Estuarine, Coastal and Shelf Science (1988) 26, 447-458

Contemporary and Historical Patterns of African Penguin Spheniscus demersus: Distribution at Sea

Rory P. Wilsona,c, Marie-Pierre T. Wilsona and David C. Duffyb Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa

Received 27 April 1987 and in revised form 29 October 1987

Keywords: distribution; penguin; population; seabirds; Spheniscus

Line transects from boats were conducted during 1984 and 1985 to determine African Penguin (Spheniscus demersus) distribution in the South Mrican Cape waters. Range limits for breeding birds were derived from information on pen­guin travelling speeds and durations of foraging trips. Over 50% of all penguins considered to be non-breeding occurred within 20 km of the coast whilst over 50% of all breeding birds occurred within 3 km of the coast. Penguin density decreased with increasing distance offshore. The most frequently encountered penguin group size was one with larger groups decreasing in incidence according to a power curve decay. There was no difference in the frequency of occurrence of different group sizes between breeding and non-breeding penguins, nor did group size distribution change with distance offshore. Data on African Penguin distribution at sea collected in 1984 and 1985 was not discemably different to equivalent data collected between 1954 and 1974. The composition of penguin group size was also the same during both periods.

Introduction

Mrican Penguins Spheniscus demersus are important members of the breeding marine avifauna of the Benguela upwelling ecosystem off southern Africa (Duffy et al., 1987 a) and changes in their numbers have been used to monitor population trends of their prey: pelagic schooling fish such as anchovy Engraulis japonicus and pilchard Sardinops ocellata (e.g. Crawford & Shelton, 1978). The breeding population of African penguins in South Mrica has decreased by about 90% during this century (Frost et al., 1976; Shelton et al., 1984). Much of this decrease can be attributed to historical factors, such as the removal of guano nesting substrata, the collecting of eggs, and oiling (for a review see Frost et at., 1976). These factors, however, do not explain the species' continuing decrease of about

Present address:• Institut fiir Meereskunde an der Universitiit Kiel, Diistembrooker Weg 20, D-2300 Kiel 1, F.R.G., and &Escuela de Ciencias Ambientales, Universidad Nacional, Heredia, Costa Rica. 'Address for correspondence: Rory P. Wilson, Instituut fiir Meereskund an der Universitiit Kiel, Diistembrooker Weg 20, D-2300 Kiel1, F.R.G.

447

0272-7714/88/040447 + 12 $03.00/0 © 1988 Academic Press Limited

448 R. P. Wilson et al.

14% per year (data for 1979-85: La Cock et al., 1987)on the westcoastofSouth Africa, the traditional centre of its range (Frost et al., 1976).

Competition with a commercial fishery has been suggested as the main cause of the continuing decrease in African Penguin abundance (e.g. Frost et al., 1976; Crawford & Shelton, 1978, 1981; Burger & Cooper, 1984). Supporting evidence for this explanation has been excessively long incubation shifts (Cooper, 1984), inference of competition from estimates of food consumption by penguins (Furness & Cooper, 1982), and the post hoc linkage of penguin population decreases with apparent changes in penguin diet and fishery landings (Crawford & Shelton, 1981). Data from a long-term study (1979-85) showed that nesting success on Marcus Island in the centre of the commercial fishing area is similar to that on St Croix Island where such fishing did not occur (La Cocket al., 1987). African Penguin juveniles have a low survival (Duffy & Cooper, 1987; La Cock et al., 1987), suggesting that the controlling factor affects birds at sea, rather than at their breeding islands.

We conducted transects to examine the distributions and group sizes of penguins at sea and compared these with earlier data (1954-74: Siegfried et al., 1975). Previous work on the foraging behaviour of breeding African Penguins (Wilson & Bain, 1984; Broni, 1985; Wilson, 1985a,b; Wilson et al., 1986a) enabled us to assess whether the birds we saw were likely to be breeding or not. We also examined how the breeding season may determine numbers and distribution of African Penguins at sea.

