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Population dynamics of the African penguin Spheniscus demersus at Marcus Island in the Benguela upwelling ecosystem: 1979–1985
Biological Conservation 40 (1987) 117-126
Population Dynamics of the African Penguin Spheniscus demersus at Marcus Island in the Benguela Upwelling Ecosystem: 1979-85 Graeme D. La Cock, David Cameron Duffy & John Cooper Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa (Received 4 June 1986; revised version received 19 September 1986; accepted 29 September 1986)
ABSTRACT Breeding attempts by the African penguin Spheniscus demersus a t a colony on Marcus Island, Saldanha Bay, South Africa, decreased by 14% per year during 1979-85. Annual adult survival was 33"3-70.4%. Nesting success at Marcus Island was similar to that at o t h e r African penguin colonies of St Croix and Dassen islands a n d a t colonies in other areas o f other penguin species not undergoing population decreases. Minimum survival of juveniles in the first year of life was only 4% of the level needed to sustain a stable population.
INTRODUCTION
Populations of the African or jackass penguin Spheniscus demersus, the only penguin species breeding in Africa, have decreased by 59% off Namibia and by 38% off South Africa between 1956 and 1978-79 (Shelton et al., 1984). The decrease has not been uniform: in South Africa, the population remained stable on the east coast, increased on the south coast, and decreased on the west coast (Shelton et al., 1984), the area with the greatest concentration of breeding penguins in the late 1950s (Rand, 1963a,b). In an attempt to assess long-term population trends on the west coast of South Africa, and reasons for such trends, a long-term study site was established at Marcus Island (33°03'S; 17°58'E), in Saldanha Bay, Cape Province, in 1977. 117 Biol. Conserv. 0006-3207/87/$3"50 © Elsevier Applied Science Publishers Ltd, England. 1987. Printed in Great Britain
118
Graeme D. La Cock, David Cameron Duffy, John Cooper
This paper presents data on the penguins' breeding performance at this study site during 1979-85, compares these with results previously obtained at other South African islands, and considers reasons for the species' population decrease.
METHODS A 3500 m 2 study area was established in February 1977 within the African penguin colony at Marcus Island (area 11 ha, 7 m above sea-level, Rand, 1963a). The island is flat with scattered boulders, under which many of the penguins nest. Between January 1979 and December 1985, 148 individually marked potential nest-sites were inspected each month, except January 1984, initially every 10-14 days, but monthly after 1982. We recorded the numbers of adults, eggs, and chicks in each nest, as well as the size of chicks, stage of moult of adults, and band numbers of adults and chicks. Eggs were considel:ed to have been laid and young to have hatched in the month when they were first encountered. Adult penguins were banded (Cooper & Morant, 1981) when they were brooding chicks, and chicks when they lost their down and moulted into juvenile plumage, between two and three months after hatching. Many penguins were banded at the site during February 1977-December 1978, and some prior to February 1977. We occasionally found it difficult to determine the number of eggs in nests under large boulders. In this event, eggs were recorded only as present. To reduce biases, we used numbers of clutches (at least one egg) in the analysis rather than numbers of eggs to determine the mean laid each month and each year. The numbers of clutches resulting in hatchlings each month and each year were calculated as percentages of the numbers of clutches laid, and mean monthly hatching success from the combined data for each month. Nestlings were considered to have fledged and successfully left the nest when they moulted into juvenile plumage and were no longer recorded in the colony. A nest was considered successful when at least one chick in a brood fledged. We determined numbers of successful breeding attempts for each month and each year, calculated as percentages of the numbers of clutches laid and as percentages of the numbers of hatchlings. The mean number of fledglings produced per breeding attempt and per successful breeding attempt were calculated. Because some pairs attempt to breed more than once in a year (La Cock et al., unpublished data), we calculated the mean fledgling production per breeding pair per year. The percentage of adults resighted or recaptured in subsequent years was calculated including those observed outside the colony. Ninety-five per cent confidence intervals were determined for percentage annual adult survival
Population dynamics of African penguin
119
from 1979 to 1985, and a contingency test was used to determine if there were significant differences between years. We calculated nest-site and mate fidelities from banded adults which bred in the colony in consecutive years; the number breeding with the same partner in both years as a percentage of the total number of breeding in both years; and numbers which bred in the same nest in both years as a percentage of the total number breeding in the colony in both years. These calculations were done for each pairing of consecutive years from 1979 to 1985. When penguins attempted to breed more than once in the same year, we compared the first attempt of the year with the last attempt of the previous year, and the last attempt of the year with" the first attempt of the following year. We calculated the percentage of banded juveniles resighted or recaptured in subsequent years and the mean time elapsed between banding and resighting. Figures for juvenile survival rates were adjusted to take into account birds recovered as adults. Fledglings banded in 1985 were not included, because they were unlikely to have returned to Marcus Island within a year of banding. We constructed a life table, based on data from this study, assuming that the age of first breeding was three years (PFIAO unpubl, data) and that all adults bred. Minimal juvenile survival was equal to adult mortality. A second life table for African penguins at St Croix Island (33°48'S, 25°46'E), Algoa Bay, eastern Cape, South Africa (Randall, 1983) is included for comparison.
