Browsing by Author "Petersen, S L"

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    Diving behaviour of African penguins: do they differ from other Spheniscus penguins?
    (National Inquiry Services Centre, 2007) Ryan, P G; Petersen, S L; Simeone, A; Grémillet, D
    African penguins Spheniscus demersus closely resemble Magellanic S. magellanicus and Humboldt S. humboldti penguins and have similar breeding and feeding ecologies. Adults feed on pelagic schooling fish in continental shelf waters, but African penguins have been reported to have shallower dive angles and remain submerged longer for dives to a given depth than their congeners. The few data for African penguins were gathered using relatively large time-depth recorders. We measured diving behaviour of 36 African penguins provisioning small chicks at three colonies near Cape Town, South Africa. Maximum and mean dive depths were 69m and 14m respectively. Diving took place mainly during the day. Although dive depths differed between colonies, there were no significant differences in dive duration or maximum, median or mean depth. Total dive duration, descent time, bottom time, ascent time and dive angle all were strongly correlated with the maximum depth attained. The diving behaviour of African penguins is similar to that of its congeners. Diving performance probably was compromised by the data-logger used in the previous study. Comparative data from Humboldt penguins also indicate potential biases in an earlier study of this species. Care is needed when comparing the diving performance of penguins measured using different loggers.
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    Populations of surface-nesting seabirds at Marion Island, 1994/5-2002/3
    (2003) Crawford, R J M; Cooper, J; Dyer, B M; Greyling, M D; Klages, N T W; Ryan, P G; Petersen, S L; Underhill, L G; Upfold, L; Wilkinson, W; De Villiers, M S; Du Plessis, S; Du Toit, M; Leshoro, T M; Makhado, A B; Mason, M S; Merkle, D; Tshingana, D; Ward, V L; Whittington, P A
    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.
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    Sightings of killer whales Orcinus orca from longline vessels in South African waters, and consideration of the regional conservation status
    (Taylor & Francis, 2009) Williams, A J; Petersen, S L; Goren, M; Watkins, B P
    Killer whales Orcinus orca are seldom reported from South African nearshore waters but, allowing for the bias of vessel attraction, observations from longline vessels suggest there is a resident offshore population of fish-eating killer whales. We present reports of killer whales made by observers on pelagic longline vessels fishing for tuna Thunnus spp., swordfish Xiphias gladius and sharks off South Africa, and on demersal longline vessels fishing for Patagonian toothfish Dissostichus eleginoides off the Subantarctic Prince Edward Islands. Off South Africa, observers reported 689 occurrences of killer whales during the period January 2002 to December 2006. Observations off South Africa peaked in January and were minimal in April–May. Most killer whale pods comprised 1–4 individuals and few were more than six. Observations were largely over the shelf edge between the Agulhas Bank and Port Elizabeth on the south-east coast of South Africa. In all, 1 843 line sets, using 3.8 million hooks, were monitored and killer whales occurred at an overall rate of 0.18 per 1 000 hooks. Killer whales depredated at an overall rate of 0.5% of the total catch. A small (maximum 12 individuals) population was recorded at longline vessels off the Prince Edward Islands, but observations were limited to August–September, October–November and May–June. We consider the conservation status of killer whales in southern African waters to be 'vulnerable', because the populations are very small and are subject to both short- and long-term impacts from the longline fisheries.