Methods

Transects Between February 1984 and September 1985, we counted penguins at sea during transects run from motor boats (12-21 m long) or yachts (10m long) travelling at constant speeds (22 km h -t and 8 km h _ ,,respectively). The observers, at an eye height of approximately 3m above the water, continuously surveyed a 90° arc of radius 100m, extending from directly ahead of the boat to perpendicular to the direction of travel. Thus, a strip 100m wide was inspected with the forward bias increasing the likelihood that birds diving due to the approach of the boat be seen. Bird sightings were recorded to the nearest minute. Time of day, group size (all penguins within 2m of other penguins were classified as a group) and associated animals were recorded for all penguin sightings.

Three transect routes were conducted in the Saldanha Bay region (33°03'S, 17°58'E) on the south-western Cape coast of South Africa. Transects started at the mouth of the bay and ran: (1) north-north-west (bearing 337°) for 20 km; (2) west (bearing 270°) for 20 km; and (3) south-south-west (bearing 157°) for 20 km (Figure 1). All transects were run during 10.00-15.30 h, when most African Penguins feed (Wilson, 1985b).

Transects were also conducted in November 1983, May and July 1984, January and June 1985, from Cape Agulhas, South Africa (34°50'S, 20°0l'E) to Liideritz, Namibia (26°39'S, 15°09'E ), using the research vessel Africana (observer eye elevation, 11m). The ship ran a box-transect pattern 2-90 km from the coast. There was no significant differ­ence between the number of penguins seen by observers on yachts and observers on motor boats (y2 = 1·85, df = 1; p > 0·05) for three transects run per vessel type in the southern Saldanha Bay section between 10.30h and 15.30h in March-April (cf. Broni, 1985; Wilson, 1985a).

The study area was divided into four sectors: Cape Agulhas to Cape Point (34°2l'S, 18°30'E), Cape Point to Dassen Island (33°25'S, 18°05'E), Dassen Island to Cape

African Penguin distribution at sea

N Transect I (N=3)

Transect 2

(N=4l

Transect 3 (N=28)

449

Figure l. Direction and number of transects conducted in the Saldanha Bay region (33°03'S, l7°58'E ).

Columbine (32°50'S, l7°51 'E), and Cape Columbine to Liideritz. The numbers of penguins seen in these areas during transects were summed and divided by the transect lengths to calculate their relative densities.

Penguin foraging constraints When African Penguins are not breeding or moulting, they remain constantly at sea (Wilson, 1985c). We assumed that the distribution of non-breeding penguins would be related primarily to prey availability. On the other hand, the foraging range of breeding penguins would be limited by their need to return to the nest.

Adults with chicks leave the nest in the early morning, spend the day fishing, and return in the evening to feed their young. The mean departure and arrival times for penguins breeding at Marcus Island, Saldanha Bay, South Africa (33°03'S , .17°58'E), are 08.00 h and 17.30h respectively, with more than 95% of the departing birds still on land at 04.40 h, and back from fishing by 20.00 h (Wilson et al., unpubl. ), so that most penguins with chicks spend less than 15.3 hat sea per foraging trip. Birds with eggs may spend up to five days at sea (unpubl. data).

Knowledge of penguin travelling speed and diving patterns led us to consider that all penguins within 18 km of any breeding island were most likely to be birds with chicks (Appendix 1). Penguins with eggs probably range further (see later). All bi.as seen farther than 20 km from the nearest island were assumed to be non-breeding. As very few penguins breed on the mainland, penguins seen close to the mainland, but at distances greater than 18 km from the nearest breeding island, were assumed to be non-breeding.

