Regional mesopelagic fish assemblages in the Indian Ocean and Bay of Bengal, and their relationships to hydrographic characteristics

Abstract

Mesopelagic fish often dominate the micronekton in the mesopelagic zone. Despite their ecological importance, information about their diversity, distribution and abundance in some areas of the Indian Ocean remains patchy and incomplete. Using data collected from three acoustic/trawl surveys spanning the western (WIO), central (CIO) and eastern (EIO) Indian Ocean regions in 2018, we investigated the composition and structure of the fish assemblages that were sampled using different pelagic trawl types, namely Macrozooplankton and Multpelt, which targeted the deep scattering layer (DSL) and shallow scattering layer (SSL). We assessed the structure of mesopelagic assemblages across the region at the level of genus and family, and analysed relationships between the samples with concurrently collected hydrographic data using multivariate statistics. Mesopelagic fish taxonomic richness varied across the region, as follows: WIO–68 genera (13 unique), 34 families (4); CIO–48 genera (6), 35 families (5); and EIO–86 genera (26), 44 families (9). Although there was a pronounced difference in the structure of the assemblages by survey region, gear type and time of day (and many interactions between these), the assemblages were significantly impacted by water temperature, dissolved oxygen and aspects of the chlorophyll a environment— which varied across the sampled areas. Intriguingly, the spatial patterns (and diversity) observed when the mesopelagic fish assemblages were examined at the genus level were likewise reflected when the data were analysed at the family level. The implications of these results are discussed.