RESULTS Within the Marcus Island study colony of African penguins, a single egglaying peak occurred in late summer-autumn (February-May), and fewest clutches were laid in spring (August-December; Fig. I). Some inter-annual variation in egg-laying peaks occurred (Fig. 2). The number of clutches laid decreased annually by 14% during 1979-85 (Fig. 3). The greatest percentage decreases were between 1983-84 and 1984-85, when 50% fewer clutches were laid. Annual percentage hatching success was 35-65%, and annual percentage fledging was 15-50% of clutches (Fig. 4). Two hatching peaks occurred from clutches laid in March and May (Fig. 1), with minima extending from August to December. Hatching success was highest for clutches laid in April-July and in October, outside the peak laying periods (Fig. 5), 30"4% (n - 365) of breeding attempts being successful overall. In 57.7% (n -- 111) two fledglings were produced, yielding a mean fledgling production of 1.58 fledglings per successful breeding attempt.
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MONTHS Numbers of African penguin nests with eggs laid, eggs hatched, or young fledged per month: January 1979-December 1985.
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Fig. 3. Annual total numbers of African penguin clutches laid, nests with eggs hatched and nests with young fledged" 1979-85.
Fig. 4. Annual percentage success of African penguin nests hatching and young fledging.
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122
Graeme D. La Cock, David Cameron Duffy, John Cooper TABLE 1
Percentage Annual Adult Survival and 95 % Upper and Lower Confidence Limits for African Penguins, based on Recaptured Individuals at Marcus Island Year
n
Mean survival
Range
1979/80 1980/81 1981/82 1982/83 1983/84 1984/85
125 113 79 57 43 24
70.4 63.7 64.6 49-3 46.5 33.3
62.4-78.4 54-8-72.6 54.1-75.1 36.3-62.3 31.6-61-4 14.4-52.2
Percentage a n n u a l adult survival (Table 1) decreased significantly f r o m 70-4% in 1980 to 33-3% in 1985 (X 2 = 17"2; P < 0.005; d f = 5). F o r all years combined, 61.7% o f adults seen in one year survived to the next. Eleven (4.7%; n = 232) fledglings b a n d e d in the colony before 1985 have been resighted or recaptured, all on M a r c u s Island. F o u r o f the birds were breeding, three in the study site at nests 15 m, 30 m a n d 45 m f r o m their birth places. Only one fledgling was resighted in the year following banding. The m e a n period between b a n d i n g and resighting was 37.8 m o n t h s (SD = 12.2 months). The m i n i m u m first-year survival for fledglings was 12.5%. Individual nesting penguins re-used their nest-site o f the previous year in 59"8% (n = 214 total nests) o f all cases. W h e n it was possible to determine the identity of mates o f those penguins in two consecutive years, 62.1% (n = 161) nested with the same mate. D e a t h or disappearance of mates occurred in 73-8% (n = 61) of m a t e changes. In 75% (n = 100) o f instances where the
TABLE 2
Life Table for African Penguins at Marcus Island (this study) and St Croix Island (Randall, 1983) (Values given in text)
Breeding adults Chicks fledged Minimum 1st year survival Minimum 2nd year survival Minimum 3rd year survival Recruits required (= adults dying) Shortage of birds compared to number required to maintain stable population:
Marcus Island
St Croix Island
1000 315 39 24 15 383
1000 189 60 55 39 89
368 (96%)
50 (56%)
Population dynamics of African penguin
123
same members of a pair bred together in consecutive years, they re-used the same nest-site. In our life-table analysis of the Marcus Island population (Table 2), we assumed 0"63 nestlings fledged pr- ~year- 1, 12"5% survival of fledging birds and 61.7% survival for adults. For the St Croix population in Algoa Bay, Randall (1983) used 0.37 nestlings fledged p r - l y e a r - 1, 32% subsequent survival in the first year of life and 90% subsequently. Using our values, the Marcus Island study area produced only 4% of the young penguins necessary for the maintenance of a stable population each year, compared to 56% for St Croix Island. DISCUSSION The African penguin population at Marcus Island has decreased, while the population of this species at St Croix Island has remained stable (Shelton et al., 1984). Since a decrease has occurred over the whole of Marcus Island during 1979-85 (personal observation), the trends reported here probably do not reflect biases resulting from a marginal habitat that might have a higher rate of decrease than more preferred areas (cf. Fretwell, 1972). The decrease in the Marcus Island population cannot be explained entirely by reduced breeding success, one among several explanations suggested by Cooper (1980) after studies at other west coast islands. At Marcus Island, 0-63 fledglings were produced per breeding pair per year, and 0.49 fledglings