Incorporating small-scale fisheries into ecosystem models of the southern Benguela

Naidoo, Lerusha

Incorporating small-scale fisheries into ecosystem models of the southern Benguela

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Despite the southern Benguela small-scale sector providing a fisheries-based livelihood for thousands of South Africans, the catches made by the small-scale sector as well as its impacts on marine living resources are presently not well quantified. The lack of catch data for the sector has previously precluded its inclusion into ecosystem models of the region, and thus limited quantitative assessment of the impacts of climate change on the small-scale fishery exist. This study addresses the overarching challenge of predicting the impacts of climate change on small-scale fishery catches by developing a local catch reconstruction of the data limited, southern Benguela small-scale fishery which was then incorporated into the Ecopath with Ecosim (EwE) and Atlantis (ABACuS v2) modelling frameworks for the southern Benguela. Various climate change scenarios were then tested to determine the possible impacts of climate change on the target species of the small-scale fishery, and subsequently on the sector itself. Catch reconstructions for the small-scale sector's key target species (i.e. snoek, yellowtail, geelbek, kob, carpenter, and west coast rock lobster) were developed by using locally available data such as management information of the small-scale sector and number of trips made by the commercial traditional linefishery. Drawing on the concept of participatory modelling, I made use of key experts to review assumptions made where data was lacking. When compared to global reconstructions - namely the Sea Around Us project and Global Fisheries Landings versions four and five - my independently developed catch reconstruction generally predicted lower catches by the small-scale fishery. Further, due to the use of the commercial traditional linefishery effort in the development of my reconstruction, the trends in catches in my reconstruction are similar to that of the commercial sector. By contrast, the global reconstructions generally exhibited different trends from my reconstruction. I then incorporated these reconstructions into the EwE and ABACuS v2 ecosystem models of the southern Benguela. I used model skill metrics to compare the various model simulations incorporating the different catch reconstructions. Overall, there were few differences between the modelled biomass by the baseline ecosystem models and the models incorporating small-scale fishery catches. Based on the model metrics comparing the simulated catches by the ecosystem models to the original reconstructions, the simulation including this study's catch reconstruction generally performed best, particularly in the EwE model. Using my catch reconstruction, climate change scenarios were tested to determine the influence of climate change on the targets of the small-scale fishery. Temperature and primary production forcings from the Geophysical Fluid Dynamics Laboratory (GFDL) and the Institut Pierre Simon Laplace (IPSL) earth system models were used to simulate climate change effects under varying Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs). Under the SSP1-RCP2.6 (high mitigation, low carbon emissions) and SSP5 RCP8.5 (business-as-usual, high carbon emissions) scenarios, there is a predicted decline in biomasses and catches of key small-scale target species, including yellowtail and sciaenids (kob and geelbek) and west coast rock lobster by the end of the century. Snoek shows a mixed response between the two ecosystem models, with ABACuS v2 indicating a decline in biomass but an increase in small-scale catches whilst EwE indicates a decline in biomass and small scale catches. Interestingly, groups which represent carpenter (medium sparids or benthic feeding demersal fish) show a decline in biomass under GFDL and an overall increase in small scale catches. Given the projected decline of target species by the end of the century under the tested climate change scenarios, there is the possible need to set up adaptation plans to diversify livelihoods. The findings of this study can be used to communicate the potential threats of climate change to the southern Benguela small-scale fishery. However, as both the data used to fit catches and climate change projections are uncertain, the findings of this study are best suited as a starting point for discussions about climate change influences on the small-scale fishery.

Keywords

Benguela small-scale sector

South Africa

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