Krill Swarms Research Articles

Novel DV-hop Method Based on Krill Swarm Algorithm Used for Wireless Sensor Network Localization

Wireless sensor network (WSN) is self-organizing network; it consists of a large number of sensor nodes with perception, calculation ability and communication ability. As we all know, the floor, walls or people moving has an effect on indoor localization, so it will result in multi-path phenomena and decrease signal strength, also the received signal strength indicator (RSSI) is unable to gain higher accuracy of positioning. When using multilateral measurement method to calculate the unknown node coordinates, it will generate big error in range-free distance vector-hop (DV-hop) localization algorithm of WSN. In order to improve the WSN positioning accuracy in indoor condition, more reasonable distribute network resources, in this paper, we firstly propose krill swarm algorithm used for WSN localization. First, we detailed analyze the multilateral measurement method in DV-hop localization algorithm. The position problem can be transformed into a global optimization problem. Then, we adequately utilize the advantage of calculating optimization problem. We apply the krill swarm algorithm into the stage of estimating unknown node coordinates in DV-hop algorithm to realize localization. Finally, the simulation experience results show that the localization with krill swarm algorithm has an obviously higher positioning precision and accuracy stability with different anchor node proportion and nodes. We also make comparison with DV-hop algorithm and the newest localization algorithm.

Krill distribution in relation to environmental parameters in mesoscale structures in the Ross Sea

Krill, a key pelagic resource of the Antarctic food web, provides an important link between primary and secondary plankton production and top predators. Since krill abundance is a crucial factor in Antarctic ecosystem functioning, its monitoring supplies vital data. Acoustic surveys are an effective approach to estimating krill abundance. An acoustic survey was conducted in the western Ross Sea in January 2014–10years after a similar survey by our team - to estimate krill abundance and biomass and monitor oceanographic conditions. Antarctic krill (Euphausia superba) was detected in the northern part of the western Ross Sea and dense swarms of ice krill (Euphausia crystallorophias) were found in its central coastal area. Data analysis revealed an inverse correlation between E. superba density and salinity in the water column, whereas a positive correlation was found between E. crystallorophias abundance and fluorescence; the latter relationship was confirmed in thematic maps of E. crystallorophias spatial distribution and fluorescence. Comparison of 2004 and 2014 biomass data showed a much greater abundance of both species in the more recent survey.

Transient stability constrained optimal power flow using oppositional krill herd algorithm

Transient stability constrained optimal power flow (TSCOPF) is becoming an effective tool for many problems in power systems since it simultaneously considers economy and dynamic stability of system operations. It is increasingly important because modern power systems tend to operate closer to the stability boundaries due to the rapid increase of electricity demand and the deregulation in power sector. TSCOPF is, however, a nonlinear optimization problem with both algebraic and differential equations which is difficult to solve even for small power network. In order to solve the TSCOPF problem efficiently, a relatively new optimization technique, named as krill herd algorithm (KHA), is employed in this paper. KHA simulates the herding behavior of krill swarms in response to specific biological and environmental processes to solve multi-dimensional, linear and nonlinear problems with appreciable efficiency. To accelerate the convergence speed and to improve the simulation results, opposition based learning (OBL) is also incorporated in the basic KHA method. The simulation results, obtained by the basic KHA method and the proposed oppositional KHA (OKHA) algorithm, are compared to those obtained by using some other recently developed methods available in the literature. In this paper, case studies conducted on 10 generator New England 39-bus system and 17 generator 162-bus system indicate that the proposed OKHA approach is much more, computationally, efficient than the other reported popular state-of-the-art algorithms including the basic KHA and the proposed method is found to be a promising tool to solve the TSCOPF problem of power systems.

Effect of prey type on the fine-scale feeding behaviour of migrating east Australian humpback whales

For terrestrial migrants, feeding at migratory stopover sites is important, with prey quality linked to future survival and reproductive success. In contrast, the importance of this behaviour to marine species is unknown. The humpback whale Megaptera novaeangliae is a marine migrant that is thought to fast while migrating; however, recent studies suggest that feeding may occur during this time. The aims of this study were to determine how the prey type available on a migratory route off Eden, New South Wales, Australia, influenced whether whales fed or not, and to study the fine-scale behaviour of the whales. Digital acoustic recording tags (DTAGs) and focal follows were used to record whale behaviour. A larger proportion of groups were determined to be feeding when krill was observed at the surface. Whales encountering fish spent a small percentage (28%) of time feeding, behaving similarly to non-feeding whales on migration, with small groups, a higher proportion of males sampled, and relatively straight tracks. In contrast, whales encountering surface swarms of krill spent significantly more time feeding (92%) and behaved similarly to whales on feeding grounds, with larger groups, more females sampled, and tracks with high turning angles. Therefore, changes in the available prey type in - fluenced the amount of time spent feeding and the social dynamics of groups. Given the link between the amount of feeding completed on migration and the future survival and reproductive success of individuals in terrestrial species, the impact of such fluctuations on marine species such as the humpback whale deserves more attention.

