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Southern Ocean myctophid fish (Family Myctophidae) are an important conduit of energy through foodwebs and between the surface layers and mesopelagic depths. Species that reside in both pelagic and near-bottom environments of continental shelves, such as Gymnoscopelus nicholsi and Gymnoscopelus bolini, may also be important in benthopelagic coupling, although their ecology and role in such processes remain unresolved. Here, we examined inter-annual variation in the depth of occurrence, biomass and population dynamics of benthopelagic G. nicholsi on the South Georgia shelf (100–350 m) using bottom trawl data collected between 1987 and 2019. Gymnoscopelus nicholsi was a regular component of the local benthopelagic community, particularly northwest of South Georgia, but was patchily distributed. It appeared to enter a benthopelagic phase at ~ 3 years, with annual growth and recruitment of year classes between ~ 3 and 5 years. However, transition of cohorts into the benthopelagic zone was not annual. There was clear inter-annual variation in G. nicholsi biomass and depth of occurrence. Shallower depth of occurrence was significantly (P G. nicholsi cohorts (~ 5 years) in the benthopelagic zone. We note that Gymnoscopelus bolini is rare in bottom trawl catches between 100 and 350 m, although Antarctic krill appears to dominate its diet from the available data. Our study provides important information on understudied myctophid species in a poorly investigated region of the water column that is relevant for Southern Ocean ecosystem studies, particularly in relation to understanding trophic connectivity between the pelagic and near-bottom realms.

Zooplankton form an integral component of epi- and mesopelagic ecosystems, and there is a need to better understand their role in ocean biogeochemistry. The export and remineralisation of particulate organic matter at depth plays an important role in controlling atmospheric CO2 concentrations. Pelagic mesozooplankton and micronekton communities may influence the fate of organic matter in a number of ways, including: the consumption of primary producers and export of this material as fast-sinking faecal pellets, and the active flux of carbon by animals undertaking diel vertical migration (DVM) into the mesopelagic. We present day and night vertical biomass profiles of mesozooplankton and micronekton communities in the upper 500 m during three visits to an ocean observatory station (P3) to the NW of South Georgia (Scotia Sea, South Atlantic) in austral spring, alongside estimates of their daily rates of ingestion and respiration throughout the water column. Day and night community biomass estimates were dominated by copepods >330 μm, including the lipid-rich species, Calanoides acutus and Rhincalanus gigas. We found little evidence of synchronised DVM, with only Metridia spp. and Salpa thompsoni showing patterns consistent with migratory behaviour. At depths below 250 m, estimated community carbon ingestion rates exceeded those of metabolic costs, supporting the understanding that food quality in the mesopelagic is relatively poor, and organisms have to consume a large amount of food in order to fulfil their nutritional requirements. By contrast, estimated community rates of ingestion and metabolic costs at shallower depths were approximately balanced, but only when we assumed that the animals were predominantly catabolising lipids (i.e. respiratory quotient = 0.7) and had relatively high absorption efficiencies. Our work demonstrates that it is possible to balance the metabolic budgets of mesopelagic animals to within observational uncertainties, but highlights the need for a better understanding of the physiology of lipid-storing animals and how it influences carbon budgeting in the pelagic.

20 pages, 3 tables, 5 figures.-- Samantha Andrzejaczek ... et al.-- Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC), Knowledge of the three-dimensional movement patterns of elasmobranchs is vital to understand their ecological roles and exposure to anthropogenic pressures. To date, comparative studies among species at global scales have mostly focused on horizontal movements. Our study addresses the knowledge gap of vertical movements by compiling the first global synthesis of vertical habitat use by elasmobranchs from data obtained by deployment of 989 biotelemetry tags on 38 elasmobranch species. Elasmobranchs displayed high intra- and interspecific variability in vertical movement patterns. Substantial vertical overlap was observed for many epipelagic elasmobranchs, indicating an increased likelihood to display spatial overlap, biologically interact, and share similar risk to anthropogenic threats that vary on a vertical gradient. We highlight the critical next steps toward incorporating vertical movement into global management and monitoring strategies for elasmobranchs, emphasizing the need to address geographic and taxonomic biases in deployments and to concurrently consider both horizontal and vertical movements, Data analysis was funded by the Bertarelli Foundation through the Marine Science program through grants to D.J.C., B.A.B., and S.A. D.J.C. is also funded through Research England, UK. S.A. and B.A.B. thank the Moore Foundation and the Packard Foundation. F.G.-M. thanks the Instituto Politecnico Nacional for fellowships (COFAA, EDI). S.B.W. thanks funding from the Darwin Initiative (DPLUS046). A.D.M.D. acknowledges funding from the Research and Conservation Budget at Georgia Aquarium, including philanthropic gifts from several anonymous donors. K.F. acknowledges funding from the Rolex Awards for Enterprise and the Whitley Fund for Nature

