Nekton community of the Scotia Sea as sampled by the RMT 25 during austral summer

The nekton community was sampled by a Rectangular Midwater Trawl (RMT 25) over the upper 1000 m of the Scotia Sea during January 1991. A total of 81 nekton and micronekton species were collected from 2 sites, one in the oceanic western Scotia Sea (Stn 1) and the other on the northwestern slope of the South Georgia shelf (Stn 2). Species composition, abundance, biomass and day/night vertical distribution were investigated. Crustaceans were the most important group in terms of species numbers (28 species) followed by mesopelagic fish (24), molluscs (15) and coelenterates (11). Species diversity increased with depth and was higher at Stn 2 (76 species) than at Stn 1 (62 species). Biomass in the upper 1000 m was considerably higher at Stn 1 (94.6 g wet wt m(-2) during daytime, 87 g wet wt m(-2) during night) than at Stn 2 (10.2 and 23.7 g wet wt m(-2), respectively), mostly due to dense concentrations of the tunicate Salpa thompsoni (41.6 g wet Mrt m(-2) during night). The other main contributors to the high biomass at Stn 1 were coelenterates (28.3 g wet wt m(-2) during night) and mesopelagic fish (4.9 g wet wt m(-2) during night). Euphausiids (Euphausia triacantha and E. superba) accounted for 1.5 g wet wt m(-2) at Stn 2 during night, with E. triacantha the more important of the two (1.4 g wet wt m(-2)). Except for Bathylagus antarcticus all common mesopelagic fishes showed a marked diurnal vertical migration (i.e. Electrona antarctica, Gymnoscopelus braueri, Krefftichthys anderssoni, Protomyctophum bolini). During daylight they stayed in the core of the Circumpolar Deep Water (CDW; 400 to 800 m) and at night they were mainly distributed in the Antarctic Surface Water (ASW; 0 to 400 m). Other species with pronounced vertical migration were the hydromedusa Calycopsis borchgrevinki, the squid Brachioteuthis ?picta, and the euphausiid Euphausia triacantha. The scyphomedusae Atolla wyvillei and Periphylla periphylla and the crustaceans Cyphocaris richardi, Gignato-cypris mulleri and Pasiphaea scotiae did not appear to migrate and remained concentrated in the CDW. Spatial variability was analysed by multivariate data analyses (clustering techniques) and related to hydrography. Four main groups, characterised by different nekton communities, were derived: (1) a lower mesopelagic nekton community from the deeper layers of the CDW, apparent at both stations, (2) an upper mesopelagic nekton community from the core of the CDW, apparent at both stations, (3) an epipelagic nekton community from the ASW over the South Georgia slope (Stn 2) and finally (4) an epipelagic nekton community from the ASW of the oceanic Scotia Sea (Stn 1). The performance of the midwater trawl is discussed as it has a substantial impact on the catchability of the nekton. The presented data provide new information on the structure and spatial variability of Antarctic nekton communities and emphasise the geographical and vertical discontinuities between communities.

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