Browsing by Author "Crawford, R J M"

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    Are Cape Cormorants (Phalacrocorax capensis) losing the competition? Dietary overlap with commercial fisheries
    (2018) Masiko, Oyena B; Makhado, A B; Crawford, R J M; Ryan, P G
    Characterisation of the diet of pelagic feeders can be regarded as key to development of ecosystem-based management plans, conservation of predators as well as understanding of ecological and trophic interactions. Therefore, long term studies on dietary changes provide insights into the nature of competition and overlap between seabirds and fisheries. The Cape Cormorant Phalacrocorax capensis breeds in the Benguela upwelling region of southern Africa. Its population has decreased by over 50% in the three most recent generations resulting in the species being listed as Endangered in the IUCN Red Data List. Its decline has previously been postulated to result from reduced prey availability. In this thesis, I examined and reported on the spatio-temporal distribution of Cape cormorant diet obtained from different colonies over two decades (1988 – 1997 and 1998 – 2007). I further investigated the extent of overlap between the diet composition and sizes of fish eaten by Cape Cormorants and those caught by the purse-seine fishery. Epipelagic fish (Anchovy, Sardine, Horse Mackerel and Redeye) dominated the diet of Cape cormorants at all sites. Anchovy was the most important in the diet in all years except in 1992 and 1993, when Sardine dominated the diet, and in 2007 when ‘other’ fish species (mainly Cape Silverside Atherina breviceps and Southern Mullet Chelon richardsonii) became the most frequently eaten fish. There were decadal variations in the relative numerical abundance of different prey species. Over both decades investigated, the diet was largely dominated by Anchovy and Sardine, with Horse Mackerel contributing more in the second than first decade. There was, however, an increase in the contribution of Anchovy relative to Sardine from the first decade to the second. At all colonies, Cape Cormorants mostly preyed on Anchovies of sizes between 5 and 11cm. Sardines eaten were larger than Anchovies (mostly 11 – 23 cm), with a bimodal distribution in the first decade. However, the few Sardine eaten in the second decade were smaller (4 – 7 cm) perhaps reflecting the length of fish available. There was an overlap in the distribution of fish sizes caught by the fishery and those found in the diet of Cape Cormorants especially in the first decade. However, increased sampling of Cape Cormorant diet is required to more fully understand the extent of competition and overlap with fisheries.
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    Biological survey of the Prince Edward Islands, December 2008
    (2009) Cooper, J; Bester, M N; Chown, S L; Crawford, R J M; Daly, R; Heynse, E; Lamont, T; Ryan, P G; Shaw, J D
    A biological survey of the Prince Edward Islands took place in December 2008. The survey repeated an earlier survey of the populations of surface-nesting seabirds on both islands and of fur seals (Arctocephalus spp.) and alien plants on Prince Edward Island in December 2001. Observations on burrowing seabirds, macro-invertebrates and plant communities on Prince Edward Island and an oceanographic survey of surrounding waters were also included. The survey confirmed many of the observations made on the earlier survey and permitted an assessment of trends in the abundance and distribution of biota since 2001.
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    Impact of flipper-banding on breeding success of African penguins Spheniscus demersus at Robben Island: comparisons among silicone rubber bands, stainless-steel bands and no bands
    (2008) Barham, P J; Underhill, L G; Crawford, R J M; Leshoro, T M; Bolton, D A
    From 2001 to 2006, two new designs of flipper bands made from silicone rubbers were tested on African penguins Spheniscus demersus at 365 nests on Robben Island, South Africa. We compared, over six years, the breeding success, from hatching to fledging, of three different groups of penguins: those with rubber bands (117 nests), with conventional stainless-steel bands (103 nests) and without bands (145 nests). There were no significant differences in breeding success between the three groups, suggesting that neither the currently used steel bands, nor either of the new rubber-band designs, were harmful during the seasons investigated. The rubber bands caused less wear of feathers and less drag on a model penguin than the steel bands. In captivity, the behaviours of African penguins fitted with rubber bands were not noticeably different to those of unbanded birds.
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    Implications of recruitment, distribution and availability of stocks for management of South Africa's Western Cape purse-seine fishery
    (1979) Crawford, R J M; Field, John G; Newman , G G
    South Africa's Western Cape purse-seine fishery is situated off the country's western seaboard in the highly productive waters of the southern Benguela Current system (Cushing 1969). Management of the multi-species resource is the responsibility of the government's Department of Industries and since 1950 a large volume of data relating to performances of contributing species has been collected. Recent analyses have indicated an oversubscription of effort, a sequential depletion of the more valuable stocks and, consequently, a present reliance upon less favourable species (Newman and Crawford 1979, Crawford et al. in press). These adverse trends have been precipitated, in part at least, by ineffective legislation, which has resulted from a past failure to appreciate that exceptionally good year classes occur infrequently (Newman and Crawford in- press). Optimal ways of harvesting strong cohorts, or of avoiding overexploitation in the event of recruitment failure, remain to be explored.
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    Mass on arrival of rockhopper penguins at Marion Island correlated with breeding success
    (2008) Crawford, R J M; Makhado, A B; Upfold, L; Dyer, B M
    For rockhopper penguins Eudyptes chrysocome at Marion Island, there were significant decreases over time in the numbers breeding and breeding success at three monitored colonies from 1985/1986 to 2006/2007, and in mass on arrival for breeding of both males and females from 1994/1995 to 2007/2008. Breeding success decreased by 0.15 chicks pair−1 y−1 over 22 years and was significantly correlated with mass on arrival of males and females. Survival of chicks at the guard stage was low in the late 1990s; hatching success decreased in the early 2000s. It is thought that an increasingly poor parental condition caused birds to abandon breeding at a progressively early stage. Parental condition is influenced by feeding opportunities at overwintering grounds, which have probably been altered by global climate.
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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.