450 R. P. Wilson et al.

Penguin distribution data for 1984-85 were compared with similar data collected during 1954-74 (Siegfried et al., 1975), by standardizing the older information to our format. Siegfried et al. (1975) present relative abundances of penguins at observation stations located at different distances offshore (Appendix 2). Knowing the total number of penguins seen at all stations, we calculated the numbers seen within each distance sector offshore (B ). Dividing this by the number of observation stations for the relevant distance sector (N), the mean number of penguins seen per observation station (B /N ) was obtained. The resultant values were then expressed in percentages (N%) and divided by the number of kilometres making up each distance sector to give the mean percentage, by number, of birds seen per kilometre within each distance limit after a standard number of observation stations (N% / N x ) (Appendix 2). These data were regressed against the mid­point of each distance sector offshore (NP). A similar procedure was performed for the data on penguin group size at sea (Appendix 3).

All data presented as percentages were arcsin-transformed before being regressed against other parameters. Areas under curves describing Mrican Penguin distributions at sea (see above) were standardized so that the integral of the curve was always 100. The integration limits for penguin group size compared to frequency of occurrence were all 0 and 100. For numerical abundance compared to distance offshore the integration limits were determined by the shape of the curve fit and were: 0 and 103 km for non-breeding penguins (the curve-fit crossed the x axis at 103); 0 and 21 km for breeding penguins (the line crossed the x axis at 21 ); and 0 and 200 km for data from Siegfried et al. (1975).

Between March 1984 and August 1985 we visited a 3500 m 2 study colony of penguins at Marcus Island, Saldanha Bay. The total number of nests containing eggs and chicks was noted monthly. The totals were compared (correlation coefficient) to the monthly total number of birds seen during transects conducted in the Saldanha Bay region.

Results

Regional abundance A total of 10 238 km of transect was surveyed during 134 days at sea. The highest density of non-breeding penguins (those more than 20 km from colonies) occurred on transects between Cape Point and Dassen Island, with a total of96 birds seen in 2974 km of transect (one bird per 31 km). Between Dassen and Cape Columbine, 51 birds were seen during 2085 km of transect (one bird per 41 km); between Cape Columbine and Liideritz, 34 birds were seen in 1804 km of transect (one bird per 53 km); and between Cape Agulhas and Cape Point, 16 birds were seen in 2679 km of transect (one bird per 167 km).

Offshore distribution Over 50% of all non-breeding penguins seen in our transects occurred within 20 km of the coast, with birds becoming less abundant with increasing distance offshore (Figure 2). The relationship between the percentage of birds (N) seen and distance from land (d ) (Ian) is given by: N =67·14-14·481n d (r 2 =0·96).

Between March and August, the normal breeding season for African Penguins Saldanha Bay (Wilson, 1985c; La Cock et al., 1987), over 50% of breeding birds (those the zone 0-18 km from nesting colonies) occurred within 3 km of the coast (Figure 3), the percentage of penguins (N) seen as a function of distance offshore (0-18 km) is by the equation:

N = 28·12-9·13ln d ( r 2 =0·88) where dis distance offshore (km).

300

c ., ., "' "' c ·:;

"' 150 c

"' c. d z 0 100 ~

50

African Penguin distribution at sea

180

160

140 c

"' "' "' 120 "' c: ·:;

"' 100 c

"' c.

0 80 t D E 60 ::1 z

40

20

r-

f.-

r

'--

----

f---

--

~ 10 20 30 40 50 60 70 80 90

Distance offshore ( km)

451

Figure 2. T otal number of non-breeding penguins along coastal areas more than 20 krn from the nearest breeding island, in relation to distance offshore.

Theoretical limit

2 14 16 18 20 Distance from mainland ( km)

Figure 3. Total number of breeding penguins at distances less than 18 km from the nearest breeding island, in relation to distance offshore.

Penguin density during the major breeding months generally decreased with increasing distance (dm) from the centre of the mouth of Saldanha Bay, irrespective of transect direction. There was great variability in mean penguin numbers seen per unit distance in the northern and western transects. Due to the low bird numbers and the relatively few transects conducted, we do not consider the data sufficient to test whether one direction was differentially favoured. The data from all Saldanha Bay transects are shown in Figure4.