Particle flux on the continental shelf in the Amundsen Sea Polynya and Western Antarctic Peninsula

Abstract We report results from a yearlong, moored sediment trap in the Amundsen Sea Polynya (ASP), the first such time series in this remote and productive ecosystem. Results are compared to a long-term (1992–2013) time series from the western Antarctic Peninsula (WAP). The ASP trap was deployed from December 2010 to December 2011 at 350 m depth. We observed two brief, but high flux events, peaking at 8 and 5 mmol C m−2 d−1 in January and December 2011, respectively, with a total annual capture of 315 mmol C m−2. Both peak fluxes and annual capture exceeded the comparable WAP observations. Like the overlying phytoplankton bloom observed during the cruise in the ASP (December 2010 to January 2011), particle flux was dominated by Phaeocystis antarctica, which produced phytodetrital aggregates. Particles at the start of the bloom were highly depleted in 13C, indicating their origin in the cold, CO2-rich winter waters exposed by retreating sea ice. As the bloom progressed, microscope visualization and stable isotopic composition provided evidence for an increasing contribution by zooplankton fecal material. Incubation experiments and zooplankton observations suggested that fecal pellet production likely contributed 10–40% of the total flux during the first flux event, and could be very high during episodic krill swarms. Independent estimates of export from the surface (100 m) were about 5–10 times that captured in the trap at 350 m. Estimated bacterial respiration was sufficient to account for much of the decline in the flux between 50 and 350 m, whereas zooplankton respiration was much lower. The ASP system appears to export only a small fraction of its production deeper than 350 m within the polynya region. The export efficiency was comparable to other polar regions where phytoplankton blooms were not dominated by diatoms.

Interannual variability in Antarctic krill (Euphausia superba) density at South Georgia, Southern Ocean: 1997–2013

Abstract Antarctic krill (Euphausia superba) are a key species in Southern Ocean ecosystems, maintaining very large numbers of predators, and fluctuations in their abundance can affect the overall structure and functioning of the ecosystems. The interannual variability in the abundance and biomass of krill was examined using a 17-year time-series of acoustic observations undertaken in the Western Core Box (WCB) survey area to the northwest of South Georgia, Southern Ocean. Krill targets were identified in acoustic data using a multifrequency identification window and converted to krill density using the Stochastic Distorted-Wave Born Approximation target strength model. Krill density ranged over several orders of magnitude (0–10 000 g m−2) and its distribution was highly skewed with many zero observations. Within each survey, the mean krill density was significantly correlated with the top 7% of the maximum krill densities observed. Hence, only the densest krill swarms detected in any one year drove the mean krill density estimates for the WCB in that year. WCB krill density (µ, mean density for the area) showed several years (1997/1998, 2001–2003, 2005–2007) of high values (µ > 30 g m−2) interspersed with years (1999/2000, 2004, 2009/2010) of low density (µ < 30 g m−2). This pattern showed three different periods, with fluctuations every 4–5 years. Cross correlation analyses of variability in krill density with current and lagged indices of ocean (sea surface temperature, SST and El Niño/Southern Oscillation) and atmospheric variability (Southern Annular Mode) found the highest correlation between krill density and winter SST (August SST) from the preceding year. A quadratic regression (r2 = 0.42, p < 0.05) provides a potentially valuable index for forecasting change in this ecosystem.

