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Les G Underhill - One of the best experts on this subject based on the ideXlab platform.
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an assessment of the impact of predation by cape fur seals arctocephalus pusillus pusillus on seabirds at dyer island south africa
Ostrich, 2013Co-Authors: Azwianewi B Makhado, Robert J M Crawford, Lauren J Waller, Les G UnderhillAbstract:At Dyer Island, South Africa, observations of predation of seabirds by Cape Fur Seals Arctocephalus pusillus pusillus were made during 2004 and 2006/07. It was estimated that seals killed about 7% of adult African Penguins Spheniscus demersus annually. This may have contributed to the penguin colony not increasing in spite of food becoming more available in its vicinity during an eastward shift of epipelagic fish resources off South Africa at the start of the twenty-first century. Adult penguins were most susceptible to mortality in the breeding season as they returned to feed chicks in the evening. Seals also killed substantial numbers (3–9%) of Cape Cormorant Phalacrocorax capensis fledglings as they left the island, mostly in the morning. The losses of Cape Cormorant fledglings accounted for about 95% of the overall mortality of seabirds attributable to seals. Mortality inflicted by seals on adults of four species of cormorant was negligible and no predation by seals on gulls and terns was observed.
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influence of local and regional prey availability on breeding performance of african penguins Spheniscus demersus
Marine Ecology Progress Series, 2013Co-Authors: Richard B Sherley, Les G Underhill, Robert J M Crawford, Bruce M Dyer, Pj Barham, Barbara J Barham, Janet C Coetzee, Mario T Leshoro, L UpfoldAbstract:Population trends of African penguins Spheniscus demersus in the Western Cape, South Africa, and their breeding success have been linked to the abundance of their main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. During the late 1990s and early 2000s, both fish species increased markedly in abundance, but after 2004, sardine biomass decreased to below average levels. In addition, adults of both stocks were principally located to the east of Cape Agulhas from 2001 to 2009 and were thus distant from seabird colonies on South Africa's West Coast. The number of African penguin pairs counted at Robben Island from 2001 to 2009 and the fledging period of chicks from successful nests increased and decreased in apparent response to the biomass of sardine prior to each breeding season, possibly linked through adult condition at the onset of breeding. Breeding success and chick-fledging rates increased during the study period and showed positive relationships with local food availability, indexed through the annual industrial catch of anchovy made within 56 km (30 nautical miles) of the colony. In addi- tion, chick-fledging rates were depressed in 2-chick broods during years when anchovy con- tributed <75% by mass to the diet of breeding birds. Previously reported relationships between the overall abundance of forage fish in South Africa and penguin breeding success were not sup- ported. Taken together, these results highlight the combined importance of ensuring adequate local food availability for seabirds during the reproductive cycle and safeguarding regional prey abundance during the non-breeding season.
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artificial nests enhance the breeding productivity of african penguins Spheniscus demersus on robben island south africa
Emu, 2012Co-Authors: Richard B Sherley, Pj Barham, Barbara J Barham, Mario T Leshoro, Les G UnderhillAbstract:Loss of nesting habitat threatens many cavity nesting birds worldwide and has contributed to the decline of several species of burrow-nesting seabirds. Replacing lost habitat with artificial nesting structures is considered to be a useful conservation intervention. Here we report on an investigation into the effectiveness of such a strategy – providing artificial nests for the endangered African Penguin (Spheniscus demersus) at a colony on Robben Island, South Africa. The re-colonisation of Robben Island by breeding African Penguins in the 1980s was partly attributed to the availability of shaded nesting habitat under introduced vegetation. However, the suitability of this habitat had not been tested empirically. In addition, artificial nests have been present at Robben Island since 2001, but whether they were a viable means of providing improved nesting habitat was not known. The reproductive output of African Penguins was monitored on Robben Island from 2001 to 2010. Breeding success varied between years but, overall, was within the range of figures previously reported for the species. Relative to pairs in nests under vegetation, birds occupying artificial nests and nests in abandoned buildings had increased nesting survival during chick-rearing, with 9 and 13% more chicks fledged per egg hatched over the study period. These artificial structures seem to offer the advantages of shelter from the weather and protection from predators, without the risks of collapse associated with natural burrows in non-guano substrates. This study supports findings from Namibia, and also supports the continued use of artificial nests as a conservation tool throughout the range of the species.
