (eds), Benguela trophic functioning. South African Journal of Marine Science 12: 881–889.

Drago M, Cardona L, Crespo EA, García N, Ameghino S, Aguilar A. 2010. Change in the foraging strategy of female South American sea lions (Carnivora: Pinnipedia) after parturition. Scientia Marina 74: 589–598.

Ducatez S, Dalloyau S, Richard P, Guinet C, Cherel Y. 2008. Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (Mirounga leonina) breeding at the Kerguelen Islands. Marine Biology 155: 413–420.

France RL. 1995. Carbon-13 enrichment in benthic compared to planktonic algae: foodweb implications. Marine Ecology Progress Series 124: 307–312 .

Germain LR, Koch PL, Harvey J, McCarthy MD. 2013. Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations. Marine Ecology Progress Series 482: 265–277.

Green DB. 2014. Foraging ecology of Cape gannets breeding at Bird Island, Algoa Bay. MSc thesis, Nelson Mandela Metropolitan University, South Africa.

Habran S, Debier C, Crocker DE, Houser DS, Lepoint G. 2010. Assessment of gestation, lactation and fasting on stable isotope ratios in northern elephant seals (Mirounga angustirostris). Marine Mammal Science 26: 880–895.

Hammer Ø, Harper DAT, Ryan PD. 2001. PAST: Palaeontological Statistics software package for education and data analysis. Palaeontologia Electronica 4(1): article no. 4 (online).

Hobson KA, Schell DM, Renouf D, Noseworthy E. 1996. Stable carbon and nitrogen isotopic fractionation between diet and tissues of captive seals: implications for dietary reconstructions involving marine mammals. Canadian Journal of Fisheries and Aquatic Sciences 53: 528–533.

Hobson KA, Sease JL. 1998. Stable isotope analyses of tooth annuli reveal temporal dietary records: an example using Steller sea lions. Marine Mammal Science 14: 116–129.

Hobson KA, Welch HE. 1992. Determination of trophic relationships within a high Arctic marine food web using 13C and 15N analysis. Marine Ecology Progress Series 84: 9–18.

Hückstädt LA, Koch PL, McDonald BI, Goebel ME, Crocker DE, Costa DP. 2012. Stable isotope analyses reveal individual variability in the trophic ecology of a top marine predator, the southern elephant seal. Oecologia 169: 395–406.

Huisamen J, Kirkman SP, van der Lingen CD, Watson LH, Cockcroft VG, Jewell R, Pistorius PA. 2012b. Diet of the Cape fur seal Arctocephalus pusillus pusillus at the Robberg Peninsula, Plettenberg Bay, and implications for local fisheries. African

Journal of Marine Science 34: 431–441. Huisamen J, Kirkman SP, Watson LH, Cockcroft VG, Pistorius PA.

2012a. Recolonisation of the Robberg Peninsula (Plettenberg Bay, South Africa) by Cape fur seals. African Journal of Marine Science 33: 453–461.

Jackson S, Duffy DC, Jenkins JFG. 1987. Gastric digestion in marine vertebrate predators: in vitro standards. Functional Ecology 1: 287–291.

Jenkins SG, Partridge ST, Stephenson TR, Farley SD, Robbins CT. 2001. Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring. Oecologia 129: 336–341.

Kernaléguen L, Cazelles B, Arnould JPY, Richard P, Guinet C, Cherel Y. 2012. Long-term species, sexual and individual variations in foraging strategies of fur seals revealed by stable isotopes in whiskers. PLoS ONE 7: e32916.

Kirkman SP, Mecenero S, Roux J-P, Meÿer MA, Kotze PGH. 2007b. Annex 2: Manual of methods for monitoring Cape fur seals in the BCLME. In: Kirkman SP (ed.), Final report of the BCLME (Benguela Current Large Marine Ecosystem) project on top predators as biological indicators of ecosystem change in the BCLME. Cape Town: Avian Demography Unit, University of Cape Town.

Kirkman SP, Oosthuizen WH, Meÿer MA, Kotze PGH, Roux J-P, Underhill LG. 2007a. Making sense of censuses and dealing with missing data: trends in pup counts of Cape fur seal Arctocephalus pusillus pusillus for the period 1972–2004. African Journal of Marine Science 29: 161–176.

Kirkman SP, Yemane D, Oosthuizen WH, Meÿer MA, Kotze PGH, Skrypzeck H, Vaz Velho F, Underhill LG. 2013. Spatio-temporal shifts of the dynamic Cape fur seal population in southern Africa, based on aerial censuses (1972–2009). Marine Mammal Science 29: 497–524.