The density of breeding penguins did not reflect the actual proportions of bird numbers at a particular distance from their breeding site. If penguins from Saldanha Bay radiate

452 R . P. Wilson et al.

.. c :;

[ d z 0 ~

Dostonce f rom centre of mouth of boy ( kml

Figure 4. Total number of penguins seen in the Saldanha Bay area during the maiD breeding season (April-July) in relation to distance from the centre of the mouth of the bay.

~ ~ 80 .. &.

" 0 .. !: .. c :; g' 40 ., .,. d z 2 20 0 1-

2 4 6 20 Dostonce from centre of mou1h of boy (km)

Figure 5. Predicted numbers of breeding penguins at different distances from the cmue of the mouth of Saldanha Bay during the main breeding season (April-July).

uniformly over a semi-circle from a point at the centre of the mouth of the bay (this is oversimplification and assumes that penguins at a specific distance from the centre of bay do not occur at higher densities when closer to land), the number of birds within area bounded by radii (r.) and (rx_ 1) is given by:

No.= (r,/7r/2- rx_12 7r/2) p

where p is the density of penguins in that sector. This model predicts that most birds occurred between 4 and 12 km from the centre

the bay (F igure 5). The distribution of these birds was approximately bilaterally cal, showing a general trend of decreasing density with increasing distance after 8 km the mouth of the bay, except for a peak at 16-20 km (Figure 5). This may be caused

African Penguin distribution at sea 453

Breeding penguins with chicks

+

-]

Distance from breedin9 island

Figure 6 . Schematic representation of the potential costs (a) and benefits (b) from penguins fishing near breeding colonies as a function of distance from land/breeding island (see text).

incubating birds, which typically forage at sea for several days at a time, but lack the potential range of non-breeding birds. Assuming that non-breeding penguins distribute themselves according to prey abundance, the highest densities of birds with eggs would be expected at the limit of the foraging range of birds with chicks, because no birds with chicks would be in the area to compete. A predicted distribution for penguins with eggs can be obtained by subtracting the distribution of breeding birds (with chicks) from the distribution of non-breeding birds (Figure 6).

Seasonal abundance The mean total number of penguins observed per transect to the south of Saldanha Bay (N = 28), was highest during May and lowest during March (Figure 7). The number of nests containing chicks in our study area was highest in May (data for months when transects were conducted). The mean number of penguins seen at sea per transect (y ) is significantly related to the number of nests in our study area containing chicks (x): y=0·29x + 5·84 (r 2 =0·43; F 6 .6 =4·53, p < 0·05). This supports the assumption that penguins found within 18 km of nesting islands are breeding birds.

Group size Penguin group-size composition did not change significantly with increasing distance from land, when comparing penguins 0-20 km and 21-40 km offshore (data grouped to satisfy x2 requirements; x2 = 0·65, p > 0·05). If all data on non-breeding penguins are combined, over 90% of groups consisted of fewer than five birds (Figure 8). The

454 R. P. Wi/so11 et al.

80 0 Penquons

70 Ill Nests

c 60 .. :!(

"' 50 = c :; i "' c ! 8. 40 20 .. ~ ~

d c 30 z

2 c :::. 0 .. ::E

20 10

10

F M A M J J A s 0 N Month

Figure 7 . Relationship between the mean number of penguins seen per transect in the Saldanha Bay area and the number of nests containing chicks at Marcus Island for various months of the year.

90

f-80

70

60

"' 0. :> 0 50 a. 0

il 40 E :>

1--z 30

20

'---10

l ..r1. ~ 3 5 7 9 II 13 15

Penguon group soze

Figure 8. Total number of penguin groups seen in relation to group size for all non­breeding penguins seen during transects.

percentage frequency of occurrence (P ) of the various penguin group sizes is approxi­mated by: P = 44·35 G - 0

.9 4 (r 2 = 0·86) where G is the group size. Figure 9(a) shows the

relative abundance of non-breeding penguin groups of different sizes at different distances from land.

