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Distribution, population structure and trophodynamics of Southern Ocean
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Gymnoscopelus (Myctophidae) in the Scotia Sea.
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Ryan A. Saunders, 2Martin A. Collins, 1Peter Ward, 1Gabriele Stowasser, 1Rachael Shreeve
and 1Geraint A. Tarling
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British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley
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Road, Cambridge, CB3 0ET, UK
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Government of South Georgia and South Sandwich Islands, Government House, Stanley,
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Falkland Islands
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Corresponding author:
Email: ryaund@bas.ac.uk, Tel: +44(0)1223 221275, Fax: +44 (0)1223 362616
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Manuscript in preparation for Polar Biology
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Abstract
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Gymnoscopelus braueri, Gymnoscopelus fraseri and Gymnoscopelus nicholsi are common in
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the Southern Ocean mesopelagic fish community. However, their ecology is poorly
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understood in the region. This study investigated spatial and temporal patterns in their
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abundance, population structure and diets at different times of year within the Scotia Sea to
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ascertain their functional role in the pelagic food web. G. braueri was the most abundant
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species (0.07-0.17 ind. 1000 m-3) throughout the Scotia Sea. G. fraseri was absent from the
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sea-ice sectors and occurred mostly around the Antarctic Polar Front (APF), comprising
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densities of 0.01-0.04 ind.1000 m-3. G. nicholsi occurred in low abundance (<0.01 ind. 1000
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m-3) throughout the region. G. braueri and G. fraseri had a life span of ~4 and 3 years,
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respectively, but spatial variation in their population structures was evident and recruitment
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appeared to occur only around the APF. G. nicholsi had a life span of >4 years. There was
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evidence of seasonal variation in depth distribution, size-related sexual dimorphism and
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vertical segregation in size-classes for each species. Overall, diets were dominated by
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copepods (Metridia spp., Rhincalanus gigas, Pleuromamma robusta) and euphausiids
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(Thysanoessa spp. and Euphausia superba), although G. fraseri did not predate E. superba.
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Regional, seasonal and ontogenetic patterns in diet were evident for all species. This study
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provides new insight into the ecology of these Gymnoscopelus species in the Scotia Sea. Such
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details contribute towards resolving how pelagic food webs are structured in the Southern
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Ocean and their sensitivity to ongoing environmental change.
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Keywords
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Myctophidae; Gymnoscopelus; Feeding ecology; Population dynamics; Scotia Sea
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Introduction
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Myctophids (Family Myctophidae) are the dominant meosopelagic fish in most of the world’s
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oceans in terms of diversity and abundance (Gjøsaeter and Kawaguchi 1980). They have an
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important function in pelagic food webs, linking primary consumers, such as copepods and
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macro-zooplankton (Pakhomov et al. 1996; Gaskett et al. 2001; Pusch et al. 2004; Shreeve et
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al. 2009), to higher predators such as marine mammals, birds and large pelagic fish (Olsson
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and North 1997; Cherel et al. 2002; Collins et al. 2007; Makhado et al. 2008). Myctophids are
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also primary vectors in the transfer of carbon from the sea surface to the mesopelagic zone
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through their extensive vertical migrations (Pakhomov et al. 1996). The ecology of this group
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of fish is therefore an important part of the operation of oceanic ecosystems.
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There is little information on the ecology of myctophids globally, but data are particularly
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scarce in remote high latitude regions, such as the Southern Ocean. The Scotia Sea (Atlantic
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sector) is one of the most productive regions of the Southern Ocean, sustaining high levels of
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secondary production, such as copepods and Antarctic krill (Euphausia superba), which in
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turn sustains major populations of whales, penguins, seals and commercially-targeted fish
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(Atkinson et al. 2001; Murphy et al. 2007b). The Scotia Sea food web is largely centred upon
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Antarctic krill, although other trophic pathways, such as myctophid fish, are important in
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seasons and regions of low krill abundance (Murphy et al. 2007b). Myctophids in the Scotia
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Sea are the primary prey of king penguins (Aptenodytes patagonicus), elephant seals
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(Mirounga leonina) and squid (Martialia hyadesi), and are dietary components of many other
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predators including fur seals (Arctocephalus gazelle), Cape petrels (Daption capense) and
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Patagonian toothfish (Dissostichus eleginoides) (Olsson and North 1997; Casaux et al. 1998;
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Brown et al. 1999; Dickson et al. 2004; Collins et al. 2007). In turn, they are predators of
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copepods, amphipods and euphausiids, including E. superba (Pusch et al. 2004; Shreeve et al.
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2009). Despite their ecological importance in the Scotia Sea, little is known of their seasonal
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distribution patterns, population dynamics and feeding ecology, primarily due to difficulties
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in sampling them appropriately in this remote region.
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The Scotia Sea is a region subject to decadal-scale environmental change. Sea surface
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temperatures in the region have increased markedly in recent years (Whitehouse et al. 2008),
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which together with evidence of substantial reductions in winter sea-ice extent (de la Mare
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1997; Curran et al. 2003) and the possibility of declining krill stocks in this sector (Atkinson
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et al. 2004), has raised concerns for the health of the Scotia Sea ecosystem (Moline et al.
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2004; Murphy et al. 2007a; Flores et al. 2012). With the possibility of further large-scale
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reductions in krill abundance due to continued ocean-warming (Hill et al. 2013), the
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importance of myctophid fish as a krill-independent trophic pathway may increase
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considerably. It is therefore a high priority to obtain more comprehensive data on all
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myctophid species in the region so that we can ascertain the extent to which these fish can
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buffer the ecosystem during sustained periods of low krill abundance, and address hypotheses
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on how myctophids might be impacted by ocean-climate change.
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In the Southern Ocean there are 33 species of myctophid fish, the majority of which are
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found in the Scotia Sea (Hulley 1990). Electrona antarctica and Electrona carlsbergi are
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considered to be the most numerically abundant and biomass dominant species in the region
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(Hulley 1981; Collins et al. 2012), and studies there have revealed a relatively high degree of
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spatial, temporal and ontogenetic variability in their distribution, population structure and
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feeding ecology (Rowedder 1979; Pakhomov et al. 1996; Pusch et al. 2004; Shreeve et al.
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2009; Saunders et al. 2014). Another important genus of myctophid fish in the Scotia Sea
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ichthyofauna is Gymnoscopelus, with Gymnoscopelus braueri, Gymnoscopelus fraseri and
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Gymnoscopelus nicholsi occurring regularly in net haul samples and in the diet of several top
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predators (Piatkowski et al. 1994; Casaux et al. 1998; Pusch et al. 2004; Ciaputa and Sicinski
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2006; Collins et al. 2008; Makhado et al. 2008; Brickle et al. 2009; Collins et al. 2012).
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However, all species within this genus are understudied and many aspects of their basic
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ecology, particularly their trophodynamics, remain unresolved (Kock et al. 2012). The
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available data suggest that most species within this genus are broadly Antarctic or sub-
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Antarctic types, with very similar morphological characteristics and broadly overlapping
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distribution patterns in the Scotia Sea (Marshall 1960; Hulley 1981; McGinnis 1982; Hulley
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1990). Most species are believed to be relatively long-lived (up to ~7 years), attain an adult
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size of between ~60-160 mm (Standard Length, SL) (Hulley 1981; Linkowski 1985;
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Pakhomov et al. 1996; Collins et al. 2008), and feed primarily on copepods, euphausiids and
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hyperiid amphipods (Shreeve et al. 2009). They therefore appear to have similar niche roles
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in the Scotia Sea region. Spatial variations in population structure have been reported for
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some species in the Scotia Sea, including G. nicholsi and G. braueri, although these studies
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were restricted by sampling over limited spatial and temporal scales, and observations were
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often confounded by comparing data from different seasons and years (Hulley 1981;
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McGinnis 1982; Linkowski 1985). Gymnoscopelus diet studies have been similarly restricted
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in their sample coverage (Kozlov and Tarverdiyeva 1989; Oven et al. 1990; Pakhomov et al.
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1996; Pusch et al. 2004; Shreeve et al. 2009) and increased quantitative data at more
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appropriate spatial/temporal scales are clearly warranted for this genus to underpin robust
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food web and ecosystem studies, both in the Scotia Sea and throughout the Southern Ocean
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(Kock et al. 2012).
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This paper presents new data on the distribution of abundance, population structure and diet
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of three of the most frequently encountered Gymnoscopelus species in the Scotia Sea, G.
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braueri, G. fraseri and G. nicholsi, using net samples collected at different times of year
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during three multi-disciplinary research surveys in the Scotia Sea (Collins et al. 2012).
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Spatial, temporal and ontogenetic patterns in their respective diets were also investigated.
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These data are the most comprehensive for these species in the Scotia Sea to date, and
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provide important parameterisations for new food web and ecosystem studies in the region in
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an ocean-climate change context. They also enable us to address how these superficially
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similar species are able to co-occur over large areas of the Southern Ocean.
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Materials and methods
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Study location
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Three research cruises were conducted in the Scotia Sea onboard RRS James Clark Ross
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during October-December 2006 (JR161, austral spring), January-February 2008 (JR177,
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austral summer) and March-April 2009 (JR200, austral autumn). The surveys covered the
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region from the ice-edge to the Antarctic Polar Front (APF), with sampling stations spread
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across the predominant water masses and frontal zones in the region (Fig. 1). Oceanographic
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(Venables et al. 2012), acoustic (Fielding et al. 2012) and biological data (Korb et al. 2012;
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Ward et al. 2012; Whitehouse et al. 2012) were collected at all stations during each survey.
