Browsing by Subject "mathematical models"

Now showing 1 - 4 of 4
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    The development of an operational management procedure for the South African west coast rock lobster fishery
    (1998) Johnston, Susan Joy; Butterworth, Doug S; Stewart, Theodor J
    This thesis considers the development of an operational management procedure (OMP) to provide scientific recommendations for commercial TAC for the South African west coast rock lobster (Jasus lalandii) fishery. This fishery has been under considerable stress in recent years as a result of overfishing and low somatic growth rates. Present catch levels, less than 2000 MT, are substantially smaller than levels recorded in the past. The present biomass (above 75mm carapace length) is estimated to be only six percent of the pristine level. At the start of this research, no long-term management strategy for the resource existed. Neither was there any robust, tested, scientific method available for setting the annual TAC for the fishery, which resulted in a time-consuming and unsatisfactory scientific debate each year in developing a series of ad hoc TAC recommendations. The work presented in this thesis is thus aimed at answering two important questions. i) Can an adequate mathematical model be developed as a basis to simulate the resource and its associated fishery? ii) Can a self-correcting robust OMP be developed for the resource? The first phase of this thesis is the development of a size-structured population model of the resource and the associated fishery. A size-structured model is necessary as lobsters are difficult to age and hence most of the data collected are on a size basis. Furthermore, important management issues, such as the legal minimum size which has changed over time, require a model able to take size-structure into account. This model is fitted to a wide range of data from the fishery, including CPUE (catch-per-unit-effort) and catch-at-size information, by maximising a likelihood function. The model is shown to fit reasonably well to all data, and to provide biologically plausible estimates for its six estimable parameters.
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    FIRST-YEAR SLOAN DIGITAL SKY SURVEY-II (SDSS-II) SUPERNOVA RESULTS: CONSTRAINTS ON NONSTANDARD COSMOLOGICAL MODELS
    (2009) Sollerman, J; Mörtsell, E; Davis, T M; Blomqvist, M; Bassett, B; Becker, A C; Cinabro, D; Filippenko, A V; Foley, R J; Frieman, J; Garnavich, P; Lampeitl, H; Marriner, J; Miquel, R; Nichol, R C; Richmond, M W; Sako, M; Schneider, D P; Smith, M; VanderPlas, J T; Wheeler, J C
    We use the new Type Ia supernovae discovered by the Sloan Digital Sky Survey-II supernova survey, together with additional supernova data sets as well as observations of the cosmic microwave backgr ...
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    Magnetic moments of T = 3 / 2 mirror pairs
    (2010) Perez, S M; Richter, W A; Brown, B A; Horoi, M
    We predict values of the magnetic moments of T=3/2 proton-rich fp-shell nuclei in the mass range A=43-53, by using known values for their neutron-rich mirrors together with shell-model estimates for small quantities. We extend the analysis to those T=3/2 sd-shell mirror pairs for which both the T{sub z}-3/2 and T{sub z}=+3/2 magnetic moments have been measured. We find that these obey the same linear relation as previously deduced for T=1/2 mirror pairs.
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    Modelling the relationship between antiretroviral treatment and HIV prevention: limitations of the Spectrum AIDS Impact Model in a changing policy environment
    (2007) Nattrass, Nattrass
    This paper shows how two publicly available epidemiological modelling packages, namely the Spectrum AIDS Impact Model and the ASSA2003 AIDS and Demographic Model, predict very different impacts from rolling out highly active antiretroviral treatment (HAART) on new HIV infections. Using South Africa as a case study, it shows that the ASSA2003 model predicts a significant drop in new HIV infections as HAART is rolled out, whereas the Spectrum model assumes that HAART does not have a preventative impact (and in fact generates a small increase in new HIV infections). Users will thus draw different conclusions about the public health benefits of HAART depending on which modelling package they use. Despite being presented as a policy-oriented modelling tool capable of exploring 'what if' questions about the impact of different policy choices, the Spectrum model is illequipped to do so with regard to a HAART rollout. Unlike Spectrum, ASSA2003 is more flexible and its assumptions are clear. Better modelling and more information (including about the relationship between HAART and sexual risk behaviour) is required to develop appropriate public-policy modelling for the HAART era.