Browsing by Author "Williams, A J"

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    Breeding success of African penguins Spheniscus demersus at Dassen Island, especially after oiling following the Apollo Sea spill
    (2008) Wolfaardt, A C; Underhill, L G; Nel, D C; Williams, A J; Visagie, J
    The reproductive success of African penguins Spheniscus demersus at Dassen Island from 1994 to 2000 was variable, but much higher than previously reported figures for the species. Breeding success was positively related to the abundance of anchovy Engraulis encrasicolus and sardine Sardinops sagax, and the high reproductive output during the study was attributed to the large biomass and high availability of these two species for much of the study period. De-oiled penguins from the Apollo Sea spill had a slightly lower hatching success but a significantly lower overall breeding success than that of un-oiled birds, driven mainly by the reduced number of chicks which fledged. Nests with two de-oiled Apollo Sea parents were less successful than nests with one de-oiled bird. There was increased mortality of chicks 40 days and older in nests with de-oiled birds. Chicks from nests with one de-oiled Apollo Sea parent grew at a similar rate to chicks from nests with no de-oiled parents. However, chicks from nests that comprised two de-oiled Apollo Sea birds had significantly slower growth rates than these other two groups. Breeding success and chick growth at nests with de-oiled birds were more negatively impacted when feeding conditions were less favourable. These results suggest that one of the main reasons for lower breeding success in de-oiled birds was their reduced ability to provision chicks, especially during the period in which the energy demands of the chicks is greatest. The rate of mate fidelity was lower in de-oiled birds than un-oiled birds, and there was a positive association between mate fidelity and breeding success.
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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.