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1.
The diet of African penguins Spheniscus demersus in Namibia consisted mainly of sardine Sardinops sagax in the 1950s. Since the collapse of pelagic fish stocks in the 1970s, birds fed mainly on bearded (pelagic) goby Sufflogobius bibarbatus, a low-energy prey species. We present diet data for African penguins breeding at Mercury Island, the largest colony for this species in Namibia, between 1996 and 2009. Bearded goby was the main prey item throughout the study period, both in terms of frequency of occurrence (67.8%; SD 31.2) and in terms of mass (59.2%; SD 31.5). Diet composition varied throughout the year as well as between years; birds occasionally fed on a variety of fish species other than bearded goby. In Namibia, poor prey abundance is considered as a major factor contributing to the decline of penguin numbers after the collapse of the sardine stocks. However, bearded goby appears to be relatively abundant along Namibia's southern coast and low prey quality rather than low abundance appears to be a key factor influencing population dynamics of African penguins and other marine top predators in southern Namibia.  相似文献   

2.
During the period 17–22 December 2001, the onshore distribution and the abundance of Antarctic fur seals Arctocephalus gazella and Subantarctic fur seals A. tropicalis were determined for Prince Edward Island. Two breeding colonies of Antarctic fur seals were located on the south-east coast of the island; the first a mixed (with Subantarctic fur seals) breeding colony with an estimated 24 pups on a vegetated promontory on the northern section of Boggel Beach, and the second, a presumably pure Antarctic fur seal breeding colony with an estimated 380 pups, at Penguin Beach. At a mean intrinsic rate of natural increase of 16.2% per year, Antarctic fur seals appear to be in the rapid recolonization phase of population growth. Breeding colonies of Subantarctic fur seals, largely found on the entire east coast, produced an estimated 15 000 pups, and the population had maintained a mean intrinsic rate of natural increase of some 9.5% per year since 1987/88.  相似文献   

3.
Four species of penguin breed regularly at South Africa's Prince Edward Islands: king penguin Aptenodytes patagonicus, gentoo penguin Pygoscelis papua, macaroni penguin Eudyptes chrysolophus and southern rockhopper penguin E. chrysocome. In December 2008, it was estimated that some 65 000 pairs of king penguins were incubating eggs at Marion Island, the larger of the two islands in the group, and 2 000 pairs at Prince Edward Island. At Marion Island from 1987 to 2008, there was no long-term trend in numbers of king penguin chicks that survived to the end of the winter period, but there was considerable fluctuation in chick production in the 1990s. It was roughly estimated that on average 88% of king penguin chicks survived the winter period (from April to September/October). Numbers of gentoo penguins at Marion Island decreased from more than 1 300 pairs in the mid-1990s to fewer than 800 pairs in 2003, and then increased to almost 1 100 pairs in 2008 as breeding success improved. Between 1994/1995 and 2008/2009, numbers of macaroni and southern rockhopper penguins at Marion Island decreased by about 30% and 70% respectively. In 2008/2009, some 290 000 pairs of macaroni penguins bred at this island, mostly in two large colonies where there was a progressive decrease in the density of nests. At both these colonies, decreases in numbers breeding followed outbreaks of disease. Inadequate breeding success has influenced the decreases of macaroni and rockhopper penguins. In 2008/2009, some 42 000 pairs of southern rockhopper penguins bred at Marion Island and 12 000 pairs of macaroni penguins and 38 000 pairs of southern rockhopper penguins at Prince Edward Island.  相似文献   

