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The extent to which marine species avoid survey vessels is the subject of considerable debate, with stimuli such as noise and pressure waves receiving particular attention. By some estimates, avoidance behaviour is a major bias in the assessment of pelagic species distribution and density. Surveyors of the marine environment are increasingly using autonomous platforms; thus the question of avoidance of these novel platforms must be considered. A long-duration, underwater glider with an integrated echo-sounder was deployed to measure the density and distribution of Antarctic krill. We show that krill reacted to the glider, despite the low-noise profile of the platform. The weighted-mean depth of krill swarms was observed to deepen when a glider came within 30 m. In addition, we observed a near-range decrease in depth-integrated acoustic backscatter and we demonstrate that this effect is not explained by the instrument's acoustic footprint. The potential triggers for these behaviours are discussed. The list of sensors available for use on a glider is increasing, with some producing sound, light or other potential stimuli for behavioural response. Our paper demonstrates that caution should be used when designing mission payloads and in the interpretation of acoustic data for animal density from gliders. A key consideration for surveyors may be the factoring in of an exclusion zone from their analyses. • Antarctic krill swarms responded to the presence of the glider, interpreted as an avoidance response. • Near-range echo was less than expected without avoidance; simulation shows sampling does not account for the difference. • Slope of swarm centre of mass suggests and effect up to 40 m below the glider. • Often thought of as quiet vehicles, glider surveys of biomass will need to account for target disturbances. [ABSTRACT FROM AUTHOR]

Antarctic krill, Euphausia superba , is a highly abundant and ecologically important zooplankton species in the Southern Ocean. Regions of elevated Antarctic krill biomass exist around Antarctica, often as a result of the concentrating effect of bathymetry and ocean currents. Such areas are considered biological hotspots and are key foraging grounds for numerous top predators in the region. A hotspot of Antarctic krill biomass exists off the southern extent of Anvers Island, Western Antarctic Peninsula, and supports a population of Adélie penguins that feed almost exclusively on it, as well as numerous other top predators. We investigated the spatio-temporal variability in Antarctic krill biomass and aggregation structure over four consecutive summer seasons, identifying environmental factors that were responsible. We identified three distinct krill aggregation types (Large-dense, Small-close and Small-sparse), and found that the relative proportion of each type to total aggregation numbers varied significantly between survey days. Large-dense aggregations occurred more frequently when westerly winds predominated and when the local mixed tide was in the diurnal regime. Small-close aggregations were also more frequent during diurnal tides and were negatively correlated with phytoplankton biomass. Small-sparse aggregations, on the other hand, were more prevalent when the mixed tide was in the semi-diurnal phase. We suggest that, under certain conditions (i.e. diurnal tides and westerly winds), the biological hotspot in the nearshore waters off Palmer Station, Anvers Island, functions as a zone of accumulation, concentrating krill biomass. Our findings provide important information on the dynamics of Antarctic krill at the local scale. [ABSTRACT FROM AUTHOR]

Following the cessation of whaling, the southwest Atlantic humpback whale (Megaptera novaeangliae) population is thought to be close to pre-exploitation size, reversing 20th century changes in abundance. Using a model-based approach applied to concurrently collected data on baleen whale abundance and Antarctic krill (Euphausia superba) biomass in the South Sandwich Islands (SSI) region, we explore ecological interactions between these taxa. Krill biomass and baleen whale density were highest to the north and northeast of the SSI, where the Antarctic Circumpolar Current (ACC) is deflected around the island chain. Humpback whale density was elevated at locations of krill biomass density >150 gm-2. Krill consumption by baleen whales was estimated at 19–29% of the available krill standing stock. We used historic whaling data to confirm the plausibility of these consumption rates and found evidence of rapid weight gain in humpback whales, such that blubber depleted during the breeding season could be restored in a much shorter period than previously assumed. Little is known about krill replenishment rates in the flow of the ACC, or about niche separation between recovering baleen whale populations; both factors may affect species carrying capacities and further monitoring will be required to inform the management of human activities in the region. • Krill biomass density distribution was modelled using a two-stage Maxent/GLM process. • Humpback whale abundance and distribution was modelled using a GAM. • Whales consume 19–29% of available krill in the South Sandwich Islands region. • On southern feeding grounds humpback whales can rapidly restore depleted blubber. • Future studies on rates of krill transport in oceanographic currents will be important. [ABSTRACT FROM AUTHOR]