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impact of the treasure oil spill on african penguins Spheniscus demersus at dassen island case study of a rescue operation
African Journal of Marine Science, 2008Co-Authors: A C Wolfaardt, Les G Underhill, Res Altwegg, J Visagie, A J WilliamsAbstract:The bulk ore carrier MV Treasure sank between Robben and Dassen islands off western South Africa in June 2000. Although releasing less oil than the Apollo Sea spill six years earlier, the number of African penguins Spheniscus demersus oiled (approximately 19 000) in the Treasure spill was almost double the number oiled in the Apollo Sea incident. The majority of oiled penguins was collected from Robben Island. The capture, stabilisation and transport of oiled penguins following the Treasure spill were substantially more successful than during the Apollo Sea rescue operation and resulted in an increase in the percentage of oiled penguins that were released back to the wild. In addition to the 19 000 oiled penguins that were caught, 19 500 un-oiled birds (12 345 from Dassen Island) were evacuated to Cape Recife, 800 km to the east, to prevent them from becoming contaminated. The evacuated birds returned rapidly to Dassen Island and many resumed breeding soon after their return. The restoration success (prop...
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implications for seabirds off south africa of a long term change in the distribution of sardine
African Journal of Marine Science, 2008Co-Authors: R J M Crawford, T. Fairweather, Les G Underhill, P S Sabarros, A C WolfaardtAbstract:From 1997 to 2005, the distribution of sardine Sardinops sagax, an important prey item for four seabirds off South Africa, shifted 400 km to the south and east, influencing its availability to breeding birds. It became progressively less available to seabirds in the Western Cape Province, where the number of African penguins Spheniscus demersus breeding decreased by 45% between 2004 and 2006, survival of adult penguins decreased and penguins established a new eastern colony in 2003. In that province, the number of Cape gannets Morus capensis breeding decreased by 38% between 2001/2002 and 2005/2006 and the contribution of sardine to the diet of gannets fell from an average of 40% during the period 1987–2003 to 5–7% in 2005 and 2006. The proportions of Cape cormorants Phalacrocorax capensis and swift terns Sterna bergii breeding in the south of the province increased as sardine moved south and east. In the Eastern Cape Province, the number of penguins breeding halved between 2001 and 2003, whereas after 20...
Robert J M Crawford - One of the best experts on this subject based on the ideXlab platform.
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bayesian inference reveals positive but subtle effects of experimental fishery closures on marine predator demographics
Proceedings of The Royal Society B: Biological Sciences, 2018Co-Authors: Richard B Sherley, Robert J M Crawford, Alistair M Mcinnes, Pj Barham, Barbara J Barham, Kate J Campbell, Jennifer Grigg, Cat Horswill, Taryn L MorrisAbstract:Global forage-fish landings are increasing, with potentially grave consequences for marine ecosystems. Predators of forage fish may be influenced by this harvest, but the nature of these effects is contentious. Experimental fishery manipulations offer the best solution to quantify population-level impacts, but are rare. We used Bayesian inference to examine changes in chick survival, body condition and population growth rate of endangered African penguins Spheniscus demersus in response to 8 years of alternating time–area closures around two pairs of colonies. Our results demonstrate that fishing closures improved chick survival and condition, after controlling for changing prey availability. However, this effect was inconsistent across sites and years, highlighting the difficultly of assessing management interventions in marine ecosystems. Nevertheless, modelled increases in population growth rates exceeded 1% at one colony; i.e. the threshold considered biologically meaningful by fisheries management in South Africa. Fishing closures evidently can improve the population trend of a forage-fish-dependent predator—we therefore recommend they continue in South Africa and support their application elsewhere. However, detecting demographic gains for mobile marine predators from small no-take zones requires experimental time frames and scales that will often exceed those desired by decision makers.