4.
The second mid-summer survey of surface-nesting seabirds at the Prince Edward Island group (Marion and Prince Edward islands) was conducted during December 2008, seven years after the initial mid-summer survey. Wandering albatrosses Diomedea exulans may have decreased slightly at Prince Edward Island, mirroring a decrease of roughly 2% per year at Marion Island from 1998 to 2005, a decline that has since reversed. Numbers of grey-headed albatrosses Thalassarche chrysostoma on Marion Island have remained stable, whereas the population on Prince Edward Island decreased by 20% from 2001 to 2008 (3% per year). The estimate of Indian yellow-nosed albatrosses T. carteri at Prince Edward Island was similar in 2001 and 2008. Counts of both sooty albatrosses Phoebetria spp. were substantially higher at Prince Edward Island in 2008, possibly as a result of better coverage compared to 2001. Dark-mantled sooty albatrosses P. fusca on Marion Island have decreased by almost 2% per year since 1996, continuing a negative trend from the early 1980s, whereas light-mantled sooty albatrosses P. palpebrata have increased by almost 6% per year at Marion Island since 1996. Counts of both giant petrels increased at Prince Edward Island (northern Macronectes halli by 44%; southern M. giganteus by 28%), whereas their numbers have remained stable at Marion Island. Current best estimates for annual breeding populations (pairs) at the two islands are 3 650 wandering albatrosses, 9 500 grey-headed albatrosses, 7 000 Indian yellow-nosed albatrosses, 2 900 dark-mantled sooty albatrosses, 800 light-mantled sooty albatrosses, 750 northern giant petrels and 2 800 southern giant petrels, confirming the global importance of the Prince Edward Islands for these seven species. Apart from the dark-mantled sooty albatross, their populations are reasonably healthy despite fishing mortality.  相似文献   

5.
The onshore distributions and the abundances of Antarctic fur seals Arctocephalus gazella and Subantarctic fur seals A. tropicalis were determined at Prince Edward Island during 16-20 December 2008. This repeats a survey conducted in December 2001 and extends the area surveyed to include the entire south-west coast of Prince Edward Island. Of the two colonies of Antarctic fur seals, the colony among Subantarctic fur seals north of Boggel Beach remained small, with increased numbers of Subantarctic fur seals and putative hybrids. The other Antarctic fur seal breeding colony at Penguin Beach remained free of Subantarctic fur seals and had expanded at a mean intrinsic rate of natural increase of 11.4% per year from 2001. With an estimated 810 pups, the Antarctic fur seal is still in the rapid recolonisation phase of population growth. The distribution of the more widespread and abundant Subantarctic fur seals also had increased, with several new breeding colonies along the east coast and one at Kent Crater on the west coast. The annual pup production was conservatively estimated at 14 130 pups. The mean intrinsic rate of natural increase has declined to ?0.3% per year over the last seven years, compared to the 9.3% per year between 1987/1988 and 2001/2002, and the population is in the mature phase of population growth.  相似文献   

6.
During the 1990s and early 2000s, populations of surface-nesting seabirds at Marion Island showed different trends, but for the majority of species numbers decreased. Reduced numbers of gentoo penguins Pygoscelis papua, eastern rockhopper penguins Eudyptes chrysocome filholi, Crozet shags Phalacrocorax [atriceps] melanogenis and probably macaroni penguins E. chrysolophus are most plausibly attributed to an altered availability of food. Decreases in numbers of dark-mantled sooty albatrosses Phoebetria fusca, light-mantled sooty albatrosses P. palpebrata, southern giant petrels Macronectes giganteus and possibly northern giant petrels M. halli may have resulted from mortality of birds in longline fisheries. However, populations of wandering Diomedea exulans and grey-headed Thalassarche chrysostoma albatrosses fluctuated around a stable level. Numbers of Subantarctic skuas Catharacta antarctica and kelp gulls Larus dominicanus breeding at Marion Island also decreased. Kerguelen Sterna virgata and Antarctic S. vittata terns remain scarce at the island. Trends for king penguins Aptenodytes patagonicus were not reliably gauged, but numbers probably remained stable or increased. There were large fluctuations in numbers of king penguin chicks surviving to the end of winter.  相似文献   

