J . Zoo/., Lond. (1987) 213, 81-94

Prey of coastal black-backed jackal Canis mesomelas (Mammalia: Canidae) in the Skeleton Coast Park, Namibia

G . AVERY, D. M . AVERY South African Museum, PO Box 61, 8000 Cape Town, RSA

s. BRAINE AND R. LOUTIT Directorate of Nature Conservation, Skeleton Coast Park, Prizwte Bag 5001, Swakopmund,

Namibia (Accepted 10 March 1987)

(With 2 plates and 4 figures in the text) Remains of prey of black-backed jackal Cunis mesomelas from 13 jackal middens situated within 25 m of the beach, which provides the most regular source of food, and one 1.3 km inland in the Skeleton Coast Park, Namibia, were examined. Middens accumulate on small sand hummocks which provide shelter from prevailing harsh environmental conditions. Canis mesomelus is shown to rely on marine resources. A total of 30 taxa was recorded. Birds were the most common prey individuals represented in the combined samples (68%), followed by mammals (17%) and fish (15%). One invertebrate was recorded. At the inland site, which is located at a colony of whitebreasted cormorants Phalucrocorux curbo, this species alone formed 78% of the total individuals. Surveys were conducted to determine the extent to which C. mesomelas is unselective in scavenging beached seabird carcasses and the results were compared with data on roosting habits. There was good correlation between both these sets of data and the contents of the jackal middens, thereby indicating that C. mesomelas is unselective in its foraging/scavenging. The drop in the jackass penguin population was also evident when middens of different ages were compared. It was shown that the data from jackal middens provide an index of seabird mortality which reflects changes in the oceanic and climatic conditions which affect fish populations. Regular monitoring of jackal middens may provide an easily accessible source of information on long- and short-term changes in fish stocks.

Contents

Introduction . . . . . . . . Methods and materials . . Results . . . . . . . . . .

Jackalmiddens . . . . Prey remains . . . . . .

Discussion . . . . . . . . Middens and hummocks Prey of Canis mesomelas Fish population estimates

Conclusion . . . . . . . . References . . . . . . . .

Beach surveys . . . . . .

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Page . . 81 . . 82 . . 83 . . 83 . . 85 . . 87 . . 87 . . 87 . . 91 . . 93 . . 93 . . 94

Introduction Shortridge (1934) noted that the black-backed jackal Canis mesomelas will scavenge on

0022-5460/87/009081+ 14 803'00 81

1987 The Zoological Society of London

82 G . A V E R Y E T A L .

NAMIBIA 1 20 Y

25 ‘I

FIG. 1. Location of research area (*) and places mentioned in the text. Shaded area is Skeleton Coast Park.

beaches for washed-up remains of marine mammals, fish and birds. More recently, studies of scats from beaches at Sandwich Harbour (Stuart, 1976) (Fig. l) , Wolf Bay and Van Reenen Bay (Stuart & Shaughnessy, 1984) and localities between Cape Cross and Torra Bay (Nel & Loutit, 1986), have led to the conclusion that C. mesomelas is an opportunistic feeder that utilizes a variety of resources, seabirds and Cape fur seal Arcrocephaluspusillus in particular, according to availability. Nel & Loutit (1986) concluded more specifically that density and predictability of food sources determine the density of C. mesomelas and the diversity of prey types taken and that, although they move considerable distances inland, C . mesomelas derive almost all of their food from the coast where predominantly marine species are taken.

This paper presents additional information obtained from a study of accumulations of the remains of prey of C. mesomelas in the Skeleton Coast Park (Fig. 1) and examines the phenomenon of reliance on marine resources. The term jackal midden is preferred to the previously used jackal kitchen (Avery, Avery, Braine & Loutit, 1984). Although the term ‘midden’ has been used to describe piles of jackal faeces (Brown & Macdonald, 1985), i t is here applied to those localized areas to which jackals regularly bring their prey for consumption, and where remnants, in the form of bones, feathers and the like, accumulate (Plates I, 11).

Methods and materials

Remains of all prey were collected from I3 jackal middens on a stretch of beach extending4 km south of the Huab River mouth, and 1 midden at an abandoned oil-drilling rig. Small numbers of shells of white mussel

PREY OF COASTAL BLACK-BACKED JACKALS 83

PLATE I . Typical vegetated dune hummocks. Note the short distance to the sea, hummocks on storm beach and gravel plain in left background.