Instantaneous movement of krill swarms in the Antarctic Circumpolar Current

Antarctic krill are known to have strong swimming capabilities, but direct observations of the speed and direction of krill‐swarm movement within their natural environment are rare. We identified and examined 4060 swarms within the main flow of the Antarctic Circumpolar Current (Scotia Sea) using a combination of an EK60 echosounder, a 153.6 kHz acoustic Doppler current profiler, and ground‐truthing nets. Net displacement magnitude (m) and net angle of deviation (d) were determined by vector subtraction from the background flow immediately below them. Values were compared against control data sets in which swarms were absent. With greater background flow, m became increasingly lower than predicted, which suggests that drag influences swarm movement. The characteristics of the flow regime influenced swarm behavior, given that both m and d varied according to the direction of background flow. Furthermore, multiple‐regression analysis indicated that swarm area, the vicinity of the sea‐ice edge, and salinity had a significant influence on m, with levels of displacement being greatest in larger swarms and in low‐salinity regions close to the ice edge. The ice edge is a key environment for Antarctic krill and swarm behavior may assist in retaining this location. Only fluorescence was found to have a significant influence on d, with deviations being greatest in regions of highest fluorescence. This agrees with laboratory observations of krill turning more frequently within food patches. We demonstrate that it is possible to measure instantaneous movement patterns in Antarctic krill swarms and, at large scales, these movements are consistent with current understanding of responses of krill to local stimuli such as sea‐ice and patches of food.

Krill Excretion Boosts Microbial Activity in the Southern Ocean

Antarctic krill are known to release large amounts of inorganic and organic nutrients to the water column. Here we test the role of krill excretion of dissolved products in stimulating heterotrophic bacteria on the basis of three experiments where ammonium and organic excretory products released by krill were added to bacterial assemblages, free of grazers. Our results demonstrate that the addition of krill excretion products (but not of ammonium alone), at levels expected in krill swarms, greatly stimulates bacteria resulting in an order-of-magnitude increase in growth and production. Furthermore, they suggest that bacterial growth rate in the Southern Ocean is suppressed well below their potential by resource limitation. Enhanced bacterial activity in the presence of krill, which are major sources of DOC in the Southern Ocean, would further increase recycling processes associated with krill activity, resulting in highly efficient krill-bacterial recycling that should be conducive to stimulating periods of high primary productivity in the Southern Ocean.

Contrasting diel patterns in vertical movement and locomotor activity of whale sharks at Ningaloo Reef

Activity patterns of animals often relate to environmental variables such as food availability and predation pressure. Technological advances are providing us with new tools to monitor and better understand these activity patterns. We used animal-attached data loggers recording acceleration and depth to compare activity patterns and vertical habitat use of whale sharks (Rhincodon typus) at Ningaloo Reef, Western Australia. Whale sharks showed a moderate reverse diel vertical migration but exhibited a clear crepuscular pattern in locomotory activity. Peak activity occurred at sunset, whereas vertical movement peaked prior to this. Typical ram surface filter feeding could be identified and occurred primarily during sunset and the first hours of night. At such times, direct observations indicated whale sharks were feeding on tropical krill swarms. Kinematic analysis of postural data and data from vertical movement suggests that whale sharks at Ningaloo spend ~8 min per day actively ram surface filter feeding. Considering the high biomass present in krill schools, it is estimated that whale sharks at Ningaloo have a similar energy intake as those at other aggregation sites. Diel patterns in activity and diving behaviour suggest that whale sharks have tuned their diving behaviour in anticipation of the formation of these high-density patches which appear to only be periodically, but predictably available at sunset. Our results confirm that diel patterns in vertical habitat selection and vertical movements do not necessarily reflect patterns in activity and foraging behaviour. Direct quantification of activity and behaviour is required in gaining accurate representation of diel activity patterns.

Antarctic krill swarm characteristics in the Southeast Atlantic sector of the Southern Ocean

Knowledge about swarm dynamics and underlying causes is essential to understand the ecology and distribution of Antarctic krill Euphausia superba. We collected acoustic data and key environmental data continuously across extensive gradients in the little-studied Southeast Atlantic sector of the Southern Ocean. A total of 4791 krill swarms with swarm descriptors includ- ing swarm height and length, packing density, swimming depth and inter-swarm distance were extracted. Through multivariate statistics, swarms were categorized into 4 groups. Group 2 swarms were largest (median length 108 m and thickness 18 m), whereas swarms in both Groups 1 and 4 were on average small, but differed markedly in depth distribution (median: 52 m for Group 1 vs. 133 m for Group 4). There was a strong spatial autocorrelation in the occurrence of swarms, and an autologistic regression model found no prediction of swarm occurrence from envi- ronmental variables for any of the Groups 1, 2 or 4. Probability of occurrence of Group 3 swarms, however, increased with increasing depth and temperature. Group 3 was the most distinctive swarm group with an order of magnitude higher packing density (median: 226 ind. m �3 ) than swarms from any of the other groups and about twice the distance to nearest neighbor swarm (median: 493 m). The majority of the krill were present in Group 3 swarms, and the absence of association with hydrographic or topographic concentrating mechanisms strongly suggests that these swarms aggregate through their own locomotion, possibly associated with migration.