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Sample collection and processing
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Mesopelagic fish were collected using a rectangular mid-water trawl net (RMT25;
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Piatkowski et al. 1994). The system incorporates two 25 m2 nets that can be opened and
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closed sequentially via an electronic downwire control unit. Each net had a 5 mm mesh size
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at the cod-end. The net was towed obliquely at ~2.5 knots for 30-60 mins in each depth zone
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and each deployment was monitored in real-time using a custom-built net-monitor system
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that also logged depth and temperature. Six nominal stations were sampled across the study
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site: Southern Scotia Sea (SSS), Mid Scotia Sea (MSS), Western Scotia Sea (WSS), Northern
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Scotia Sea (NSS), Georgia Basin (GB) and the Polar Front (PF) (Fig. 1). At each station,
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depth-stratified hauls were undertaken at 0-200 m, 200-400 m, 400-700 m and 700-1000 m.
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These zones were sampled by day and night during the spring and summer, but sampling was
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conducted only during hours of darkness in the autumn. Additional net hauls were undertaken
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opportunistically on acoustically detected mesopelagic fish aggregations. These hauls were
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primarily undertaken at the Polar Front and were omitted from calculations of fish density
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and biomass.
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Net haul samples were sorted onboard to the lowest taxonomic level possible (Hulley 1990).
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Total catch weights per fish species were recorded using a motion-compensated balance. All
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fish were measured to the nearest mm using standard length (SL). Where possible, sex and
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gonad maturity status was recorded from a subsample, with maturity recorded as: 1) Juvenile
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(gonad absent), 2) Immature (gonad visible, but immature and reduced), 3) Developing
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(gonad visible and maturing), 4) Mature (gonad fully developed), 5) Gravid female (ovaries
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full of oocytes and ready to spawn, and 6) Spawned female (ovaries large, but no/few oocytes
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visible). Stomachs were dissected from a random subsample of 25 fish per net haul or from
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each fish where catches were small. All stomachs were frozen for subsequent microscopic
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analyis back at the laboratory. Concurrent Longhurst-Hardy Plankton Recorder (LHPR),
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RMT8 and Bongo net samples (with mesh sizes of 200 µm, 3mm and 53 µm, respectively)
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were examined for myctophid larval stages that might have been missed by the RMT25 net.
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Stomach contents analysis
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Following Shreeve et al. (2009), fish stomach contents were thawed and sorted to the lowest
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taxonomic level the state of digestion would allow. Individual prey items were enumerated
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and weighed. If the prey was highly disaggregated, the weights of component species were
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estimated as a proportion of the weight of the total contents. Prey items that were completely
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undigested were considered to represent trawl feeding and were removed from the analysis.
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Diet was expressed using four measures: 1) percentage frequency of occurrence (%F), 2)
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percentage mass (%M), 3) percentage number (%N) and 4) percentage Index of Relative
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Importance (%IRI) (Cortes 1997). The %IRI was calculated for prey species and %IRIDC was
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calculated for prey categories (Main et al. 2009; Shreeve et al. 2009). Initial prey categories
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used in the analysis were: Amphipods, Copepods, Euphausiids, Ostracods, Molluscs,
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Urochordata, Unidentified crustaceans and Other taxa. A more detailed analysis was also
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performed for the nine most numerically dominant prey categories: Copepods Metridia spp.,
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Pleuromamma robusta, Rhincalanus gigas, Other copepods, Euphausiids E. superba,
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Thysanoessa spp., Ostracoda, Hyperiid amphipods Themisto gaudichaudi and Other taxa
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(Mollusca Urochordata, Unidentified crustaceans). The %IRI was calculated as:
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%𝐼𝑅𝐼𝑖 =
(%𝑁𝑖 + %𝑀𝑖 ) ×
𝑛
∑𝑖=1(%𝑁𝑖 + %𝑀𝑖 )
%𝐹𝑖
× 100
× %𝐹𝑖
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where i is prey item.
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The %IRI for each prey category was calculated in this way for all three myctophid species to
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investigate variations in diet between fish pooled by region, cruise (a proxy for season), size
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class, and sex and maturity status. Myctophid size classes were derived from the composite
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length-frequency distributions. G. braueri and G. fraseri individuals were categorised as
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those below µ-σ (smallest size class), those within µ±σ (mid size class), and those above µ+σ
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(largest size class), and the size classes for G. nicholsi were defined as those above and below
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µ due to relatively low numbers to stomachs. The ±95% confidence limits for the mean %IRI
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of each prey category were calculated using a bootstrapping technique, whereby each species
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dataset (individual stomachs) was re-sampled (with replacement) 1000 times (Main et al.
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2009).
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Length-frequency and population structure analysis
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A series of Kolmogorov-Smirnov (K-S) tests were conducted on the pooled length-frequency
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distributions (5 mm bins) to investigate possible differences in population structure between
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regions. These tests were performed for each species at stations where there were >50
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individuals. Component-fitting software (CMIX)(de la Mare 1994) was then used to fit
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normal distributions to the composite length-frequency data and identify modes in each
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season following the approach detailed in Saunders et al. (2007). In brief, a series of runs
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were performed during the analysis based on the presence of one to four cohorts in the data.
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The best component fit to the observed data was determined using a Chi-squared test. No
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constraints were placed on the mean length, variance or proportions expected within each
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component when fitting the mixed distributions. Spatial differences between gender sizes and
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depth zones were investigated using a series of Students t-tests performed on data aggregated
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across all surveys.
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The timing of spawning and larval ecology for all Gymnoscopelus species in the Southern
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Ocean is presently unresolved, but mature and gravid females for some species, such as G.
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fraseri and G. braueri, have been observed in winter (June-August) around the Subtropical
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Front (Hulley 1981), and eggs and larvae of these species were observed in the southwest
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Atlantic at a similar time (Efremenko 1986). Based on these observations, we assumed that
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spawning for all species was timed so that hatching coincided with the main phytoplankton
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bloom in spring (September/October). We therefore considered individuals as belonging to a
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0-group from the time of hatching until 31 October the following year, to a I-group from 1
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November to 31 October the next year, and so on. Identification of cohorts based on modal
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size relative to the timing of hatching was aided by published growth rates of high latitude
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and temperate myctophids (Smoker and Pearcy 1970; Gjøsaeter 1978; Linkowski 1985;
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Greely et al. 1999). Although the data were not collected in consecutive seasons, and
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therefore interannual effects cannot be accounted for, our analyses provide the most
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comprehensive synopsis of seasonal variations in Gymnoscopelus population structure to
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date.
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Results
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Oceanographic context
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During each survey, stations in the SSS were situated in the sea-ice sectors south of the
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Southern Boundary of the Antarctic Circumpolar Current (SB-ACC; Fig. 1), where mean
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surfaces temperatures ranged from -1.6 to 1.5°C. Further north, stations in the WSS and MSS
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lay close to the South Antarctic Circumpolar Current Front (SACCF). Mean surface
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temperatures were around -0.2 to 2.1°C in these regions. The NSS and GB stations were
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situated between the SACCF and the APF, where surface temperatures were around 1.5 to
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4.0°C. All PF stations were situated north of the southern Antarctic Polar Front (S-PF) and
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these waters had the warmest surface temperatures on all surveys (~4.0 to 5.5°C). The
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physical oceanography is detailed more comprehensively in Venables et al. (2012) and
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Whitehouse et al. (2012).
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Distribution
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A total of 143 non-targeted net hauls were conducted during the three surveys (Table 1). All
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stations were sampled repeatedly to a varying degree during the study period, except for the
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WSS where sampling was predominantly confined to the spring survey (Fig. 1). The main
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Gymnoscopelus species caught during the study were G. braueri, G. fraseri, G. nicholsi, G.
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opisthopterus and G. piabilis. G. bolini and G. hintonoides were also caught in low numbers
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(n< 3), along with a species that could not be identified by morphological examination,
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termed here as Gymnoscopelus sp. (n= 9).
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G. braueri: The species occurred throughout the Scotia Sea and was the most abundant
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Gymnoscopelus species encountered at most stations (Fig. 1; Table 1). G. braueri comprised
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survey mean densities and biomass between 0.07-0.17 ind. 1000 m-3 and 0.55-1.13 g 1000 m-
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and summer (mean: ~0.12 ind. 1000 m-3), but abundance was greatest in regions south of the
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SACCF in the MSS and SSS during autumn (mean: 0.24 ind. 1000 m-3). Correspondingly, G.
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braueri biomass was highest in the GB during spring (mean: 0.78 g 1000 m-3) and summer
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(mean: 0.84 g 1000 m-3), but in autumn its biomass was greatest at the PF station (mean: 1.37
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g 1000 m-3).
, respectively (Tables 1 and 2). G. braueri abundance was highest in the GB during spring
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G. fraseri: This species was distributed predominantly north of the SACCF between the NSS
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and PF, and was mostly absent from the SSS (Fig. 1). G. fraseri comprised a mean abundance
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of around 0.02 ind.1000 m-3 and a mean biomass of 0.02-0.08 g 1000 m-3 (Table 1 and 2).