7.
From 1987 to 2005, numbers of African penguins Spheniscus demersus breeding in South Africa's Western Cape Province increased by about 50%. Numbers decreased at the four northernmost colonies in the region: Lambert's Bay and the three colonies in Saldanha Bay, although at Jutten Island the decrease is inferred from an estimate for 1987, derived from interpolation. Numbers also decreased at Geyser Rock and Dyer Island on the South Coast. At five colonies between Saldanha Bay and Dyer Island there were large increases. At a sixth colony in that region, Seal Island, where Cape fur seals Arctocephalus pusillus pusillus limit breeding space, numbers remained stable. At two colonies that were initiated in the early 1980s, Robben Island and Boulders, increases were initially rapid (>20% per annum) and matched growth of the South African stock of sardine Sardinops sagax. Strong growth at Dassen and Vondeling islands, between Robben Island and Saldanha Bay, was observed from about 1996–2002, when there was a large increase in the biomass of pelagic fish off South Africa. Increases at colonies between Saldanha Bay and Boulders slowed after 2002, whereas the colony at Dyer Island stabilised at that time. In 2003, a new colony was initiated east of Dyer Island at De Hoop Nature Reserve. These latter trends followed an eastward shift in the distribution of sardine. Small penguin colonies may act as foci for growth in a period when the distribution of prey is changing. Hence, it is important that they be maintained, especially those that, if lost, would increase the isolation of regional populations. Some of the small colonies are less susceptible to oil spills than colonies in the proximity of harbours, and for that reason also are important.  相似文献   

8.
The biology of the redspotted tonguesole Cynoglossus zanzibarensis, a common African cynoglossid inhabiting the Agulhas Bank, South Africa, is described. Growth studies based on sectioned sagittal otoliths revealed that C. zanzibarensis is relatively fast-growing and long-lived, attaining ages >8 years. Growth in length was rapid in immature fish, fish attaining 56% of their maximum size within their first year. By sexual maturity, fish had attained 28% of their maximum age and 68% of their maximum length. Total length-at-age was best described by the Von Bertalanffy growth model with combined-sex growth described as Lt = 354.78(1?e?0.43 (t+1.17)) mm TL. Sexually dimorphic growth patterns were evident, females attaining larger lengths, but at a slower growth rate than males. Despite the similar mean size of adult fish, the trawl-sampled adult population was dominated by females, with a sex ratio of 1 male:2.4 females. Female C. zanzibarensis mature in their second year of life (275 mm TL), after which they spawn small, pelagic eggs throughout the year. Approximations of the rates of total, natural and fishing mortality were estimated to be 0.62, 0.48 and 0.14 year?1 respectively.  相似文献   

9.
African penguins Spheniscus demersus live in the Benguela and western Agulhas ecosystems off southern Africa. Their numbers decreased throughout the 20th century from at least 1.5 million to about 0.18 million adults, although different regional trends were apparent. They feed to a large extent on shoaling epipelagic fish, notably anchovy Engraulis capensis and sardine Sardinops sagax, and regional trends in the abundance of penguins are associated with trends in the abundance and distribution of these prey fish. Many first-time breeders emigrate from colonies where feeding or other conditions at the time are unfavourable to more favourable breeding localities. This has led to both the extinction and formation of colonies. Food now may limit colonies at relatively small sizes, a fact attributable to industrial fisheries reducing the densities of forage fish. African penguins share their habitat with several other predators, with which they compete for food and breeding space. One of these, the Cape fur seal Arctocephalus p. pusillus, increased through the 20th century to 1.5 – 2 million animals at its close. Reported observations of predation by fur seals on seabirds have increased in recent decades and threaten the continued existence of small colonies of penguins. Stochastic modelling suggests that colonies of 10 000 pairs have a 9% probability of extinction in 100 years, so smaller populations should be regarded as "Vulnerable". However, in a period of prolonged food scarcity off southern Namibia, the regional population decreased from more than 40 000 pairs in 1956 to about 1 000 pairs in 2000, and many colonies numbering less than 1 000 pairs became extinct. The minimum viable population for African penguins is currently considered to be >40 000 pairs, likely of the order of 50 000 pairs, a figure equivalent to its level in 2000. The chance of survival of the species through the 21st century is tenuous.  相似文献   

10.
The body condition of altricial seabird chicks is directly related to the amount of food received from their parents, which depends on feeding conditions at sea. An index of body condition is therefore a useful tool for assessing variation in food availability between different breeding colonies. Quantile regression techniques were used to develop a body condition index for African penguin Spheniscus demersus chicks, based on maximum and minimum growth relative to structural size. This is considered the most appropriate index for comparing the body condition of groups of chicks, varying in size and of unknown age, between breeding colonies. Based on this study, recommendations are made with regard to morphometric measurements, sample size and the sampling interval required to compare effectively the condition of African penguin chicks between breeding colonies throughout their distributional range.  相似文献   

