Esther Fondo

The biodiversity of the western Indian Ocean is known mostly from inshore habitats, but little is known from the most extensive habitat of the region, the soft, unconsolidated sediments. Demersal trawl data from Kenya, Tanzania, Mozambique, South Africa and Madagascar were analysed, using average taxonomic distinctness in association with various environmental factors, to determine the biodiversity, the dominant genera that characterise the community assemblages and the areas where biodiversity was highest. A decreasing trend in biodiversity was observed from Kenya to Mozambique, including Madagascar, but South Africa had the highest average biodiversity. The continental slope was more biodiverse than the shelf, and biodiversity increased with depth to approximately 500 m, whereafter it declined. Depth and turbidity were the factors that best accounted for differences in community assemblages. The deep-water fish genera, Chlorophthalmus and Chaunax, dominated the slope assemblages, while the shelf communities were dominated by the shallow-water shrimp genera, Penaeus and Metapenaeus. Areas that have higher diversity, in general, included off Durban, South Africa, the Delagoa Bight off Maputo, Mozambique, and most of the area off southern Kenya. The information presented has improved the knowledge of soft sediment habitats in the WIO, thereby supporting decision-makers when deciding on planning options.

The Western Indian Ocean (WIO) hosts substantial marine biodiversity, which is better known from inshore and coastal hab- itats. The diversity from soft sediments of the shelf and slope is much less known regionally, which was the premise for the WIO-Benth Project. Existing seabed information and bottom trawl catch data were compiled for five countries (Kenya, Tanza- nia, Mozambique, South Africa, and Madagascar), to characterize seabed habitats and benthic (demersal trawl) communities of the continental shelf and upper slope in the western part of the WIO. Soft sediment (sand, mud) habitats were spatially dominant (54 %) relative to rock/mixed habitats; associated trawled fauna from these habitats were dominated by Chordata (fishes) and Arthropoda (mainly crustaceans). This overview of the special issue on the WIO-Benth Project provides a summary of the general results, greater details on which are provided in the issue’s constituent papers. In the process of compiling infor- mation and data from the region, the WIO-Benth Project has provided new knowledge to assist with marine spatial planning and decision-making in the WIO.

The Western Indian Ocean (WIO) hosts substantial marine biodiversity, which is better known from inshore and coastal hab- itats. The diversity from soft sediments of the shelf and slope is much less known regionally, which was the premise for the WIO-Benth Project. Existing seabed information and bottom trawl catch data were compiled for five countries (Kenya, Tanza- nia, Mozambique, South Africa, and Madagascar), to characterize seabed habitats and benthic (demersal trawl) communities of the continental shelf and upper slope in the western part of the WIO. Soft sediment (sand, mud) habitats were spatially dominant (54 %) relative to rock/mixed habitats; associated trawled fauna from these habitats were dominated by Chordata (fishes) and Arthropoda (mainly crustaceans). This overview of the special issue on the WIO-Benth Project provides a summary of the general results, greater details on which are provided in the issue’s constituent papers. In the process of compiling infor- mation and data from the region, the WIO-Benth Project has provided new knowledge to assist with marine spatial planning and decision-making in the WIO.

The biodiversity of the western Indian Ocean is known mostly from inshore habitats, but little is known from the most extensive habitat of the region, the soft, unconsolidated sediments. Demersal trawl data from Kenya, Tanzania, Mozambique, South Africa and Madagascar were analysed, using average taxonomic distinctness in association with various environmental factors, to determine the biodiversity, the dominant genera that characterise the community assemblages and the areas where biodiversity was highest. A decreasing trend in biodiversity was observed from Kenya to Mozambique, including Madagascar, but South Africa had the highest average biodiversity. The continental slope was more biodiverse than the shelf, and biodiversity increased with depth to approximately 500 m, whereafter it declined. Depth and turbidity were the factors that best accounted for differences in community assemblages. The deep-water fish genera, Chlorophthalmus and Chaunax, dominated the slope assemblages, while the shelf communities were dominated by the shallow-water shrimp genera, Penaeus and Metapenaeus. Areas that have higher diversity, in general, included off Durban, South Africa, the Delagoa Bight off Maputo, Mozambique, and most of the area off southern Kenya. The information presented has improved the knowledge of soft sediment habitats in the WIO, thereby supporting decision-makers when deciding on planning options.

Bottom trawling is a common fishing method that targets bottom-dwelling fisheries resources. It is non-selective and large amounts of by-catch are discarded, raising serious sustainability and ecosystem conservation concerns. In this study, a shallow-water bottom-trawl fishery was evaluated using logbook catch data between 2011 and 2019 and the species composition data collected by fisheries observers between 2016 and 2019. The logbook data showed a twenty-fold increase in the annual catches with a ten-fold increase in fishing effort and an increase in the proportion of retained catch from 2011 to 2019. The observer data showed that for prawn, the by-catch ratio ranged from 1:3 to 1:9 during the four years. Multivariate analysis revealed significant differences between the compositions of retained and discarded catches mainly attributed to Pellona ditchela, Nematopalaemontenuipes, and Secutor insidiator. There was no significant decline in species diversity and the trophic level of the catches over the 4-year observer period indicating no marked impact of trawling on the stock at the current level of fishing effort. This study provides baseline information on the prawn trawl fishery against which the performance of the management regulations may be evaluated towards the Ecosystem Approach to Fisheries management.

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