Abstract
Vertical distribution patterns and relative abundance of mesopelagic fish species and other major taxonomic groups were investigated through vertically stratified trawl sampling and hydroacoustic analyses along the subtropical convergence zone from 52° W to 70° W in the oligotrophic Sargasso Sea. Persistent stationary layers and several migrating components of different scattering characteristics were detected. The results reveal varying vertical migration patterns, including different times of onset of diel vertical migration in different depths and a migrant pathway emerging daily from the lower deep scattering layer (DSL) at dusk and migrating through the upper DSL without affecting its composition. Fish species identification was made based on morphological characteristics and confirmed by genetic barcoding analyses of subsamples. In total, 5022 fish specimens from 27 families, 62 genera and 70 species were caught. In terms of relative abundance (A) and biomass (M), catches were dominated by species of the families Myctophidae (A=59.1%, M=47.4% of total fish catch) and Melamphaidae (A=22.5%, M=17.1%). Myctophidae and Stomiidae were the most species-rich families with 31 and 12 species, respectively. Catches at the two easternmost stations were dominated by Scopelogadus mizolepis and Nannobrachium cuprarium, while Bolinichthys photothorax and Ceratoscopelus warmingii were the most abundant species in catches from the two westernmost stations. This study provides insights into distribution and vertical migration behaviour of mesopelagic fish in the Sargasso Sea and adds to our understanding of the mesopelagic community in this large oceanic area.
Highlights
Mesopelagic fish, living in depths of 200 to 1000 m, are dominating the fish biomass in large parts of the world’s oceans
Stratified catches were compared with hydroacoustic data, in order to assess the vertical migration behaviour of mesopelagic fish and to characterize the composition of deep scattering layer, providing insights into the horizontal and vertical distribution of fish species over this large oceanic area
Sea surface temperatures varied between 22.5 and 24.9°C, with the lowest temperatures being measured at northern, western- and easternmost stations (Fig. 1)
Summary
Mesopelagic fish, living in depths of 200 to 1000 m, are dominating the fish biomass in large parts of the world’s oceans. Besides environmental factors like primary production, oxygen concentration and light intensity (Irigoien et al 2014; Klevjer et al 2016; Aksnes et al 2017), the distribution of pelagic fish in the Sargasso Sea may be influenced by distinct temperature fronts in the upper 150 m of the ocean. These fronts may form zoogeographic boundaries for certain mesopelagic fish (Backus et al 1969) and cephalopod species (Lischka et al 2017) caused by temperature-related changes in e.g. productivity or stratification. Stratified catches were compared with hydroacoustic data, in order to assess the vertical migration behaviour of mesopelagic fish and to characterize the composition of deep scattering layer, providing insights into the horizontal and vertical distribution of fish species over this large oceanic area
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