!a l

African Penguin distribution at sea

Probability of occurrence ( x 10- 2)

!c l

Probabil ity of occurrence (X 10- 2)

455

Probobi I i ty of occurrence 15 ( X 10-2)

Figure 9 . Relationship between the proportion of penguins in particular group sizes and distance from the mainland for: (a) non-breeding birds, 1984-85 (data from this study); (b) breeding birds, 1984-85 (data from this study and Wilson et al., 1986a); (c) breeding and non-breeding birds (1954-74) data from Siegfried et al. , 1975).

Data from Wilson et al. ( 1986b) on the frequency of occurrence of various group sizes of breeding penguins at sea do not differ significantly from our data for non-breeding penguins (x2 = 10·25, p > 0·05; for group sizes of 1-7 grouped to satisfy requirements of x2) . The percentage frequency of occurrence (P ) of various group sizes of breeding penguins is: P = 49·36 G - 0

.98 (r 2 = 0·92). Figure 9(b) shows the relative abundance of

breeding penguin groups of different sizes in relation to distance from land.

Earlier records Between 1954 and 1974, African Penguins showed much the same inshore distribution at sea as was observed in this study. The relationship between the percentage of total numbers of penguins seen (N ) and the distance from land (d ) is: N = 76·58 d - 0

"8 7 (r 2 =

0·98). From data collected during 1954-74, the percentage frequency of occurrence (P ) of the various penguin group sizes was: P = 45 ·52 a -o-so (r 2 = 0·97). The relationship between the frequency of occurrence of various penguin group sizes in relation to distance from land is shown in Figure 9(c).

Discussion

Historical changes No major change could be discerned in penguin distribution at sea between 1954-7 4 and 1984-85. Although Siegfried et al. (1975) were not able to derive potential range limi­tations between breeding and non-breeding penguins, their data showed high densities of

456 R. P. Wilson et al.

birds close to land with a power curve decay in density with increasing distance from the coast. A similar pattern would be obtained if the logarithmic decays in densities of breed­ing and non-breeding penguins, obtained in the present study, were added together (cf. Figure9).

The composition of penguin group sizes also appears to have been the same during botb periods and independent of bird density or distance from land. Prey capture is apparently sometimes facilitated by communal feeding (Ryan et al., 1987), although there are practi­cal limits to the size of the flock because more than about 10 birds cannot synchronize dives (Wilson et al., 1986a). Flock size may also reflect the size of prey patches (Duffy, 1983).

If penguin distribution and group size at sea reflect fish distribution and school size, there would appear to have been little change in penguin prey ecology during the last 40 years. It would also appear that penguins have not changed their diet during this period. Duffy et al. (1987b) conclude that breeding African Penguins are unaffected by commer­cial overfishing (e.g. Crawford & Shelton, 1978) because chick growth is not correlated with fishery catch. In this study, half the breeding penguins seen at sea occurred within 3 km of the coast and, in a similar study, Broni (1985) concluded that 95% of penguins are seen within 4 km of the coast. Fishing vessels rarely operate within 15 km of Saldanha Bay (Broni, 1985), so breeding penguins and purse seiners search for fish in spatially distinct areas, and thus do not appear to compete directly for the same schools.

Non-breeding penguins are found much further from the coast and so are more likely to compete directly with the fishing industry. Adult African Penguin mortality on the West coast of southern Africa is higher than that of other penguin species and higher thaD conspecifics of the south and east coasts where extensive commercial fishing does not

occur (La Cock et al., 1987). Similarly, juvenile African Penguins, which also have ID

offshore distribution (Duffy & Cooper, 1987) and which are thought to be less efficient II capturing prey than adults (Rand, 1960; Wilson, 1985a; Ryan et at., 1987), have a first year mortality of over 95% on the west coast compared to 68% on the east coast (La Cock et 1987). This suggests that, although the overall distribution of African Penguin prey not have changed as a result of commerical fishing, prey density may be ternoclra:riiY reduced in the fishing areas and this may be responsible for the penguin popu1al:1U11

decrease on the west coast of southern Africa.