Dona.r serra and brown mussel Pernapema, which only occurred on low hummocks on the storm beach, were discounted because it was impossible to determine whether they were deposited by jackals, kelp gulls Larus dominicanus or storms. Furthermore, although Stuart (1976) and Nel & Loutit (1986) recorded fragments of shell of these molluscs in scats. their ingestion may have been accidental.

All identifiable remains were used to calculate the minimum numbers of individuals representing the identified taxa. The numbers of seabirds washed up on measured stretches of beach in the Skeleton Coast Park were recorded at various times of the year to provide an estimate of their availability. Arctocephalus pusillus carcasses are regularly beached along the coast (Bridgeford, 1985) and, although numbers were not recorded during the beach surveys, a figure is availablc for the 6-month period prior to the sampling of the middens.

Results

Jackal middens

The middens occur on small sand hummocks that form around Zygophyllum claoatum, Psiiocaukon sulicornioides and Sulsola sp. plants, and grow as the vegetation clump expands to consolidate and trap more sand (Giess, 1971). As more sand is trapped, middens may be covered

84 G . A V E R Y ET A L .

PLAE 11. Accumulation o f bones. feathers and skin on jackal midden 8.

and stratified beneath later midden accumulations. The hummocks vary in area, ranging from as little as 2 m2 to approximately 20 m2 and in height from 0.4 m to approximately 2 m. Jackals take their prey on to the vegetation, where they often feed on the leeward side. All but one of the jackal middens sampled were situated less than 25 m from the sea on the storm beach and edge of the gravel plain, although hummocks occur up to 500 m inland. The last sample was taken from a midden at an oil-drilling rig 1.3 km inland. Here there is a burrow 20 m away and derelict machinery provides additional shelter for the jackals. The main supply of food is provided by a colony of whitebreasted cormorants Phalacrocorax carho, which has remained stable since it was established in 1980 with 54 nests and over 130 birds on the derrick above (RL, pers. obs.). The jackals from this site also forage on the beach over an area which includes the sampled middens.

The number of remains and minimum number of individuals of all species represented varied between middens, although a general similarity exists, particularly for the larger samples (Table I) . Differences in their composition are, however, reflected by the Shannon-Wiener indices of general diversity (H) and their components dominance (c) and evenness (e) (Odum, 1971) (Tables 11. 111). From these data, it is apparent that samples from the beach, which have relatively high indices of general diversity, low dominance and high evenness, are distinct from the inland midden where the reverse is true.

85 PREY OF COASTAL B L A C K - B A C K E D J A C K A L S

T A B L E I Prc~r 01 hluck-hacked iuckds irnd tirinitiiiini nurnhers o/' incliriduuls froni juckul niiddens sunipled in rhc

Skeleron Cocrsr Park

Prey species

Puro /orii!..s lit tlerlulei Delphinidae Mysticeti A rc~tocep1iulu.s pusilllrs

Total mammals Splieniscus denrersus Dionledeu crulons D. CNUIU

D . nielanophr is Mocronecre.s spp. Duprion cupense Puchyptilu sp . Proccll~rriu uc~quinoctiuli.s Pufinus gri.reii.s Morrts c~upensis Phulucroc~orrrs c,urho P. capensis Pliiriulis squururolu S/er~orrrrius puru.ri/icu.s Lurus sohini Sterna sandticensis S. hirundo

Total birds Chondrichthyes Tuc1ij.suru.s ,felicep.s Trtdiurus s p . A rgyrosonlus ho~o~cpirk~tus Litlrognurhus s p . Rlrahdosurgus glohiceps Di od on t i dae '? Cyclich 1hy.s Indet.