Kurle CM. 2002. Stable-isotope ratios of blood components from captive northern fur seals (Callorhinus ursinus) and their diet: applications for studying the foraging ecology of wild otariids. Canadian Journal of Zoology 80: 902–909.

Kurle CM, Worthy GAJ. 2002. Stable nitrogen and carbon isotope ratios in multiple tissues of the northern fur seal Callorhinus ursinus: implications for dietary and migratory reconstructions. Marine Ecology Progress Series 236: 289–300.

Mahan DC, Shields RG. 1998. Essential and nonessential amino acid composition of pigs from birth to 145 kilograms of body weight, and comparison to other studies. Journal Animal Science 76: 513–521.

Mecenero S, Roux J-P, Underhill LG, Bester MN. 2006a. Diet of Cape fur seals Arctocephalus pusillus pusillus at three mainland breeding colonies in Namibia. 1. Spatial variation. African Journal of Marine Science 28: 57–71.

Mecenero S, Roux J-P, Underhill LG, Kirkman SP. 2006b. Diet of Cape fur seals Arctocephalus pusillus pusillus at three mainland breeding colonies in Namibia. 2. Temporal variation. African Journal of Marine Science 28: 73–88.

Meyer M, Smale MJ. 1991. Predation patterns of demersal teleosts from the Cape south and west coasts of South Africa. 2. Benthic and epibenthic predators. South African Journal of Marine Science 11: 409–442.

Mhlongo N, Coetzee J, Shabangu F, Merkle D, Hendricks M, Geja Y. 2013. Results of the 2013 spawner biomass survey. Unpublished Report. Fisheries Management Branch Department of Forestry and Fisheries, Cape Town.

Michalik A, McGill RAR, Furness RW, Eggers T, van Noordwijk HJ, Quillfeldt P. 2010. Black and white – does melanin change the bulk carbon and nitrogen isotope values of feathers? Rapid Communication in Mass Spectrometry 24: 875–878.

Michener RH, Kaufman L. 2007. Stable isotope ratios as tracers

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius342

in marine food webs: an update. In: Michener R, Lajtha K (eds), Stable isotopes in ecology and environmental science. Singapore: Blackwell Publishing Ltd. pp 238–282.

Moseley C, Grémillet D, Connan M, Ryan PG, Mullers RHE, van der Lingen CD, Miller TW, Coetzee JC, Crawford RJM, Sabarros P, McQuaid CD, Pichegru L. 2012. Foraging ecology and ecophysiology of Cape gannets from colonies in contrasting feeding environments. Journal of Experimental Marine Biology and Ecology 422–423: 29–38.

Newsome SD, Clementz MT, Koch PL. 2010. Using stable isotope biogeochemistry to study marine mammal ecology. Marine Mammal Science 26: 509–572.

R Development Core Team. 2009. R: a language and environment for statistical computing. Vienna: R Foundation for Statistical Computing. Available at www.r-project.org.

Rand RW. 1956. The Cape fur seal Arctocephalus pusillus (Schreber) – its general characteristics and moult. Investigational Report, Division of Fisheries, South Africa.

Rand RW. 1959. The Cape fur seal (Arctocephalus pusillus). Distribution, abundance and feeding habits off the south western coast of the Cape Province. Investigational Report, Division of Fisheries, South Africa.

Reed JZ, Tollit DJ, Thompson PM, Amos W. 1997. Molecular scatology: the use of molecular genetic analysis to assign species, sex and individual identity to seal faeces. Molecular Ecology 6: 225–234.

Reid K. 1995. The diet of Antarctic fur seals (Arctocephalus gazella Peters 1875) during winter at South Georgia. Antarctic Science 7: 241–249.

Roy C, van der Lingen CD, Coetzee JC, Lutjeharms JRE. 2007. Abrupt environmental shift associated with changes in the distribution of Cape anchovy Engraulis encrasicolus spawners in the southern Benguela. African Journal of Marine Science 29: 309–319.

Sauer WHH, Downey NJ, Lipiński M, Roberts MJ, Smale MJ, Shaw P, Glazer J, Melo Y. 2013. Loligo reynaudi, Chokka squid. In: Rosa R, O’Dor R, Pierce G (eds), Advances in squid biology,

ecology and fisheries. Part I – Myopsid squids. New York: Nova Publishers. pp 33–72.

