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

Abstract

The nekton community was sampled by a Rectangular Midwater Trawl (RMT 25) over the upper 1000 m of the Scoha Sea dunng January 1991 A total of 81 nekton and rmcronekton species were collected from 2 sites, one In the oceanlc western Scoha Sea (Stn 1) and the other on the northwestern slope of the South Georgla shelf (Stn 2). Specles composibon, abundance, blomass and d a y h g h t verbcal distnbubon were investigated. Crustaceans were the most Important group in terms of species numbers (28 species) followed by mesopelagic fish (24), molluscs (15) and coelenterates (1 1) Species diversity Increased with depth and was hlgher at Stn 2 (76 species) than at Stn 1 (62 specles) Biomass in the upper 1000 m was considerably hlgher at Stn 1 (94 6 g wet wt m-2 dunng daytime, 87 g wet wt m-' dunng rught) than at Stn 2 (10 2 and 23 7 g wet wt m-', respectively), mostly due to dense concentrahons of the tunicate Salpa thompsonl (41 6 g wet wt m-' dunng night) The other main contnbutors to the high biomass at Stn 1 were coelenterates (28 3 g wet wt m-' dunng night) and mesopelagic fish (4 9 g wet wt m-' dunng night) Euphausilds (Euphausia tnacantha and E superba) accounted for 1 5 g wet wt m-' at Stn 2 dunng nlght, w ~t h E tr~acantha the more Important of the two (1 4 g wet wt m-') Except for Bathylagus antarchcus all common mesopelaglc fishes showed a marked diurnal vertical nugration (l e Electrona antarctlca, Gymnoscopelus brauen, Krefftichthys anderssoni, Protomyctophum bolinl) Dunng dayhght they stayed m the core of the Clrcumpolar Deep Water (CDW, 400 to 800 m) and at night they were mainly dlstnbuted in the Antarctic Surface Water (ASW, 0 to 400 m) Other species wlth pronounced vertical mgrabon were the hydromedusa Calycops~s borchgrevmlu, the squid Brachioteuttus ?picta, and the euphauslid Euphaus~a tr~acantha The scyphomedusae Atolla wynllei and Penphylla penphylla and the crustaceans Cyphocans nchard~, Gigantocypnsmullen and Paslphaea scohae did not appear to nugrate and remained concentrated in the CDW. Spatial variability was analysed by multivanate data analyses (clustenng techniques) and related to hydrography Four main groups, charactensed by different nekton communibes, were denved (1) a lower mesopelaglc nekton communlty from the deeper layers of the CDW, apparent at both stations, (2) an upper mesopelaglc nekton communlty from the core of the CDW, apparent at both stations, (3) an epipelagic nekton commumty from the ASW over the South Georgla slope (Stn 2) and flnally (4) an epipelaglc nekton communlty from the ASW of the oceanic Scoha Sea (Stn 1) The performance of the midwater trawl is discussed as ~t has a substanhal Impact on the catchabihty of the nekton. The presented data provlde new information on the structure and spatial vanabihty of Antarchc nekton comrnunibes and emphasise the geographcal and vertical discontinuihes between communibes.