were produced per breeding attempt. In contrast, at St Croix Island only 0.38 fledglings were produced per breeding pair per year (Randall, 1983). Differences in breeding habitat appear to be important in determining fledgling production. Most penguins are surface-nesters at St Croix Island (Randall, 1983), but at the Marcus Island study colony most breed under boulders (personal observation). Values for fledgling production of surfacenesting and boulder-nesting African penguins at Dassen Island (Frost et al., 1976) were also very similar to those obtained at St Croix (Randall, 1983). Kelp gull Larus dominicanus predation, insolation, and exposure to inclement weather may reduce the success of birds breeding in open nests (Frost et al., 1976; Cooper, 1980). Fledgling production at Marcus Island was similar to values obtained for other penguin species studied elsewhere: e.g., little penguin Eudyptula minor (0"50 fledglings per breeding attempt and 0"71 fledglings per pair per year; Reilly & Cullen, 1981); macaroni penguin Eudyptes chrysolophus (0"43 fledglings per pair; Williams, 1980); rockhopper penguin E. chrysocome (0.45 fledglings per pair; Williams, 1980); and Magellanic penguin Spheniscus magellanicus (0"45 fledglings per pair; Scolaro, 1980). None of these species appears to be decreasing in abundance.
124
Graeme D. La Cock, David Cameron Duffy, John Cooper
Oiling poses a threat to the African penguin population (Westphal & Rowan, 1971; Morant et al., 1981), but during this study no large-scale oiling occurred on the west coast of South Africa. Other possible explanations for the observed population decrease were mass breeding failures and coastal 'wrecks', but none was reported on the west coast during 1979-85 (La Cock, 1986). The 1982-83 warm event may have been associated with increased adult mortality (Duffy et aL, 1984; Avery, 1985), but this does not explain the persistent, inter-annual, downward population trend. The decrease in the annual adult survival to only 33.3% in 1984-85 apparently played a major role in the reduction of the Marcus Island study population. There was a strong E1 Nifio-like event off Nambia in 1983-84 which led to a recruitment failure of Cape anchovy Engraulis japonicus (Boyd et al., 1985), the major food of African penguins (Rand, 1960; Duffy et al., 1984). If adult African penguins from Marcus Island disperse northwards to Namibian waters when not breeding, this could account for the increased mortality. Whatever the cause of the decrease in adult survival in 1984-85, the survival rate in all years was still below that of the African penguins at St Croix Island (Randall, 1983) and at Dyer Island (34°41'S; 19°25'E) on the south coast (La Cock & Hanel, in prep.), and lower than survival rates of other penguin species studied elsewhere (61-70% for Ad61ie penguins Pygoscelis adeliae to 95% for emperor penguins Aptenodytes forsteri: cf. Croxall, 1981). Low post-fledgling survival of juvenile African penguins appears to be another, and perhaps the most important, reason ~for the decrease of the Marcus Island population. First-year survival of juveniles at St Croix Island was 32% for 'normal' years, although in two 'abnormal' years, only 3.9% and 5-1% of fledglings survived (Randall, 1983). At St Croix Island, 90% of survivors were resighted within three years after fledging, and no fledglings were resighted for the first time more than four years after banding (Randall, 1983). At Marcus Island, however, only 4.7% of banded fledglings have been resighted. It is unlikely that the difference between Marcus and St Croix islands can be attributed solely to lack of search effort. For example, Marcus Island was visited at least monthly, but Dyer Island was visited for only one-three weeks a year from 1982 to 1985. Yet, at Dyer Island, 13% (n = 512) of fledglings banded in 1978 have been resighted (La Cock & Hanel, in prep.). Rand (1960) and Wilson (1985) have suggested that fledgling penguins feed on relatively slow-moving prey such as larval fish. The low survival of fledgling African penguins may be an indication of the low availability of such larvae. The unusual oceanographic events of 1982-83 and 1983-84 could also have reduced survival of first-year penguins at sea, but there are too few recoveries to examine annual mortality patterns. The recruitment of
Population dynamics of African penguin
125
young penguins into the Marcus Island population appears to be so low that the colony is unlikely to survive. If the levels are indicative of what is happening on other west coast islands, then the population is in danger of extinction. While recruitment appears to be better on islands on the south coast of South Africa, a future restriction of the breeding population to one or two sites would make the species extremely vulnerable to a single major oiling disaster.