Mesopelagic fish are an important component of marine ecosystems, and their contribution to marine biogeochemical cycles is becoming increasingly recognized. However, major uncertainties remain in the rates at which they remineralize organic matter. We present respiration rate estimates of mesopelagic fish from two oceanographically contrasting regions: the Scotia Sea and the Benguela Current. Respiration rates were estimated by measuring the enzyme activities of the electron transport system. Regression analysis of respiration with wet mass highlights regional and inter-specific differences. The mean respiration rates of all mesopelagic fish sampled were 593.6 and 354.9 µl O2 individual−1 h−1 in the Scotia Sea and Benguela Current, respectively. Global allometric models performed poorly in colder regions compared with our observations, underestimating respiratory flux in the Scotia Sea by 67–88%. This may reflect that most data used to fit such models are derived from temperate and subtropical regions. We recommend caution when applying globally derived allometric models to regional data, particularly in cold (

The oceanic mesopelagic zone, 200–1000 m below sea level, holds abundant small fishes that play central roles in ecosystem function. Global mesopelagic fish biomass estimates are increasingly derived using active acoustics, where echosounder-generated signals are emitted, reflected by pelagic organisms and detected by transducers on vessels. Previous studies have interpreted a ubiquitous decline in acoustic reflectance towards the Antarctic continent as a reduction in mesopelagic fish biomass. Here, we use empirical data to estimate species-specific acoustic target strength for the dominant mesopelagic fish of the Scotia Sea in the Southern Ocean. We use these data, alongside estimates of fish relative abundance from net surveys, to interpret signals received in acoustic surveys and calculate mesopelagic biomass of the broader Southern Ocean. We estimate the Southern Ocean mesopelagic fish biomass to be approximately 274 million tonnes if Antarctic krill contribute to the acoustic signal, or 570 million tonnes if mesopelagic fish alone are responsible. These quantities are approximately 1.8 and 3.8 times greater than previous net-based biomass estimates. We also show a peak in fish biomass towards the seasonal ice-edge, corresponding to the preferred feeding grounds of penguins and seals, which may be at risk under future climate change scenarios. Our study provides new insights into the abundance and distributions of ecologically significant mesopelagic fish stocks across the Southern Ocean ecosystem.

Myctophids (Family Myctophidae, commonly known as the lanternfishes) are critical components of open ocean food webs and an important part of the ocean biological carbon pump, as many species actively transport carbon to the deep ocean through their diel vertical migrations. Estimating the magnitude of myctophids’ contribution to the biological carbon pump requires knowledge of their metabolic rate. Unfortunately, data on myctophid metabolic rates are sparse, as they rarely survive being captured and placed in a respirometer. Because of this limitation, many studies estimate myctophid metabolic rates indirectly from body mass and temperature scaling relationships, often extrapolating regressions from global data sets to regional scales. To test the validity of these estimates, we employed a newly developed proxy for mass-specific field metabolic rate (Cresp: the proportion of metabolically derived carbon in the otolith) based on the stable carbon isotope composition (δ13C) of otolith aragonite. We recovered estimates of Cresp for individuals of 6 species of myctophids from the Scotia Sea, giving a range in Cresp values from 0.123-0.248. We found that ecological and physiological differences among species are better predictors of variation in Cresp values than body mass and temperature. We compared our results to estimates of metabolic rates derived from scaling relationships and from measurements of electron transport system activity. When considering myctophids as a whole, we found that estimates of oxygen consumption from different methods are broadly similar; however, there are considerable discrepancies at the species level. Our study highlights the usefulness of metabolic proxies where respirometry is currently unavailable, and provides valuable information on field metabolic rates of myctophids.

This study examines age and growth of Brauer's lanternfish Gymnoscopelus braueri and rhombic lanternfish Krefftichthys anderssoni from the Scotia Sea in the Southern Ocean, through the analysis of annual growth increments deposited on sagittal otoliths. Otolith pairs from 177 G. braueri and 118 K. anderssoni were collected in different seasons from the region between 2004 and 2009. Otolith-edge analysis suggested a seasonal change in opaque and hyaline depositions, indicative of an annual growth pattern, although variation within the populations of both species was apparent. Age estimates varied from 1 to 6 years for G. braueri (40 to 139 mm standard length; LS ) and from 0 to 2 years for K. anderssoni (26 to 70 mm LS ). Length-at-age data were broadly consistent with population cohort parameters identified in concurrent length-frequency data from the region for both species. The estimated values of von Bertalanffy growth curves for G. braueri were L∞ = 133.22 mm, k = 0.29 year-1 and t0 = -0.21 year and the values for K. anderssoni were L∞ = 68.60 mm, k = 0.71 year-1 and t0 = -0.49 year. There were no significant (P > 0.05) differences in growth between sexes for either species, suggesting that males and females have similar growth and development trajectories in the Scotia Sea. A positive allometric relationship between LS and wet mass was found for each species, as well as a significant (P < 0.0001) linear relationship between otolith size and LS . Growth performance (Ф') was similar between the two species and congruent with other myctophid species across the Southern Ocean. This study provides important parameters for future Southern Ocean ecosystem studies in a resource management context.

