Sean T. Fennessy

Knowledge of deeper marine ecosystems is limited relative to that from coastal regions but is nevertheless important for broader decisionmaking purposes. Limited access to suitable vessels in KwaZu...

In the Western Indian Ocean (WIO) elasmobranchs play important roles in ecosystems as apex and mesopredators and contribute high socio-economic value to coastal communities. Their diversity in the WIO is amongst the highest globally yet few studies have assessed the occur- rence of various species in catches. Demersal trawl catches from Kenya, Tanzania, Mozambique, Madagascar and South Africa (East Coast) were analysed using Generalised Linear Models to esti- mate the probabilities of non-zero catches of sharks and batoids. The data were further subjected to non-metric multidimensional scaling to establish clusters of various communities and to sim- ilarity percentage analyses to determine the dominant taxa. The probability of catching sharks and batoids in trawl nets was high. While communities were not highly separated within factors, it was depth that showed the greatest influence on community structures. The genus Squalus dom- inated the shark catches as depth increased while species dominance of batoids changed from stingrays (Dasyatidae) to guitarfishes (Rhinobatidae) and to skates (Rajidae) as depth increased. This study underlines the need for improved species identification of elasmobranch catches and for marine protected areas on soft sediment habitats to protect these important species.

Marine protected areas (MPAs) have become a crucial conservation strategy in response to anthropogenic threats impacting the world’s oceans. However, some MPAs have conflicting goals that compromise conservation effectiveness. To guide future MPA development, the conservation value of current MPAs need to be accurately quantify using diversity metrics, such as species richness and rarity. This study focused on the demersal soft-sediment fauna of the Western Indian Ocean (WIO) and explored their environmental predictors and distributions. Hotspot areas of high species richness and rare taxa were identified, comparing them to existing MPAs, to evaluate their value as a conservation strategy for these fauna. The results show that depth, which negatively correlates with water temperature, and latitude, which positively correlates with longitude, are the most influential predictors of demersal species richness and rarity. Various other environmental factors are significantly associated with these two diversity measures. Hotspots of species richness and rarity occur in deeper, cooler waters away from the coastline, which are less exposed to disturbances. Higher richness was found outside of MPAs, likely because most WIO MPAs are in shallow waters, while the studied demersal fauna favour deeper conditions. These findings underscore the need for MPAs to comprise a mosaic of habitat types to effectively conserve the diversity of demersal species.

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 biodiversity of the east coast of South Africa differs markedly from that of the rest of the marine environment of the country. Data from commercial crustacean trawls were used in conjunction with environmental data to investigate biodiversity patterns of offshore habitats off the KwaZulu-Natal coast. Biodiversity was measured using taxa observed (genus and species) and average taxonomic distinctness. Community assemblages were determined through clustering, analysis of similarities and similarity percentage analysis. Factors influencing biodiversity were position on the continental margin (littoral/slope), location along the coast, turbidity and temperature. These were the same factors that influenced the community assemblages. Crustaceans were the most frequently occurring taxa followed by teleost fishes. This reflected the nature of the local trawl fisheries that target crustacean species. The community assemblages at the locations included in the study were highly dissimilar to each other and clear transitions in assemblages were obvious as depth increased. Substantial disparities in sampling effort reduced options for in depth analysis. This leaves scope to expand this work both through increased sampling in under-sampled areas but also to investigate areas with higher sampling effort more fully

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.

Marine biodiversity research in Madagascar has mostly focused on coastal habitats and deep-sea ecosystems, with fewer studies on the shelf and upper slope. This study contributes to knowledge of the demersal communities on the west coast of Madagascar using data collected from 54 bottom trawls by the research vessel Dr Fridtjof Nansen in 2008 and 2009. The analyses focused on two factors: location along the coast and continental shelf versus slope. A total of 239 genera were identified, consisting of 31,194 individuals with a total weight of 3,335.61 kg. The Shannon-Wiener diversity index showed high diversity in the northwest. Diversity was higher on the slope than shelf. The genus Upeneus dominated all shelf catches (19.35 %) and Rexea (2.18 %) was strongly dominant on the slope. A clear distinction existed between the shelf and the slope communities shown by nMDS plots and ANOSIM tests, with R values of 0.67 for the presence/ absence dataset, 0.061 for the abundance dataset, and 0.57 for the biomass dataset. According to SIMPER analysis, the genera driving the dissimilarity between these two ecosystems are Rexea and Chlorophthalmus. These results suggest that depth is the most influential factor in determining community assemblages

While the Western Indian Ocean (WIO) hosts a large diversity of marine and coastal organisms, less is known about the biodiversity of deeper habitats, particularly from soft sediments. Consequently, there is less known about the vulnerability of these habitats to disturbance and degradation. This paper is a first attempt to assess this at the regional scale, using data on trawled fauna. In the absence of life history information, alternative indices of sensitivity were used: mobility, distribution, presence of cnidaria and porifera, and presence of IUCN Threatened species. Central and southern Mozambique and the east coast of South Africa have the most extensive areas of potentially highest sensitivity in soft sediment habitats, with smaller patches off central Tanzania and Kenya. Greater confidence in these findings requires greater sampling coverage and improved identification of fauna. Nevertheless, these preliminary findings draw attention to the relative sensitivity of these soft sediment habitats and provide a basis for assisting countries in identifying potential soft sediment areas for protection, contributing towards their Sustainable Development Goals (SDGs) and other marine conservation targets.

Marine biodiversity research in Madagascar has mostly focused on coastal habitats and deep-sea ecosystems, with fewer studies on the shelf and upper slope. This study contributes to knowledge of the demersal communities on the west coast of Madagascar using data collected from 54 bottom trawls by the research vessel Dr Fridtjof Nansen in 2008 and 2009. The analyses focused on two factors: location along the coast and continental shelf versus slope. A total of 239 genera were identified, consisting of 31,194 individuals with a total weight of 3,335.61 kg. The Shannon-Wiener diversity index showed high diversity in the northwest. Diversity was higher on the slope than shelf. The genus Upeneus dominated all shelf catches (19.35 %) and Rexea (2.18 %) was strongly dominant on the slope. A clear distinction existed between the shelf and the slope communities shown by nMDS plots and ANOSIM tests, with R values of 0.67 for the presence/ absence dataset, 0.061 for the abundance dataset, and 0.57 for the biomass dataset. According to SIMPER analysis, the genera driving the dissimilarity between these two ecosystems are Rexea and Chlorophthalmus. These results suggest that depth is the most influential factor in determining community assemblages