Acknowledgements

We thank the Department of Environment Affairs for logistic support and access breeding islands, and S . Jackson, P. King, W. Leill-Cock, P. Lorber, H. Veldhuis C. Walter for technical help, discussion and data. The National Geographic Endangered Wildlife Trust and Benguela Ecology Programme of the South National Committee for Oceanographic Research supported the project.

References Broni, S . C. 1985 Social and spatial foraging patterns of the Jackass Penguin S pheniscus demersus.

African J ournal of Z oology 201 241- 245. Burger, A. E. & Cooper, J . 1984 The effects of fisheries on seabirds in South Africa and Namibia. In,

Birds: Their Feeding Ecology and Commercial Fisheries Relationships Nettleship, D . N ., Sanger, C. Springer, P. F. eds). Canadian Wildlife Service Special Publication, Onawa, pp. 150- 160.

Cooper, J. 1984 Changes in resource division among four breeding seabirds in the Benguela system. Proceedings of the Pan-African Ornithological Congress, pp. 217- 230.

Crawford, R . J. M . & Shelton, P. A. 1978 Pelagic fish and seabird interrelationships off the coast of West and South Africa. Biological Conservation 14,85-109.

African Penguin distribution at sea 457

Crawford, R. J. M. & Shelton, P. A. 1981 Population trends for some southern African seabirds related to fish availability. In Proceedings of the Symposium on the Birds of the Sea and Shore, 1979 (Cooper, J. ed.). Mrican Seabird Group, Cape Town, pp. 15-41.

Duffy, D . C. 1983 The foraging ecology of Peruvian seabirds. Auk 100, 800-810. Duffy, D . C. & Cooper, J. 1987 Survival of first-year Jackass Penguins at sea: an explanation of popular

trends. South African Journal of Science, in press. Duffy, D . C., Siegfried, W. R. & Jackson, S. 1987a Seabirds as consumers in the southern Benguela

ecosystem. South African Journal of Man"ne ScienceS, 771- 790. Duffy, D . C., Wilson, R. P., Ricklefs, R. E., Broni, S.C. & Veldhuis, H. 1987b Penguins and purse-seiners:

competition or coexistence? National Geographic R esearch 3, 480-488. Frost, P. G. H., Siegfried, W. R. & Cooper, J. 1976 Conservation of the Jackass Penguin (Spheniscus

demersus). Biological Conservation 9, 79-99. Furness, R. W. & Cooper, J . 1982 Interactions between breeding seabirds and pelagic fish populations in the

southern Benguela region. Marine Ecology Progress Series 8, 243-250. La Cock, G. D., Duffy, D . C. & Cooper, J . 1987 Population dynamics of the Mrican Penguin Spheniscus

demersus at Marcus Island in the Benguela upwelling ecosystem: 1979-1985. Biological Conservation 40, 117-126.

Rand, R. W. 1960 The distribution, abundance and feeding habits of the Cape Penguin (Spheniscus demersus) off the South-western coast of the Cape Province. Investigational Reporr of the Division of Fishen"es of the Union of South Africa 41.

Ryan, P. G., Wilson, R. P. & Cooper, J. 1987 Intra-specific mimicry and status signals in juvenile Mrican Penguins. Behavioural Ecology and Sociobiology 20,69-76.

Shelton, P. A., Crawford, R. J. M., Cooper, J. & Brooke, R. K. 1984 Distribution, population size and con­servation of the Jackass Penguin Spheniscus demersus. South African Journal of Marine Science 2, 217-257.

Siegfried, W . R., Frost, P. G. H ., Kinahan, J. B. & Cooper, J. 1975 Social behaviour of Jackass Penguins at sea. Zoologica Africana 10, 87-100.