Shaughnessy PD. 1985. Interactions between fisheries and Cape fur seals in southern Africa. In: Beddington JR, Beverton RJH, Lavigne DM (eds), Marine mammals and fisheries. London: George Allen and Unwin. pp 119–134.

Smale MJ, Sauer W, Roberts M. 2001. Behavioural interactions between predators and spawning chokka squid off South Africa: towards quantification. Marine Biology 139: 1095–1105.

Smale MJ, Watson G, Hecht T. 1995. Otolith atlas of southern African marine fishes. Ichthyological Monographs of the JLB Smith Institute of Ichthyology 1.

Stewardson CL. 2001. Biology and conservation of the Cape (South African) fur seal Arctocephalus pusillus pusillus (Pinnipedia: Otariidae) from the Eastern Cape coast of South Africa. PhD thesis, Australian National University , Australia.

Szteren D, Naya DE, Arim M. 2004. Overlap between pinniped summer diet and artisanal fishery catches in Uruguay. Latin American Journal of Aquatic Mammals 3: 119–125.

Trites AW, Joy R. 2005. Dietary analysis from fecal samples: how many scats are enough? Journal of Mammalogy 86: 704–712.

Weimerskirch H, Inchausti P, Guinet C, Barbraud C. 2003. Trends in bird and seal populations as indicators of a system shift in the Southern Ocean. Antarctic Science 15: 249–256.

Wickens P, York AE. 1997. Comparative population dynamics of fur seals. Marine Mammal Science 13: 241–292.

Wilhelm MR, Roux J-P, Moloney CL, Jarre A. 2013. Data from fur seal scats reveal when Namibian Merluccius capensis are hatched and how fast they grow. ICES Journal of Marine Science 70: 1429–1438.

Wright BE, Riemer SD, Brown RF, Ougzin AM, Bucklin KA. 2007. Assessment of harbor seal predation on adult salmonids in a Pacific northwest estuary. Ecological Applications 17: 338–351.

Yonezaki S, Kiyota M, Baba N, Koido T, Takemura A. 2003. Size distribution of the hard remains of prey in the digestive tract of northern fur seal (Callorhinus ursinus) and related biases in diet estimation by scat analysis. Mammal Study 28: 97–102.

Manuscript received May 2014, revised June 2014, accepted July 2014

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

African Journal of Marine Science 2014, 36(3): 331–344 343

TissueStableisotope

(‰)

Mean (SD)Summer Winter

Adult females (n 4)(weight 68.4 kg; SD 5.3)

Pups (n 13)(weight 13.2 kg; SD 2.5)

Adult females (n 5)(weight 73.8 kg; SD 9.2)

Pups (n 2)(weight 23.2 and 22.3 kg)

Blood 13C15N

14.9 (0.4)15.7 (0.5)

15.1 (0.4)17.0 (0.5)

15.0 (0.4)15.5 (0.5)

15.0/–14.817.3/17.4

Guard hairs 13C 15N

13.3 (0.5)15.9 (0.4)

13.9 (0.3)17.4 (0.5)

13.4 (0.5)15.8 (0.7)

14.1/–13.817.8/17.7

Whiskers 13C 15N

13.0 (0.4)16.1 (0.6)

13.3 (0.3)18.1 (0.5)

12.9 (0.4)15.8 (0.6)

13.6/–13.517.8/18.1

Appendix 1: Carbon and nitrogen stable isotope values of blood, guard hairs and whiskers of adult female and pup Cape fur seals collected on Black Rocks, Algoa Bay, South Africa