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The greatest concentrations of abundance occurred in autumn at the PF (mean: 0.04 ind.1000
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m-3), but its biomass was greatest in the GB during spring and summer (mean: ~0.11 g 1000
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m-3). A single specimen was caught in the SSS on the autumn survey during a relatively short
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haul, which resulted in a relatively high mean biomass for the region (mean: 0.13 g 1000 m-3)
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due to its large body size and the low volume filtered by the net. Excluding this individual, its
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mean biomass was greatest around the PF during this time (0.07 g 1000 m-3).
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G. nicholsi: The species occurred in low abundance (mean: <0.01 ind. 1000 m-3) at most
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stations during the surveys (Fig. 1; Table 1). However, it comprised a relatively high overall
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mean biomass (0.13-0.28 g 1000 m-3), with the greatest concentrations occurring in the MSS
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during both spring (mean: 0.30 g 1000 m-3) and summer (mean: 0.70 g 1000 m-3), and in the
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SSS during autumn (mean: 0.19 g 1000 m-3; Table 2).
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G. opisthopterus: This species was caught mainly on the summer survey where it occurred in
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low abundance (mean: <0.01 ind. 1000 m-3) throughout the Scotia Sea (Table 1 and Fig. 1).
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Its overall mean biomass ranged between 0.01-0.16 g 1000 m-3 and the greatest
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concentrations occurred in the SSS in summer (mean: 0.36 g 1000 m-3; Table 2). The species
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was found only south of the SACCF in spring and autumn.
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G. piabilis: The species was caught only at the MSS and PF during the spring survey (Fig. 1),
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where the species comprised a relatively low mean abundance and biomass of <0.01 ind.
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1000 m-3 and ~0.04 g 1000 m-3, respectively.
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Vertical distribution
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G. braueri: Daytime net catches were consistently low and the species was seldom caught
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above 400 m during this time (Fig. 2a). The species was confined to 0-400 m at night in
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spring, but it occurred as deep as 700 m in summer and autumn. At night, numerical
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abundance was greatest between 0-200 m on all surveys, particularly in autumn, whilst
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biomass was typically highest between 201-400 m.
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G. fraseri: This species occurred predominantly in the upper 400 m at night (Fig. 2b). During
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this time, its abundance and biomass was greatest between 0-200 m in spring and summer
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and between 201-400 m in autumn. Overall daytime catches were low in spring and the
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species was only caught between 401-700 m. However, greater concentrations were caught
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above 400 m in summer, particularly at 201-400 m. There was evidence of some diel vertical
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migration during this season, with peak daytime concentrations in abundance moving up
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from 201-400 m to 0-200 m at night.
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G. nicholsi: The species was caught mostly between 401-700 m during the daytime (Fig. 2c).
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At night, the species occurred predominantly in the upper 400 m in spring and summer, with
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peaks in abundance and biomass typically occurring at 201-400 m. Both abundance and
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biomass in autumn were comparatively low at each depth interval and the population
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appeared to be spread between 0-700 m.
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G. opisthopterus: This species seldom occurred above 400 m at night during all seasons and
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the greatest concentrations of abundance and biomass occurred mostly at 701-1000 m (Fig.
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2d). The species was seldom caught shallower than 700 m during the daytime.
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G. piabilis: The species was only caught in low abundance during the night in spring, where
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its depth distribution was spread between 0-700 m.
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Other species: Both G. bolini and Gymnoscopelus sp. were caught between 201-1000 m
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during the surveys and the single G. hintonoides specimen was caught between 401-700 m.
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Population structure
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G. braueri: The species ranged from 35-135 mm in size and its overall life span was at least
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four years (Fig. 3a). Length-frequency histograms showed that three size-, and presumably
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age-, classes were present in the spring population: the newly recruited II-group (mode: 49
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mm), III-group (mode: 73 mm) and IV+ group (mode: 105 mm). Newly spawned larvae and
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the 0-group were absent in all biological net samples during all three surveys. Approximately
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14% of the population were juvenile at this time and, of the adult component of the
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population, ~80% had developing gonads and ~18% had immature gonads. The new I-group
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(mode 50 mm) was first evident in the population in summer and both the II-group (mode: 70
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mm) and III-group (mode: 89 mm) evident in spring had increased in size by this time. A
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reduction in the proportion of IV+ individuals (mode: 112 mm) was evident from springtime,
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suggesting that a proportion of this group may have died out of the population. Around 36%
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of the summer population was juvenile and almost all adults had developing gonads (~96%).
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By autumn, there was evidence of growth for the I-group (mode: 52 mm), II-group (mode: 77
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mm) and III-group (mode: 109 mm), but the remaining IV+ group evident in spring had
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either died out from the population, or was now indistinguishable in size from the III-group
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individuals. These three cohorts would presumably over-winter and recruit into the II-group,
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III-group and IV+ group the following spring. Maturity status of the autumn population was
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very similar to that in summer. In all seasons, the juvenile I-group (modes: ~50 mm) occurred
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almost exclusively at the PF and NSS stations and was markedly absent from the population
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further south in the Scotia Sea (Fig. 4a). This spatial variation in population structure was
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supported by a series of K-S tests which showed that composite length-frequency
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distributions at the SSS, MSS and GB stations were significantly different (P <0.01) from
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those in the NSS and PF on each survey. The overall ratio of adult females to adult males was
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approximately 1:1 during all surveys and in all regions.
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G. fraseri: This species had an overall size range between 40-115 mm and a life span of
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approximately three years (Fig. 3b). The number of samples was low in spring and only one
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age-class, the III-group (mode: 71 mm) was evident in the population. There was no evidence
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of larvae or the 0-group during any of the surveys. Three age-classes were present in the
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summer population: the I-group (mode: 50 mm), II-group (mode: 66 mm) and the III-group
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(mode: 83 mm) that had increased in size from spring. Around 15% of the population were
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juvenile at this time and all adults had developing gonads. The population size and maturity
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structure was very similar in autumn and there was little growth from the summer period for
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the I-group (mode: 49 mm), II-group (mode: 70 mm), and III-group (modes: 83 mm). Our
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data showed that juvenile I-group (<55 mm) was largely confined to the PF station during the
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surveys (Fig. 4b). The length-frequency histograms indicated that G. fraseri cohorts older
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than I-group were smaller than those of G. braueri and G. nicholsi in comparative seasons
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indicating lower rates of growth. The overall ratio of adult females to adult males was
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consistently around 4:1.
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G. nicholsi: The overall size range of the species during the surveys was 30-165 mm (Fig 3c).
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Only data from the summer survey was used to provide an estimate of the species population
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dynamics, as too few samples were obtained in spring and autumn. There were three age-
365
classes present in the summer population: the I-group (mode 45 mm), III-group (mode 110
366
mm) and IV+ group (mode 146 mm). Individuals between 50-105 mm in size, presumably
367
belonging to the II-group, were markedly absent from the population at this time. The 0-
368
group and larvae were also absent. Juveniles comprised ~11% of the population in summer
369
and all adults had developing gonads. Overall, the species appeared to have a life span of
13
370
more than four years and rates of adult growth (III- and IV-group) were notably greater than
371
for G. braueri and G. nicholsi, as they had attained a greater size in comparative seasons. The
372
overall ratio of adult females to adult males was ~2:1.
373
374
G. opisthopterus: There was only sufficient data from the summer survey to describe the
375
population structure for this species (Fig. 3d). The species had a size range of 55-195 mm and
376
four age-classes were evident in the summer population: the II-group (modes: 62 mm), the
377
III-group (mode: 81 mm) IV-group (mode: 114 mm) and V+ group (mode: 157 mm). Both
378
the 0-group and I-group (<50 mm) were absent from the population, but an approximate
379
lifespan of at least five years was apparent for the species.
380
381
Other species: Population and size analyses were not possible for G. piabilis, G. bolini, G.
382
hintonoides, or Gymnoscopelus sp. The size range of G. piabilis and G. bolini was 80-149
383
and 178-208 mm, respectively. Gymnoscopelus sp. had a size range of 37-106 mm and the G.
384
hintonoides specimen measured 170 mm.
385
386
Gender- and depth- based size differences
387
G. braueri: Overall, adult females (mean: 99.9 mm) were significantly larger in size than
388
adult males (mean 87.8 mm) (P<0.001). Adult males did not differ in size between regions
389
(P>0.05), but females in the SSS (mean: 104.6 mm) were significantly (P<0.01) larger than
390
those in any other region by ~9 mm. There was a distinct vertical stratification in mean size
391
for the species (Fig. 5a). Females were greater in size at depth (701-1000 m) than those in the
392
surface zone (0-200 m) by 22.7 mm. The mean size of males was also greater below 400 m
393
than in the 0- 200 m zone by 8.1 mm, although the largest G. braueri males (mean: 94.7 mm)
394
tended to occur at 201-400 m.
395
396
G. fraseri: Adult females had a significantly (P<0.05) larger mean size than adult males (68.2
397
mm compared to 60.8 mm). Regional variation in mean size was not apparent for males or
398
females from the available data. There was some evidence that specimens were larger in size
14
399
at depth (401-700 m), but few data were collected in this zone (n= 5) and there was no
400
significant (P>0.05) difference in mean size between 0-200 m and 201-400 m (Fig. 5b).