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an assessment of the impact of predation by cape fur seals arctocephalus pusillus pusillus on seabirds at dyer island south africa
Ostrich, 2013Co-Authors: Azwianewi B Makhado, Robert J M Crawford, Lauren J Waller, Les G UnderhillAbstract:At Dyer Island, South Africa, observations of predation of seabirds by Cape Fur Seals Arctocephalus pusillus pusillus were made during 2004 and 2006/07. It was estimated that seals killed about 7% of adult African Penguins Spheniscus demersus annually. This may have contributed to the penguin colony not increasing in spite of food becoming more available in its vicinity during an eastward shift of epipelagic fish resources off South Africa at the start of the twenty-first century. Adult penguins were most susceptible to mortality in the breeding season as they returned to feed chicks in the evening. Seals also killed substantial numbers (3–9%) of Cape Cormorant Phalacrocorax capensis fledglings as they left the island, mostly in the morning. The losses of Cape Cormorant fledglings accounted for about 95% of the overall mortality of seabirds attributable to seals. Mortality inflicted by seals on adults of four species of cormorant was negligible and no predation by seals on gulls and terns was observed.
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influence of local and regional prey availability on breeding performance of african penguins Spheniscus demersus
Marine Ecology Progress Series, 2013Co-Authors: Richard B Sherley, Les G Underhill, Robert J M Crawford, Bruce M Dyer, Pj Barham, Barbara J Barham, Janet C Coetzee, Mario T Leshoro, L UpfoldAbstract:Population trends of African penguins Spheniscus demersus in the Western Cape, South Africa, and their breeding success have been linked to the abundance of their main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. During the late 1990s and early 2000s, both fish species increased markedly in abundance, but after 2004, sardine biomass decreased to below average levels. In addition, adults of both stocks were principally located to the east of Cape Agulhas from 2001 to 2009 and were thus distant from seabird colonies on South Africa's West Coast. The number of African penguin pairs counted at Robben Island from 2001 to 2009 and the fledging period of chicks from successful nests increased and decreased in apparent response to the biomass of sardine prior to each breeding season, possibly linked through adult condition at the onset of breeding. Breeding success and chick-fledging rates increased during the study period and showed positive relationships with local food availability, indexed through the annual industrial catch of anchovy made within 56 km (30 nautical miles) of the colony. In addi- tion, chick-fledging rates were depressed in 2-chick broods during years when anchovy con- tributed <75% by mass to the diet of breeding birds. Previously reported relationships between the overall abundance of forage fish in South Africa and penguin breeding success were not sup- ported. Taken together, these results highlight the combined importance of ensuring adequate local food availability for seabirds during the reproductive cycle and safeguarding regional prey abundance during the non-breeding season.
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An assessment of the impact of predation by Cape fur seals Arctocephalus pusillus pusillus on seabirds at Dyer Island, South Africa
NISC Pty Ltd, 2013Co-Authors: Makhado, Azwianewi B, Robert J M Crawford, Waller, Lauren J., Underhill, Les GAbstract:At Dyer Island, South Africa, observations of predation of seabirds by Cape Fur Seals Arctocephalus pusillus pusillus were made during 2004 and 2006/07. It was estimated that seals killed about 7% of adult African Penguins Spheniscus demersus annually. This may have contributed to the penguin colony not increasing in spite of food becoming more available in its vicinity during an eastward shift of epipelagic fish resources off South Africa at the start of the twentyfirst century. Adult penguins were most susceptible to mortality in the breeding season as they returned to feed chicks in the evening. Seals also killed substantial numbers (3–9%) of Cape Cormorant Phalacrocorax capensis fledglings as they left the island, mostly in the morning. The losses of Cape Cormorant fledglings accounted for about 95% of the overall mortality of seabirds attributable to seals. Mortality inflicted by seals on adults of four species of cormorant was negligible and no predation by seals on gulls and terns was observed.Keywords: African Penguin, Cape Cormorant, Cape fur seal, Crowned Cormorant, predation mortality, White-breasted CormorantOSTRICH 2013, 84(3): 191–19
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an altered carrying capacity of the benguela upwelling ecosystem for african penguins Spheniscus demersus
Ices Journal of Marine Science, 2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). - ICES Journal of Marine Science, 64: 570-576.The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.
Lg Underhill - One of the best experts on this subject based on the ideXlab platform.