Total fish Plagusia chuhrus Total invertebrates

Grand Totals

~. ~~

Jackal middens Code* 1-10 12a I2b 13 14 ODR** ALL '%

a

b

C

d

f h

c

I

j k n P r S

t

- - I - - 3 - I - - 1 - - -

17 I I 4 5 7

21 I I 6 5 7

-

2 2 8

31

-

1 3

48

2 12 -

- I - I I - I I - I I - I - - - I -

3 2 - 1 I - 9 7 8 1 1 -

I2 6 I 1 - I -

I

2 2 1 1

34 36 23

- -

- - -

- -

I - - - - I - - - -

22 10 8 3 2 I - - - - 1 - - - -

92 55 50 31 34

-

I

2 3

-

I 35 6

45

- I 2

-

50

I 0.3 5 1.6 I 0.3

46 14.7

53 17.0

15 4.8 I 0.3 2 0.6 2 0.6 2 0.6 2 0.6 I 0.3 8 2.6 5 1.6

38 12.2 37 11.9 81 26.0

1 0.3 I 0.3 I 0.3 4 1.3

10 3.2 211 67.6

I 0.3 29 9.3

3 1.0 6 1.9 2 0.6 2 0.6 I 0.3 3 1.0

47 15.1

I 0.3 1 0.3

312

* Codes for species names are used in Fig. 2 ** ODR = oil-drilling platform

Prey remains

Birds constitute the most common prey over all, the Cape cormorant Phalacrocorax capensis being the most numerous (Table 1). There is a certain amount of variation in percentage

86 G . AVERY ET AL.

TABLE 11

Indices of general ditiersity ( H ) , dominance (c) and evenness (e l calculated from all taxa from the larger jackal midden samples taken

in the Skeleton Coast Park and the total sample

12a 13

Midden 12a 13 14 14 ODR 12b Total

n 55 31 34 120 50 50 312 S 17 7 7 25 10 20 30 H 2.34 1.64 1.56 2.17 1.18 2.56 2.44 C 0.13 0.23 0.27 0.18 0.51 0.11 0.13 e 0.83 0.84 0.80 0.67 0.51 0.85 0.72

n = total individuals s = no. species

TABLE I 1 1 Relatiire proportions (%) and indices of general diversity ( H ) , dominance (c) and evenness (e l calculated,from bird taxa,from the larger,jackalmidden samples taken in

the Skeleton Coast Park and the total sample

12a 12b 13 14 ODR ALL 12a/13/14

Spheniscus demersus Diomedea exulans D. cauta D . melanophris Macronecres s p p . Daption capense Pachyptila sp. Procellaria aequinoctialis Puffinus griseus Morus capensis Phalacrocorax carbo P. capensis Pluvialis squatarola Stercorarius parasiticus Lurus sabini Sterna sandvicensis S. hirundo n

H

e

S

C

5.9

2.9 2.9 2.9 2.9

8.8 2.9

26.5 2.9

35.3

-

-

-

- - -

5.9 34 1 1

1.89 0.2 I 0.79

33.3 2.8 2.8 2.8 2.8 - 2.8 5.6 2.8

19.4 2.8

16.7 2.8 - - - 2.8 36 13

2.04 0.19 0.80

- -

- - -

- -

4.0 4.0

20.0

60.0 -

-

-

- -

12.0 25 5

1.14 0.42 0.7 I

7. I 0.5 0.9 0.9 0.9

2.2 0.9 0.5 3.8 2.4

2.2 18.0 77.8 17.5 13.3 38.4

0.5 0.5

2.2 0.5 2.2 1.9

4.7 45 211 6 17

0.80 1.91 0.62 0.22 0.45 0.67

-

- -

-

-

-

-

-

- -

-

2.4

1.2 1.2 I .2 I .2

4.9 2.4

26.8 I .2

46.3

1.2

2.4 7.3 80 13

1.57 0.3 I 0.6 1

-

-

-

-

P R E Y OF COASTAL BLACK-BACKED JACKALS 87

occurrence of seabirds in the larger samples (Table 111, Fig. 2). The oil-drilling rig (ODR) is distinguished by an overwhelming proportion of Phulacrocorax curbo, as would be expected, whereas midden 12b contains a high proportion ofjackass penguin Spheniscus demersus, while in middens 12a, 13 and 14, P . cupensis is the best represented species. General diversity ranges from 0.80 at ODR to 2.04 in midden 12b (Table III) , caused in the former by very high dominance and in the latter by a greater number of species.

Mammals were the next most common prey (Table I). Of these, the Cape fur seal Arcrocephafus pusillus was the most common. Other marine mammals were dolphins, probably the Heaviside’s dolphin Cephalorhynchus heavisidii or dusky dolphin Lugenorhynchus obscurus, and baleen whale (Mysticeti). Littledale’s whistling rat, Parotomys littfedafei, was the only terrestrial mammal. Fish were the third most abundant prey (Table I). Of these, sea-catfish Tachysurusfeficeps was the most common. The only invertebrate recorded was an intertidal crab Pfagusia chabrus. No plant remains were found.