Abstract: A one-dimensional, temperature-dependent model was used to simulate the descent–ascent cycle of the embryos and early larval stages of Antarctic krill to determine which regions of the western Antarctic Peninsula (wAP) continental shelf support successful completion of this cycle under present environmental conditions and those projected to occur as a result of climate change. The transport and fate of the embryo and larva under present and modified conditions was investigated with Lagrangian particle tracking simulations. The two modeling studies were implemented using temperature and density (embryo–larva model) and circulation distributions (Lagrangian particle tracking) obtained from a high resolution version of the Regional Ocean Modeling System configured for the wAP shelf region. Additional simulations used temperature and circulation distributions obtained from simulations that were forced with increased wind speed and increased transport of the Antarctic Circumpolar Current (ACC), both projected to possibly occur with climate change in the wAP region. Simulations using present conditions showed that successful completion of the descent–ascent cycle occurred along the outer shelf and on the shelf in regions with bottom depths of 600–700m. Estimated residence times for the shelf regions that support success of the embryo and larva were 20–30 days. Thus, krill spawned in the mid and inner shelf regions can be retained in these regions through development to the first feeding stage (calyptopis 1). Increased winds and ACC transport resulted in more onshelf transport of Circumpolar Deep Water (CDW), which increased the volume of warm (1–1.5°C) water at depth. These conditions supported a moderate increase in success of the krill embryo and larva, but only for limited areas of the shelf where hatching depths decreased by 10–30m (<5%) and development time to the calyptopis 1 stage decreased by 15–20%. The modified circulation conditions also supported increased advection of krill larvae into areas of the shelf that would experience the largest reduction of sea ice, especially in winter. Projected changes in wind strength and CDW transport may potentially enhance larval survival and advection onto the wAP shelf, but recruitment may be decreased by modifications to local sea ice distributions that would impede survival of Antarctic krill that overwinter on the shelf. [Copyright &y& Elsevier]

Abstract: Net-based data on the abundance, distribution, and demographic patterns of Antarctic krill are quantified from a contemporaneous two ship survey of the Antarctic Peninsula during austral summer 2011. Two survey areas were sampled focussed on Marguerite Bay in the south, and the tip of the Antarctic Peninsula in the north. Data from 177 stations showed that the highest concentrations of krill were found in the southern sampling area. Differences between areas were associated with a few large catches of one year old krill found in anomalously warm and productive waters in Marguerite Bay, and small krill catches in the less-productive, offshore waters in the north. Estimated krill density across the survey area was 3.4krillm−2, and was low compared to the long-term average of 45krillm−2 for the Elephant Island area. Overall recruitment between the two survey regions was similar, but per capita recruitment was about 60% lower than historical mean recruitment levels measured at Elephant Island since the late 1970s. Demographic patterns showed small krill concentrated near the coast, and large krill concentrated offshore on the shelf and slope all along the survey area. The offshore distribution of adult krill was delineated by the warm (∼1°C), low salinity (33.8) water at 30m, suggesting that most krill were present shoreward of the southern boundary of Antarctic Circumpolar Current Front. Distributions of larvae indicated that three hotspot areas were important for the production of krill: slope areas outside Marguerite Bay and north of the South Shetland Islands, and near the coast around Antarctic Sound. Successful spawning, as inferred from larval abundance, was roughly coincident with the shelf break and not with inshore waters. Given the rapid changes in climate along the Antarctic Peninsula and the lower per capita recruitment observed in recent years, studies comparing and contrasting production, growth, and recruitment across the Peninsula will be critical to better understand how climate change will impact krill populations and their dependent predators in the Scotia Sea. [Copyright &y& Elsevier]