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A condition index for African penguin Spheniscus demersus chicks
'Japanische Hegel-Gesellschaft', 2014Co-Authors: Lubbe A, Lg Underhill, Lj Waller, Veen JAbstract: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.Keywords: African penguin chick, body condition, food availability, quantile regressionAfrican Journal of Marine Science 2014, 36(2): 143–15
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Surviving off junk: low-energy prey dominates the diet of African penguins Spheniscus demersus at Mercury Island, Namibia, between 1996 and 2009
'Japanische Hegel-Gesellschaft', 2010Co-Authors: Ludynia K, Roux J-p, Kemper J, Jones R, Lg UnderhillAbstract:The diet of African penguins Spheniscus demersus in Namibia consisted mainly of sardine Sardinops 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.Keywords: African penguin, bearded (pelagic) goby, Benguela upwelling system, diet, Mercury Island, prey quality, Spheniscus demersus, Sufflogobius bibarbatusAfrican Journal of Marine Science 2010, 32(3): 563–57
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Breeding and moult phenology of African penguins Spheniscus demersus at Dassen Island
'Japanische Hegel-Gesellschaft', 2009Co-Authors: A C Wolfaardt, Lg Underhill, Visagie JAbstract:The breeding and moult phenology of African penguins Spheniscus demersus at Dassen Island form part of a variable annual cycle. Between 1994 and 2006, African penguins bred throughout the year. Most of this period was characterised by favourable feeding conditions. There were definite peaks in breeding activity, but these varied between years and between colonies on the island, with no obvious seasonal pattern. Synchronisation of breeding activities was greater in colonies with higher nest densities, suggesting that breeding phenology was influenced more by social interaction between individuals than by a wider-scale seasonal effect. Moult phenology of African penguins at Dassen Island was more synchronous and seasonal than breeding activities. Most penguins moulted between September and January. A large proportion (30%) of moult records was of birds moulting in nest sites rather than at landing beaches along the shore. The incidence of nest moulting was greatest from August to October, when sea conditions were generally rougher. The interval between successive moults ranged from 221 days to 546 days, with a mean of 349 days. The much greater degree of synchronisation in moult compared with breeding patterns suggests that moult, and not breeding, may represent the key driver of the annual cycle. Keywords: African penguin, annual cycle, breeding phenology, Dassen Island, moult phenology, Spheniscus demersusAfrican Journal of Marine Science 2009, 31(2): 119–13
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Comparison of moult phenology of African penguins Spheniscus demersus at Robben and Dassen islands
'Japanische Hegel-Gesellschaft', 2009Co-Authors: A C Wolfaardt, Lg Underhill, Crawford RjmAbstract:Dassen and Robben islands are approximately 50 km apart, and currently support the largest and third largest populations of African penguins Spheniscus demersus respectively. At both islands, moult is the most synchronised and seasonal activity of the annual cycle. The main difference in moult phenology between the two islands is the degree of synchronisation. The peak moult period at Dassen Island is broader (less synchronous) than at Robben Island. At both islands, juvenile penguins moulted less synchronously than adults. Juvenile moult was less synchronous at Dassen Island than at Robben Island. Interannual variation in moult phenology was similar at Robben and Dassen islands. Islandlevel moult phenology was more synchronous in the year following the Treasure spill than was the case after the Apollo Sea spill. This was attributed to the larger number and proportion of birds that were affected by the Treasure spill. The temporal duration of the disruption to moult phenology was also greater following the Treasure spill, especially at Robben Island. Moult counts provide a relatively reliable method of estimating the size of the adult population at Robben Island, but not at Dassen Island where the large number of birds moulting away from the shoreline results in the population size being significantly underestimated. Although the number of active nest sites continued to increase up until 2004 at both islands, the numbers of adult moulters counted declined by 55% between 2003 and 2005 at Robben Island, and by 50% between 2002 and 2005 at Dassen Island. The decrease in the number of moulters coincided with a decreased availability of food in the region. Keywords: African penguin, Dassen Island, moult phenology, population trend, Robben Island, Spheniscus demersusAfrican Journal of Marine Science 2009, 31(1): 19–2
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Review of the rescue, rehabilitation and restoration of oiled seabirds in South Africa, especially African penguins Spheniscus demersus and Cape gannets Morus capensis, 1983–2005