Acoustic determination of the distribution of fish and krill across the Scotia Sea in spring 2006, summer 2008 and autumn 2009

Multifrequency acoustic backscatter data were examined from transects at eight stations across the Scotia Sea, from the South Orkneys to the north west of South Georgia. These transects were repeated in austral spring (November 2006), summer (December 2007/January 2008) and autumn (March 2009). A dB identification window ( S v120-38 ) identified two types of scatterers dominating the acoustic backscatter; a positive S v120-38 indicative of Antarctic krill ( Euphausia superba) and a negative S v120-38 indicative of mesopelagic fish with swim bladders, such as myctophids. Acoustic backscatter attributed to schooling myctophid fish was greater at stations in the north of the Scotia Sea across all seasons and found to increase significantly with increasing mean sea temperature and absolute dynamic height. A schools analysis identified more fish schools at the stations with high fish acoustic backscatter. The number of fish schools increased, were distributed further south, and occurred deeper from spring through to autumn. Krill density, calculated using a target strength model, was similar in spring (27.7 g m −2) and summer (27.2 g m −2), but lower (2.2 g m −2) in autumn 2009. An analysis of swarm characteristics showed that during the spring cruise the high mean krill density resulted from a small number of swarms containing a large number of small krill, particularly in the south of the survey region. In contrast in the summer cruise the high mean krill density resulted from a large number of swarms containing a low number of large krill. Finally, in autumn the low krill density resulted from a small number of swarms containing a small number of large krill. Krill swarms were located at a mean depth of 58 m in spring, 94 m in summer and 123 m in autumn and were generally further apart, deeper, contained fewer krill and smaller in the north of the survey region. Large-scale shifts in the latitudinal and vertical distribution of krill and schooling mesopelagic fish in the Scotia Sea were observed. This seasonal reorganisation in both horizontal and vertical distribution will alter the role of these prey items in the ecosystem.

Shapes of Krill Swarms and Fish Schools Emerge as Aggregation Members Avoid Predators and Access Oxygen

Many types of animals exhibit aggregative behavior: birds flock, bees swarm, fish shoal, and ungulates herd. Terrestrial and aerial aggregations can be observed directly, and photographic techniques have provided insights into the behaviors of animals in these environments and data against which behavioral theory can be tested. Underwater, however, limited visibility can hamper direct observation, and understanding of shoaling remains incomplete. We used multibeam sonar to observe three-dimensional structure of Antarctic krill shoals acoustically. Shoal size and packing density varied greatly, but surface area:volume ratios (roughnesses) were distributed narrowly about ∼3.3 m(-1). Shoals of clupeid fish (e.g., sardine, anchovy) from geographically and oceanographically diverse locations have very similar roughnesses. This common emergent shape property suggests common driving forces across diverse ecosystems. Group behavior can be complex, but a simple tradeoff--that we model--in which individual fish and krill juggle only their access to oxygen-replete water and exposure to predation can explain the observed shoal shape. Decreasing oxygen availability in a warming world ocean may impact shoal structure: because structure affects catchability by predators and fishers, understanding the response will be necessary for ecological and commercial reasons.

Swarm characteristics of Antarctic krill Euphausia superba relative to the proximity of land during summer in the Scotia Sea

We studied the relationship between the proximity of land and the distribution and swarming characteristics of Antarctic krill across the Scotia Sea in January and February 2003. Krill swarms identified with a Simrad EK60 (38 kHz, 120 kHz) echosounder were grouped into 4 categories according to distance from shoreline: 0 to 50 km, 50 to 100 km, 100 to 200 km and >200 km. Cross-sectional areas of swarms were significantly larger inshore, with a mean value of 120 m(2) in the 0 to 50 km zone compared to < 80 m(2) further offshore. The packing concentration of krill within inshore swarms was also significantly greater, with an average density of 95 ind. m(-3) compared to between 24 and 31 ind. m(-3) elsewhere. A large proportion of the biomass was concentrated into a small number of large, dense swarms throughout the survey area, and this trend increased with decreasing distance from shore. The highest median number of swarms per km and krill acoustic biomass per km was found in the 50 to 100 km zone. However, a significantly greater number of large, biomass-rich swarms occurred in the 0 to 50 km zone compared to all other zones. Swarms in the 0 to 50 km zone were also significantly further apart. The majority of swarms were located in the upper 50 m during the daytime although they were marginally deeper in the night in offshore regions. Krill are likely to move between inshore and offshore environments continuously over their lifetimes. The change in krill behaviour between environments could be a response to local predatory threats over short spatial and temporal scales.