401
402
G. nicholsi: There were insufficient data for regional comparisons of male and female sizes
403
for G. nicholsi. Adult females were significantly (P<0.01) greater in size than males (means:
404
141.4 mm compared to 126.8 mm). Vertical separation in size classes was apparent, with an
405
increase in mean size of ~15-22 mm between the upper- and lower-most depth zones for both
406
sexes (Fig 5c).
407
408
G. opisthopterus: Comparisons of gender size were not possible for this species. There was
409
no significant (P>0.05) difference in mean body size between the 401-700 m and 701-1000 m
410
depth zones, but the numbers of measurements per depth strata were relatively low (n< 40).
411
412
Patterns in diet
413
G. braueri: A total of 372 stomachs were examined for G. braueri. The species had a
414
relatively broad diet predating several species of amphipods, copepods, euphausiids,
415
ostracods and molluscs (Table 3). Copepods dominated the diet (60% IRI), with Metridia
416
spp. (47 %IRI), R. gigas (7% IRI) and P. robusta (5% IRI) the most predated species in this
417
group. Euphausiids also formed a substantial part of the overall diet (35% IRI), particularly
418
Thysanoessa spp. (19 %IRI). E. superba and unidentified euphausiids formed a
419
comparatively small proportion (~4% IRI) of the diet. Other important prey species included
420
T. gaudichaudii (~5% IRI).
421
422
There were clear spatial, temporal and ontogenetic variations in G. braueri diet (Fig. 6).
423
Predation on Metridia spp. was greatest in the MSS and SSS (32-58% IRI), but these
424
copepods decreased in the diet with decreasing latitude towards the PF (6% IRI; Fig. 6a).
425
Other copepod species, comprising mostly Paraeuchaeta spp. and Calanus simillimus, also
426
increased in prevalence along this cline from the MSS, and P. robusta and R. gigas were
427
predated most around the GB and PF (~7-20% IRI). E. superba was predated in small
15
428
quantities (<5% IRI) in all regions, except the GB where it formed a substantial part of the
429
diet (34% IRI). Predation on Thysanoessa spp. was highest between the SSS and NSS (25-
430
47% IRI) and T. gaudichaudii occurred mostly in the diet in the NSS (21% IRI).
431
432
From a temporal perspective, the species fed mostly on Metridia spp. during autumn (50%
433
IRI) and R. gigas occurred predominantly in the diet during the spring (Fig. 6b). T.
434
gaudichaudii and E. superba were most prevalent in the diet during summer. P. robusta and
435
ostracods were also predated mostly in spring and summer.
436
437
Specimens <58 mm and between 58-105 mm took the greatest proportions of Metridia spp.
438
(37-46% IRI), whilst those >105 mm only predated these copepods in small proportions (5%
439
IRI; Fig. 6c). Only specimens >58 mm predated E. superba and the proportion of
440
Thysanoessa spp. in the diet increased with increasing size. The proportion of ostracods in the
441
diet also increased with increasing size. All size classes took T. gaudichaudii and R. gigas,
442
but those <58 mm predated the greatest proportions of P. robusta (11% IRI).
443
444
G. fraseri: A total of 103 G. fraseri stomachs were examined during the study. The species
445
fed predominantly on copepods (79% IRI), particularly Metridia spp. (58% IRI), R. gigas
446
(10% IRI) and P. robusta (4% IRI; Table 3). The species also predated euphausiids (19%
447
IRI), but only Thysanoessa spp. and not E. superba. Stomach samples were collected
448
predominantly from just two regions, the GB and the PF, but there was clear evidence of
449
variation in diet between these two stations (Fig. 7a). Greater proportions of Metridia spp.,
450
occurred in the diet in the GB (67% IRI) than at the PF (30% IRI), but the proportion of
451
Thysanoessa spp. was notably lower in the region (15% compared to 41% IRI). Other
452
copepod species, comprising mostly C. simillimus and Calanoides acutus, were also more
453
prominent in the diet at the PF.
454
455
There was some evidence of temporal variation in diet, as R. gigas comprised a greater
456
proportion of the diet in spring (27% IRI) than summer and autumn (3-7% IRI), and
16
457
Thysanoessa spp. were most prominent in the diet in autumn (40% IRI; Fig. 7b). The largest
458
specimens (>80 mm) predated T. gaudichaudii, but this size class took the fewest Metridia
459
spp. (27% IRI; Fig. 7c). R. gigas was absent from the diet in fish <53 mm.
460
461
G. nicholsi: The total number of stomachs examined for this species was 40. The diet was
462
dominated by copepods (64% IRI), with Metridia spp. (34% IRI) and R. gigas (30% IRI) and
463
P. robusta (10% IRI) the main species targeted in this group (Table 3). Euphausiids
464
accounted for around 35% IRI and the main species predated in this group was E. superba
465
(15% IRI). All other prey items, including amphipods and ostracods, represented only minor
466
components of the species diet (<1% IRI).
467
468
Due to the low number of samples per region, data were spatially aggregated to compare the
469
diet north and south of the SACCF (Fig. 8a). The species predated greater proportions of R.
470
gigas and P. robusta in regions north of the SACCF, whilst Metridia spp. and E. superba
471
were taken most in regions south of this front. T. gaudichaudii was seldom predated in
472
regions south of the SACCF. Clear temporal variations in diet were apparent, with G. nicholsi
473
predating R. gigas and P. robusta more in spring than in summer and autumn (Fig. 8b).
474
Fewer Metridia spp. were present in the diet during this time and predation on these copepods
475
was highest in autumn. E. superba and T. gaudichaudii seldom occurred in the diet in spring
476
and predation on E. superba was highest in summer.
477
478
There were clear size-related differences in diet, as specimens <125 mm took predominantly
479
R. gigas and P. robusta and those >125 mm predated more of the other copepods, particularly
480
Metridia spp. (Fig. 8c). Only specimens >125 mm took E. superba and T. gaudichaudii.
481
482
G. opisthopterus: Only 5 stomachs were obtained for this species from the spring survey.
483
From the available data, its diet was dominated by euphausiids (47% IRI), copepods (35%
484
IRI) and unidentified amphipods (14% IRI). Ostracods (3% IRI) and unidentified prey items
485
(<1% IRI) were also present in the diet. E. superba (23% IRI) and Thysanoessa spp. (9% IRI)
17
486
were the main euphausiid species predated, and the dominant copepods were C. acutus (13%
487
IRI) and Metridia spp. (11% IRI).
488
489
Discussion
490
Gymnoscopelus braueri
491
G. braueri was spread throughout the Scotia Sea and was the most biomass-dominant
492
Gymnoscopelus species encountered during the investigation. This is in accordance with
493
previous studies which report that G. braueri is one of the most abundant myctophids in the
494
Southern Ocean (Hulley 1981; McGinnis 1982; Piatkowski et al. 1994; Pusch et al. 2004;
495
Collins et al. 2008; Donnelly and Torres 2008; Flynn and Williams 2012). The species has
496
been described as a broadly Antarctic type, with a distribution pattern extending from the
497
Antarctic Continent to the Subtropical Front (STF) and the greatest concentrations generally
498
in regions south of the APF (Hulley 1981). During all surveys, the species was mostly
499
distributed in the upper 400 m at night, but was seldom caught above 700 m during the
500
daytime, indicative of net avoidance and possibly some diurnal vertical migration (DVM) by
501
the species. G. braueri occupied all of the major water masses in the region (Circumpolar
502
Deep Water (CDW): >200 m, Winter Water (WW): ~100-200 m, and Antarctic Surface
503
Waters (ASW): <100 m) (Venables et al. 2012). Similar patterns in depth distribution and
504
DVM have been reported previously for G. braueri in the Scotia Sea area (Piatkowski et al.
505
1994; Pusch et al. 2004; Collins et al. 2008; Donnelly and Torres 2008). However, the
506
species has also been observed to occur principally at depths below 700 m in regions towards
507
the STF and over the Macquarie Ridge (Pacific sector) (Hulley 1981; Flynn and Williams
508
2012). These studies suggested that increased surface temperatures near the STF, and
509
increased current velocities in the upper water column along the Macquarie Ridge, may have
510
caused the population to occur deeper in the water column to avoid thermal stress and
511
increased advection forces in these two regions, respectively. Consistent with other studies,
512
ontogenetic patterns in vertical distribution were apparent for G. braueri, particularly
513
females, with larger fish occurring progressively deeper in the water column (Collins et al.
514
2008).
515
18
516
Our results showed that G. braueri had an overall life cycle of at least four years in the study
517
region, but there were clear differences in population structure between regions associated
518
with the APF and those situated further south in the Scotia Sea. Four cohorts (I-IV+ groups)
519
were consistently present in the APF population and seasonal growth and recruitment was
520
evident, despite the absence of gravid and post-spawning females, eggs, and larvae (0-group)
521
in all biological sampling devices (including RMT8, LHPR and Bongo netsused at the same
522
stations throughout the surveys. However, the juvenile I-group was markedly absent from the
523
Scotia Sea population south of the APF and only two cohorts were prevalent in this region;
524
one presumably comprised of II-group specimens and the other comprised of III-group
525
specimens and greater. This pattern in population structure is consistent with data from South
526
Georgia (Collins et al. 2008) and the South Shetland Islands (Pusch et al. 2004), and supports
527
the notion that spawning and recruitment is confined to waters north of the APF (Hulley
528
1981; McGinnis 1982; Linkowski 1985; Efremenko 1986) in the Southern Ocean. These
529
previous studies also reported distinct spatial variations in body size and ontogeny for G.