Abstract: Pathways and rates of ocean flow near the Antarctic Peninsula are strongly affected by frontal features, forcings from the atmosphere and the cryosphere. In the surface mixed layer, the currents advect material from the northwestern Weddell Sea on the eastern side of the Peninsula around the tip of the Peninsula to its western side and into the Scotia Sea, connecting populations of Antarctic krill (Euphausia superba) and supporting the ecosystem of the region. Modelling of subsurface drifters using a particle tracking algorithm forced by the velocity fields of a coupled sea ice-ocean model (ORCA025-LIM2) allows analysis of the seasonal and interannual variability of drifter pathways over 43 years. The results show robust and persistent connections from the Weddell Sea both to the west into the Bellingshausen Sea and across the Scotia Sea towards South Georgia, reproducing well the observations. The fate of the drifters is sensitive to their deployment location, in addition to other factors. From the shelf of the eastern Antarctic Peninsula, the majority enter the Bransfield Strait and subsequently the Bellingshausen Sea. When originating further offshore over the deeper Weddell Sea, drifters are more likely to cross the South Scotia Ridge and reach South Georgia. However, the wind field east and southeast of Elephant Island, close to the tip of the Peninsula, is crucial for the drifter trajectories and is highly influenced by the Southern Annular Mode (SAM). Increased advection and short travel times to South Georgia, and reduced advection to the western Antarctic Peninsula can be linked to strong westerlies, a signature of the positive phase of the SAM. The converse is true for the negative phase. Strong westerlies and shifts of ocean fronts near the tip of the Peninsula that are potentially associated with both the SAM and the El Niño-Southern Oscillation restrict the connection from the Weddell Sea to the west, and drifters then predominantly follow the open paths to South Georgia and the east. Over the 43-year time series, the number of drifters advected into the Bellingshausen Sea decreases significantly by 23% and the travel time to South Georgia shortens significantly by 19% which corresponds to 56 days. We propose that these trends are linked, at least in part, to the increasingly positive trend in the SAM and, as such, this suggests an additional anthropogenic source of change to the regional ecosystem. [Copyright &y& Elsevier]

Abstract: Despite much research on Euphausia superba, estimates of their total biomass and production are still very uncertain. Recently, circumpolar krill databases, combined with growth models and revisions in acoustics have made it possible to refine previous estimates. Net-based databases of density and length frequency (KRILLBASE) yield a summer distributional range of ∼19×106 km2 and a mean total abundance of 8×1014 post-larvae with biomass of 379 million tonnes (Mt). These values are based on a standardised net sampling method but they average over the period 1926–2004, during which krill abundance has fluctuated. To estimate krill biomass at the end of last century we combined the KRILLBASE map of relative krill density around Antarctica with an acoustics-derived biomass estimate of 37.3Mt derived for the Scotia Sea area in 2000 by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Thus the CCAMLR 2000 survey area contains 28% of the total stock, with total biomass of ∼133Mt in January–February 2000. Gross postlarval production is estimated conservatively at 342–536Mtyr−1, based on three independent methods. These are high values, within the upper range of recent estimates, but consistent with the concept of high energy throughput for a species of this size. The similarity between the three production estimates reflects a broad agreement between the three growth models used, plus the fact that, for a given population size, production is relatively insensitive to the size distribution of krill at the start of the growth season. These production values lie within the envelope of what can be supported from the Southern Ocean primary production system and what is required to support an estimated predator consumption of 128–470Mtyr−1. Given the range of recent acoustics estimates, plus the need for precautionary management of the developing krill fishery, our net-based data provide an alternative estimate of total krill biomass. [Copyright &y& Elsevier]