The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS., SEA-DNA, SI

Funding Information: We acknowledge the financial support of the Portuguese Foundation for the Science and Technology (FCT / MCTES) through a PhD grant to Jose Seco (SRFH/PD/BD/113487) and CESAM (UIDP/50017/2020+UIDB/50017/2020), through national funds. The Institut Universitaire de France is acknowledged for its support to P. Bustamante as a Senior Member. This research was also within strategic program of MARE (MARE - UID/MAR/04292/2020). The work is a contribution to the Ecosystems component of the British Antarctic Survey Polar Science for Planet Earth Programme, funded by the Natural Environment Research Council, which is part of UK Research and Innovation. Funding Information: We thank the officers, crew and scientists aboard RSS James Clark Ross during cruises JR177 and JR16003 for their assistance in collecting samples. We also thank Giulia Pompeo for her help with the Hg analysis. We are grateful to G. Guillou from the “Plateforme Analyses Isotopiques” of LIENSs for his assistance during stable isotope analyses at the University of La Rochelle. We acknowledge the financial support of the Portuguese Foundation for the Science and Technology ( FCT / MCTES ) through a PhD grant to José Seco ( SRFH/PD/BD/113487 ) and CESAM ( UIDP/50017/2020+UIDB/50017/2020 ), through national funds. The Institut Universitaire de France is acknowledged for its support to P. Bustamante as a Senior Member. This research was also within strategic program of MARE (MARE - UID/MAR/04292/2020). The work is a contribution to the Ecosystems component of the British Antarctic Survey Polar Science for Planet Earth Programme, funded by the Natural Environment Research Council , which is part of UK Research and Innovation . Publisher Copyright: © 2021 Elsevier Ltd Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Biomagnification of mercury (Hg) in the Scotia Sea food web of the Southern Ocean was examined using the stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) as proxies for trophic level and feeding habitat, respectively. Total Hg and stable isotopes were measured in samples of particulate organic matter (POM), zooplankton, squid, myctophid fish, notothenioid fish and seabird tissues collected in two years (austral summers 2007/08 and 2016/17). Overall, there was extensive overlap in δ13C values across taxonomic groups suggesting similarities in habitats, with the exception of the seabirds, which showed some differences, possibly due to the type of tissue analysed (feathers instead of muscle). δ15N showed increasing enrichment across groups in the order POM to zooplankton to squid to myctophid fish to notothenioid fish to seabirds. There were significant differences in δ15N and δ13C values among species within taxonomic groups, reflecting inter-specific variation in diet. Hg concentrations increased with trophic level, with the lowest values in POM (0.0005 ± 0.0002 μg g−1 dw) and highest values in seabirds (3.88 ± 2.41 μg g−1 in chicks of brown skuas Stercorarius antarcticus). Hg concentrations tended to be lower in 2016/17 than in 2007/08 for mid-trophic level species (squid and fish), but the opposite was found for top predators (i.e. seabirds), which had higher levels in the 2016/17 samples. This may reflect an interannual shift in the Scotia Sea marine food web, caused by the reduced availability of a key prey species, Antarctic krill Euphausia superba. In 2016/17, seabirds would have been forced to feed on higher trophic-level prey, such as myctophids, that have higher Hg burdens. These results suggest that changes in the food web are likely to affect the pathway of mercury to Southern Ocean top predators. Changes in foodweb dynamics influence Hg bioaccumulation in top predators. Postprint

Fish morphometric relationships are key tools for fisheries science and studies of food web dynamics and predator foraging behaviour, but parameterisations are limited for Southern Ocean myctophids (Family Myctophidae). New standard length (LS) to total mass (MT) relationships are therefore described for the 12 biomass-dominant myctophid fish species living in the Scotia Sea, Southern Ocean, using the most comprehensive data collected in the region to date. New linear regressions for otolith size (length; OL and width; OW) and LS are also described. Significant (p < .01) LS–MT relationships were established for all species using simple non-linear regression. Significant (p < .01) relationships between LS and both OL and OW were also determined for all species, with OW being the best predictor of LS in all but one species. Our study provides a comprehensive tool for reconstructing the myctophid component of marine predator diets that will improve future food web, predator behaviour and ecosystem studies in the Scotia Sea. info:eu-repo/semantics/publishedVersion

The applicability of macroecological rules to patterns in body size varies between taxa. One of the most examined is Bergmann’s rule, which states that body size increases with decreasing temperature and increasing latitude, although the rule is not universal and the proposed mechanisms underpinning it are multifarious and lack congruence. This study considers the degree to which Bergmann’s rule applies to the Southern Ocean mesopelagic fish community. We studied patterns in body size, temperature, and latitude across a 12° latitudinal gradient within the Scotia-Weddell sector. Intraspecific Bergmann’s rule was found to apply to 8 of the 11 biomass-dominant species in the family Myctophidae. The rule was also apparent at an interspecific level. Our study suggests that greater attainable body size in this community is a necessary attribute to reach colder regions further south. The adherence of these taxa to Bergmann’s rule enables such species to act as sentinels for identifying the drivers and consequences of ocean warming for the Southern Ocean ecosystem.