11.
Predation by Cape fur seals Arctocephalus pusillus pusillus on seabirds has previously been attributed to a few individuals, mainly males. Scat samples were collected at three mainland breeding colonies of seals in Namibia to determine the extent of seabird predation. Mainly females were sampled. No scats from the van Reenen Bay and Atlas-Wolf Bay colonies, and only two from Cape Cross, contained feather remains, giving an overall frequency of occurrence of feathers in scats of 0.1%. The feathers found were of African penguin Spheniscus demersus and a species of cormorant (Phalacrocorax sp.).  相似文献   

12.
Abundance and exploitation patterns of monkfish Lophius vomerinus and L. vaillanti were investigated for use as inputs into a stock assessment framework to be used for management of the Namibian monkfish resource. Total numbers of monkfish caught per size-class were estimated using industry records of tail products classified into six commercial categories. The proportions per category varied each year. Analysis of the commercial data suggests that large numbers of juvenile monkfish are harvested annually. Catch-per-unit-effort (cpue) data of vessels targeting monkfish and sole (the two species are combined in terms of Namibian fleet legislation) for the period 1991–1999 were analysed using two different methods to construct indices of abundance. Both indices, one standardized on vessel horsepower and the other standardized by means of a Generalized Linear Model, showed an increase in catch rate of monkfish from 1991 to 1994 and between 1996 and 1998, but a decline from 1994 to 1996 as well as during 1999. Conversion factors for landed or tail weight to whole weight for four different tail products of monkfish were estimated. Results obtained from the study suggest that the factor of 3.04 currently applied in Namibia to all tail-weight classes is not appropriate for the current fishery and needs to be amended. For management purposes it is also suggested that four different conversion factors, one for each monkfish tail product, be implemented.  相似文献   

13.
Populations of the African penguin Spheniscus demersus have decreased dramatically over the past century, due in part to competition for food with commercial fisheries, and the species is now endangered as a result. Economic arguments are used to favour fisheries over the needs of penguins, but penguins have direct value to the South African economy thanks to penguin-based tourism at several breeding colonies. We estimated the value of African penguins at the most important tourist site for African penguins: Boulders on the Cape Peninsula, South Africa. As a mainland colony close to Cape Town, Boulders is accessible to large numbers of visitors; gate revenues in 2009/2010 alone were R14.5 million (US$2 million). A zonal travel-cost analysis revealed an average consumer surplus among Cape Town residents of some R20 per visit. Penguin-based tourism thus forms an integral part of the R25 billion Western Cape tourism sector. Given that the future survival of this valuable resource is dependent, among others, on the availability of sufficient prey, competition with South Africa's purse-seine fishing fleet should be limited through management strategies that lessen the potential effects of fishing on prey available at the local scale.  相似文献   

14.
The main impact of the fishery for Tristan rock lobster Jasus tristani on seabirds at the Tristan archipelago and Gough Island is through night strikes, when petrels collide with a ship after being disorientated by its lights. Tristan fishery observers have kept records of night strikes on the MV Edinburgh since the 2010/2011 fishing season. Over the last three years, 723 seabirds from nine species were recorded coming aboard the fishing vessel, with at least 39 (5.4%) birds dying as a result. Birds killed were broad-billed prions Pachyptila vittata (41%), common diving petrels Pelecanoides urinatrix (23%), and storm petrels (Pelagodroma marina and Fregetta grallaria/tropica 36%). All these species are listed as Least Concern globally, and the numbers killed per year are <0.1% of the island populations. The captain and crew of the Edinburgh are aware of the problem posed by deck lights at night, and attempt to keep external lighting to a minimum. As a result, the numbers of birds coming aboard vessels in this fishery have decreased from an average of 130 birds per night in 1989 to less than two birds per night in 2010–2013. Currently, most incidents occur during exceptional events when circumstances require deck lights to be lit at night. Consideration should be given to banning fishing operations at night, at least on misty nights.  相似文献   