ACKNOWLEDGEMENTS We thank the following people: J. C. Sinclair set up the study area; S. C. Broni, P. A. R. Hockey, D. and W. Suter, A. J. Williams, R. P. Wilson, and many other members of the FitzPatrick Institute checked the colony; A. A. Crowe and L. G. Underhill assisted with data analysis; and S. Hunter, W. R. Siegfried, and R. P. Wilson provided valuable discussion or comments on the manuscript. The Sea Fisheries Research Institute provided access to Marcus Island. T. B. Oatley, South African Bird Ringing Unit, provided invaluable assistance with banding data. The project was supported by the Benguela Ecology Programme of the South African National Committee for Oceanographic Research, the Department of Environment Affairs, Endangered Wildlife Trust, the National Geographic Society, and the South African Nature Foundation.
REFERENCES Avery, G. (1985). Results of patrols for beached seabirds conducted in southern Africa in 1983. Cormorant, 13, 3-15. Boyd, A. J., Hewitson, J. D., Kruger, I. & Le Clus, F. (1985). Temperature and salinity trends off Namibia from August 1982 to August 1984, and their relation to plankton abundance and the reproductive success of pelagic fish. Colin scient. Pap. int. Commn SE. Atl. Fish. 1985, 53 8. Cooper, J. (1980). Breeding biology of the jackass penguin with special reference to its conservation. Proc. Pan-Afr. orn. Congr., 4th, 227-31. Cooper, J. & Morant, P. D. (1981). The design of stainless steel flipper bands for penguins. Ostrich, 52, 119-23. Croxali, J. R. (1981). Aspects of the population demography of Antarctic and Subantarctic seabirds. Comm. Natl. Franc. Rech. Antarct., 51,479 88. Duffy, D. C., Berruti, A., Randall, R. M. & Cooper, J. (1984). Effects of the 1982 3 warm water event on the breeding of south African seabirds. S. Afr. J. Sei., 80, 65 9. Fretwell, S. D. (1972). Populations in a seasonal environment. Monogr. Pop. Biol., 5. Princeton, Princeton University Press.
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Frost, P. G. H., Siegfried, W. R. & Cooper, J. (1976). Conservation of the jackass penguin Spheniscus demersus (L.). BioL Conserv., 9, 79-99. La Cock, G. D. (1986). The Southern Oscillation, environmental anomalies, and mortality of two southern African seabirds. Climatic Change, 8, 173-84. Morant, P. D., Cooper, J. & Randall, R. M. (1981). The rehabilitation of oiled jackass penguins Spheniscus demersus, 1970-1980. In Proceedings of the symposium on birds of the sea and shore, 1979, ed. by J. Cooper, 267-301. Cape Town, African Seabird Group. Rand, R. W. (1960). The biology of guano-producing seabirds: The distribution, abundance and feeding habits of the Cape penguin Spheniscus demersus off the south-western coast of the Cape Province. InvestlRep. Div. Fish. Un. S. Afr., 41, 1-28.
Rand, R. W. (1963a). The biology of guano producing seabirds, 4. Composition of colonies on the Cape Islands. Investl Rep. Div. Fish. Rep. S. Aft., 43, 1-32. Rand, R. W. (1963b). The biology of guano producing seabirds, 5. Composition of colonies on the South West African islands. Investl Rep. Div. Fish. Rep. S. Afr., 46, 1-26. Randall, R. M. (1983). Biology of the Jackass penguin Spheniscus demersus (L.) at St Croix Island, South Africa. PhD thesis, University of Port Elizabeth, South Africa. Reilly, P. N. & Cullen, J. M. (1981). The little penguin Eudyptula minor in Victoria, I. Breeding. Emu, 81, 1-19. Scolaro, J. A. (1980). El pinguino de Magallanes Spheniscus magellanicus, 6. Dinamica de la poblacirn de juveniles. Hist. Nat. (Mendoza), 1, 173-8. Shelton, P. A., Crawford, R. J. M., Cooper, J. & Brooke, R. K. (1984). Distribution, population size and conservation of the jackass penguin Spheniscus demersus. S. Afr. J. mar. Sci., 2, 217 57. Westphal, A. & Rowan, M. K. (1971). Some observations on the effects of oil pollution on the jackass penguin. Ostrich (SuppL), 8, 521-6. Williams, A. J. (1980). Offspring reduction in macaroni and rockhopper penguins. Auk, 97, 754-9. Wilson, R. P. (1985). The jackass penguin Spheniscus demersus as a pelagic predator. Mar. Ecol. Prog. Ser., 25, 219 27.
