Browsing by Author "Müller, Andrea"
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- ItemOpen AccessApplication of a photo-identification based assessment model to southern right whales in South African waters(2012) Brandão, Anabela; Butterworth, Doug S; Müller, Andrea; Best, Peter BThe three-mature-stages (receptive, calving and resting) model of Cooke et al. (2003) is applied to photo-identification data available from 1979 to 2010 for southern right whales in South African waters. The 2010 number of females having reached the age at first parturition is estimated to be 1 205, the total population (including males and calves) 4 725, and the annual population growth rate 6.8%. The probability (average 11%) that a resting mature whale rests for a further year appears to vary annually, whereas the probability (7%) that a receptive whale rest (or aborts) rather than calves the next year appears to be constant. Information from resightings of grey blazed calves as adults with calves allows estimation of first year survival rate of 0.914 (compared to a subsequent annual rate of 0.987, and an age at 50% maturity of 6.4 years. However this suggests also that 27% (s.e. 6%) of grey blazed calves lose their marking before becoming adults. In contrast, the relative proportions of grey blazed animals amongst calves and amongst calving adults suggest rather a value of 10% (s.e. 8%). If the proportion losing markings is in fact 10%, first year survival rates estimate drops to 0.837 and the population growth rate to 6.4% pa.
- ItemOpen AccessBayesian assessments of the southern hemisphere humpback whale breeding stock B using three different models for stock-structure(2010) Müller, Andrea; Butterworth, Doug S; Johnston, Susan JThree different models are considered for Southern Hemisphere humpback breeding stock B: Model 1, a single fully-mixed stock; Model 2, reproductively independent stocks B1 off Gabon and B2 of the southern African coastline; and Model 3 with two stocks as in Model 2 but with B1 consisting of two sub-stocks, one of which migrates to Gabon along the southern African coastline. The models are fitted to mark-recapture data (both photo-ID and genetic) available for the Gabon and South African regions. The purpose of the paper is one of illustrating the stock-structure models and associated methods of analysis, anticipating that further discussion and selection of specific model input assumptions will take place during the Scientific Committee meeting.
- 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 AccessHumpback whales, rock lobsters and mathematics : exploration of assessment models incorporating stock-structure(2011) Müller, Andrea; Butterworth, Doug SThis thesis presents four marine resource assessments; three concern the Southern Hemisphere (SH) humpback whale (Megaptera novaeangliae) and one the South African east coast rock lobster Palinurus delagoae. It also sets out the statistical background to the methodology employed in the assessments, including an outline of the Bayesian approach, Bayes' theorem, and the sampling-importance re-sampling (SIR) as well as the Markov Chain Monte Carlo (MCMC) methods.
- ItemOpen AccessIs the length distribution of C1 and C3 humpback whale catches consistent with an age-structured version of the resident model(2009) Müller, Andrea; Butterworth, Doug S; Johnston, Susan JA sex- and age-structured BALEEN II population model is fitted to population abundance and trend as well as photo-id capture-recapture data for the C1 and C3 humpback whale breeding sub-stocks. The model is of the “Resident” type, i.e. no interchange between breeding grounds, though the whales do mix on the feeding grounds. Uniform selectivity on the 1+ population is assumed for both regions. A particular aim is to address the question of whether length distribution differences between the two regions are a reflection of different levels of past exploitation. Comparison with length distribution data for both regions does however indicate a greater proportion of larger males than anticipated in the C3 catches, and the reverse effect for both males and females in the C1 catches.
- ItemOpen AccessIs the length distribution of C1 and C3 humpback whale catches consistent with an age-structured version of the resident model(2009) Müller, Andrea; Butterworth, Doug S; Johnston, Susan JA sex- and age-structured BALEEN II population model is fitted to population abundance and trend as well as photo-id capture-recapture data for the C1 and C3 humpback whale breeding sub-stocks. The model is of the “Resident” type, i.e. no interchange between breeding grounds, though the whales do mix on the feeding grounds. Uniform selectivity on the 1+ population is assumed for both regions. A particular aim is to address the question of whether length distribution differences between the two regions are a reflection of different levels of past exploitation. Comparison with length distribution data for both regions does however indicate a greater proportion of larger males than anticipated in the C3 catches, and the reverse effect for both males and females in the C1 catches.