Beach surveys A total of 408 carcasses was recorded from 3489 km of beach covered during 1983-1985 (Table

IV). This represents an overall density of 0.12 carcasses/km, with monthly means ranging from 0.03 to 0.22 (Fig. 3). Only seven out of 69 beach surveys failed to produce bird carcasses, which represents a 90% reliability for this source of food. The most abundant species was Phalucrocorax capensis, followed by Morus capensis; all other species comprised less than 10% of the total (Fig. 4).

The various indices for the combined 1983-85 data (Table IV) are very similar to those for the combined midden samples 12a, 13 and 14 (Table III), which are recent accumulations. The same general pattern of relative abundance remained similar for each of the samples for individual years. I t is, however, apparent that there tended to be differences in the minor species represented. The index of general diversity shows a peak of 1 a85 in 1984, with the 1983 value much closer to this than is the 1985 value (Table IV). The relatively low value for 1985, resulting from a mass mortality of Phafacrocorax cupensis, caused the value for this sample to be more closely comparable to that from the jackal midden ODR where P. carbo is the dominant species.

The nearest regular source of live Arctocephuluspusiflus is the mainland colony at Cape Cross, some 100 km south of the Huab River. In the study area, therefore, Cunis mesomefus must rely on beached carcasses. In August 1982, Bridgeford (1 985) recorded 22 seal carcasses on a 50 km stretch of beach north of Mowe Bay. In September of the same year one of us (SB) recorded a total of 170 A. pusillus beached between the Huab and Ugab Rivers (40 km). This yields a density of 2.13 carcasses per km covered. This mortality was exceptional, however, and a more reasonable estimate is perhaps provided by 1979/1980 figures of 0.83 for the fishing season and 0.59 for the closed season (Shaughnessy, 1985). At the time of sampling, two buried carcasses were being fed on by jackals in the vicinity of the middens. During 1982, 14 Cephaforhynchus heuvisidii were washed up on the beach after being trapped in fishing nets and killed by fishermen who had set their nets within 500 m of the shore. Three carcasses still remained at the time the middens were sampled.

Discussion

Middens and hummocks The middens tend to be concentrated close to the most regular source of food which, in this

88 G . AVERY ET A L .

.! a v)

JM12a JM12a/l3/14 40.

I J

v)

'H v)

e JM12b 5 30- 3 v)

n P .- 9 .I- m aJ n -

601 JM14

20 40LJl 0 rrrrrr a c e g i k m o q s u w a c 8 g i k m o q s u - w

Species Species

FIG. 2. Relative proportions (%) of seabird species in the larger jackal middens. See Tables I1 and IV for species represented by codes a, b, etc. Sample sizes: JM12a-34; JM13-23; JMI425; JM 12a/13/IC82; JM 12b36; ODR45 .

context, is the beach. Virtually all of the remains studied were of marine origin and a number of species, particularly of birds, was taken. Canis mesomelus is not, however, the only predator that forages on the shore. The brown hyaena Hyaena brunnea regularly scavenges Arctocephalus pusillus

PREY OF COASTAL BLACK-BACKED JACKALS 89

0.20

E $ e ii 0.10

0 J

FIG. 3. Mean

i

'@\

F M A M J J A S O N D Months

number of seabird carcasses per km recorded on beach surveys between 1983 and 1985.

E a I

a c e g i k m o q s u w Species

FIG. 4. Relative proportions (Yo) of seabird species recovered during surveys of beached carcasses and breeding populations of common species on islands off the Namibian coast. See Tables I1 and IV for species represented by codes a, b. etc. Beach survey data (n =408). Breeding populations + S. hirundo (n = 240500).

90 G . AVERY ET A L .

TABLE IV Birds recorded between 1983 and 1985 on beach surveys in the Skeleton Coast Park and

indices of general diversity ( H), dominance (c) and eoenness ( e l

Species Code* 1983 Year I984 1985 Total

Spheniscus demersus Diomedea melanophris D . chlororynchos Diomedea spp. Daption capense Pachyptilu sp. Procellaria aequinoctialis Puffinus gracis P. griseus Morus capensis Phalacrocurax carho P. capensis Charadrius marginatus Plurialis squaiarola Calidris ferruginea Recurvirosira aiwseita Stercorarius parasiticus k r u s dominicanus Sierna sandvicensis S. hirundo S . paradisea S. halaenarum Chlidonias niger Totals (n) n (excluding Diomedea spp.)