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14

Connan, Hofmeyr, Smale and Pistorius344

Fixed effect

Response13C 15N

Estimate SE t- or z-value p-value Estimate SE t- or

z-value p-value

(Intercept) 0.0673 0.0007 93.00t <0.001*** 0.0638 0.0007 86.94t <0.001***Stage(Pup) 0.0008 0.0008 −0.99t 0.324 −0.0050 0.0008 −5.96t <0.001***Tissue(Guard hairs) 0.0083 0.0006 12.99t <0.001*** −0.0006 0.0007 −0.97t 0.330Tissue(Whiskers) 0.0098 0.0006 15.23t <0.001*** −0.0014 0.0006 −2.22t 0.026*Season(Winter) −0.0007 0.0010 −0.67t 0.502 0.0009 0.0010 0.86t 0.388Stage(Pup):Tissue(Guard hairs) −0.0027 0.0007 −3.64t <0.001*** −0.0006 0.0007 −0.82t 0.414Stage(Pup):Tissue(Whiskers) −0.0007 0.0007 −0.95t 0.340 −0.0020 0.0007 −2.68t 0.007**Stage(Pup):Season(Winter) 0.0014 0.0015 0.93t 0.351 −0.0021 0.0015 −1.4t 0.160Tissue(Guard hairs):Season(Winter) −0.0001 0.0009 −0.13t 0.897 −0.0005 0.0009 −0.63t 0.530Tissue(Whiskers):Season(Winter) 0.0010 0.0009 1.12t 0.261 0.0001 0.0009 0.07t 0.945Stage(Pup):Tissue(Guard hairs):Season(Winter) −0.0010 0.0013 −0.77t 0.444 0.0004 0.0013 0.35t 0.730Stage(Pup):Tissue(Whiskers):Season(Winter) −0.0034 0.0013 −2.59t 0.010** 0.0014 0.0012 1.15t 0.250Guard hairs:Whiskers 0.0015 0.0007 2.26z 0.061 −0.0008 0.0006 −1.25z 0.426Adult.Summer.Guard hairs:Adult.Summer.Blood 0.0083 0.0006 12.99z <0.001*** −0.0006 0.0007 −0.97z 0.997Adult.Summer.Whiskers:Adult.Summer.Blood 0.0098 0.0006 15.23z <0.001*** −0.0014 0.0006 −2.22z 0.464Adult.Summer.Whiskers:Adult.Summer.Guard hairs 0.0015 0.0007 2.26z 0.436 −0.0008 0.0006 −1.25z 0.977Adult.Winter.Guard hairs:Adult.Winter.Blood 0.0082 0.0006 14.48z <0.001*** −0.0012 0.0006 −2.01z 0.618Adult.Winter.Whiskers:Adult.Winter.Blood 0.0108 0.0006 18.75z <0.001*** −0.0014 0.0006 −2.35z 0.375Adult.Winter.Whiskers:Adult.Winter.Guard hairs 0.0026 0.0006 4.32z <0.001*** −0.0002 0.0006 −0.34z 1.000Pup.Summer.Guard hairs:Pup.Summer.Blood 0.0056 0.0004 15.49z <0.001*** −0.0012 0.0004 −3.53z 0.017*Pup.Summer.Whiskers:Pup.Summer.Blood 0.0091 0.0004 24.55z <0.001*** −0.0034 0.0003 −9.84z <0.01**Pup.Summer.Whiskers:Pup.Summer.Guard hairs 0.0035 0.0004 9.58z <0.001*** −0.0022 0.0003 −6.74z <0.01**Pup.Winter.Guard hairs:Pup.Winter.Blood 0.0045 0.0009 5.17z <0.001*** −0.0014 0.0008 −1.64z 0.859Pup.Winter.Whiskers:Pup.Winter.Blood 0.0067 0.0009 7.55z <0.001*** −0.0019 0.0008 −2.32z 0.395Pup.Winter.Whiskers:Pup.Winter.Guard hairs 0.0022 0.0009 2.39z 0.348 −0.0006 0.0008 −0.68z 1.000Pup.Summer.Blood:Adult.Summer.Blood −0.0008 0.0008 −0.99z 0.997 −0.0050 0.0008 −5.95z <0.01**Pup.Summer.Guard hairs:Adult.Summer.Guard hairs −0.0035 0.0009 −4.06z 0.002** −0.0056 0.0008 −6.77z <0.01**Pup.Summer.Whiskers:Adult.Summer.Whiskers −0.0015 0.0009 −1.77z 0.790 −0.0070 0.0008 −8.47z <0.01**Pup.Winter.Blood:Adult.Winter.Blood 0.0005 0.0012 0.45z 1.000 −0.0071 0.0012 −5.91z <0.01**Pup.Winter.Guard hairs:Adult.Winter.Guard hairs −0.0031 0.0012 −2.49z 0.290 −0.0073 0.0012 −6.10z <0.01**Pup.Winter.Whiskers:Adult.Winter.Whiskers −0.0035 0.0013 −2.79z 0.146 −0.0076 0.0012 −6.42z <0.01*** p < 0.05** p < 0.01*** p < 0.001

Appendix 2: Generalised linear mixed model results followed by Tukey HSD post hoc tests with Stage (Adult/Pup), Tissue (Blood/Guard hairs/Whiskers) and Season (Summer/Winter) as fixed factors and Individuals as a random factor

Dow

nloa

ded

by [

Port

Eliz

abet

h M

useu

m],

[M

s D

Pitm

an]

at 0

7:00

01

Nov

embe

r 20

14