Modelling regime shifts in the southern Benguela : a frame-based approach

Master Thesis


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University of Cape Town

Small pelagic fish populations in productive upwelling systems are characterised by long-term patterns of alternating dominance. Sardine and anchovy are the most important small pelagic species in the southern Benguela ecosystem, which has been shown to have undergone regime shifts during the past 50 years. Modelling regime shifts at an ecosystem scale can be of great importance in fisheries management, to aid in long-term planning and fishing strategy evaluation. Frame-based modelling has been successfully applied to regime shift dynamics in terrestrial ecosystems. The pattern of abrupt shifts between quasi-stable regimes suggests the usefulness of a frame-based approach in the southern Benguela ecosystem, with separate frames describing each small pelagic fish dominance pattern. Frame-based modelling is applied to sardine/anchovy dynamics under the influence of climate variability involving interaction with the small pelagic fishery. Four frames are used in the model: Both Species High; Sardine High / Anchovy Low; Anchovy High / Sardine Low; and Both Species Low. Switching rules for transition between the frames are described. Rapid prototyping is used to construct and test first- and second-generation prototypes of a frame-based model. A sensitivity analysis of the model is performed, and the model is found to be sensitive to the frame switching rules. The model is also reasonably sensitive to the sardine population model parameters, and the influence of juvenile sardine bycatch is noticeable in the "Sardine Low" frames. The model behaviour is relatively insensitive to climate variability, but the inherent degree of stochasticity in the sardine recruitment calls for continuous population monitoring and adjustment of fishing levels to avoid crashing the modelled sardine stock. Frame behaviour in the model is sensitive to sardine fishing activity. The model is exercised in a variety of scenario analyses, and confidence in the model is strengthened by the observed parallels to the real world. The use of the model as a "test platform" is explored to improve understanding of fishing impact on the dynamics of small pelagic populations. Previously identified advantages of the frame-based modelling technique include their particular usefulness in inter-disciplinary teams and the ease with which a frame-based model can be expanded and modified, and the experiences of this project support these findings. The use of frames as indicators adds information about the condition of the modelled stock at a given point beyond what can be inferred by population levels alone. Frame-based modelling is also found to be an approach well-suited to the development and maintenance of the computer software which encapsulates the model, and as a common interface between biologists, programmers and non-specialist model users. Suggested applications of the model include deriving "probabilities of change" for use in an expert system to predict long-term ecosystem changes. Applications of the model in understanding the impact of survey data error and compliance issues are discussed.

Includes bibliographical references (leaves 83-88).