Spatial association between hotspots of baleen whales and demographic patterns of Antarctic krill Euphausia superba suggests size-dependent predation

We examined the spatial association between baleen whales and their principal prey, Antarctic krill Euphausia superba near the South Shetland Islands (Antarctic Peninsula) using data collected by the US Antarctic Marine Living Resources (AMLR) program during January surveys from 2003 through 2007. Whale distributions were determined using ship-based visual surveys, while data on krill distribution, abundance, and demographic characteristics were derived from net hauls. Approximately 25 000 km of transects and 500 net hauls were sampled over 5 yr. We defined hotspots based on statistical criteria to describe persistent areas of occurrence of both whales and krill. Hotspots were identified, and whales and krill length-maturity classes exhibited distinct spatial seg- regation in their distribution patterns. We found that baleen whales aggregated to krill hotspots that differed in size structure. Humpback whales Megaptera novaeangliae were associated with small ( 45 mm) mature krill located offshore. Overlapping these size-dependent krill dis- tributions, Antarctic minke whales B. bonaerensis were associated with intermediate sized krill (35-44 mm). The correlation among different whale species and krill swarms of differing size com- position presents an intriguing pattern that deserves further study.

Swarms of diversity at the gene cox1 in Antarctic krill

The Antarctic krill, Euphausia superba, is an abundant and key species found in the Southern Ocean that forms dense, discrete swarms. Despite over three decades of research on Antarctic krill, the genetics of individual swarms is yet to be specifically investigated. In this study, we address the genetic diversity, population structure and demographic history of nine Antarctic krill swarms by sequencing 1173 bases of the gene cytochrome c oxidase subunit I (cox1, COI) from 504 individuals. Both haplotype diversity (h=0.9974-1.0000) and nucleotide diversity (pi=0.010275-0.011537) of Antarctic krill swarm samples was consistently high compared with populations of other species reported in the literature. Analysis of molecular variance did not show any significant genetic structure, thus implying that the sampled swarms do not appear to reflect discrete genetic units. Fu's Fs and Bayesian Skyride analyses provided strong evidence for a large increase in the population size of Antarctic krill, or selection favouring a particular mitochondrial lineage, within the last few 100,000 years (Pleistocene). The swarm-level results presented in this study not only further our understanding of Antarctic krill biology but, because of the economical importance of this species, also provide data to consider for future krill stock management.

Three-dimensional observations of swarms of Antarctic krill (Euphausia superba) made using a multi-beam echosounder

Abstract Antarctic krill (Euphausia superba) aggregate in dense swarms. Previous investigations of krill swarms have used conventional single- or split-beam echosounders that, with post-processing, provide a two-dimensional (2-D) view of the water column, leaving the third dimension to be inferred. We used a multi-beam echosounder system (SM20, 200 kHz, Kongsberg Mesotech Ltd, Canada) from an inflatable boat (length=5.5 m) to sample water-column backscatter, particularly krill swarms, directly in 2-D and, with post-processing, to provide a three dimensional (3-D) view of entire krill swarms. The study took place over six days (2–8 February 2006) in the vicinity of Livingston Island, South Shetland Islands, Antarctica (62.4°S, 60.7°W). An automatic 3-D aggregation detection algorithm resolved 1006 krill swarms from the survey data. Principal component analyses indicated that swarm morphology metrics such as length, surface area and volume accounted for the largest between swarm variance, followed by echo energy, and finally swarm geographic location. Swarms did not form basic cylindrical or spherical shapes, but had quite consistent surface area to volume ratios of 3.3 m−1. Swarms were spatially segregated, with larger sizes (mean north-south length=276 m, at least double that of two other swarm classifications), found to the northwest of the survey area. The apparent clustering of swarm types suggests that krill biomass surveys and ecosystem investigations may require stratified survey design, in response to varying 3-D swarm morphology, variation that may be driven in turn by environmental characteristics such as bathymetry.

