Pelagic fishes in the Marguerite Bay region of the West Antarctic Peninsula continental shelf

Pelagic fishes in the Marguerite Bay region of the western Antarctic Peninsula (WAP) continental margin were sampled using a 10-m2 MOCNESS as part of the Southern Ocean Global Ocean Ecosystems Dynamics (SO GLOBEC) program. Sixty-two tows were completed during the course of four cruises conducted during the austral fall and winter, 22 each during the austral fall, and 9 each during the austral winter. Six thousand and sixty individuals of 34 species representing 13 families were collected in the fall, while 672 individuals of 22 species from 10 families were collected in the winter. Nearly all of the notothenioid specimens collected (families Artedraconidae, Bathydraconidae, Channichthyidae, and Nototheniidae) were either larvae or young juveniles (0–2 years). Conversely, except for the paralepidid Notolepis coatsi and the occasional juveniles of the bathylagid Bathylagus antarcticus, the gonostomatid Cyclothone kobayashii, or the myctophid Electrona antarctica, the non-notothenioid specimens collected were predominantly adults. In the fall, the nototheniids Pleuragramma antarcticum and Trematomus scotti, and the myctophid E. antarctica numerically dominated the overall assemblage, collectively accounting for 89.7% of the total catch. In the winter, E. antarctica, Cyclothone microdon, and B. antarcticus were the numerical dominants, each contributing 14–20% of the total.The pelagic fish community within the Marguerite Bay region of the WAP continental shelf is a variable mixture of mesopelagic and neritic fauna. At one extreme is an oceanic assemblage exhibiting high-diversity indices and characterized by the genera Electrona, Gymnoscopelus, Protomyctophum, Bathylagus, Cyclothone, and Notolepis. Minor components of this group include numerous less common mesopelagic genera (e.g., Paradiplospinus, Lampanyctus, Benthalbella, Borostomias) and the occasional larval/juvenile notothenioid. At the other extreme is a coastal assemblage with low-diversity indices dominated by larval and juvenile notothenioids, particularly Pleuragramma antarcticum This assemblage is also characterized by a numerically low but consistent liparid and zoarcid component, with the latter group often contributing disproportionately to the total biomass. The degree of overlap between the two assemblages and the relative dominance of representative species is directly related to local hydrographic conditions, in particular the presence of Circumpolar Deep Water (CDW). The unique hydrographic conditions of the WAP shelf and the accompanying spatial heterogeneity in pelagic ichthyofauna provide a striking contrast to the continental margin areas of the Ross, Weddell, Davis, and Dumont d’Urville Seas where sharp temperature gradients near the shelf break result in a clear separation of oceanic and coastal assemblages.