Population Dynamics of the African Penguin Spheniscus demersus at Marcus Island in the Benguela Upwelling Ecosystem: 1979-85 Graeme D. La Cock, David Cameron Duffy & John Cooper Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa (Received 4 June 1986; revised version received 19 September 1986; accepted 29 September 1986)
ABSTRACT Breeding attempts by the African penguin Spheniscus demersus a t a colony on Marcus Island, Saldanha Bay, South Africa, decreased by 14% per year during 1979-85. Annual adult survival was 33"3-70.4%. Nesting success at Marcus Island was similar to that at o t h e r African penguin colonies of St Croix and Dassen islands a n d a t colonies in other areas o f other penguin species not undergoing population decreases. Minimum survival of juveniles in the first year of life was only 4% of the level needed to sustain a stable population.
INTRODUCTION
Populations of the African or jackass penguin Spheniscus demersus, the only penguin species breeding in Africa, have decreased by 59% off Namibia and by 38% off South Africa between 1956 and 1978-79 (Shelton et al., 1984). The decrease has not been uniform: in South Africa, the population remained stable on the east coast, increased on the south coast, and decreased on the west coast (Shelton et al., 1984), the area with the greatest concentration of breeding penguins in the late 1950s (Rand, 1963a,b). In an attempt to assess long-term population trends on the west coast of South Africa, and reasons for such trends, a long-term study site was established at Marcus Island (33°03'S; 17°58'E), in Saldanha Bay, Cape Province, in 1977. 117 Biol. Conserv. 0006-3207/87/$3"50 © Elsevier Applied Science Publishers Ltd, England. 1987. Printed in Great Britain
118
Graeme D. La Cock, David Cameron Duffy, John Cooper
This paper presents data on the penguins' breeding performance at this study site during 1979-85, compares these with results previously obtained at other South African islands, and considers reasons for the species' population decrease.
METHODS A 3500 m 2 study area was established in February 1977 within the African penguin colony at Marcus Island (area 11 ha, 7 m above sea-level, Rand, 1963a). The island is flat with scattered boulders, under which many of the penguins nest. Between January 1979 and December 1985, 148 individually marked potential nest-sites were inspected each month, except January 1984, initially every 10-14 days, but monthly after 1982. We recorded the numbers of adults, eggs, and chicks in each nest, as well as the size of chicks, stage of moult of adults, and band numbers of adults and chicks. Eggs were considel:ed to have been laid and young to have hatched in the month when they were first encountered. Adult penguins were banded (Cooper & Morant, 1981) when they were brooding chicks, and chicks when they lost their down and moulted into juvenile plumage, between two and three months after hatching. Many penguins were banded at the site during February 1977-December 1978, and some prior to February 1977. We occasionally found it difficult to determine the number of eggs in nests under large boulders. In this event, eggs were recorded only as present. To reduce biases, we used numbers of clutches (at least one egg) in the analysis rather than numbers of eggs to determine the mean laid each month and each year. The numbers of clutches resulting in hatchlings each month and each year were calculated as percentages of the numbers of clutches laid, and mean monthly hatching success from the combined data for each month. Nestlings were considered to have fledged and successfully left the nest when they moulted into juvenile plumage and were no longer recorded in the colony. A nest was considered successful when at least one chick in a brood fledged. We determined numbers of successful breeding attempts for each month and each year, calculated as percentages of the numbers of clutches laid and as percentages of the numbers of hatchlings. The mean number of fledglings produced per breeding attempt and per successful breeding attempt were calculated. Because some pairs attempt to breed more than once in a year (La Cock et al., unpublished data), we calculated the mean fledgling production per breeding pair per year. The percentage of adults resighted or recaptured in subsequent years was calculated including those observed outside the colony. Ninety-five per cent confidence intervals were determined for percentage annual adult survival
Population dynamics of African penguin
119
from 1979 to 1985, and a contingency test was used to determine if there were significant differences between years. We calculated nest-site and mate fidelities from banded adults which bred in the colony in consecutive years; the number breeding with the same partner in both years as a percentage of the total number of breeding in both years; and numbers which bred in the same nest in both years as a percentage of the total number breeding in the colony in both years. These calculations were done for each pairing of consecutive years from 1979 to 1985. When penguins attempted to breed more than once in the same year, we compared the first attempt of the year with the last attempt of the previous year, and the last attempt of the year with" the first attempt of the following year. We calculated the percentage of banded juveniles resighted or recaptured in subsequent years and the mean time elapsed between banding and resighting. Figures for juvenile survival rates were adjusted to take into account birds recovered as adults. Fledglings banded in 1985 were not included, because they were unlikely to have returned to Marcus Island within a year of banding. We constructed a life table, based on data from this study, assuming that the age of first breeding was three years (PFIAO unpubl, data) and that all adults bred. Minimal juvenile survival was equal to adult mortality. A second life table for African penguins at St Croix Island (33°48'S, 25°46'E), Algoa Bay, eastern Cape, South Africa (Randall, 1983) is included for comparison.
