Browsing by Author "Ghebrehiwet, Dawit Yemane"

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    An investigation of spatial-temporal diel changes in Loligo reynaudii catch rates in the commercial squid jig fishery of South Africa
    (2025) Botes, Peet; Githaiga-Mwicigi, Jean; Ghebrehiwet, Dawit Yemane
    Loligo reynaudii (chokka) squid has been jigged commercially on the South African South-southeast coast since 1982. Starting in 2019, a chokka-directed hydroacoustic survey programme has formed part of scientific efforts to best inform the management of this fishery, mainly focussed on surveying at daytime during October-November, on the main fishing grounds between Plettenberg Bay and Port Alfred. However, it has so far remained unclear whether this programme may be improved in terms of spatial-temporal focus, to better survey acoustically detectable aggregations of adult squid throughout the year. Considering that commercial jig catches similarly depend on the prevalence, intensity and duration of aggregations; and assuming standardised Catch Per Unit Effort (CPUE) to be a representative measure of local abundance and proportional representation of regional population distribution: this study employed statistical modelling to specifically investigate whether, on the commercial squid jig fishing grounds of South Africa, there is: (1) an effect of diel period on chokka CPUE; (2) seasonal variation in any effects of diel period on CPUE; and (3) spatial variation in any effects of diel period on CPUE between seasons and years. The data used were DFFE (South African Department of Forestry, Fisheries and the Environment) commercial session-level squid jig catch logbook records. A tweedie generalized additive model was applied following an iterative semi-inductive modelling approach, for an optimal account of data non-linearity and non-normality when estimating CPUE as a function of relevant space-time variables. The data were limited, inter alia, to 2006-2022 sessions conducted purely at day/night between 20-29°E. The final model explained 24.4 % deviance and all terms were statistically significant in their effect on the response (p<0.05). There were three main findings when estimating relevant terms' marginal effects. Firstly, chokka concentrations are greater at day than night. Secondly, there is clear seasonality in the aggregated proportion with a single greater daytime and lesser nighttime peak in annual spawning concentrations, around October-December. Thirdly, CPUEs involved varying spatial-temporal complexity while being greater at day than night across most of the fishing grounds for most of the average year. The main takeaway was a dismissal of any notions that acoustically surveyable, large aggregations of squid may likely be found consistently in areas and at times other than when-where acoustic efforts have typically been expended in late spring so far, on inshore spawning grounds targeted by the commercial jigging fleet. Recommendations for future research include accounting for session-level variations in relevant oceanographic parameters when modelling session-level jig CPUE.
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    Open Access
    Assessing the effects of fishing on fish communities using South African case studies : empirical and theoretical approaches
    (2007) Ghebrehiwet, Dawit Yemane; Field, John G; Leslie, Rob W; Shin, Yunne-jai
    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.