This study examines age and growth of Brauer's lanternfish Gymnoscopelus braueri and rhombic lanternfish Krefftichthys anderssoni from the Scotia Sea in the Southern Ocean, through the analysis of annual growth increments deposited on sagittal otoliths. Otolith pairs from 177 G. braueri and 118 K. anderssoni were collected in different seasons from the region between 2004 and 2009. Otolith-edge analysis suggested a seasonal change in opaque and hyaline depositions, indicative of an annual growth pattern, although variation within the populations of both species was apparent. Age estimates varied from 1 to 6 years for G. braueri (40 to 139 mm standard length; L

Within the twilight of the oceanic mesopelagic realm, 200–1000 m below sea level, are potentially vast resources of fish. Collectively, these mesopelagic fishes are the most abundant vertebrates on Earth, and this global fish community plays a vital role in the function of oceanic ecosystems. The biomass of these fishes has recently been estimated using acoustic survey methods, which rely on echosounder-generated signals being reflected from gas-filled swimbladders and detected by transducers on vessels. Here, we use X-ray computed tomography scans to demonstrate that several of the most abundant species of mesopelagic fish in the Southern Ocean lack gas-filled swimbladders. We also show using catch data from survey trawls that the fish community switches from fish possessing gas-filled swimbladders to those lacking swimbladders as latitude increases towards the Antarctic continent. Thus, the acoustic surveys that repeatedly show a decrease in mesopelagic fish biomass towards polar environments systematically overlook a large proportion of fish species that dominate polar seas. Importantly, this includes lanternfish species that are key prey items for top predators in the region, including king penguins and elephant seals. This latitudinal community switch, from gas to non-gas dominance, has considerable implications for acoustic biomass estimation, ecosystem modelling and long-term monitoring of species at risk from climate change and potential exploitation.

Authors acknowledge the financial support of the Portuguese Foundation for the Science and Technology (FCT) through PhD grants to José Seco (SRFH/PD/BD/113487), Francesca Coppola (SFRH/BD/118582/2016) and Ângela Almeida (SFRH/BD/110218/2015), and the Portuguese Polar Program PROPOLAR. This study also benefited from the strategic program of MARE (Marine and Environmental Sciences Centre) financed by FCT (UIDB/04292/2020). João Pedro Coelho was funded by the Integrated Program of SR&TD ‘Smart Valorization of Endogenous Marine Biological Resources Under a Changing Climate’ (Centro-01-0145-FEDER-000018), co-funded by Centro 2020 program, Portugal 2020 and the European Regional Development Fund. Thanks are due for the financial support to CESAM (UIDB/50017/2020+UIDP/50017/2020) and REQUIMTE (UIDB/50006/2020). The Institut Universitaire de France (IUF) is acknowledged for its support to P. Bustamante as a Senior Member. RAS, SF, GS and GT were supported by the British Antarctic Surveys Ecosystem Programme, which is part of NERC and UKRI. Indicators of oxidative stress and metabolic capacity are key factors in understanding the fitness of wild populations. In the present study, these factors were evaluated in the pelagic Southern Ocean taxa Antarctic krill (Euphausia superba) and myctophid fish (Electrona antarctica, Gymnoscopelus braueri and G. nicholsi) to establish a baseline record for future studies. Mercury (Hg) concentrations were also analysed to evaluate its potential impacts on species biochemical performance. E. superba had higher metabolic activity than most of the myctophid species, which may explain the comparatively lower energy reserves found in the former. The activity of antioxidant enzymes showed, generally, a lower level in E. superba than in the myctophid species. The lack of any relationship between Hg concentrations and organisms' antioxidant and biotransformation defence mechanisms indicate that levels of Hg accumulated in the studied species were not high enough to affect their biochemical processes adversely. Postprint

