Browsing by Author "Crawford, RJM"
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- ItemOpen AccessBreeding production of Cape gannets Morus capensis at Malgas Island, 2002-03(2006) Staverees, Linda; Underhill, Les; Crawford, RJMIncludes bibliographical references.
- ItemOpen AccessCompetition for anchovy (Engraulis capensis) and sardine (Sardinops sagax) between the Cape Gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and the purse-seine fishery on the west coast of South Africa(2002) Pillay, Pavitray; Field, John G; Crawford, RJM; Oosthuizen, WHCompetition for pelagic fish resources was investigated by assessing the overlap in the food base of three land-based predators: Cape gannet (Morus capensis), Cape fur seal (Arctocephalus pusillus pusillus) and humans operating via a purse-seine fishery. Multivariate analysis the diet composition of gannets and seals and the catch composition of the pelagic fishery indicated that there were three ""feeding regimes"" during the 21-year study period: (1978-1998), an anchovy-dominated regime (1978-1983), an intermediate regime (1984-1990) and a sardine-abundant regime (1991-1998). It further showed that anchovy (Engraulis capensis) and sardine (Sardinops sagax) contributed substantially to the catches of all three top predators. Analysis of size-distributions of catches showed that the three predators generally caught the same sizes of anchovy and sardine, reflecting very little resource partitioning. The analysis demonstrated stronger competition between gannets and the purse-seiners than between gannets and seats, especially during seasons of poor recruitment. Furthermore, seasonal differences in the catches of the three predators are related to the recruitment, growth and migration of the prey species. The study confirmed previous observations that gannets prefer sardine to anchovy by showing gannet exploitation of sardine in the early 19905, when anchovy was still abundant. The gannets, which are species-specific feeders, may be impacted by substantial removals of pelagic fish resources by a large seal population and a large fishery.
- ItemOpen AccessThe diet of the Cape fur seal Arctocephalus pusillus pusillus in Namibia : variability and fishery interactions(2005) Mecenero, Silvia; Underhill, Les; Roux, Jean-Paul; Crawford, RJM; Bester, MNIncludes bibliographical references.
- ItemOpen AccessInvestigation of the impact of fur seals on the conversation status of seabirds at islands off South Africa and the the Prince Edward Islands(2009) Makhado, Azwianewi Benedict; Underhill, Les; Crawford, RJM; Bester, MNThis research project investigated the impact of predation by the Cape fur seals Arctocephalus pusillus pusillus on the conservation status of seabirds at islands off the coast of South Africa and at the Prince Edward Islands. At Malgas Island, the impact of predation was investigated over three breeding seasons (2000/01, 2003/04 and 2005/06), and at Dyer Island over two breeding season (2004 and 2006/07). At Marion Island (Prince Edwards Islands) and at other South African islands, such as Dassen Island, Bird Island at Lambertâs Bay and Robben Island, observations were collected opportunistically. Cape fur seals were estimated to have killed some 6 000 Cape gannet Morus capensis fledglings around Malgas Island in the 2000/01 breeding season, 11 000 in 2003/04 and 10 000 in 2005/06. This amounted to an estimated 29%, 83% and 57% of the overall production of fledglings at the island in these breeding seasons respectively. Preliminary modelling suggests this predation is not sustainable. There was a 25% reduction in the size of the colony, the second largest of only six extant Cape gannet colonies, between 2001/02 and 2005/06. At Dyer Island, it was estimated that seals killed up to 7% of African penguin Spheniscus demersus adults annually. The present mortality attributable to seals is considered unsustainable. Seals also killed a substantial proportion, 4â8%, of Cape cormorant Phalacrocorax capensis fledglings as they left the island. Although this level of mortality may be sustainable for Cape cormorants, this species have also been affected by disease. iv Predation on seabirds by Cape fur seals was demonstrated to occur at several other localities in southern Africa but observations were insufficient to enable the effect of such predation to be quantified. Several hundred Cape gannet fledglings were killed annually at Bird Island (Lambertâs Bay) and threatened penguins and cormorants were attacked at other localities. At subantarctic Marion Island, Antarctic fur seals A. gazella fed on king Aptenodytes patagonicus and macaroni Eudyptes chrysolophus penguins. However, predation there on seabirds by subantarctic fur seals A. tropicalis has not yet been observed. King penguins were mostly killed in winter and macaroni penguins (which are absent from the island in winter) in summer. Predation of seabirds by seals has recently increased in southern Africa and may be doing so at Marion Island. In both regions fur seal populations are expanding. The influences of environmental factors on the rate of predation were investigated at both Malgas and Dyer Islands. Of environmental factors considered, time of the day had the most important influence on predation by Cape fur seals on fledglings of Cape gannets and Cape cormorants and adult African penguins. Fledglings of gannets and cormorants were mainly killed between mid morning and late afternoon, coinciding with the time they left the islands and were in the water. Most adult African penguins were killed as they returned in the evening from foraging at sea. Wind speed and direction, sea state and tide had a variable and lesser influence on predation rates. Although these variables might be used to interpolate predation rates through periods when observations are not conducted, their contribution is limited and there will remain considerable uncertainty in the estimate of actual numbers of fledglings killed. Uncertainty will best be decreased by extending the period of observations so as to reduce the amount of days for which predation rates are interpolated. v In six years between 1999 and 2008, 141 Cape fur seals that were feeding