Description: An RMT 25 opening/closing net was used to sample the nekton comunity at 2 stations in the ice free zone (IFZ) of the Scotia Sea (Stn 1 approximately 10 nautical miles south of the Antarctic Polar Front, Stn 2 on the edge of the South Georgia Shelf Break to the northwest of the island) Oblique hauls sampled 200 m depth layers to 1000 m during both day and night. Total and individual volumes of each species in each 200 m layer were measured by displacement The data were used to generate biomass and numerical spectra for day and night at each station for the whole water column to 1000 m. At both stations the relationship between log10 biomass density (B/A) and log10 individual body mass (M) were strongly positive. Slopes of the biomass spectra were not significantly different among the day and night stations and an overall regression showed that biomass density scaled as M061. Analysis of biomass spectra revealed that although the species composition and biomass density vaned between the 2 stations, energy turnover in the nekton community in the 2 areas was similarly dominated by animals of larger size. Considering energy turnover in terms of taxonomic groups revealed that Stn 1 turnover was dominated by tunicates (salps) followed by fish and cnidarians and at Stn 2 turnover was dominated by crustaceans followed similarly by fish and cnidarians. Use of biomass spectra in this case study was shown to enhance insight into the comparative function of 2 pelaglc systems obtained using a conventional taxonomc approach The analysis of biomass spectra in the absence of taxonomic data would have had limited value as it would not have emphasised the major difference between the 2 stations: the domination by tunicates, an energetic dead end, at Stn 1 and crustaceans, which are available to predators, at Stn 2.

Description: Juvenile cuttlefish Sepia offcinalis were hatched and reared in the laboratory at six ration levels and two temperatures. At 12.5 °C there was little growth whereas at 17.5 °C growth rate responded strongly to ration level. Nucleic acids were extracted into buffer containing 0.01% SDS and assayed by fluorescence using ethidium bromide and bis-benzimide and by absorbance at 260 nm. The sensitivity of nucleic acid assay using ethidium bromide was increased by shortening the excitation wavelength to 305 nm. RNA:DNA ratio was strongly correlated with growth rate in cuttlefish held at 17.5 °C; at 12.5 °C, the relationship was similar but not statistically significant. At low growth rates, RNA:DNA ratios were similar to those reported for larval fish; nucleic acid ratios at higher growth rates were higher than reported so far. This may be related to the very high growth rates achieved by many cephalopods.

Description: The nekton community was sampled by a Rectangular Midwater Trawl (RMT 25) over the upper 1000 m of the Scotia Sea dunng January 1991. A total of 81 nekton and micronekton species were collected from 2 sites, one in the oceanlc 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 specles). 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 wt 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 brauen, 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 distnbuted 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, Gigantocypris 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 Antarchc nekton communities and emphasise the geographical and vertical discontinuities between communities.

Description: In the Atlantic sector of the Southern Ocean, data from vertebrate predators and commercial fisheries suggests that the distribution of the ommast rephid squid Martialia hyadesi is related to the Antarctic Polar Frontal Zone, but it spreads further to the north in some years (Rodhouse, in press). A mass stranding of M. hyadesi occurred on Macquarie Island in 1971 (O'Sullivan et al. 1983) suggesting that its distribution is c ircumpolar (Rodhouse and Yeatman 1990). However, apart from a single beak collected from the s tomach of a wandering albatross at Mar ion Island (Imber and Berruti 1981) its presence has not, until now, been confirmed in the Indian Ocean sector and in particular it is not included in the list of cephalopods from the Kerguelenian Province (Lu and Mangold 1978). M. hyadesi is a major prey item of the grey-headed albatross, Diomedea chrysostoma, and the southern elephant seal, Mirounoa leonina, at South Georgia (Rodhouse et al. 1990; Rodhouse et al., unpublished data) and is present in the diet o f several other predators in the Scotia Sea area including the wandering albatross, Diomedea exulans (Rodhouse et al. 1987) and the giant petrels, Macronectes halli and M. gioanteus (Hunter 1983). It occasionally occurs as a significant by-catch in the lllex argentinus fishery on the Pa tagonian Shelf and has been taken during commercial squid jigging trials in the Scotia Sea at the Antarctic Polar Frontal Zone (Rodhouse, in press). It appears to have potential for commercial exploitation in the sub-Antarctic waters of the Atlantic sector of the Southern Ocean (Rodhouse 1990). In view of the ecological importance of M. hyadesi to Antarctic predators, and the likelihood that it will be commercially exploited in the future, it is important to thoroughly establish its geographical range, and in particular to confirm its circumpolar distribution.