530
braueri in the southwest Atlantic, with small juveniles (<40 mm) tending to dominate
531
populations in waters towards the STF, and progressively larger and older specimens more
532
prevalent in populations further south. G. braueri eggs and larvae also appear to be rare south
533
of the APF (Efremenko 1986). It has therefore been hypothesised that G. braueri undertakes
534
specific ontogenetic migrations to waters north of the APF to spawn and then subsequently
535
migrate away from the newly spawned cohort to the Scotia Sea to feed (Hulley 1981;
536
McGinnis 1982; Linkowski 1985; Zasel'sliy et al. 1985; Efremenko 1986; Collins et al.
537
2008).
538
539
G. braueri predated mostly copepods with Metridia spp., R. gigas, and P. robusta dominating
540
this component of the prey field. Thysanoessa spp. formed an important part of the diet, but
541
E. superba was only predated in relatively small amounts. Another important prey species
542
was T. gaudichaudii. Comparisons of diet with data from previous studies are difficult
543
primarily due to the low sample sizes (Pakhomov et al. 1996; Gaskett et al. 2001; Pusch et al.
544
2004; Flynn and Williams 2012), size selective net sampling and semi-quantitative analyses
545
(Kozlov and Tarverdiyeva 1989), and spatially and temporally restricted sampling (Shreeve
546
et al. 2009). In general, our results contrasted markedly with those of Shreeve et al. (2009)
547
and Kozlov & Tarverdiyeva (1989) who reported, respectively, that G. braueri predated
19
548
mostly T. gaudichaudii in the northern Scotia Sea and E. superba in the Lazarev Sea and
549
Kosmonavtov Sea (Indian sector). Spatial, temporal and ontogenetic variation in G. braueri
550
diet was apparent in our study. Predation on Metridia spp. decreased northwards to the APF,
551
whilst predation on R. gigas, P. robusta and other copepods increased along this cline. The
552
species only predated T. gaudichaudii in the northern regions of the Scotia Sea, whilst
553
predation on Thysanoessa spp. was greatest in the more southern regions. These trends
554
corresponded with the general regional distributional patterns of these prey species during the
555
surveys (Ward et al. 2012; Saunders et al. 2014). However, predation on E. superba was
556
greatest around the GB in summer, a region where this prey species was caught in relatively
557
low abundance during the surveys. The apparent seasonal variation in diet did not correspond
558
well with the observed temporal variation in the abundance of its main prey species in the
559
region, and the reasons for these trends were unclear from our data. Seasonal variations in
560
diet may be related to changes in prey ontogeny or behaviour (Shreeve et al. 2009), although
561
more data at an increased spatial and temporal resolution are clearly required to address such
562
hypotheses. The largest sized specimens predated the greatest amounts of E. superba,
563
Thysanoessa spp. and T. gaudichaudii suggesting size-related prey selectivity within this
564
myctophid species. The ability to predate larger sized organisms, such as E. superba and T.
565
gaudichaudii could be controlled by gape size and body size, such that only the larger sized
566
fish are able to capture and consume these animals (Karpouzi and Stergiou 2003).
567
Consequently, smaller sized fish may selectively target smaller Metridia spp. because they
568
are of a more suitable size to predate(Bradford-Grieve et al. 1999).
569
570
Gymnoscopelus fraseri
571
G. fraseri was found predominantly in waters around the GB and APF, but seldom occurred
572
south of the SACCF and, except for one individual, was absent from the sea-ice sector. G.
573
fraseri has been described as a sub-Antarctic species with its centre of distribution occurring
574
in regions north of the APF and not extending south of this front in waters less than 1.5-2.0°C
575
(Hulley 1981). In the Scotia Sea, G. fraseri has been reported previously south of the APF at
576
South Georgia (Piatkowski et al. 1994; Collins et al. 2008), and around the South Shetland
577
Islands (Pusch et al. 2004), although only two specimens were caught in the latter study.
578
Studies around the Kerguelen Islands (Indian sector) showed further that G. fraseri has a
20
579
distribution pattern that is highly associated with the APF, where the species is an integral
580
component of the mesopelagic fish community (Duhamel 1998).
581
582
In our study, G. fraseri occurred predominantly in the upper 400 m during the night, with the
583
greatest concentrations occurring between 0-200 m in spring and summer, but slightly deeper
584
at 201-400 m in autumn, indicative of a seasonal deepening in depth distribution that may
585
have been a response to a seasonal deepening of its main copepod prey species at this time
586
(Ward et al. 2012). The species still inhabited ASW, WW and CDW despite being largely
587
confined to 0-400 m (Venables et al. 2012), but, unlike many other myctophid fish, it was
588
caught in relatively high abundance in the upper water column during the daytime in summer,
589
suggesting a relatively low degree of daylight net avoidance at this time (Duhamel et al.
590
2000; Collins et al. 2012). There was also evidence of some diel vertical migration for the
591
species similar to that observed previously at South Georgia (Collins et al. 2008) and around
592
the Kerguelen Islands (Duhamel et al. 2000). There was no difference in size between depth
593
zones over the species main vertical range (0-400 m) and this is in accordance with Collins et
594
al. (2008). However, this could have been a function of the coarse vertical resolution of the
595
net sampling over the species vertical range.
596
597
The available data showed that G. fraseri had a life span of around three years and it
598
appeared to have slightly lower rates of adult growth than G. braueri and G. nicholsi. There
599
were distinct regional differences in population structure as the juvenile I-group (<55 mm)
600
was present in waters associated with the APF, but was clearly absent in the colder waters of
601
the Scotia Sea. Larval stages (0-group) were also absent during the three surveys. Our results
602
are in accordance with previous observations from around South Georgia that also showed an
603
absence of juveniles in the Scotia Sea, supporting the notion that this species does not spawn
604
and recruit in regions south of the APF (Collins et al. 2008). Similar to G. braueri, G. fraseri
605
appears to have ontogenetically separated populations in the southwest Atlantic, with adults
606
principally distributed around the APF and juveniles (<45 mm) occurring much further north
607
of this front (by 10-15°) towards the STF (Hulley 1981; McGinnis 1982). Gravid females
608
also tended to occur in the more northern regions of the southwest Atlantic during summer
609
(November-December), but not at the APF or in regions south of this front (McGinnis 1982),
21
610
further suggesting that this predominantly sub-Antarctic species is either an expatriate in
611
Antarctic waters such as the Scotia Sea, or undertakes distinct ontogenetic (e.g. spawning)
612
migrations between regions north and south of the APF (Hulley 1981; McGinnis 1982;
613
Zasel'sliy et al. 1985; Efremenko 1986; Collins et al. 2008).
614
615
The diet of G. fraseri was dominated by copepods and the species predated mostly Metridia
616
spp., R. gigas and P. robusta. Thysanoessa spp. and T. gaudichaudii were also predated, but
617
E. superba was notably absent from the diet. Few studies have focussed on the diet of G.
618
fraseri in the Southern Ocean and the data available prior to our investigation were limited to
619
relatively small sample sizes (<53 stomachs) making it difficult to substantiate the species
620
main dietary components (Gaskett et al. 2001; Shreeve et al. 2009). Our data are, however,
621
consistent with those available from the northern Scotia Sea (Shreeve et al. 2009). Regional,
622
seasonal and ontogenetic variations in G. fraseri diet were apparent during the study, as the
623
species took more Metridia spp. in the GB than at the PF. This trend corresponded broadly
624
with an increase in abundance of these copepods in the GB region (Ward et al. 2012). Other
625
copepods, such as C. simillimus and Paraeuchaeta spp., occurred more in the diet at the PF,
626
which also corresponded to their increased abundance in the region compared to the GB
627
(Ward et al. 2012). However, Thysanoessa spp. abundance was similar between the two
628
stations (Saunders et al. 2014), suggesting that G. fraseri may actively target these
629
euphausiids more when Metridia copepods are less readily available in the water column.
630
Seasonal changes in diet did not match seasonal changes in prey abundance in the region
631
(Ward et al. 2012), indicating that temporal variations in diet might not just be related to
632
temporal variations in prey abundance. Only the largest specimens took T. gaudichaudii and
633
the smallest specimens did not predate the relatively large sized copepod R. gigas
634
(Boltovskoy 1999), suggesting size-related predation for this myctophid species similar to
635
that discussed for G. braueri..
636
637
Gymnoscopelus nicholsi
638
G. nicholsi occurred in low abundance at most stations throughout the Scotia Sea, with the
639
greatest concentrations of biomass generally occurring in regions south of the SACCF. G.
22
640
nicholsi is considered a broadly Antarctic species and has a circumpolar distribution that
641
extends from the Antarctic continent to the STF at around 40 °S (Hulley 1981; McGinnis
642
1982). In the Scotia Sea region, the species has been reported to occur mostly around the APF
643
(Hulley 1981; McGinnis 1982), but it is also common around South Georgia (Collins et al.