Wilson, R. P . 1985a The Jackass Penguin ( Spheniscus demersus) as a pelagic predator. Marine Ecology Progress Sen"es 25,219-227.

Wilson, R. P. 1985b Diurnal foraging panerns of the Jackass Penguin. Ostrich 56, 212-214. Wilson, R. P. 1985c Seasonality in diet and breeding success of the Jackass Penguin Spheniscus demersus.

Journal fur Ornithologie 126, 53-62. Wilson, R. P. & Bain, C. A. R. 1984 An inexpensive speed meter for penguins at sea. Journal of Wildlife

Man~ement48, 1360- 1364. Wilson, R. P., Wilson, M.-P. & McQuaid, L. 1986a Group size in foraging African Penguins Spheniscus

demersus). Ethology 72,338-341. Wilson, R. P., Grant, W. S. & Duffy, D. C. 1986b Recording devices on free-ranging marine animals: does

measurement affect foraging performance? Ecology 67, 1091-1093.

Appendix 1

African Penguins travel by alternating periods underwater with periods on the surface (Wilson, 1985a): The travelling speed= [(dive time x underwater speed) + (pause timex surface speed)]/(dive time + pause time).

As the dive time of travelling (but not foraging) penguins is 22·3 s (Wilson, 1985b), the pause time is 17·8 s (Siegfried et al., 1975), and the underwater and surface speeds are 7·7 km h -I (Wilson et al., 1986b) and 1·5 km h - I (Wilson, 1985a), respectively, the overall travelling speed is [(22·3 x 7·7) +(17·8+ 1·5)]/(17·8 +22·3)= 4·95 km h -•.

Penguins breeding at Marcus Island generally fish outside the mouth of Saldanha Bay, 5 km from the island (Wilson, 1985a ), so they spend approximately 2 h day - 1 commuting to the fishing area. If penguins spend a maximum of 15·3 h day - 1 at sea, they have 13·3 h in which to forage. The distance covered during this time can be calculated by knowing their foraging speed. The mean dive time of foraging African Penguins is 146 s (Wilson, 1985a) and the mean pause time is 420 s (unpubl. data). Foraging African Penguins travel at 7·7 km h - 1 (Wilson et al., 1986b). Surface-swimming penguins do not appear to travel between foraging dives. We have assumed that foraging African Penguins have a maxi­mum surface speed of 1 km h - •. Therefore, the foraging speed is ca. [(7·7 x 146)+ (l·Ox420)]/(420+146)=2·74kmh- 1

, so a foraging penguin travels 36·4km in 13·3h, obtaining a maximum range ofless than 18·2 km from the entrance of Saldanha Bay.

458 R. P. Wilson et al.

Appendix2

Distance (km) category*

0-12 13-24 25-36 37-49 50-74 75-99 100-124 125-149 150-190

Mid-point between distance limits (MP), (km) 6 18·5 30·5 43 62 87 112 137 170

No. ofkm within limits (N ) 12 11 11 12 24 24 24 24 40 Abundance* (%) No. of stations* (N ) Total b irds seen (B) B /N N % N %/Nx

79 7 5 2 5 1 0·5 0 0·5 201 78 122 89 171 102 107 22 91

1261 112 80 32 80 16 8 0 8 6·27 1·44 0·66 0·36 0·47 0·16 0·08 0 0·09

65·95 15·10 6·89 3·78 4·92 1·65 0·79 0 0·8 5·50 1·37 0·63 0 ·32 0·21 0 ·07 0·03 0 0

*Data taken directly from Siegfried et al. (1975). All other data are derived from the present work (see text).

Appendix3

Group size

2-5 6-10 11-20 21- 50 51-210

No. of birds within limits of categories 1 3 4 9 29 159 Mid-point of categories 1 3·5 8 15·5 36 131 Frequency at midpoint (%) 29·0 13·3 4·0 0·8 0·2 0·02