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 tot al of 81 nekton and micronekton species were collected from 2 sites, o ne in the oceanic western Scotia Sea (Stn 1) and the other on the nort hwestern slope of the South Georgia shelf (Stn 2). Species composition , abundance, biomass and day/night vertical distribution were investig ated. Crustaceans were the most important group in terms of species nu mbers (28 species) followed by mesopelagic fish (24), molluscs (15) an d coelenterates (11). Species diversity increased with depth and was h igher 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) du ring 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 coelente rates (28.3 g wet wt m(-2) during night) and mesopelagic fish (4.9 g w et wt m(-2) during night). Euphausiids (Euphausia triacantha and E. su perba) 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 mar ked diurnal vertical migration (i.e. Electrona antarctica, Gymnoscopel us braueri, Krefftichthys anderssoni, Protomyctophum bolini). During d aylight they stayed in the core of the Circumpolar Deep Water (CDW; 40 0 to 800 m) and at night they were mainly distributed in the Antarctic Surface Water (ASW; 0 to 400 m). Other species with pronounced vertic al migration were the hydromedusa Calycopsis borchgrevinki, the squid Brachioteuthis ?picta, and the euphausiid Euphausia triacantha. The sc yphomedusae Atolla wyvillei and Periphylla periphylla and the crustace ans Cyphocaris richardi, Gignato-cypris mulleri and Pasiphaea scotiae did not appear to migrate and remained concentrated in the CDW. Spatia l variability was analysed by multivariate data analyses (clustering t echniques) and related to hydrography. Four main groups, characterised by different nekton communities, were derived: (1) a lower mesopelagi c 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 (St n 1). The performance of the midwater trawl is discussed as it has a s ubstantial impact on the catchability of the nekton. The presented dat a provide new information on the structure and spatial variability of Antarctic nekton communities and emphasise the geographical and vertic al discontinuities between communities.