'Japanische Hegel-Gesellschaft', 2009Co-Authors: A C Wolfaardt, A J Williams, Lg Underhill, Crawford Rjm, P A WhittingtonAbstract:South Africa is a global hotspot for oil pollution. The regional oiled seabird cleaning centre, the South African Foundation for the Conservation of Coastal Birds (SANCCOB), has handled over 50 000 seabirds from its inception in 1968 until 2005. The majority of seabirds oiled in South Africa are African penguins Spheniscus demersus, followed by Cape gannets Morus capensis, both of which are classified as Vulnerable to extinction. On the basis of the proportion of the population that has been affected, the African penguin is considered to have suffered more from oiling than any other seabird species globally. The rehabilitation success (proportion of birds known to have survived for at least one month in the wild) and restoration success (proportion of rehabilitated birds attempting to breed) of de-oiling penguins and gannets are higher than has been reported for any other species. The financial costs of de-oiling African penguins are substantially lower than the costs of de-oiling seabirds in the Northern Hemisphere. De-oiling contaminated birds is thus a valuable conservation intervention for these species, both of which are relatively localised in areas within or close to major shipping routes and ports, where a single spill can threaten a large proportion of the global population. There are, however, long-term effects of oiling on penguins and gannets. De-oiled gannets survive slightly less well than un-oiled birds, but the difference is similar to inter-colony differences in survival. Approximately 27% of rehabilitated African penguins are unable to breed following their release. In addition, oiling has a long-term negative impact on the breeding productivity and cost of reproduction in de-oiled birds. The primary objective should therefore be to prevent or reduce oil spills in the first place. However, future oil spills are inevitable and the authorities need to ensure that they have plans in place and the required capacity to respond rapidly to spills when they do occur. One of the ways to reduce the number of penguins becoming oiled during a spill is to evacuate birds from the affected area. The continued capture and cleaning of penguins and gannets that do become oiled is justified on conservation grounds. Thus, de-oiling should be a twin objective to prevention in South Africa’s oil spill management strategy, and every effort should be made to further improve both of these aspects. Keywords: African penguin; Cape gannet; Dassen Island; de-oiled; Morus capensis; oil spills; rehabilitation; rescue; restoration; SANCCOB; Spheniscus demersusAfrican Journal of Marine Science 2009, 31(1): 31–5
Bruce M Dyer - One of the best experts on this subject based on the ideXlab platform.
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influence of local and regional prey availability on breeding performance of african penguins Spheniscus demersus
Marine Ecology Progress Series, 2013Co-Authors: Richard B Sherley, Les G Underhill, Robert J M Crawford, Bruce M Dyer, Pj Barham, Barbara J Barham, Janet C Coetzee, Mario T Leshoro, L UpfoldAbstract:Population trends of African penguins Spheniscus demersus in the Western Cape, South Africa, and their breeding success have been linked to the abundance of their main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. During the late 1990s and early 2000s, both fish species increased markedly in abundance, but after 2004, sardine biomass decreased to below average levels. In addition, adults of both stocks were principally located to the east of Cape Agulhas from 2001 to 2009 and were thus distant from seabird colonies on South Africa's West Coast. The number of African penguin pairs counted at Robben Island from 2001 to 2009 and the fledging period of chicks from successful nests increased and decreased in apparent response to the biomass of sardine prior to each breeding season, possibly linked through adult condition at the onset of breeding. Breeding success and chick-fledging rates increased during the study period and showed positive relationships with local food availability, indexed through the annual industrial catch of anchovy made within 56 km (30 nautical miles) of the colony. In addi- tion, chick-fledging rates were depressed in 2-chick broods during years when anchovy con- tributed <75% by mass to the diet of breeding birds. Previously reported relationships between the overall abundance of forage fish in South Africa and penguin breeding success were not sup- ported. Taken together, these results highlight the combined importance of ensuring adequate local food availability for seabirds during the reproductive cycle and safeguarding regional prey abundance during the non-breeding season.
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an altered carrying capacity of the benguela upwelling ecosystem for african penguins Spheniscus demersus
Ices Journal of Marine Science, 2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). - ICES Journal of Marine Science, 64: 570-576.The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.
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an altered carrying capacity of the benguela upwelling ecosystem for african penguins Spheniscus demersus
Ices Journal of Marine Science, 2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). - ICES Journal of Marine Science, 64: 570-576.The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.
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An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus)
2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5–3.0 million adult birds in the 1920s to just 10–20 % of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruit-ment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability
L Upfold - One of the best experts on this subject based on the ideXlab platform.
