Latitudinal and Bathymetric Patterns in the Distribution and Abundance of Mesopelagic ﬁsh in the Scotia Sea

Deep-Sea Research II 59-60 (2012) 189–198 Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Latitudinal and bathymetric patterns in the distribution and abundance of mesopelagic fish in the Scotia Sea Martin A. Collins a,b,n, Gabriele Stowasser a, Sophie Fielding a, Rachel Shreeve a, Jose´ C. Xavier c, Hugh J. Venables a, Peter Enderlein a, Yves Cherel d, Anton Van de Putte e a British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 0ET, UK b Oceanlab, Aberdeen University, Newburgh, Aberdeenshire AB41 6AA, UK c Institute of Marine Research, University of Coimbra, 3001-401 Coimbra, Portugal d Centre d’Etudes Biologiques de Chize´, UPR 1934 du CNRS, 79360 Villiers-en-Bois, France e Katholieke Universiteit Leuven, Laboratory of Animal Diversity and Systematics, Ch. Deberiotstraat 32, B-3000 Leuven, Belgium article info abstract Available online 27 July 2011 Mesopelagic fish are a key component of the pelagic ecosystem throughout the world’s oceans. Opening Keywords: and closing nets were used to investigate patterns in the distribution and abundance of mesopelagic Myctophid fish from the surface to 1000 m on a series of transects across the Scotia Sea from the ice-edge to the Myctophidae Antarctic Polar Front. A total of 141 non-target net hauls were undertaken during three cruises (Nov Bathylagidae 2006, Jan 2008 and Mar 2009), with 7852 teleost fish captured, representing 43 species in 17 families. A Biomass further 1517 fish were caught in targeted net hauls. The dominant families were the Myctophidae Southern Ocean (6961 specimens; 21 species) and Bathylagidae (1467 specimens; 4 species). Few fish were caught in the upper 400 m during daylight, which was attributed to a combination of net avoidance and diurnal vertical migration. Species composition was linked to depth and location and was closely associated with oceanographic features. Diversity was lowest in cold water at the most southerly stations, which were dominated by Electrona antarctica, Gymnoscopelus braueri and Bathylagus antarcticus. Further north, diversity increased with the addition of species such as Krefftichthys anderssoni, Protomyctophum bolini and Electrona carlsbergi. The depth integrated biomass of myctophids was similar across the latitudinal transect and produced an estimate of 4.5 million tonnes in the Scotia Sea. Bathylagids were patchily distributed, but were abundant in the lower mesopelagic zone (4400 m) and are potentially significant zooplankton consumers. Given the biomass of the myctophids and bathylagids coupled with the vertical migrations of many species, these fish are likely to play a significant role in carbon export from the surface waters to the deep ocean. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction 1997), and are a significant component of the diet of many other higher predators such as toothfish, elephant and fur seals and Mesopelagic fish, which occupy the upper 1000 m of the macaroni penguins (Collins et al., 2007; Reid et al., 2006). Guinet world’s oceans, play a key role linking primary consumers such et al. (1996) estimated that the population of king penguins on the as copepods and macro-zooplankton to higher predators such as Crozet Islands consume 745,000 tonnes of lantern fish per year. birds, large pelagic fish and marine mammals. Lantern fish Southern Ocean myctophids are themselves predators of macro- (Myctophidae) are one of the dominant mesopelagic fish families zooplankton such as copepods, amphipods and euphausids, including (see Gjøsaeter and Kawaguchi, 1980) and are ubiquitous in the Antarctic krill (Euphausia superba)(Pakhomov et al., 1996; Pusch world’s oceans, with high diversity and abundance. et al., 2004; Shreeve et al., 2009), with evidence of dietary specialisa- In the Southern Ocean the myctophid fauna includes 33 species tion in different myctophid species (Shreeve et al., 2009). (Hulley, 1981; McGinnis, 1982), with an estimated biomass of The Scotia Sea, which includes 2 million km2 of the Atlantic sector 70–130 million tonnes (Lubimova et al., 1987). They are a key dietary of the Southern Ocean (Hewitt et al., 2004), is a region of complex component of king penguins (Cherel et al., 2002; Olsson and North, bathymetryandoceanography.Itisboundedonthreesidesbythe Scotia Arc (which includes South Georgia, the South Sandwich Islands and the South Orkneys), and is open to the Drake Passage to the west n Corresponding author. Present address: Government of South Georgia and the (Fig. 1). The Scotia Sea is influenced by two oceanographic regimes: South Sandwich Islands, Government House, Stanley, Falkland Islands, FIQQ 1ZZ. Tel.: þ500 28214. the generally eastward flowing Antarctic Circumpolar Current (ACC) E-mail address: [email protected] (M.A. Collins). and the cyclonic circulation of the Weddell Gyre, which meet in the 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.07.003 190 M.A. Collins et al. / Deep-Sea Research II 59-60 (2012) 189–198 Fig. 1. Map of the Scotia Sea, with the locations of non-target stratified RMT25 net hauls indicated for each cruise. The mean locations of the principal fronts are indicated. Weddell-Scotia Confluence. The ACC, which is steered by the bathy- this variability is linked to climate variability (Murphy et al., metry of the Scotia Sea, includes high velocity currents associated 2007), with larval krill highly dependent on winter sea ice and with four major thermohaline fronts. The Sub-Antarctic Front (SAF) krill swarms advected through the region by the high velocity, but separates the ACC from temperate waters to the north, with the extremely variable, currents. In seasons and regions of low krill Southern Boundary (SB) the southern limit of the ACC. The Southern abundance, normally krill-dependent predators switch to alter- Antarctic Circumpolar Current Front (SACCF) crosses the central native food sources, such as the pelagic amphipod Themisto Scotia Sea and wraps around the eastern end of South Georgia, before gaudichaudii and mesopelagic fish (e.g. Reid et al., 2006). These retroflecting to the north and east of the island (Meredith et al., 2003). krill-independent pathways may increase in importance with The Polar Front (PF) lies between the SACCF and SAF and separates evidence of a long-term decline in krill stocks, which is poten- waters with a subsurface temperature minimum to the south from tially linked to climate change (Murphy et al., 2007). warmer waters to the north. Within this area of complex oceano- The role of mesopelagic fish in the Scotia Sea ecosystem is graphy different water masses may becharacterisedbydifferent linked to both the abundance and bathymetric, geographic and fauna, with fronts potentially providing elevated productivity and seasonal distribution of the key species. Studies have been under- putative barriers to stenothermal fauna. taken over limited spatial scales in the Southern Ocean, but here, In contrast to other parts of the Southern Ocean, the Scotia Sea is using opening and closing nets and as part of a broader study of an area of high nutrient concentration and high primary productiv- seasonal and latitudinal patterns in the operation of the Scotia ity (Holm-Hansen et al., 2004), which sustains abundant consumers Sea, we examine broad scale patterns in the abundance and such as copepods and particularly Antarctic krill (Murphy et al., distribution of the mesopelagic fish fauna. 2007). Indeed the Scotia Sea is believed to support around half of the circumpolar krill population (Atkinson et al., 2004). This in turn sustains major populations of higher predators, such as whales, 2. Methods penguins and seals, in short efficient food chains (Murphy et al., 2007). The Scotia Sea is also the primary location of the Antarctic 2.1. Study location and timing krill fishery and also supports fisheries for Patagonian toothfish (Dissostichus eleginoides) and mackerel icefish (Champsocephalus Three cruises were undertaken on the R.R.S. James Clark Ross in gunnari)(Agnew, 2004; Constable et al., 2000). the Scotia Sea, covering the region from the seasonal ice-edge to Whilst krill is particularly abundant in the Scotia Sea region, its the Polar Frontal Zone (Fig. 1). Cruise JR161 was in the austral abundance varies regionally, seasonally and inter-annually and spring (Nov 2006), JR177 in the austral summer (Jan 2008) and M.A. Collins et al. / Deep-Sea Research II 59-60 (2012) 189–198 191 JR200 in the austral autumn (Mar 2009). A winter cruise to the using the SIMPROF procedure and the SIMPER procedure was southern Scotia Sea would be very difficult due to the sea-ice and applied to identify the key species. hence no comparative winter sampling was undertaken. During each cruise oceanographic (Venables et al., 2012), acoustic (Fielding et al., 2012) and biological data were collected at a 3. Results series of stations occupying a transect from the edge of the sea-ice to the Polar Front (PF). Stations were selected to sample the 3.1. Oceanographic context of different stations/locations different water-masses and frontal zones. During each cruise, an oceanographic transect ran from south of 2.2. Oceanographic background the Southern Boundary, within the Seasonal Ice Zone northwards across the Scotia Sea, crossing the Southern Boundary and SACCF Details of the oceanographic transects are provided in (Fig. 2, Venables et al., 2012). The North Scotia Sea (NSS) station Venables et al. (2012). showed characteristics of the very southern edge of the PF, especially in JR161. The Georgia Basin (GB), downstream of South 2.3. Fishing methods and locations Georgia, is north of the Scotia Ridge but, due to a retroflection of the flow, it had a slightly lower dynamic height than the NSS station Mesopelagic fish and pelagic invertebrates were sampled using and was more similar, by water mass properties, to the stations in an opening and closing 25-m2 rectangular mid-water trawl the middle of the Scotia Sea.

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