644
2008), the South Shetland Islands (Linkowski 1985; Lubimova et al. 1987; Pusch et al. 2004)
645
and the Antarctic Peninsula (Donnelly and Torres 2008), where it comprises a relatively high
646
biomass within the myctophid fish community.
647
648
Our data showed that G. nicholsi had a life span in excess of four years and the overall
649
population structure of this species was consistent with other observations in the region
650
(Linkowski 1985; Collins et al. 2008). Linkowski (1985) reported that the species attains a
651
maximum age of around seven years and it seems apparent from our data that it has slightly
652
higher rates of adult growth than G. braueri and G. nicholsi. There was insufficient data to
653
examine spatial patterns in population structure across the Scotia Sea, but such variation has
654
been observed previously (Hulley 1981; Linkowski 1985; Lubimova et al. 1987) and is
655
apparent from other data sets (Pusch et al. 2004; Collins et al. 2008; Donnelly and Torres
656
2008). Again, the underlying trend appears to be that G. nicholsi populations tend to contain
657
multiple cohorts and are dominated by smaller juvenile stages (<80 mm, SL) in regions north
658
of the APF, but south of this front, populations contain fewer cohorts and are comprised of
659
predominantly larger specimens (~110-165 mm, SL). This trend is particularly apparent in
660
the sea-ice sectors where only single cohorts of large (mode: ~140 mm; range: 120-160 mm,
661
SL) and older (5-7 years) adults occur in the population (Linkowski 1985; Pusch et al. 2004;
662
Donnelly and Torres 2008). Linkowski (1985) observed that small juvenile stages did not
663
occur south of 60 °S in the Scotia Sea and Efremenko (1986) reported that G. nicholsi eggs
664
were largely confined to the waters around the APF.
665
666
Collectively, the available data indicate that G. nicholsi is most likely a sub-Antarctic species
667
similar to G. braueri and G. fraseri, and that the species is possibly an expatriate in Antarctic
668
waters, particularly in the sea-ice sectors where there is no evidence of growth or recruitment.
669
Behavioural migrations appear to be an important control on the species population
670
dynamics, as there is some evidence of a switch from pelagic to benthopelagic mode for G.
23
671
nicholsi that could be related to ontogeny (Hulley 1981; McGinnis 1982; Linkowski 1985;
672
Lubimova et al. 1987; Pusch et al. 2004; Collins et al. 2008; Donnelly and Torres 2008).
673
Linkowski (1985) reported a marked absence of II-group (~60-90 mm, SL) specimens in
674
Antarctic waters compared to regions north of the APF, a trend that was also evident in our
675
study. It was therefore hypothesised that the APF region was a recruitment zone for the
676
species in the southwest Atlantic and that II-group (~2-3 year olds) specimens transitioned
677
from pelagic to benthopelagic mode and then migrated south to form the older (~5-7 year
678
olds), deep-shelf stocks observed at higher latitudes. In our study, G. nicholsi occurred
679
predominantly in the upper 400 m of the water column, regardless of body size, and there was
680
no evidence of a benthopelagic distribution between 0-1000 m. However, our net sampling
681
was confined largely to deep oceanic regions and few shelf-break regions were sampled, so it
682
was not possible to examine potential pelagic-benthic interactions for the species in this
683
study. Large sized (mean: ~130 mm) G. nicholsi specimens are often caught in large numbers
684
in bottom trawls (~200-400 m) around South Georgia (Collins, unpublished data), which
685
supports the notion that the species becomes benthopelagic in the latter stages of its lifecycle.
686
Our data showed evidence of DVM for the species within the pelagic realm that was
687
consistent with other studies (Duhamel et al. 2000; Pusch et al. 2004; Collins et al. 2007).
688
689
G. nicholsi had a diet that comprised mostly the copepods Metridia spp., R. gigas and P.
690
robusta. Euphausiids were also an important component of the diet, particularly E. superba.
691
Overall, our results were consistent with those from previous studies in the region (Pakhomov
692
et al. 1996; Pusch et al. 2004; Shreeve et al. 2009). Our analyses showed that the diet of G.
693
nicholsi varied between regions north and south of the SACCF. R. gigas and P. robusta were
694
predated less in regions south of the SACCF and this trend corresponded with a marked
695
decrease in abundance of these prey species south of this front (Ward et al. 2012). Predation
696
on E. superba was also greatest in regions south of the SACCF, where its abundance was
697
greatest in the study area (Fielding et al. 2012). Again, the drivers of the apparent seasonal
698
variation in diet were unclear from the available data, but seasonal variations in prey
699
behaviour and ontogeny are possible factors that warrant further investigation (Shreeve et al.
700
2009). Our results showed that only the largest sized (>125 mm) specimens took E. superba
701
suggesting a certain degree of size-related prey selectivity that has been observed previously
702
for the species (Kozlov and Tarverdiyeva 1989; Pakhomov et al. 1996; Pusch et al. 2004;
24
703
Shreeve et al. 2009). However, this size group preferred smaller Metridia spp. copepods to
704
the larger sized R. gigas and P. robusta, suggesting that prey selectivity within this
705
component of the prey field was not just related to size. Shreeve et al. (2009) hypothesised
706
that variations in prey ontogeny and behaviour, such as DVM, winter diapause and motility,
707
could be important factors in myctophid prey selectivity, as well as size.
708
709
Gymnoscopelus niche separators
710
Differences in diets, vertical distribution, population dynamics and behaviour are important
711
niche separators in pelagic ecosystems that enable several species to co-exist within the same
712
area (Barange 1990). Southern Ocean Gymnoscopelus species, such as G. braueri, G. fraseri,
713
G. nicholsi and G. opisthopterus, are ostensibly similar based on their general morphology,
714
and they have distribution patterns that overlap in the Scotia Sea (Marshall 1960; Hulley
715
1981). They also share some common ecological traits, such as increased
716
spawning/recruitment north of the APF, latitudinal variations in size/population structure,
717
size-related sexual dimorphism and vertical stratification of size-classes. However, the
718
available data indicate that they have different niche roles that may explain how these species
719
are able to avoid direct competition for resources and co-exist in the Scotia Sea region. G.
720
braueri, for example, appears to be a relatively large and long-lived species that is found
721
throughout the Scotia Sea including the sea-ice sectors. It is distributed between 0-1000 m
722
and eats mostly Metridia spp. and Thysanoessa spp. By contrast, G. fraseri is a smaller
723
species that occurs predominantly in the warmer waters north of the APF and it appears not to
724
occur south of the SACCF in the sea-ice sectors. The species is found mostly between 0-400
725
m and is a predominant copepod feeder. The niche role of G. nicholsi appears to differ from
726
both G. braueri and G. fraseri in the region in that this species attains a larger body size and
727
predates mostly R. gigas and E. superba. This species occurs between 0-700 m, but may also
728
switch from pelagic to benthopelagic mode during its lifecycle. G. opisthopterus is a
729
particularly understudied species in the Southern Ocean, but the available data indicate that it
730
is a long-lived (>5 years), deep-dwelling (<500 m) species, with a circumpolar distribution
731
south of the APF (Hulley 1981). The species eats mostly the euphausiids E. superba and
732
Thysanoessa spp. (Pakhomov et al. 1996) and therefore appears to has a different niche role
733
to that of G. braueri, G. fraseri and G. nicholsi in the Scotia Sea. Overall, these observations
25
734
are supported by biochemical studies that also indicate strong niche segregation in terms of
735
diet and habitat type within the Gymnoscopelus genus in the Southern Ocean (Cherel et al.
736
2010; Stowasser et al. 2012).
737
738
Conclusions
739
This study provides important information on the ecology and diet of Gymnoscopelus species
740
in the Scotia Sea, one of the most productive regions of the Southern Ocean, but one that is
741
also presently experiencing a rapid increase in surface temperatures. Such change is likely to
742
be altering the distribution and behaviour of myctophid prey-species, but it is unclear how
743
these fish, and in turn, the whole ecosystem will respond. Our study enables the trophic
744
positioning of this important component of the pelagic ecosystem to be examined in the
745
region, as well as their predatory impact on the zooplankton community.
746
747
Acknowledgements
748
This work was carried out as part of the British Antarctic Survey’s Discovery 2010
749
Programme that was funded by the Natural Environment Research Council. We thank the
750
officers, crew and scientists of the RRS James Clark Ross for their assistance during the three
751
research cruises. We also thank Emma Foster for assisting with lab work.