RESULTS Within the Marcus Island study colony of African penguins, a single egglaying peak occurred in late summer-autumn (February-May), and fewest clutches were laid in spring (August-December; Fig. I). Some inter-annual variation in egg-laying peaks occurred (Fig. 2). The number of clutches laid decreased annually by 14% during 1979-85 (Fig. 3). The greatest percentage decreases were between 1983-84 and 1984-85, when 50% fewer clutches were laid. Annual percentage hatching success was 35-65%, and annual percentage fledging was 15-50% of clutches (Fig. 4). Two hatching peaks occurred from clutches laid in March and May (Fig. 1), with minima extending from August to December. Hatching success was highest for clutches laid in April-July and in October, outside the peak laying periods (Fig. 5), 30"4% (n - 365) of breeding attempts being successful overall. In 57.7% (n -- 111) two fledglings were produced, yielding a mean fledgling production of 1.58 fledglings per successful breeding attempt.
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Fig. I.
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MONTHS Numbers of African penguin nests with eggs laid, eggs hatched, or young fledged per month: January 1979-December 1985.
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Year
Fig. 3. Annual total numbers of African penguin clutches laid, nests with eggs hatched and nests with young fledged" 1979-85.
Fig. 4. Annual percentage success of African penguin nests hatching and young fledging.
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122
Graeme D. La Cock, David Cameron Duffy, John Cooper TABLE 1
Percentage Annual Adult Survival and 95 % Upper and Lower Confidence Limits for African Penguins, based on Recaptured Individuals at Marcus Island Year
n
Mean survival
Range
1979/80 1980/81 1981/82 1982/83 1983/84 1984/85
125 113 79 57 43 24
70.4 63.7 64.6 49-3 46.5 33.3
62.4-78.4 54-8-72.6 54.1-75.1 36.3-62.3 31.6-61-4 14.4-52.2
Percentage a n n u a l adult survival (Table 1) decreased significantly f r o m 70-4% in 1980 to 33-3% in 1985 (X 2 = 17"2; P < 0.005; d f = 5). F o r all years combined, 61.7% o f adults seen in one year survived to the next. Eleven (4.7%; n = 232) fledglings b a n d e d in the colony before 1985 have been resighted or recaptured, all on M a r c u s Island. F o u r o f the birds were breeding, three in the study site at nests 15 m, 30 m a n d 45 m f r o m their birth places. Only one fledgling was resighted in the year following banding. The m e a n period between b a n d i n g and resighting was 37.8 m o n t h s (SD = 12.2 months). The m i n i m u m first-year survival for fledglings was 12.5%. Individual nesting penguins re-used their nest-site o f the previous year in 59"8% (n = 214 total nests) o f all cases. W h e n it was possible to determine the identity of mates o f those penguins in two consecutive years, 62.1% (n = 161) nested with the same mate. D e a t h or disappearance of mates occurred in 73-8% (n = 61) of m a t e changes. In 75% (n = 100) o f instances where the
TABLE 2
Life Table for African Penguins at Marcus Island (this study) and St Croix Island (Randall, 1983) (Values given in text)
Breeding adults Chicks fledged Minimum 1st year survival Minimum 2nd year survival Minimum 3rd year survival Recruits required (= adults dying) Shortage of birds compared to number required to maintain stable population:
Marcus Island
St Croix Island
1000 315 39 24 15 383
1000 189 60 55 39 89
368 (96%)
50 (56%)
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123
same members of a pair bred together in consecutive years, they re-used the same nest-site. In our life-table analysis of the Marcus Island population (Table 2), we assumed 0"63 nestlings fledged pr- ~year- 1, 12"5% survival of fledging birds and 61.7% survival for adults. For the St Croix population in Algoa Bay, Randall (1983) used 0.37 nestlings fledged p r - l y e a r - 1, 32% subsequent survival in the first year of life and 90% subsequently. Using our values, the Marcus Island study area produced only 4% of the young penguins necessary for the maintenance of a stable population each year, compared to 56% for St Croix Island. DISCUSSION The African penguin population at Marcus Island has decreased, while the population of this species at St Croix Island has remained stable (Shelton et al., 1984). Since a decrease has occurred over the whole of Marcus Island during 1979-85 (personal observation), the trends reported here probably do not reflect biases resulting from a marginal habitat that might have a higher rate of decrease than more preferred areas (cf. Fretwell, 1972). The decrease in the Marcus Island population cannot be explained entirely by reduced breeding success, one among several explanations suggested by Cooper (1980) after studies at other west coast islands. At Marcus Island, 0-63 fledglings were produced per