H

e

s

C

a

b

d e f g h I

j k I

m n

P 9

t U V W

0

S

94 93 13 I .69 0.28 0.66

I 4 I I -

-

2 1 I

29 3

38 -

-

3 7 2

1

I12 I08 17 I .85 0.26 0.65

-

3 1 I 4 I I 2

2 26

3 41

I 3

-

~

-

3 I

10 I

2 202 202

14 1.07 0.54 0.40

-

2 -

- I - - 27

2 I45

1 2 I I I 1 2

I 5

I

408 403

-

-

22 I .52 0.38 0.49

6 5 2 5 I I 5 I 3

82 8

230 2 5 I I 1 4 6

32 3 I 3

I .5 I .2 0.5 I .2 0.2 0.2 1.2 0.2 0.7

20. I 2.0

56.4 0.5 I .2 0.2 0.2 0.2 1 .o I .5 7.8 0.7 0.2 0.7

* Codes for species names also appear in Table I and Fig. 2

carcasses, birds and other carrion on beaches (Shortridge, 1934; Skinner & van Aarde, 198 I ; J. D. Skinner, pers. comm., 1983; Avery et al., 1984; Siegfried, 1984; Stuart & Shaughnessy, 1984), and lions Panthera leo occasionally take A . pusillus, Phalacrocorax carbo and P. capensis from beaches (Bridgeford, 1985). That the accumulations in the Skeleton Coast Park are produced by C. mesomelas is confirmed by direct observation of the animals, and by spoor and scats at the middens. Furthermore, the nature of damage on the bones is distinct from that caused by H. brunnea (Avery et al., 1984; GA, pers. obs.), while P . leo was not observed to drag carcasses on to hummocks. It is considered unlikely, therefore, that they would contribute significantly, if at all, to accumulations in this context.

The coastal environment of Namibia is exposed; seasonal fogs and cold onshore winds lower the temperatures, and conditions are generally extremely harsh (Bridgeford, 1985). The hummocks provide vantage points and protection from the elements, in addition to being close to a variety of predictable resources on the beaches. Furthermore, the vegetation on the hummocks traps heat, and surfaces under the vegetation were noticeably warmer than those of the gravel plains, thereby

P R E Y OF COASTAL BLACK-BACKED JACKALS 91

reducing the chill factor (Nel, In prep.). For all these reasons, the hummocks provide ideal places to which C. mesomelas bring prey and thereby accumulate middens.

Prey of Canis mesomelas

Studies of prey remains and those of scat contents are not directly comparable. Both have drawbacks such as differential preservation and representation of various types of food and problems of quantification, although they can be complementary. Canis mesomelas does not totally consume prey, even birds, and observations indicate that some identifiable remnant is usually left. I t must be accepted, though, that in an analysis of accumulated prey remains, small items, such as the smaller birds, rodents, invertebrates and plant foods, may be under-represented. This problem is demonstrated by the diversity of such species recorded from scats (Stuart, 1976; Stuart & Shaughnessy, 1984; Nel & Loutit, 1986), although Nel & Loutit (1986) suggest that such sources of food are not important on the coast.

Not all birds or marine mammals scavenged from the beach are taken to the middens; many carcasses were observed to have been eaten where they were found on the beach and the remnants left there. Particularly in the case of marine mammals, only part of a carcass may have been carried to the midden. During the mass mortality of Phalacrocorax capensis in July 1985, few carcasses were removed to the hummocks (RL, pers. obs.). It was noteworthy, too, that during this period of ‘surplus’ food availability, C. mesomelas ate very selectively on carcasses ( e g part of breast only), and that over 95% of the individuals not taken to the middens were moved higher up the beach and placed on wrack left after a flood earlier in the year. A few carcasses were consumed on the gravel plains between hummocks and further inland. Because it is not possible to determine how much was consumed, the actual intake of food could not be estimated. It does, however, indicate the range of prey taken and the importance of marine resources to C . mesomelas.

