Demersal trawl surveys show ecological gradients in Southwest Indian Ocean slope fauna

Abstract

We assessed the richness, diversity and community structure of demersal fish and benthic invertebrates caught by trawl nets along the deep shelf and upper continental slope of the Southwest (SW) Indian Ocean. Four depth-stratified surveys were undertaken in 2011-2012, in Kenya, Tanzania, Mozambique and southwestern Madagascar. The effects of region (north vs south), country (proxy for latitude) and depth stratum on catch composition were considered. Of 243 genera identified from 206 trawls, the majority were teleosts (55%), followed by crustaceans (18%), elasmobranchs (12%) molluscs (10%), and other invertebrates (5%). Species richness was highest in Mozambique, and in the 300-399 m depth stratum. Shannon’s diversity was greatest at 200-299 m depth in Kenya, decreasing southwards along a latitudinal gradient. Genera contributing most to the dissimilarity between the north and south were greeneye fishes, Chlorophthalmus, and knife prawns, Haliporoides. By country, Madagascar and Tanzania were most dissimilar, with blackchin fishes, Neoscopelus, greeneye fishes, and nylon shrimps, Heterocarpus, the main drivers. By depth, the 200-299 m and 500-599 m strata were most dissimilar, based on blackchin fishes, knife prawns and greeneye fishes. Our study shows a clear difference in community structure of slope fauna across the Mozambique Channel, reflecting sampling of two ecologically different soft-sediment communities. The information provided is new to the SW Indian Ocean region, and an important step towards understanding diversity trends, for the prioritization of conservation needs and development of deep-water fisheries management strategies in the region.