on, or thought to be feeding on, Cape gannet fledglings as they left Malgas Island, South Africa, were shot and collected. Examination of the stomachs of 93 of these seals showed that Cape gannets contributed an average of 70% by mass to the diet, and known prey items of Cape gannets, which may have been obtained from the alimentary tract of the fledglings, a further 5%. Hence, when the seals were culled, they were subsisting mainly on the fledglings. Other prey items of the seals included rock lobster Jasus lalandii and common octopus Octopus vulgaris. All the collected seals were bulls. Their ages were estimated from measures of their total length and previously published information on ages at length. All were 10 years old or less, and their mean age was 4.5 years. The hunting behaviour of Cape fur seals feeding on seabirds at Malgas and Dyer Islands was investigated in 2003/04 and 2005/06; and 2004 and 2006/07 respectively. At these islands, attacks on seabirds were identified mainly through the presence of other birds overhead and sometimes by the thrashing of a victim in, or throwing of it from, the water. Most attacks occurred beyond the surf zone at distances of 20â100 m from the island. Seals hunted in groups or solitarily and usually attacked birds from underneath or behind. Usually most, or a substantial portion of the carcass was utilized but some surplus killing was observed. On average, attacks lasted 11 min for both Cape gannets and Cape cormorants and 16 min for African penguins. Due to the increase in seal predation, interventive management such as culling individual fur seals seen preying on seabirds was implemented at Malgas Island. It was anticipated that removal of the small number of individual seals which target seabirds would reduce vi mortality on the seabirds because this is a learned behaviour. In the 2006â07 breeding season of Cape gannets at Malgas Island, the removal of 61 Cape fur seals that preyed on gannet fledglings when they left to sea significantly reduced the mortality rate of these fledglings in the short term. However, because seals learned to avoid the boat used for their removal, it was not possible to remove all the seals that killed gannet fledglings and some mortality continued. There was a decrease over time in both the maximum and the mean age of seals culled. Sustained removal of these animals may reduce this feeding behaviour.
- ItemOpen AccessStatistical relationships between palagic fish catches and long-term series of environmental conditions in the southern Benguela region(1991) Villacastin-Herrero, Carlos A; Field, John G; Crawford, RJMThree different techniques are used to established statistical relationships between annual pelagic fish species catches viz. pilchard, horse mackerel and chub mackerel, and monthly environmental indices. The three techniques are Spearman's Rank correlation, multiple regression and cross-correlations of the Box-Jenkins time series approach. The first method yielded interesting results but only in terms of the nature of the relationship not in terms of the response effect. In the same way the multiple regression analysis showed inherent problems in the interpretation of the results due to serial correlation in all fish catches and environmental data series. The Time-Series analysis yielded more coherent results, presumably due to the fact that the data series were pre- whitened to remove serial correlation. Monthly means of sea surface temperature, north-south wind component, west- east wind component and sea level were used as environmental indices. Over the period 1950 to 1985, annual pilchard catches are found to be negatively correlated to sea surface temperature in the Namaqualand and Agulhas Bank areas, but are positively related to sea surface temperature in the south western Cape area. With regard to wind components, northerly and westerly winds in the Namaqualand and south western Cape areas are found to be significantly correlated to annual pilchard catch, whereas in the Agulhas Bank southeasterly - and southwesterly wind are found to be predominant. Warmer waters improved horse mackerel catches in all areas, immediately in the Agulhas Bank area but affecting catch the following year in the other two areas. In the Namaqualand area predominantly northwesterly winds are found to favour horse mackerel catch immediately and this response is probably due to availability. The same wind orientation is found in the south western Cape area but with one-year-lag. In the Agulhas Bank area the favourable wind orientation is southerly with a zero lag and westerly with one-year-lag. Cooler waters influence chub mackerel catches favourably in all areas and in all seasons. In the Namaqualand area the effect of the sea surface temperature is immediate in all but the winter season. Northwesterly winds are most important in this area having an immediate effect from winter to summer. Cooler temperatures in the south western Cape area the previous year improve the annual chub mackerel catch. Northerly and easterly winds favour catch immediately during summer while northwesterly winds are most important during spring. In the south coast area, southwesterly winds improve catches immediately during spring and summer but south-westerly winds during autumn are related to improved catches the following year. These results are not easy to interpret because of problems in the quality of the catch data. These problems may be identified as constraints due to the effect of the different age-classes to the catch and the variability in the effort exerted by the fishery onto the resource. Furthermore, age-length distributions have changed recently for all three species, so that it is difficult to establish relationships which link up to specific effects of the environment on specific age- classes. It is assumed, however, that a zero-lag in the relationships between fish catches and environmental variables represents availability rather than environmental effects on spawning, recruitment or stock abundance.
- ItemOpen AccessSurvival and movements of African Penguins, especially after oiling(2002) Whittington, Phil, 1958-; Underhill, Les; Crawford, RJMBibliography: p. 273-286.