15.
Numbers of gentoo penguins Pygoscelis papua and Crozet shags Phalacrocorax [atriceps] melanogenis breeding annually at Marion Island, one of South Africa's Prince Edward Islands in the South-West Indian Ocean, were strongly correlated over 19 split-years from 1994/1995 to 2012/2013. Both species decreased between the mid-1990s and the mid-2000s, exhibited a partial recovery in the late-2000s and then decreased to their lowest recorded levels in 2012/2013. In both instances, the partial recoveries in the late-2000s were associated with improved breeding success. At a colony of gentoo penguins, breeding success was negatively correlated with the date of arrival of adults to breed. Gentoo penguins and Crozet shags are demersal feeders in inshore waters around Marion Island and there is considerable overlap in the composition of their diets. Therefore, trends in their populations may be driven by food availability, which is likely to be influenced by benthic production around the island. We propose that, in South Africa, and based on the current assessment, the Crozet shag, which elsewhere breeds only at the Crozet Islands, is now Critically Endangered, and the more widely ranging gentoo penguin is Endangered.  相似文献   

16.
Once one of the most numerous seabirds of the Benguela upwelling system, the population of Cape cormorants Phalacrocorax capensis has decreased by 60% in the past three decades and the species is listed as Near Threatened. Declines in prey availability and/or abundance brought about by recent changes in the distribution of pelagic fish stocks and industrial purse-seine fishing are hypothesised to be a key driver of seabird population decreases in the southern Benguela. We investigated the foraging behaviour of breeding Cape cormorants by deploying GPS and temperature–depth recorders on 24 breeding adults from three islands off the coast of South Africa, two of them to the north of Cape Point and a third farther south on the western Agulhas Bank. This provided the first measures of foraging dispersal by a cormorant in the Benguela system, and enabled a comparison of foraging behaviour between birds from these islands. Foraging trips of Cape cormorants lasted between 17 min and >7 h, at a maximum distance of between 2 and 58 km away from their colony. Foraging effort was significantly greater for birds from farther north off the West Coast in terms of trip duration, distance travelled, number of dives and time spent flying compared to those from the southernmost island (Dyer), which is probably a response to low prey availability in the north. Coastal reserves that exclude pelagic fishing from inshore feeding grounds around Cape cormorant breeding colonies may result in increased local prey availability, which would benefit Cape cormorant populations.  相似文献   

17.
The Cape gannet is endemic to the southern African coast where it currently breeds at six islands: Mercury, Ichaboe and Possession off South West Africa and Bird (Lambert's Bay), Malgas and Bird (Algoa Bay) off South Africa. Previously, breeding also occurred at Hollams Bird, Halifax and Seal (False Bay) Islands. Equivocal records for Marcus, Dassen and Dyer Islands are not accepted. Off South West Africa, gannets were breeding at Hollams Bird, Mercury and Ichaboe Islands at least as early as 1828, but they only occupied Halifax and Possession Islands sometime between that date and c. 1885, possibly as a result of displacement of gannets from Ichaboe Island during exploitation of accumulated guano deposits in the early 1840s. Gannets bred at Hollams Bird Island until at least 1938, but had ceased breeding at Halifax Island by 1928. Off South Africa the earliest records of breeding are 1648, 1687, 1755 and 1912 for Malgas, Seal (False Bay), Bird (Port Elizabeth) and Bird (Lambert's Bay) Islands respectively. Gannets have not been reported at Seal Island since the late 17th century. On the west coast of Africa the Cape gannet is a regular nonbreeding winter visitor as far north as 4°20′N 6°00′E, but west of 6°E it is rare. On the east coast of Africa it is a common winter visitor as far north as Delagoa Bay, but farther north it is rare. Within its normal range the Cape gannet seldom occurs farther off shore than 100 km; it hardly ever moves inland. Aerial censuses of Cape gannets at breeding islands in 1967, 1969, 1978, 1980 and 1981 are compared with an aerial census conducted in 1956 and other published estimates of abundance. Between 1956 and 1980 the estimated number of breeding pairs at all colonies decreased from c. 150 000 to c. 80 000 and numbers decreased at all three extant gannetries off South West Africa. These decreases are attributed to a greatly diminished food resource following the collapse of the South West African pilchard stock after the late 1960s. The number of gannets decreased at Bird (Lambert's Bay) and Malgas Islands between 1956 and the late 1960s but subsequently increased, trends that are related to performances of the Western Cape pilchard and anchovy resources respectively. At Bird Island (Algoa Bay) gannets were up to 3,5 times more abundant in the late 1970s than in 1956. Other marine resources located east of Cape Point have shown similar large increases in recent years. Rates of increase of gannets at islands off South Africa during the 1970s would have required an unrealistically high survival for the first year had other population parameters remained constant. It is possible that birds emigrated from the South West African Islands. Few gannets have been reported oiled, and conservation of the species seems to be mainly affected by greatly reduced prey availability and injudicious guano harvesting. Human exploitation of juvenile gannets off the West African coast is difficult to assess.  相似文献   