Abstract: Antarctic krill, Euphausia superba Dana, has a heterogeneous circumpolar distribution in the Southern Ocean. Krill have a close association with sea ice which provides access to a critical food source and shelter, particularly in the early life stages. Advective modelling of transport pathways of krill have until now been on regional scales and have not taken explicit account of sea ice. Here we present Lagrangian modelling studies at the circumpolar scale that include interaction with sea ice. The advection scheme uses ocean velocity output from the Ocean Circulation and Climate Advanced Modelling (OCCAM) project model together with satellite-derived sea ice motion vectors to examine the potential roles of the ocean and sea ice in maintaining the observed circumpolar krill distribution. We show that the Antarctic Coastal Current is likely to be important in generating the large-scale distribution and that sea ice motion can substantially modify the ocean transport pathways, enhancing retention or dispersal depending upon location. Within the major krill region of the Scotia Sea, the effect of temporal variability in both the ocean and sea ice velocity fields is examined. Variability in sea ice motion increases variability of influx to South Georgia, at times concentrating the influx into pulses of arrival. This variability has implications for the ecosystem around the island. The inclusion of sea ice motion leads to the identification of source regions for the South Georgia krill populations additional to those identified when only ocean motion is considered. This study indicates that the circumpolar oceanic circulation and interaction with sea ice is important in determining the large-scale distribution of krill and its associated variability. [Copyright &y& Elsevier]

Abstract: A time-dependent, size-structured, physiologically based krill growth model was used in conjunction with a circulation model to test the hypothesis that Antarctic krill (Euphausia superba) populations at South Georgia are sustained by import of individuals from upstream regions. Surface phytoplankton concentrations along the simulated drifter trajectories were extracted from historical Coastal Zone Color Scanner (CZCS) measurements and sea ice biota concentrations were calculated from sea ice concentration and extent extracted along drifter trajectories from Special Sensor Microwave/Imager measurements. As additional food sources, a time series of heterotrophic food was constructed from historical data, and time series of detritus concentrations were calculated using phytoplankton concentrations extracted from CZCS measurements together with measured particulate organic carbon to chlorophyll a ratios. These food resources along specified drifter trajectories were then input to the krill growth model to determine the size and viability of krill during transport from the source region to South Georgia. The krill growth model simulations showed that no single food source can support continuous growth of krill during the 58–306 days needed for transport to South Georgia. However, under the current assumptions results indicate that combinations of food sources during the transport time enhanced krill survival, with heterotrophic food and detritus being particularly important during periods of low phytoplankton concentrations. The growth model simulations also showed that larval and juvenile krill originating along the western Antarctic Peninsula can grow to (14–36mm) and (26–45mm) age and size classes observed at South Georgia during the time needed for transport to this region. Krill originating in the Weddell Sea need 20 months for transport, which allows retention in a potentially high food environment, provided by sea ice, for almost 1 year. Krill then complete transport to South Georgia in the following year and larval and juvenile krill grow to (26–45mm) and (35–60mm) age and size classes during transport. The results of this study show that the successful transport of krill to South Georgia depends on a multitude of factors, such as the location of the spawning area and timing of spawning, food concentrations during transport, predation, and variations in the location of the Southern Antarctic Circumpolar Current Front (SACCF) and in sea ice extent. [Copyright &y& Elsevier]

A particle tracking scheme that uses velocity output from an interannually varying forced run of a global ocean circulation model (Parallel Ocean Climate Model; POCM_4C) allows variability in the transport pathways across the Scotia Sea to South Georgia to be examined for the first time. The time-variant surface fluxes introduce realistic variability into the model velocity fields. This causes large variations in near-surface, mixed-layer transport from the Antarctic Peninsula region to South Georgia, an island in the southwest Atlantic sector of the Southern Ocean. The variability occurs on a variety of timescales with seasonal and longer periods of variability apparent in the 18 year time series of results. A quasi-four year period of variability is evident across the region in the sea surface temperature fields of POCM_4C and appears in the particle tracking results. This period, noted in other Southern Ocean data sets and ascribed to the Antarctic Circumpolar Wave, has been observed in the reproductive success of higher marine predators breeding on the island. The predicted oceanographic variability is likely to be significant for the South Georgia ecosystem by affecting the influx into the region of Antarctic krill (Euphausia superba), the main prey of the higher predators. [Copyright &y& Elsevier]