Assessing the effects of fishing on fish communities using South African case studies : empirical and theoretical approaches
Currently heavy ﬁshing is recognized as one of the major threats to the structural and functional organization of marine ecosystems in many coastal nations. The threat is mainly the result of the inherent nature of the various ﬁshing activities: size selectivity, habitat destruction, biomass removal, and uncertainty in resource status and management of the resource. Thus this thesis investigates structural changes that result from ﬁshing. This thesis aims to answer whether there were changes in the structure of fish communities off the of South Africa using two case studies, to explore the response of fish communities to the proposed creation of Marine Protected Areas and to investigate the alternate application of spatially uniform and heterogeneous ﬁshing mortalities. The research questions of the thesis are answered through empirical analysis of landing data for the line ﬁshery and analysis of demersal trawl survey data from the south coast of South Africa, and analysis of output of the Individual Based Model OSMOSE applied to the southern Benguela. Structural changes in the landings from the line ﬁshery and south coast survey data are assessed using a variety of ecosystem indicators believed to capture such changes: size-based indicators {mean size, slope and height of the size spectra, mean Lmax7, proportion of size classes), species-based indicators (ordination by multidimensional scaling, and dendrograms, various diversity indices, dominance curves). Inferences are based on the reference directions of the indicators, according to the expected response of indicators to heavy fishing. Structural changes in the ﬁsh communities are observed, over the spatial and temporal bounds of the two case studies, to be the most likely cause of the observed changes is heavy fishing, although the influence of environmental factors cannot he ruled out. investigation of alternative implementation ﬁshing mortality using the simulation model OSMOSE showed that the system and species biomass do differ between the two implementations, but the variability in the system remains the same. The modelled response of ﬁsh communities to the introduction of Marine Protected Areas is an overall increase in relative biomass of large predatory fishes and a decline in the biomass of prey and competitor species.
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