breeding pair per year, and 0.49 fledglings were produced per breeding attempt. In contrast, at St Croix Island only 0.38 fledglings were produced per breeding pair per year (Randall, 1983). Differences in breeding habitat appear to be important in determining fledgling production. Most penguins are surface-nesters at St Croix Island (Randall, 1983), but at the Marcus Island study colony most breed under boulders (personal observation). Values for fledgling production of surfacenesting and boulder-nesting African penguins at Dassen Island (Frost et al., 1976) were also very similar to those obtained at St Croix (Randall, 1983). Kelp gull Larus dominicanus predation, insolation, and exposure to inclement weather may reduce the success of birds breeding in open nests (Frost et al., 1976; Cooper, 1980). Fledgling production at Marcus Island was similar to values obtained for other penguin species studied elsewhere: e.g., little penguin Eudyptula minor (0"50 fledglings per breeding attempt and 0"71 fledglings per pair per year; Reilly & Cullen, 1981); macaroni penguin Eudyptes chrysolophus (0"43 fledglings per pair; Williams, 1980); rockhopper penguin E. chrysocome (0.45 fledglings per pair; Williams, 1980); and Magellanic penguin Spheniscus magellanicus (0"45 fledglings per pair; Scolaro, 1980). None of these species appears to be decreasing in abundance.
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Oiling poses a threat to the African penguin population (Westphal & Rowan, 1971; Morant et al., 1981), but during this study no large-scale oiling occurred on the west coast of South Africa. Other possible explanations for the observed population decrease were mass breeding failures and coastal 'wrecks', but none was reported on the west coast during 1979-85 (La Cock, 1986). The 1982-83 warm event may have been associated with increased adult mortality (Duffy et aL, 1984; Avery, 1985), but this does not explain the persistent, inter-annual, downward population trend. The decrease in the annual adult survival to only 33.3% in 1984-85 apparently played a major role in the reduction of the Marcus Island study population. There was a strong E1 Nifio-like event off Nambia in 1983-84 which led to a recruitment failure of Cape anchovy Engraulis japonicus (Boyd et al., 1985), the major food of African penguins (Rand, 1960; Duffy et al., 1984). If adult African penguins from Marcus Island disperse northwards to Namibian waters when not breeding, this could account for the increased mortality. Whatever the cause of the decrease in adult survival in 1984-85, the survival rate in all years was still below that of the African penguins at St Croix Island (Randall, 1983) and at Dyer Island (34°41'S; 19°25'E) on the south coast (La Cock & Hanel, in prep.), and lower than survival rates of other penguin species studied elsewhere (61-70% for Ad61ie penguins Pygoscelis adeliae to 95% for emperor penguins Aptenodytes forsteri: cf. Croxall, 1981). Low post-fledgling survival of juvenile African penguins appears to be another, and perhaps the most important, reason ~for the decrease of the Marcus Island population. First-year survival of juveniles at St Croix Island was 32% for 'normal' years, although in two 'abnormal' years, only 3.9% and 5-1% of fledglings survived (Randall, 1983). At St Croix Island, 90% of survivors were resighted within three years after fledging, and no fledglings were resighted for the first time more than four years after banding (Randall, 1983). At Marcus Island, however, only 4.7% of banded fledglings have been resighted. It is unlikely that the difference between Marcus and St Croix islands can be attributed solely to lack of search effort. For example, Marcus Island was visited at least monthly, but Dyer Island was visited for only one-three weeks a year from 1982 to 1985. Yet, at Dyer Island, 13% (n = 512) of fledglings banded in 1978 have been resighted (La Cock & Hanel, in prep.). Rand (1960) and Wilson (1985) have suggested that fledgling penguins feed on relatively slow-moving prey such as larval fish. The low survival of fledgling African penguins may be an indication of the low availability of such larvae. The unusual oceanographic events of 1982-83 and 1983-84 could also have reduced survival of first-year penguins at sea, but there are too few recoveries to examine annual mortality patterns. The recruitment of
Population dynamics of African penguin
125
young penguins into the Marcus Island population appears to be so low that the colony is unlikely to survive. If the levels are indicative of what is happening on other west coast islands, then the population is in danger of extinction. While recruitment appears to be better on islands on the south coast of South Africa, a future restriction of the breeding population to one or two sites would make the species extremely vulnerable to a single major oiling disaster.