Authors acknowledge the Portuguese Foundation for the Science and Technology (FCT) for the PhD grant to JS (SRFH/PD/BD/113487). JPC is funded by CESAM (UID/AMB/50017/2019) and the Integrated Program of SR&TD ‘Smart Valorization of Endogenous Marine Biological Resources Under a Changing Climate’ (Centro-01-0145-FEDER-000018), co-funded by Centro 2020 program, Portugal 2020 and the European Regional Development Fund. The Institut Universitaire de France (IUF) is acknowledged for its support to PB as a Senior Member. We also thank MARE (MARE - UID/MAR/04292/2019) for the support to JX. Ecosystems Programme at the British Antarctic Survey supported GAT, GS, RAS and SF. Myctophids are the most abundant fish group in the Southern Ocean pelagic ecosystem and are an important link in the Antarctic marine food web. Due to their major ecological role, evaluating the level of mercury (Hg) contamination in myctophids is important as a step towards understanding the trophic pathway of this contaminant. The concentrations of total Hg were determined in muscle, gill, heart and liver tissue of 9 myctophid species to quantify tissue partitioning variability between species. Organic Hg concentration and proportion in muscle was also determined. Hg concentrations were higher in the liver and heart than in muscle and gills, but the proportion of organic Hg was almost 100% in muscle, indicating that the main uptake route for Hg is through the diet. Most of the species analysed have similar vertical and horizontal distributions, and similar feeding modes and prey. Geographical and temporal variability of Hg concentrations was examined using samples from 3 different years (2007/08, 2015/16 and 2016/17) and 2 locations (South Georgia and South Orkneys Islands). Our results appear to indicate a decreasing trend in Hg contamination over the last decade, particularly gill tissue, which is in agreement with a previous study on squid from the same region. There was no significant variability in Hg concentration between the different sampling locations. Hg levels were consistent with values reported previously for myctophids around the world, indicating low global-scale geographic variability. A positive relationship between fish size and Hg concentration was found for most species, with the exception of Electrona antarctica females, which may be explained through Hg elimination by egg laying. We estimate that myctophids collectively comprise a Southern Ocean mercury ‘reserve’ of ≈1.82 metric tonnes. Postprint

Mesopelagic fish have recently been highlighted as an important, but poorly studied component of marine ecosystems, particularly regarding their role in the marine pelagic food webs and biogeochemical cycles. Myctophids (Family Myctophidae) are one of the most biomass-dominant groups of mesopelagic fishes, and their large vertical migrations provide means of rapid transfer of carbon to the deep ocean where it can be sequestered for centuries or more. In this study, we develop a simple regression for the respiration rate of myctophid fish using literature-based wet mass and habitat temperature data. We apply this regression to net haul data collected across the Scotia-Weddell sector of the Southern Ocean to estimate respiration rates of the biomass-dominant myctophid species. Electrona carlsbergi, Electrona antarctica, and Gymnoscopelus braueri made a high contribution (up to 85%) to total myctophid respiration. Despite the lower temperatures of the southern Scotia Sea (-1.46 to 0.95°C), total respiration here was as high (reaching 1.1 mg C m-2 d-1) as in the warmer waters of the mid and northern Scotia Sea. The maximum respiratory carbon flux of the vertically migrating community was 0.05 to 0.28 mg C m-2 d-1, equivalent to up to 47% of the gravitational particulate organic carbon flux in some parts of the Scotia-Weddell region. Our study provides the first baseline estimates of respiration rates and carbon flux of myctophids in the Southern Ocean. However, direct measurements of myctophid respiration, and of mesopelagic fish generally, are needed to constrain these estimates further and incorporate these fluxes into carbon budgets.

This study investigated spatial and temporal patterns in distribution, population structure and diet of Bolin's lanternfish Protomyctophum bolini, Tenison's lanternfish Protomyctophum tenisoni and gaptooth lanternfish Protomyctophum choriodon in the Scotia Sea using data collected by midwater trawl during spring, summer and autumn. Protomyctophum bolini was the most abundant species of the genus encountered throughout the Scotia Sea with the greatest concentrations occurring around the Antarctic Polar Front (APF). This species had a life cycle of 2+ years, but spatial differences in population structure were apparent as the I-group was absent from all regions south of the APF, suggesting that the species does not recruit in the Scotia Sea. Protomyctophum tenisoni occurred mostly in waters characteristic of the APF and was absent from the southern Scotia Sea. It had a limited size range, but there was clear size-related sexual dimorphism with males significantly larger than females. The species had a life cycle of c. 2 years, but the I-group (c. 1 year old, 1 November to 31 October the next year) occurred only in regions close to the APF suggesting that recruitment is restricted to these waters. A seasonal southward migration for P. choriodon is likely as the species occurred mostly to the south-west of South Georgia in summer, but extended to the sea-ice sectors in autumn. Protomyctophum choriodon had a life cycle of 4+ years in the Scotia Sea and the population was dominated by age classes >3 years old. Larval stages were absent during the surveys for all species. Diurnal variations in vertical distribution were apparent for all three species. Interspecific variations in diet were evident, but all species were primarily copepod feeders, with Metridia spp., Rhincalanus gigas and Calanus simillimus generally dominating their diet. Small euphausiids, principally Thysanoessa spp., were also an important component of their diets, particularly for P. choriodon which had the largest body size. The spatial and temporal variations in diet for both P. bolini and P. tenisoni were broadly consistent with underlying abundance patterns within the mesozooplankton community.