- ItemOpen AccessPreliminary results for a combined assessment of all seven southern hemisphere humpback whale breeding stocks(2010) Müller, Andrea; Butterworth, Doug S; Johnston, Susan JThis paper reports preliminary results from the development of an assessment process that aims to include all seven Southern Hemisphere humpback whale breeding stocks in a single joint assessment, with the purpose of allowing highlatitude historic catches (i.e. catches taken south of 40˚S, where mixing amongst the populations occurs), to be allocated to breeding stocks in proportion to abundance, rather than on set ratios. The approach can be broadened to allow for uncertainties in the placement of the boundaries assumed to link high latitude catches to breeding stocks. Because of the interaction between populations arising from the procedure to allocate high latitude catches amongst breeding stocks, the conventional SIR-based Bayesian approach proved impractical to expand. Instead uniform priors on the various preexploitation level (K) parameters were assumed with the intent to later iteratively adjust these to account for their being informative about the values of the intrinsic growth rate (r) parameters. Initial results (which will need later refinement) are presented purely for the purposes of illustrating the application of the approach.
- ItemOpen AccessPreliminary results for a photo-identification-based assessment of southern right whales in South African waters(2011) Butterworth, Doug S; Brandão, Anabela; Müller, Andrea; Best, Peter BThis working paper presents preliminary results of a photo-id based assessment of southern right whales in South African waters using the three-mature-stages (receptive, calving, resting) model of Cooke et al. (2003). The application of the approach is near-identical to that of Cooke et al. (2003), except that here the starting population is not assumed to reflect a steady age-structure corresponding to the Leslie matrix model describing the population dynamics. The photo-id data for grey-blazed female calves, which are identifiable when giving birth themselves, are used to link the dynamics of the mature females with the output of their reproduction by allowing for estimation of parameters for first-year mortality and the maturity ogive.
- ItemRestrictedPreliminary results for a photo-identification-based assessment of southern right whales in South African waters(University of Cape Town, 2011) Butterworth, Doug S; Brandão, Anabela; Müller, Andrea; Best, Peter BThis working paper presents preliminary results of a photo-id based assessment of southern right whales in South African waters using the three-mature-stages (receptive, calving, resting) model of Cooke et al. (2003). The application of the approach is near-identical to that of Cooke et al. (2003), except that here the starting population is not assumed to reflect a steady age-structure corresponding to the Leslie matrix model describing the population dynamics. The photo-id data for grey-blazed female calves, which are identifiable when giving birth themselves, are used to link the dynamics of the mature females with the output of their reproduction by allowing for estimation of parameters for first-year mortality and the maturity ogive.
- ItemOpen AccessPreliminary results from an assessment of the South African P. delagoae rock lobster resource to investigate the recovery of the resource between two periods of experimental trap-fishing(2010) Müller, Andrea; Butterworth, Doug S; Johnston, Susan JData from two trap-fishing experiments carried out on the east coast of South Africa targeting the P.delagoae rock lobster, along with trawl-catch information for the years 1985-2009, are used to develop a population assessment for this species. The assessment aims to investigate the extent, if any, of the recovery of the rock lobster between the two periods of fishing, as well as assess the current stock level and potential future sustainable catch. The model is an age-structured model and includes ageto-length conversions in order to assess the fit of the model-predicted catches to length data available from the trap experiments.
- ItemOpen AccessPrior incoherence within a Bayesian assessment of the Southern Hemisphere humpback whale breeding stock B population(2010) Müller, Andrea; Butterworth, Doug SIn very simple terms, a Bayesian analysis involves drawing estimatable parameter values from some prior distribution, computing population dynamics and assigning a likelihood value to each combination based on comparisons to data containing information on population size and/or trend. A posterior distribution may then be constructed and conclusions drawn about the parameter estimates. In Model Ia (see Appendix) r B1 , r B2 , ( ) 1 arg ~ ln B Nt , ( ) 2 arg ~ ln B Nt are the parameter values drawn from priors for the intrinsic growth rate and the log of the recent abundance for the two populations under consideration.