Estimating the abundance of common dolphins on the southern coast of South Africa


Show simple item record Best, Peter B Meyer, Michael A de Decker, Michelle Müller, Andrea Sekiguchi, Keiko 2016-02-26T13:31:03Z 2016-02-26T13:31:03Z 2009
dc.identifier.citation Best, Peter B; Meyer, Michael A; de Decker, Michelle; Müller, Andrea; Sekiguchi, Keiko. (2009). Estimating the abundance of common dolphins on the southern coast of South Africa,IWC Paper SC/61/SM33. en_ZA
dc.description.abstract Sightings made on an aerial survey in December 1982 and on a ship-based survey in January/February 1983 have been used to assess the size of the population of common dolphins (Delphinus capensis) occurring over the continental shelf south of South Africa. Thirteen sightings (12 primary) were made in 2,445.7 n. miles flown on the aerial survey and 10 sightings (6 primary) in 1,772.2 n. miles steamed on the ship-based survey. Sightings and effort in both surveys have been stratified by water depth (0-100 m, 100- 200 m) and geographical region (west coast, south coast). Because of difficulties in accurately estimating the size of schools in this highly gregarious species, numbers of individuals were counted in composite aerial photographs taken of the school. Radial distance and angle estimates to sightings from the ship were smeared to allow for estimation errors. Assuming g(0) = 1.0, both data sets resulted in roughly similar estimates of the number of schools (52-58 for aerial, 40-59 for ship-based across a range of sensitivity tests), but mean school size estimates differed significantly (454 SE 90 for aerial, 159 SE 27 for ship-based). As the aerial estimates were based on counts of animals in composite vertical photographs, they are considered more reliable than the ship-based estimates that were made from a lower vantage point and at a greater angle. Given the small number of primary sightings on each survey, it was considered preferable to produce a combined estimate using school density estimates from both surveys weighted by their inverse variances but applying the mean school size from the aircraft. The resultant population estimate of 49 schools (CV = 0.29) and 22200 individuals (CV = 0.35) is discussed in relation to known or estimated incidental mortalities in South African waters. en_ZA
dc.language eng en_ZA
dc.title Estimating the abundance of common dolphins on the southern coast of South Africa en_ZA
dc.type Working Paper en_ZA 2016-02-26T11:03:19Z
uct.type.publication Research en_ZA
uct.type.resource Research paper en_ZA
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Science en_ZA
dc.publisher.department Marine Resource Assessment and Management Group en_ZA
uct.type.filetype Text
uct.type.filetype Image

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