The samples contain oceanic and pelagic seabirds such as various albatrosses, petrels and shearwaters, the arctic skua Stercorarius parasiticus, Sabine’s gull Larus sahinii, Spheniscus demersus and Morus capensis, which rarely occur ashore unless sick or dead. This confirms other observations that C. mesomelas regularly takes beached birds which would provide a predictable supply of food, as has been shown by observations in the western Cape Province of South Africa (Avery, 1984, 1985; Avery & Underhill, 1986) and the Skeleton Coast Park (RL, SB, pers. obs.). This food supply is both well distributed along the coast and available throughout the year, and, depending on season, oceanic factors and coastal topography, densities of beached birds can be even higher than the mean figures given above. Although C. mesomelas takes live birds when the opportunity arises (Stuart, 1976; Nel& Loutit, 1986), theextent to which this occurs is not known. Roosts are not, however, likely to be as consistent a source of food as beached birds, unless the birds regularly use a particular locality, as, for example, terns use the Huab River lagoon.

From the data presented in Fig. 2, it is clear that, while there is variation, certain species such as Phalacrocorax cupensis, Morus capensis and Sterna hirundo regularly occur in the jackal middens. This is not surprising, given the large numbers of these species that occur along the Namibian coast. Estimates of the relative breeding population sizes of species that commonly breed on islands off the coast of Namibia, and sometimes the mainland, as well as of populations of Sterna hirundo, which is a palearctic visitor, are shown in Fig. 4 (Underhill & Whitelaw, 1977; Brooke, Cooper, Shelton & Crawford, 1982; Cooper, Brooke, Shelton & Crawford, 1982; Crawford, Shelton, Cooper & Brooke, 1983; Shelton, Crawford, Cooper & Brooke, 1984). If these data are compared with the proportions of species represented in the overall beach sample, the similarity is

92 G . A V E R Y E T A L

evident, and confirms the fact that the availability ofcarcasses, monitored in beach surveys, closely reflects the relative proportions of common coastal seabirds. The relative commonness of P. capensis and S. hirundo is also indicated by the regular occurrence of large flocks roosting on the beach, particularly during summer.

The absence of Spheniscus demersus from all the small samples is in keeping with the beach survey and population data, and penguins do not make up more than about 5% of any sample. The figure of 33% in sample 12b is an exception. This sample is from an older midden, stratified below 12a. The population sizes of all common offshore breeding species found on the middens have dropped considerably over the last 30 years as a result of over-fishing of their prey (Cooper et al., 1982; Crawford et al., 1983). The decline of S . demersus, which was also exploited for its eggs, has been even more marked (Shelton et a[., 1984). Sample 12b, therefore, was probably accumulated prior to 1956 and the major decline by some 80% of the S . demersus population. Conversely, the absence of this species from samples 13 and 14 may indicate that these samples are the most recent, since S. demersus is seldom found on the Skeleton Coast today (RL, SB, pers. obs.; Shelton et al., 1984).

At the only inland accumulation, that at the oil-drilling rig, the situation is different from that at the beach. This is because C. mesomelas concentrates on the Phalacrocorax carbo breeding colony which provides a reliable supply of food. In an observation made at the oil-drilling rig during 1985, R. Braby (Nature Conservation Officer) counted 12 immature P. carbo carcasses beneath and around the colony and jackal burrow. He also recorded the observation of J. Jozefowitz, who was filming C. mesomelas there, that an average of one P. carbo was taken every two days and that C. mesomelas was quick to take advantage of regurgitations by the cormorants when disturbance took place. The very large number of P. carbo, and relatively low proportion of remains of prey which would have been scavenged on the beach, support the suggestion that the presence of the colony reduces the necessity for opportunistic scavenging on the beach.

Seals, except new-born pups, are probably immune to attack by C. mesomelas, but carcasses of all ages are regularly beached and ensure a ready supply of meat. Moreover, a single carcass may remain relatively intact for several months. Even when the carcass is covered by sand, edible portions are exposed by digging, and sometimes detached pieces are carried off to the hummocks. The same would apply to dolphins and whales. The single Parotomys littledalei from midden 12b may have come from an individual killed but left intact (RL, pers. obs.) or a decomposed scat, or have resulted from the natural mortality of an individual burrowing in the hummock.