The biomass of mesopelagic fish in the Southern Ocean is one of the largest of any ocean region and is dominated (both in terms of diversity and biomass) by myctophids (lantern-fish). Despite their high ecological importance in this region, and globally, our understanding of the life-cycles and distribution of myctophids remains limited. We examined length-frequency data from trawl-nets collected across a major sector of the Southern Ocean (the Scotia-Weddell sector) in different seasons to determine patterns of recruitment and growth. There was an absence of larval myctophids, of any species, in net-catches, while larger, older individuals became increasingly dominant with increasing latitude. Very few specimens were found to contain mature gonads, indicating that individuals do not reach reproductive condition in this region. Most myctophid species that occurred within the survey regions neither recruited locally nor were self-supporting. Myctophids are prey for a large number of higher predators (penguins, seals and cetaceans) in the Scotia Sea and are a major predator of zooplankton and krill. We show that this vital part of the Southern Ocean food-web is dependent on mass immigration from lower latitudes in the region. By implication, the sensitivities of this system depend not only on local conditions but also on levels of connectivity to other oceanic regions.

In this paper, we describe the variation in myctophid schools characteristics at South Georgia using multi-frequency acoustic data collected annually between November 2007 and January 2012. We studied the relationship between the proximity of land and the distribution and schooling characteristics of myctophid fish. We also examined patterns in schooling behaviour in relation to oceanographic data. Fish schools were identified using a dual-frequency dB identification method (Sv120−38 kHz), where negative Sv120−38 kHz is indicative of gas-bearing organisms, such as swimbladdered fish. Available net data were used to provide information on the meosopelagic fish community in the region. School morphometrics (e.g. length, thickness, area) data were extracted and pooled according to their distance from the shore (0–30 km, 30–60 km, 60–90 km, 90–120 km). A total of 578 schools were detected in the survey region and there was high variation in school backscattering strength (NASC), length, height, perimeter, depth and horizontal distribution between surveys, including distinct inter-annual variation in these parameters when surveys were conducted in the same season (2009–2012). Schools were distributed predominantly on-shelf (0–500 m isobaths) during some summer surveys (2007 and 2012) and predominantly off-shelf on others (>500 m isobaths; 2009, 2010 and 2011). There was also evidence of bathymetric separation in the horizontal distribution of schools. Schools observed in the late-season in 2008 had the greatest NASC, largest height and the deepest depth distribution. Most schools occurred at depths between ∼60 and 300 m, but they did not occupy the same water masses during each survey. Schools became progressively thinner, shallower and acoustically weaker with increasing distance to land, whilst school length and perimeter increased correspondingly. The change in fish schooling behaviour between environments could be a response to a combination of local predatory threats over short spatial/temporal scales and differences in oceanographic conditions, such as current velocity. The trend could also be evidence of spatial habitat partitioning of myctophids, with different schooling species, or different life-stages, orientating along a bathymetric gradient at South Georgia.

Saunders, R. A., Royer, F., and Clarke, M. W. 2011. Winter migration and diving behaviour of porbeagle shark, Lamna nasus, in the Northeast Atlantic. – ICES Journal of Marine Science, 68: 166–174. The porbeagle is one of the top marine predators in the North Atlantic. However, little is known about its biology, abundance, or spatial ecology there. Results are presented on the migration and behaviour of three porbeagles tagged with archival pop-up tags off Ireland between September 2008 and January 2009. One shark migrated >2400 km to the northwest of Morocco, residing around the Bay of Biscay for approximately 30 days. The other two remained more localized in off-shelf regions around the Celtic Sea/Bay of Biscay and off western Ireland. The sharks occupied a broad vertical depth range (0–700 m) and a relatively limited temperature range (∼9–17°C), with notable variations in diving behaviour between individual sharks. There were distinct day–night differences in depth distribution, each shark being positioned higher in the water column by night than by day. Night-time depth distribution also appeared to be driven by the lunar cycle during broad-scale migration through oceanic waters. Our results show that porbeagles occupy and traverse regions of high fishing activity where they are potentially vulnerable to population depletion. Such large-scale movement outside the ICES Area underlines the need for international coordination in their assessment and management.