752
753
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924
Figure legends
925
Fig. 1 Map of the Scotia Sea and the distribution of Gymnoscopelus density during the
926
surveys. The size of the pies is proportional to the log total of fish densities per net. Crosses
927
denote net haul positions. Sampling stations are: Southern Scotia Sea (SSS), Western Scotia
928
Sea (WSS), Mid-Scotia Sea (MSS), North Scotia Sea (NSS), Georgia Basin (GB) and Polar
929
Front (PF). Mean frontal positions determined during the cruises from dynamic height data
930
(Venables et al. 2012) are: northern Antarctic Polar Front (N-PF), southern Antarctic Polar
931
Front (S-PF), South Antarctic Circumpolar Current Front (SACCF) and Southern Boundary
932
of the Antarctic Circumpolar Current (SB-ACC). The heavy black line shows the position of
933
the 15% ice-edge cover for 24/10/2006 and for 15/01/2008. The ice-edge occurred well south
934
of the transect during autumn 2009 (JR200). Bathymetry data are from the GEBCO_08 grid
935
(version 20091120, www.gebco.net)
936
937
Fig. 2 Mean vertical distribution of (a) Gymnoscopelus braueri, (b) Gymnoscopelus fraseri,
938
(c) Gymnoscopelus nicholsi and Gymnoscopelus opisthopterus density and biomass across
939
the Scotia Sea by day (open bars) and by night (filled bars). No net hauls were collected
940
during the daytime in autumn (JR200)
941
942
Fig. 3 Seasonal length-frequency distributions (SL, mm) of (a) Gymnoscopelus braueri, (b)
943
Gymnoscopelus fraseri, (c) Gymnoscopelus nicholsi and (d) Gymnoscopelus opisthopterus in
944
the Scotia Sea
945
946
Fig. 4 Length-frequency distributions (mm, SL) of (a) Gymnoscopelus braueri and (b)
947
Gymnoscopelus fraseri in waters around the Antarctic Polar Front and in the Scotia Sea. For
948
each species, data were aggregated across all seasons to illustrate the same spatial trends that
949
were apparent on each survey
950
951
Fig. 5 Mean standard length (±2 standard error) of (a) Gymnoscopelus braueri, (b)
952
Gymnoscopelus fraseri and (c) Gymnoscopelus nicholsi in each depth zone sampled by the
32
953
RMT25 in the Scotia Sea. Females and males are denoted by filled circles and triangles,
954
respectively. The number of samples measured in each zone is also given
955
956
Fig. 6 Variations in Gymnoscopelus braueri diet in the Scotia Sea by (a) region, (b) survey
957
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
958
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
959
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
960
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
961
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
962
963
Fig. 7 Variations in Gymnoscopelus fraseri diet in the Scotia Sea by (a) region, (b) survey
964
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
965
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
966
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
967
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
968
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
969
970
Fig. 8 Variations in Gymnoscopelus nicholsi diet in the Scotia Sea by (a) region, (b) survey
971
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
972
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
973
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
974
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
975
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
976
33
977
Tables
Spring (JR161)
Region
n
Mean
Range
Gymnoscopelus braueri
SSS
17
0.02
0.00-0.14
WS
8
0.11
0.00-0.41
MSS
4
0.09
0.00-0.30
NSS
8
0.06
0.00-0.24
GB
5
0.12
0.00-0.26
PF
8
0.09
0.00-0.40
All
50
0.07
0.00-0.41
19
1
9
8
10
10
57
0.06
0.00
0.05
0.05
0.11
0.00
0.07
0.00-0.36
0.00
0.00-0.43
0.00-0.28
0.00-0.43
0.00
0.00-0.43
10
0
12
4
2
8
36
0.24
0.24
0.04
0.03
0.09
0.17
0.00-0.69
0.00-1.26
0.00-0.11
0.01-0.04
0.00-0.26
0.00-1.26
Gymnoscopelus fraseri
SSS
17
0.00
WS
8
0.00
MSS
4
0.00
NSS
8
<0.01
GB
5
0.01
PF
8
0.01
All
50
<0.01
0.00
0.00
0.00
0.00-0.02
0.00-0.04
0.00-0.03
0.00-0.04
19
1
9
8
10
10
57
0.00
0.00
0.00
0.00
0.04
0.00
0.01
0.00
0.00
0.00
0.00
0.00-0.15
0.00
0.00-0.15
10
0
12
4
2
8
36
<0.01
0.01
0.02
0.01
0.04
0.02
0.00-0.01
0.00-0.07
0.00-0.08
0.00-0.01
0.00-0.26
0.00-0.26
Gymnoscopelus nicholsi
SSS
17
<0.01
WS
8
<0.01
MSS
4
0.01
NSS
8
0.01
GB
5
<0.01
PF
8
0.01
All
50
0.01
0.00-0.04
0.00-0.02
0.00-0.03
0.00-0.06
0.00-0.01
0.00-0.05
0.00-0.06
19
1
9
8
10
10
57
0.01
0.00
<0.01
<0.01
0.01
0.00
0.01
0.00-0.03
0.00
0.00-0.03
0.00-0.01
0.00-0.08
0.00
0.00-0.08
10
0
12
4
2
8
36
<0.01
<0.01
0.01
0.00
<0.01
<0.01
0.00-0.02
0.00-0.02
0.00-0.02
0.00
0.00-0.01
0.00-0.02
Gymnoscopelus opisthopterus
SSS
17
<0.01
0.00-0.02
WS
8
0.00
0.00
MSS
4
0.00
0.00
NSS
8
0.00
0.00
GB
5
0.00
0.00
PF
8
0.00
0.00
All
50
<0.01
0.00-0.02
19
1
9
8
10
10
57
0.02
0.00
0.01
0.00
0.01
0.00
0.01
0.00-0.2
0.00
0.00-0.04
0.00
0.00-0.03
0.00
0.00-0.2
10
0
12
4
2
8
36
<0.01
0.00
0.00
0.00
0.00
<0.01
0.00-0.01
0.00
0.00
0.00
0.00
0.00-0.01
n
Summer (JR177)
Mean
Range
n
Autumn (JR200)
Mean
Range
978
979
Table 1 Mean abundance (individuals 1000 m-3) of the four most abundant Gymnoscopelus
980
species in the Scotia Sea. The numbers of nets hauls per survey and per region are denoted by
981
n
34
Spring (JR161)
Region
n Mean
Range
Gymnoscopelus braueri
SSS
17
0.36
0.00-3.69
WS
8
0.38
0.00-2.25
MSS
4
0.67
0.00-2.00
NSS
8
0.75
0.00-3.02
GB
5
0.78
0.00-3.83
PF
8
0.45
0.00-2.42
All
50
0.55
0.00-3.83
19
1
9
8
10
10
57
0.49
0.00
0.61
0.46
0.84
0.00
0.62
0.00-2.95
0.00
0.00-2.87
0.00-1.72
0.00-2.46
0.00
0.00-2.95
10
0
12
4
2
8
36
1.08
1.28
0.26
0.54
1.37
1.13
0.00-4.36
0.00-14.63
0.03-0.50
0.32-0.75
0.00-6.87
0.00-14.63
Gymnoscopelus fraseri
SSS
17
0.00
WS
8
0.00
MSS
4
0.00
NSS
8
0.03
GB
5
0.11
PF
8
0.03
All
50
0.02
0.00
0.00
0.00
0.00-0.28
0.00-0.55
0.00-0.22
0.00-0.55
19
1
9
8
10
10
57
0.00
0.00
0.00
0.00
0.12
0.00
0.04
0.00
0.00
0.00
0.00
0.00-0.50
0.00
0.00-0.50
10
0
12
4
2
8
36
0.13
0.05
0.04
0.06
0.07
0.08
0.00-1.40
0.00-0.36
0.00-0.10
0.00-0.11
0.00-0.53
0.00-1.40
Gymnoscopelus nicholsi
SSS
17
0.17
WS
8
0.08
MSS
4
0.30
NSS
8
0.14
GB
5
0.05
PF
8
0.04
All
50
0.13
0.00-1.83
0.00-0.66
0.00-0.99
0.00-0.94
0.00-0.32
0.00-0.25
0.00-1.83
19
1
9
8
10
10
57
0.22
0.00
0.70
0.10
0.17
0.00
0.28
0.00-1.30
0.00
0.00-7.73
0.00-0.94
0.00-1.74
0.00
0.00-7.73
10
0
12
4
2
8
36
0.19
0.03
0.13
0.00
0.13
0.11
0.00-1.32
0.00-0.24
0.00-0.53
0.00
0.00-1.44
0.00-1.44
Gymnoscopelus opisthopterus
SSS
17
0.06
0.00-0.76
WS
8
0.00
0.00
MSS
4
0.00
0.00
NSS
8
0.00
0.00
GB
5
0.00
0.00
PF
8
0.00
0.00
All
50
0.03
0.00-0.76
19
1
9
8
10
10
57
0.36
0.00
0.01
0.00
0.10
0.00
0.16
0.00-3.49
0.00
0.00-0.16
0.00
0.00-1.15
0.00
0.00-3.49
10
0
12
4
2
8
36
0.04
0.00
0.00
0.00
0.00
0.01
0.00-0.43
0.00
0.00
0.00
0.00
0.00-0.43
n
Summer (JR177)
Mean
Range
n
Autumn (JR200)
Mean
Range
982
983
Table 2 Mean biomass (g 1000 m-3) of the four most abundant Gymnoscopelus species in the
984
Scotia Sea. The numbers of nets hauls per survey and per region are denoted by n
35
Gymnoscopelus braueri
Gymnoscopelus fraseri
Gymnoscopelus nicholsi
Prey
%F
%M
%N
%IRI
%F
%M
%N
%IRI
%F
%M
%N
%IRI
Amphipoda
Themisto gaudichaudii
8.06
15.84
2.98
4.97
10.68
16.44
1.14
1.70
22.50
5.53
0.95
1.77
Primno macropa
0.54
0.67
0.18
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Cyphocaris richardi
0.27
0.95
0.09
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Vibilia spp.