Angling in the Skeleton Coast Park is only allowed for a short period each year at certain points, the nearest being some 40 km to the south. Although fishermen do discard some species, particularly sea-catfish Tachysurus,feliceps, and heads and other offal, this is unlikely to affect the situation at the Huab River. Natural mortality, including that caused by periodic deoxygenation of nearby waters and floods (Nel & Loutit, 1986), may account for most beached fish. Mortality caused by pelagic trawlers, sometimes within 500 m of the shore, could have a localized effect. Canis mesomelas also takes flathead mullet Mugil cephalus trapped in the shallows of desiccating lagoons (Nel & Loutit, 1986). The low proportions of fish and invertebrates in the ODR sample compared to the scats examined by Nel & Loutit (1986) is not surprising as remains of smaller items are more likely to be found in scats. The high proportion of fish in the scats may derive from the fact that C. mesomelas takes regurgitated fish dropped by Phalacrocorax carbo. Nel & Loutit (1986) recorded active foraging on unknown marine organisms in the intertidal zone by C. mesomelas. They further suggested (Nel & Loutit, 1986) that the source of shell remains in the scats was marine bivalves dropped on to the rocks by Larus dominicanus in order to break the shells

PREY OF COASTAL BLACK-BACKED JACKALS 93

and gain access to the animal within (Siegfried, 1977). It is unlikely, therefore, that any edible remnant would be available for C. mesomelas unless cleptoparisitism, which is not mentioned, takes place. It is to be expected, as observed by Nel & Loutit (l986), that terrestrial invertebrates would be taken more frequently further inland.

Fish population estimates

Regular surveys of beached seabirds in the western Cape Province of South Africa have provided an index of seabird mortality in response to changing oceanic and climatic conditions. These conditions also correlate with the fish populations which form the basis of the pelagic fishing industry and are prey of seabirds such as Spheniscus demersus, Morus cupensis and, particularly, Phalucrocorax capensis (Shannon, Crawford & Duffy, 1984; Avery, 1985). Since C. mesomelas is shown to sample beached seabirds unselectively and to remove most carcasses to the hummocks, sampling of jackal middens could provide Namibian fisheries biologists with an easily accessible source of information on short- and long-term changes in anchovy Engraulis capensis populations as an alternative to regular beach surveys.

Conclusion

This study supports previous observations that, when on the coast, Canis mesomelas takes a variety of marine resources and virtually no terrestrial prey. The close similarity between the prey remains in the middens, the results of the beach surveys, and estimates of living seabird populations clearly demonstrates that C. mesomelas utilizes a wide variety of marine resources according to their availability. In the area studied, beached seabirds provide the most predictable resource, while marine mammals, particularly Arrtocephalus pusillus, provide periodic but long- lasting local abundances of food on a regular basis. The location of a breeding den at the oil- drilling rig, where a large colony of Phalucrocorux carbo provides a reliable source of food, confirms the role of local abundances in determining the subsistence, and possibly breeding, strategies of C. mesomelas. I t also supports the proposition that the jackal is essentially an opportunistic feeder.

Both at the coast and further inland, the need for shelter from the harsh environmental conditions is an important factor in determining the location of resting/eatingand breeding places. Such shelter must, however, be relatively close to the source of food, as is shown by the fact that, although some hummocks occur several kilometres inland, C . mesomelas apparently only uses those close to the coast as eating places.

The data collected from jackal middens also have the potential to assist in the monitoring of changes in the size of fish populations, through the evidence of birds that prey on the fish.

GA and DMA gratefully acknowledge the Directorate of Nature Conservation, SWA/Namibia, for permission to conduct research in the Skeleton Coast Park, logistical support while in the field and support of the beach surveys. Beach surveys were organized by R. Loutit and S. G . Braine assisted by R. J. Brady, P. Bridgeford, M. Dreyer, E. Ganuseb, F. Hoeb, L. Johannes, J. Knowles, F. Nuab, J . Paterson, P. Tarr, L. Braine, S. Clements, B. Loutit, I . Stutterheim and J . Tarr. This project forms part of a larger project on the taphonomy of Late Quaternary bone accumulations which is supported financially by the Council for Scientific and Industrial Research, Foundation for Research Development, and the Council of the South African Museum. Sorting and processing of the samples was undertaken by P. F. K. van Straaten and V. M.

94 G. AVERY E T AL.

van Zyl, typing by S. Saven and M. Scheiner. J. A. J. Nel, M. L. Wilson and W. J. J. van Rijssen are thanked for comments on an early draft of this paper.

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