Myctophid fish play an important role in the Southern Ocean pelagic food web. The lanternfish Krefftichthys anderssoni is one of the most common myctophids in the region, but its ecology is poorly known. This study examines spatial and temporal patterns in the species distribution of density, life cycle, population structure and diet using samples collected by mid-water trawl nets deployed in different seasons across the Scotia Sea. Virtually absent from the sea-ice zone, the species was most abundant in the northern Scotia Sea around the Georgia Basin at depths below 400 m that are associated with the Circumpolar Deep Water. The species migrated during night from waters deeper than 700 m to waters above the 400 m following their main prey species: the copepods Rhincalanus gigas and Calanoides acutus and euphausiids of the Thysanoessa genus. Larvae length distribution and post-larvae length-frequency analyses suggested a life cycle of ~ 3 years with spawning and recruitment strongly connected with APF and the South Georgia shelf. Our results show that species spatial distribution, population structure and diet changed both seasonally and ontogenetically. This study is the most comprehensive examination of the ecology of K. anderssoni in the Southern Ocean to date and contributes to resolving how pelagic food webs and ecosystems operate in the region.

Myctophids are the most abundant mesopelagic fishes in the Southern Ocean, although their trophic role within the predominantly krill-based food web in regions south of the Antarctic Polar Front (APF) is poorly resolved. This study therefore examined the diets of 10 species of myctophid fishes: Electrona antarctica, E. carlsbergi, Gymnoscopelus braueri, G. fraseri, G. nicholsi, Krefftichthys anderssoni, Protomyctophum bolini, P. tenisoni, P. choriodon and Nannobrachium achirus, in the Scotia Sea, together with their predatory impact on the underlying zooplankton community. Myctophids and their prey were sampled in different seasons by scientific nets deployed across the Scotia Sea from the sea-ice zone to the APF. Based on the percentage index of relative importance, myctophids had high overlap in their diets, although the data indicate dietary specialisation in some species. There was also a distinct switch in diet, from copepods to euphausiids and amphipods, with increasing myctophid size. Myctophid predation impacted daily copepod production by between 0.01 and 5%, with Calanus simillimus being most impacted. Total annual consumption of copepods was around 1.5 million t (Mt) per year. All myctophids preyed upon the euphausiid Thysanoessa spp., consuming ~12% of its daily productivity and around 4 Mt per year. However, only larger myctophid species preyed upon Antarctic krill Euphausia superba, consuming 2% of its daily productivity, which could amount to as much as 17 Mt per year. Themisto gaudichaudii was also an important dietary component, with 4% of its daily productivity being consumed, amounting to around 2 Mt per year. This study demonstrates that myctophids link secondary productivity to higher predators both through krill-dependent and krill-independent trophic pathways.

Gymnoscopelus braueri, Gymnoscopelus fraseri and Gymnoscopelus nicholsi are common in the Southern Ocean mesopelagic fish community. However, their ecology is poorly understood in the region. This study investigated spatial and temporal patterns in their abundance, population structure and diets at different times of year within the Scotia Sea to ascertain their functional role in the pelagic food web. G. braueri was the most abundant species (0.07–0.17 ind. 1,000 m−3) throughout the Scotia Sea. G. fraseri was absent from the sea-ice sectors and occurred mostly around the Antarctic Polar Front (APF), comprising densities of 0.01–0.04 ind. 1,000 m−3. G. nicholsi occurred in low abundance (4 years. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and vertical segregation in size classes for each species. Overall, diets were dominated by copepods (Metridia spp., Rhincalanus gigas, Pleuromamma robusta) and euphausiids (Thysanoessa spp. and Euphausia superba), although G. fraseri did not predate E. superba. Regional, seasonal and ontogenetic patterns in diet were evident for all species. This study provides new insight into the ecology of these Gymnoscopelus species in the Scotia Sea. Such details contribute towards resolving how pelagic food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change.

Upward-looking acoustic Doppler current profilers (ADCPs) (300 kHz) and echosounders (125 kHz) were deployed on moorings at South Georgia to measure abundance of Antarctic krill continuously over several months. Echoes from krill were identified using the theoretical difference in echo intensity at 300 and 125 kHz and scaled to krill density using target strengths appropriate for krill in the region: krill size was determined from diet samples from fur seals and penguins foraging near the moorings. A method using water flow past the moorings was developed to convert time-based acoustic observations of krill to area-based abundance estimates. Flow past the stationary moorings was treated analogously to motion along-track of a research vessel through a nominally stationary body of water during a conventional survey. The moorings thus provide a Eulerian view of variation in krill abundance. This is ecologically instructive for South Georgia, where krill are generally passive drifters on currents and where temporal fluctuations in abundance have significant consequences for krill-dependent predators. Moorings were positioned on routine research vessel survey transects, and validity of the mooring method was assessed by comparison of mooring and vessel observations. Krill density estimates from the moorings were not statistically different from vessel estimates in adjacent time periods. A time series of krill density from a mooring revealed step-changes that were not seen during short-term vessel surveys. Moorings deliver data over time scales that cannot be achieved from research vessels and provide insight on environmental factors associated with variation in krill abundance at South Georgia. Mooring data may aid ecosystem-based management.