18.
The jackass penguin, endemic to southern Africa, is the only member of the Sphenisciformes now breeding in Africa. Its breeding distribution extends from Sylvia Hill, South West Africa (Namibia), to Bird Island, Algoa Bay, South Africa. The non-breeding range extends from Sette Cama, Gabon, to Inhaca Island, Moçambique. Jackass penguins do not usually occur far off shore. The minimum total breeding population in 1978/79 was estimated to be approximately 134 000 birds. Numbers halved between 1956 and 1978 alone as a result of collapses in the South and South West African pilchard populations. Numerical decreases at Possession and Dassen Islands were particularly severe. Prior to 1956 numbers had also decreased, primarily because of large collections of eggs. Sealing activities and guano harvests may also have influenced trends. Only colonies at the periphery of the breeding distribution have increased numerically in recent times, and such increases appear to be related to the availability of suitable prey. Although the species breeds in captivity and is unlikely to become extinct, its continued survival in the wild probably relies on the continued good state of the pelagic fish resources.  相似文献   

19.
During 1997, considerable scientific differences arose about the status of the Namibian hake (Merluccius capensis and M. paradoxus) resource, and as to whether the hake Total Allowable Catch (TAC) should be substantially decreased or increased. These differences revolved primarily around whether or not abundance estimates from the swept-area trawl surveys by the Norwegian research vessel Dr Fridtjof Nansen should be considered as reliable measures of biomass in absolute (as distinct from relative) terms. The paper relates the computations underlying the Interim Management Procedure (IMP) approach that was put forward at that time as a basis to resolve this impasse. The anticipated performance, in terms of catch and risk of resource depletion, of a number of simple candidate IMPs for the Namibian hake resource is evaluated. The IMPs depend on two parameters, whose values are to be chosen by decision-makers, and adjust the TAC up or down from one year to the next according to whether trends in recent commercial catch rate and survey indices of abundance are positive or negative. Performances are evaluated across the then current wide range argued for resource abundance and status. Trade-offs in performance across the candidates considered are discussed. One of the candidates was subsequently selected by a joint meeting of scientists, industry and Ministry officials in February 1998 and served as the basis for scientific recommendations for the TAC for the hake resource for the following three years.  相似文献   

20.
Laboratory experiments were performed on the food ecology of four congeneric species of free-living plathelminths, Promesostoma caligulatum, P. marmoratum, P. rostratum, and P. meixneri, all inhabiting an intertidal sandflat near the island of Sylt (North Sea). Their prey spectrum is within the microcrustaceans: P. caligulatum preferred ostracods, while the other three species favoured copepods, with species-specific differences for copepod species and size classes. Daily consumption of prey species varied with the size of both the predator and the prey. On average, P. marmoratum consumed 0.76 Harpacticus flexus per day while this rate decreased to 0.06 in P. meixneri, the smallest predator. When these Promesostoma species were fed with Tachidius discipes, a smaller prey species, their predation rates were about 25% higher. While the larger predators preferred the larger harpacticoids as prey, the small P. meixneri preferred small cyclopoids over larger harpacticoids. In terms of biomass, P. marmoratum's mean consumption of T. discipes per day was about half the predator's own weight. This average varied with prey density and temperature. A comparison of these consumption rates with the field densities of the predators and their prey shows that the plathelminth predators may consume as much as 10% per day of their copepod prey populations, thus strongly influencing these prey populations on these sandflats. The predation pressure of P. caligulatum on ostracods was about 1% per day of the prey population. Since ostracods usually have fewer generations per year, the total effect on the population dynamics may be similar to that on copepods. Therefore, nocturnal swimming of copepods in the water column may be interpreted as an attempt to escape plathelminth predators.  相似文献   

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