0.27
0.44
0.09
0.00
0.97
0.62
0.06
0.01
0.00
0.00
0.00
0.00
Unidentfied amphipod
0.54
0.07
0.18
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Total
9.41
17.98
3.50
2.32
11.65
17.06
1.20
1.45
22.50
5.53
0.95
0.94
Copepoda
Aetidius spp.
0.54
0.04
0.18
0.00
0.97
0.02
0.06
0.00
2.50
0.03
0.09
0.00
Calanoides acutus
2.15
0.23
0.79
0.07
15.53
1.36
1.65
0.42
22.50
0.52
1.71
0.61
Calanus propinquus
1.08
0.18
0.44
0.02
4.85
0.44
0.29
0.03
12.50
0.37
0.57
0.14
Calanus simillimus
6.18
0.57
3.24
0.77
18.45
1.97
3.02
0.83
10.00
0.07
0.57
0.08
Clausocalanus spp.
0.00
0.00
0.00
0.00
0.97
0.02
0.06
0.00
0.00
0.00
0.00
0.00
Candacia sp.
3.49
0.43
1.23
0.19
6.80
0.30
0.40
0.04
17.50
0.42
1.04
0.31
Drepanopus forcipatus
0.54
0.03
0.26
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Eucalanus spp.
0.27
0.03
0.09
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Gaidius spp.
2.15
0.12
0.70
0.06
0.00
0.00
0.00
0.00
5.00
0.04
0.19
0.01
Heterorhabdus spp.
2.15
0.15
0.70
0.06
3.88
0.22
0.23
0.02
7.50
0.10
0.47
0.05
Metridia spp.
34.95
3.94
37.22
47.06
80.58
18.57
60.55
57.68
80.00
2.59
32.73
34.38
Oncaea spp.
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
5.00
0.02
0.19
0.01
Paraeuchaeta spp.
7.80
2.79
2.80
1.43
10.68
0.99
0.63
0.16
25.00
1.07
1.61
0.82
Pleuromamma robusta
15.86
1.87
7.97
5.11
43.69
5.08
6.39
4.53
42.50
3.01
16.13
9.90
Rhincalanus gigas
15.32
4.55
8.76
6.67
48.54
11.48
10.60
9.70
52.50
10.62
35.77
29.63
Scolecithricella spp.
0.27
0.03
0.09
0.00
0.97
0.05
0.06
0.00
0.00
0.00
0.00
0.00
Unidentified copepods
4.03
0.52
1.31
0.15
0.00
0.00
0.00
0.00
2.50
0.15
0.09
0.01
Total
64.25
15.50
65.76
59.97
93.20
40.52
83.92
78.98
90.00
19.01
91.18
63.84
Euphausiacea
Euphausia frigida
2.42
3.78
1.40
0.41
0.00
0.00
0.00
0.00
2.50
2.69
0.66
0.10
Euphausia superba
5.38
20.11
2.01
3.89
0.00
0.00
0.00
0.00
20.00
61.81
1.33
15.36
Euphausia triacantha
1.88
9.54
0.70
0.63
0.00
0.00
0.00
0.00
2.50
1.48
0.09
0.05
Thysanoessa spp.
23.39
14.70
9.72
18.69
52.43
38.73
10.95
23.56
45.00
7.90
3.70
6.35
Unidentified euphausiids
9.68
9.98
3.24
4.19
3.88
0.20
0.34
0.02
5.00
0.79
0.19
0.06
Total
40.86
58.12
17.08
35.30
54.37
38.92
11.29
18.59
67.50
74.66
5.98
35.04
Chordata
Unidentified fish
1.08
0.31
0.35
0.01
0.97
0.02
0.06
0.00
0.00
0.00
0.00
0.00
Total
1.08
0.31
0.35
0.01
0.97
0.02
0.06
0.00
0.00
0.00
0.00
0.00
Ostracoda
Unidentified ostracods
14.52
1.18
6.57
3.68
29.13
1.14
2.74
1.02
20.00
0.11
0.95
0.26
Total
14.52
1.18
6.57
1.29
29.13
1.14
2.74
0.77
20.00
0.11
0.95
0.14
Mollusca
Unidentified pteropods
3.49
2.57
1.31
0.40
0.97
0.10
0.06
0.00
5.00
0.13
0.19
0.02
Total
3.49
2.57
1.31
0.16
0.97
0.10
0.06
0.00
5.00
0.13
0.19
0.01
Urochordata
36
Salps
1.34
1.41
0.61
0.09
0.00
0.00
0.00
0.00
2.50
0.33
0.28
0.02
Total
1.34
11.83
1.41
2.79
0.61
3.85
0.03
1.40
0.00
0.97
0.00
0.10
0.00
0.06
0.00
0.00
2.50
5.00
0.33
0.20
0.28
0.19
0.01
0.02
11.83
2.79
3.85
0.90
0.97
0.10
0.06
0.00
5.00
0.20
0.19
0.01
Other taxa
Appendicularian
0.27
0.04
0.53
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Chaetognatha
0.81
0.06
0.26
0.01
10.68
2.15
0.68
0.27
7.50
0.03
0.28
0.03
Siphonophora
0.27
0.04
0.09
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Gelantinous mass
0.27
0.01
0.09
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Total
0.15
0.96
0.02
10.68
103
2.15
0.68
0.21
7.50
40
0.03
0.28
0.02
No. full stomachs
1.61
372
No. empty stomachs
124
Unidentified crustacean
Total
1
1
985
986
987
988
989
Table 3 All prey items identified from Gymnoscopelus braueri, Gymnoscopelus fraseri and
Gymnoscopelus nicholsi stomachs collected in the Scotia Sea. Note that %IRI is not additive,
so the sum of the individual species’ %IRI values is not the same as the prey categories
%IRIDC value (Hansson 1998)
990
991
37
992
Figures
993
994
Fig. 1 Map of the Scotia Sea and the distribution of Gymnoscopelus density during the
38
995
surveys. The size of the pies is proportional to the log total of fish densities per net. Crosses
996
denote net haul positions. Sampling stations are: Southern Scotia Sea (SSS), Western Scotia
997
Sea (WSS), Mid-Scotia Sea (MSS), North Scotia Sea (NSS), Georgia Basin (GB) and Polar
998
Front (PF). Mean frontal positions determined during the cruises from dynamic height data
999
(Venables et al. 2012) are: northern Antarctic Polar Front (N-PF), southern Antarctic Polar
1000
Front (S-PF), South Antarctic Circumpolar Current Front (SACCF) and Southern Boundary
1001
of the Antarctic Circumpolar Current (SB-ACC). The heavy black line shows the position of
1002
the 15% ice-edge cover for 24/10/2006 and for 15/01/2008. The ice-edge occurred well south
1003
of the transect during autumn 2009 (JR200). Bathymetry data are from the GEBCO_08 grid
1004
(version 20091120, www.gebco.net)
39
1005
Fig. 2 Mean vertical distribution of (a) Gymnoscopelus braueri, (b) Gymnoscopelus fraseri, (c) Gymnoscopelus nicholsi and Gymnoscopelus
1006
opisthopterus density and biomass across the Scotia Sea by day (open bars) and by night (filled bars). No net hauls were collected during the
1007
daytime in autumn (JR200)
40
1008
1009
Fig. 3 Seasonal length-frequency distributions (SL, mm) of (a) Gymnoscopelus braueri, (b)
1010
Gymnoscopelus fraseri, (c) Gymnoscopelus nicholsi and (d) Gymnoscopelus opisthopterus in
1011
the Scotia Sea
41
1012
Fig. 4 Length-frequency distributions (mm, SL) of (a) Gymnoscopelus braueri and (b)
1013
Gymnoscopelus fraseri in waters around the Antarctic Polar Front and in the Scotia Sea. For
1014
each species, data were aggregated across all seasons to illustrate the same spatial trends that
1015
were apparent on each survey
42
1016
1017
Fig. 5 Mean standard length (±2 standard error) of (a) Gymnoscopelus braueri, (b)
1018
Gymnoscopelus fraseri and (c) Gymnoscopelus nicholsi in each depth zone sampled by the
1019
RMT25 in the Scotia Sea. Females and males are denoted by filled circles and triangles,
1020
respectively. The number of samples measured in each zone is also given
43
1021
1022
Fig. 6 Variations in Gymnoscopelus braueri diet in the Scotia Sea by (a) region, (b) survey
1023
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
1024
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
1025
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
1026
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
1027
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
44
1028
1029
Fig. 7 Variations in Gymnoscopelus fraseri diet in the Scotia Sea by (a) region, (b) survey
1030
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
1031
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
1032
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
1033
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
1034
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
45
1035
1036
Fig. 8 Variations in Gymnoscopelus nicholsi diet in the Scotia Sea by (a) region, (b) survey
1037
and (c) size. Diet is expressed as mean %IRI of the dominant prey categories (%IRIDC) with
1038
95% confidence intervals (error bars). MET: Metridia spp., PLE: Pleuromamma robusta,
1039
RHI: Rhincalanus gigas, OTH: other copepods, EUP: Euphausia superba, THY:
1040
Thysanoessa spp., OST: ostracods, OTH: other taxa (e.g. Mollusca, unidentified crustacean),
1041
THE: Themisto gaudichaudii. The number of stomachs is given in parentheses
1042
46