ACKNOWLEDGEMENTS We thank the following people: J. C. Sinclair set up the study area; S. C. Broni, P. A. R. Hockey, D. and W. Suter, A. J. Williams, R. P. Wilson, and many other members of the FitzPatrick Institute checked the colony; A. A. Crowe and L. G. Underhill assisted with data analysis; and S. Hunter, W. R. Siegfried, and R. P. Wilson provided valuable discussion or comments on the manuscript. The Sea Fisheries Research Institute provided access to Marcus Island. T. B. Oatley, South African Bird Ringing Unit, provided invaluable assistance with banding data. The project was supported by the Benguela Ecology Programme of the South African National Committee for Oceanographic Research, the Department of Environment Affairs, Endangered Wildlife Trust, the National Geographic Society, and the South African Nature Foundation.
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Frost, P. G. H., Siegfried, W. R. & Cooper, J. (1976). Conservation of the jackass penguin Spheniscus demersus (L.). BioL Conserv., 9, 79-99. La Cock, G. D. (1986). The Southern Oscillation, environmental anomalies, and mortality of two southern African seabirds. Climatic Change, 8, 173-84. Morant, P. D., Cooper, J. & Randall, R. M. (1981). The rehabilitation of oiled jackass penguins Spheniscus demersus, 1970-1980. In Proceedings of the symposium on birds of the sea and shore, 1979, ed. by J. Cooper, 267-301. Cape Town, African Seabird Group. Rand, R. W. (1960). The biology of guano-producing seabirds: The distribution, abundance and feeding habits of the Cape penguin Spheniscus demersus off the south-western coast of the Cape Province. InvestlRep. Div. Fish. Un. S. Afr., 41, 1-28.
Rand, R. W. (1963a). The biology of guano producing seabirds, 4. Composition of colonies on the Cape Islands. Investl Rep. Div. Fish. Rep. S. Aft., 43, 1-32. Rand, R. W. (1963b). The biology of guano producing seabirds, 5. Composition of colonies on the South West African islands. Investl Rep. Div. Fish. Rep. S. Afr., 46, 1-26. Randall, R. M. (1983). Biology of the Jackass penguin Spheniscus demersus (L.) at St Croix Island, South Africa. PhD thesis, University of Port Elizabeth, South Africa. Reilly, P. N. & Cullen, J. M. (1981). The little penguin Eudyptula minor in Victoria, I. Breeding. Emu, 81, 1-19. Scolaro, J. A. (1980). El pinguino de Magallanes Spheniscus magellanicus, 6. Dinamica de la poblacirn de juveniles. Hist. Nat. (Mendoza), 1, 173-8. Shelton, P. A., Crawford, R. J. M., Cooper, J. & Brooke, R. K. (1984). Distribution, population size and conservation of the jackass penguin Spheniscus demersus. S. Afr. J. mar. Sci., 2, 217 57. Westphal, A. & Rowan, M. K. (1971). Some observations on the effects of oil pollution on the jackass penguin. Ostrich (SuppL), 8, 521-6. Williams, A. J. (1980). Offspring reduction in macaroni and rockhopper penguins. Auk, 97, 754-9. Wilson, R. P. (1985). The jackass penguin Spheniscus demersus as a pelagic predator. Mar. Ecol. Prog. Ser., 25, 219 27.