The Scotia Sea is one of the most productive regions of the Southern Ocean, but its surface waters are experiencing a rapid increase in temperature, which may be changing the behaviour and distribution of many myctophids and their prey species. Electrona antarctica and Electrona carlsbergi are two of the most abundant myctophids in the region, but their ecology is poorly understood and their response to ongoing environmental change is difficult to determine. This study investigated spatial and temporal patterns in their abundance, population structure and diets using mid-water trawl nets deployed across the Scotia Sea during spring, summer and autumn. E. antarctica was the most numerically abundant species (0.09–0.21 ind. 1,000 m−3), with greatest concentrations occurring in the sea-ice sectors. E. carlsbergi occurred in more northern regions, comprising densities of 0.02–0.11 ind. 1,000 m−3. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and size-specific vertical stratification for both species. Latitudinal trends in sex ratio and female body size were apparent for E. antarctica. Its diet varied between regions, seasons and size classes, but overall, Euphausia superba, Metridia spp. and Themisto gaudichaudii were the dominant prey items. E. carlsbergi appeared not to recruit in the Scotia Sea. Its diet was dominated by copepods, particularly Rhincalanus gigas and Metridia spp., but regional, seasonal and ontogenetic variations were evident. This study contributes to our understanding of how mid-water food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change.

Euphausiids are an important component of the northern North Atlantic ecosystem and several species are found in the Irminger Sea. However, data on euphausiids in this region are few, particularly for Thysanopoda acutifrons, Thysanoessa inermis and Nematobranchion boöpis. In this paper, we present the first data since the 1930s on the seasonal distribution and population dynamics of these species from net haul data collected in the Irminger Sea during winter, spring and summer 2001–2002. Thysanoessa inermis was the most numerically abundant (0.63–26.62 ind. 1000 m−3) of the three species in the region and comprised a biomass of 3.92–41.74 mg 1000 m−3. The species was largely found in the upper regions of the water column (0–400 m) and was distributed in the more on-shelf/shelf-break regions around East Greenland and Iceland. Growth rates were around 0.03 mm d−1for T. inermis and there was some evidence that either the timing of spawning was delayed, or larval development was prolonged in the region. Thysanopoda acutifrons was predominantly distributed below 400 m in more oceanic regions and had a low abundance (1.23–1.64 ind. 1000 m−3) throughout the Irminger Sea. However, the species comprised a relatively high proportion of biomass (19.39–31.33 mg 1000 m−3) due to its large body size. Our data showed that the species had low rates of growth (0.04 mm d−1) and development throughout the year, and that the reproductive season occurred during the overwintering period (November/December) once individuals had reached two years of age. Nematobranchion boöpis mainly occurred below 400 m at low abundance (0.06–0.18 ind.1000 m−3) levels throughout the region. The species was largely found where Atlantic waters prevailed in the Irminger Current and its growth rates were variable (0.02–0.06 mm d−1). Nematobranchion boöpis was a year-round spawner and the species had fairly rapid rates of post-larval development, with the newly spawned 0-group reaching sexual maturity within the first 6 months. Data presented in this paper provide useful baselines for understanding the possible impacts of long-term, broad-scale environmental change on the ecology of euphausiid communities in the Irminger Sea.

Saunders, R. A., O'Donnell, C., Korneliussen, R. J., Fässler, S. M. M., Clarke, M. W., Egan, A, and Reid, D. 2012. Utility of 18-kHz acoustic data for abundance estimation of Atlantic herring (Clupea harengus) – ICES Journal of Marine Science, 69: 1086–1098. Current acoustic survey protocols for Atlantic herring (Clupea harengus) abundance estimation are principally dependent upon 38-kHz backscatter data. This can constitute a substantial problem for robust stock assessment when 38-kHz data are compromised. Research vessels now typically collect multifrequency data during acoustic surveys, which could be used to remediate such situations. Here, we investigate the utility of using 18- and 120-kHz data for herring abundance estimation when the standard 38-kHz approach is not possible. Estimates of herring abundance/biomass in the Celtic Sea (2007–2010) were calculated at 18, 38, and 120 kHz using the standard 38-kHz target-strength (TS) model and geometrically equivalent TS models at 18 and 120 kHz. These estimates were compared to assess the level of coherence between the three frequencies, and 18-kHz-derived estimates were subsequently input into standard 38-kHz-based population models to evaluate the impact on the assessment. Results showed that estimates of herring abundance/biomass from 18 and 38 kHz acoustic integration varied by only 0.3–5.4%, and acoustically derived numbers-at-age estimates were not significantly (p > 0.05) different from 1:1. Estimates at 120 kHz were also robust. Furthermore, 18-kHz-derived estimates did not significantly change the assessment model output, indicating that 18-kHz data can be used for herring stock assessment purposes.