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influence of local and regional prey availability on breeding performance of african penguins Spheniscus demersus
Marine Ecology Progress Series, 2013Co-Authors: Richard B Sherley, Les G Underhill, Robert J M Crawford, Bruce M Dyer, Pj Barham, Barbara J Barham, Janet C Coetzee, Mario T Leshoro, L UpfoldAbstract:Population trends of African penguins Spheniscus demersus in the Western Cape, South Africa, and their breeding success have been linked to the abundance of their main prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. During the late 1990s and early 2000s, both fish species increased markedly in abundance, but after 2004, sardine biomass decreased to below average levels. In addition, adults of both stocks were principally located to the east of Cape Agulhas from 2001 to 2009 and were thus distant from seabird colonies on South Africa's West Coast. The number of African penguin pairs counted at Robben Island from 2001 to 2009 and the fledging period of chicks from successful nests increased and decreased in apparent response to the biomass of sardine prior to each breeding season, possibly linked through adult condition at the onset of breeding. Breeding success and chick-fledging rates increased during the study period and showed positive relationships with local food availability, indexed through the annual industrial catch of anchovy made within 56 km (30 nautical miles) of the colony. In addi- tion, chick-fledging rates were depressed in 2-chick broods during years when anchovy con- tributed <75% by mass to the diet of breeding birds. Previously reported relationships between the overall abundance of forage fish in South Africa and penguin breeding success were not sup- ported. Taken together, these results highlight the combined importance of ensuring adequate local food availability for seabirds during the reproductive cycle and safeguarding regional prey abundance during the non-breeding season.
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an altered carrying capacity of the benguela upwelling ecosystem for african penguins Spheniscus demersus
Ices Journal of Marine Science, 2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). - ICES Journal of Marine Science, 64: 570-576.The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.
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an altered carrying capacity of the benguela upwelling ecosystem for african penguins Spheniscus demersus
Ices Journal of Marine Science, 2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:Crawford, R. J. M., Underhill, L. G., Upfold, L., and Dyer, B. M. 2007. An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus). - ICES Journal of Marine Science, 64: 570-576.The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5-3.0 million adult birds in the 1920s to just 10-20% of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruitment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability.
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An altered carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus)
2007Co-Authors: Robert J M Crawford, Les G Underhill, L Upfold, Bruce M DyerAbstract:The carrying capacity of an ecosystem for a species is an input parameter that is required for some models that assess population viability. It may be changed by an altered structure or functioning of the ecosystem, e.g. as a consequence of changes brought about by fishing or environmental change. Hence, it cannot necessarily be assumed that the pristine level of abundance of a species reflects the present-day carrying capacity of the ecosystem for that species. Historical and modern information on abundance and density-dependent responses is used to investigate changes in the carrying capacity of the Benguela upwelling ecosystem for African penguins (Spheniscus demersus), a species categorized as vulnerable. The carrying capacity was estimated to decrease from 1.5–3.0 million adult birds in the 1920s to just 10–20 % of this value from 1978 to 2006, as a result inter alia of increased competition for food with purse-seine fisheries and fur seals (Arctocephalus pusillus pusillus). From 1988/1989 to 2005/2006, the per capita recruit-ment of young penguins to a colony where nesting space was not limiting was inversely related to the size of that colony, suggestive of a density-dependent response perhaps related to food availability
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initial impact of the treasure oil spill on seabirds off western south africa
African Journal of Marine Science, 2000Co-Authors: R J M Crawford, Les G Underhill, R M Randall, L Upfold, Tm Leshoro, S A Davis, R T Harding, L F Jackson, Michael A Meyer, A P Van DalsenAbstract:On 23 June 2000, the bulk ore carrier MV Treasure sank off western South Africa between Dassen and Robben islands, which individually currently support the largest and 3rd largest colonies of African penguins Spheniscus demersus. Subsequently, more than 19 000 penguins were oiled, almost twice the previous highest number of seabirds oiled during a single event in southern Africa (10 000 penguins after the sinking of the Apollo Sea in June 1994). About 19 000 oiled penguins were collected for cleaning and care and about 150 oiled adults died in the wild. Some 19 500 unoiled penguins were caught at Dassen and Robben islands and relocated to Port Elizabeth, 800 km to the east, to remove them from waters affected by the oil. Of all penguins caught, which amounted to 20% of the total species population, less than 2 000 died within the first month, considerably less than in the Apollo Sea spill. This can be attributed to improved transport of penguins and the rapid arrival at rescue centres of experts able to a...
