Browsing by Author "de Decker, Michelle"
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- ItemOpen AccessEstimating the abundance of common dolphins on the southern coast of South Africa(2009) Best, Peter B; Meyer, Michael A; de Decker, Michelle; Müller, Andrea; Sekiguchi, KeikoSightings 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.
- ItemOpen AccessHake Data: problems, solutions and GLM CPUE sensitivity to alternate scenarios(2009) Fairweather, Tracey; Glazer, Jean; Leslie, Rob; de Decker, Michelle; Johnston, Susan; Butterworth, Doug SThe checks carried out on the hake offshore trawl catch and effort data, which provide the basis to split the catches by species and to obtain GLM-standardised CPUE indices of abundance by species, have focused on two separate steps. The first was a re-extract of the data from the original files to check for possible errors in the earlier extracts (this applies to the subset of the data for which size composition information can be unambiguously extracted). The second step was to increase the proportion of the data utilised (and at the same time to check for possible selection biases in the existing sample) by assigning size composition to drags for which this information was not directly available, on the basis of the average for other vessels for which this was available for the area and time at which the fishing concerned took place. The first step revealed a problem with extractions over the last few years which at times for certain vessels had linked the accumulated catch for the day with only the effort for the last trawl of the day, rather than for all trawls that day combined.