Biogeneration of chromophoric dissolved organic matter by bacteria and krill in the Southern Ocean

Chromophoric dissolved organic matter (CDOM), the optically active fraction of dissolved organic matter, is primarily generated by pelagic organisms in the open ocean. In this study, we experimentally determined the quantity and spectral quality of CDOM generated by bacterioplankton using two different substrates (with and without photoproducts) and by Antarctic krill Euphausia superba and evaluated their potential contributions to CDOM dynamics in the peninsular region of the Southern Ocean. CDOM was generated by bacteria in all experiments, and the presence of photoproducts influenced both the quantity and the spectral quality of the resultant CDOM. We confirmed a direct link between bacterial production and CDOM generation, which yielded in situ CDOM duplication times from 31 to 33 d. Antarctic krill as a direct source of CDOM was also confirmed experimentally. We estimated that CDOM generation by krill would lead to CDOM duplication times from 0.48 to 0.80 d within krill swarms. Our findings highlight the potential significance of bacteria and Antarctic krill swarms in the generation of CDOM and underscore the dynamic nature of CDOM in this area.

Egg-release behaviour in Antarctic krill

The process of egg release is a complex and crucial step in the life cycle of euphausiids, especially with regards mortality and recruitment success. We examined this process in Antarctic krill (Euphausia superba) in terms of the functioning of the female genital apparatus and associated swimming behaviour. A tethering technique combined with video analysis was used to make observations of three females during the release process. We found eggs were steadily extruded over a period of up to 10 h, during which time the krill released between 1,600 and 4,000 eggs and the ovary reduced by a half in length and a third in height. Eggs were mainly released individually or, less commonly, in batches of between 2 and 4 eggs. Release rates were between 5 and 30 eggs min−1. The steady release of eggs fits well with histological evidence that mature oocytes must pass individually through tight genital ducts with narrow apertures, before coming in to close contact with the sperm plug for fertilisation, and then expulsion from the thelycum as fertilised eggs. During spawning, the female alternated between slow and rapid rates of pleopod beating with egg release occurring at the moment of beat acceleration. At the point of release, the descent of the egg was accelerated through downward beats of the 7th thoracic leg. The cyclic pattern in pleopod beat-rate during spawning may alter swimming performance and contribute to the widely reported sex- and maturity-based biases within krill swarms.

Multibeam echosounder observations reveal interactions between Antarctic krill and air-breathing predators

A multibeam echosounder (MBE) was deployed on an inflatable boat (length = 5.5 m) to observe swarms of Antarctic krill Euphausia superba in the nearshore environment off Livingston Island, South Shetland Islands, Antarctica. Visual observations of air-breathing predators, including penguins and fur seals, were made from the boat at the same time. MBEs extend the 2-dimensional acoustic observations that can be made with conventional vertical echosounders to 3 dimensions, enabling direct observation of the surface areas and volumes of entire krill swarms. Krill swarms exhibited a wide range of various size metrics (e.g. height, length and width) but only a narrow range of surface-area-to-volume ratios or 'roughnesses', suggesting that krill adopt a consistent group behavior to maintain swarm shape. The variation in R was investigated using generalized additive models (GAMs). GAMs indicated that the presence of air-breathing predators influenced swarm shape (R decreased as the range to predators decreased, and the swarms became more spherical), as did swarm nearest-neighbor distance (R decreased with increasing distance) and swarm position in the water column (R decreased in the upper 70% of the water column). Therefore, swarm shape appears to be influenced by a combination of behavioral responses to predator presence and environ- mental variables. MBEs have the potential to contribute much to studies of krill, and can provide data to improve our understanding of the behavior of krill in situ.

Influence of individual state on swimming capacity and behaviour of Antarctic krill Euphausia superba

Swarms of Antarctic krill are frequently biased towards certain body sizes, sexes, maturi- ties and physiological states. However, the mechanisms causing such biases remain unclear, with some speculating on differential responses to chemical or visual cues. This study examines the influence of individual state on swimming behaviour and performance, which, in turn, may cause biases in swarm composition. The frequency and strength of pleopod beats as well as the overall swimming capacity of Antarctic krill Euphausia superba were measured using a tethering technique. Individual state was con- sidered in terms of body size, sex, maturity, moult stage and stomach fullness. Significant differences in the rate and strength of pleopod strokes as well as in the overall swimming capacity were found between individuals in different states. Body size had one of the strongest influences, with beat rate decreasing and beat strength increasing with increasing size. The sex and maturity of an individual altered this pattern, with adult females having faster but weaker pleopod beats than males of equivalent size. The difference in swimming style of females may impose a different lifestyle to that of males. Moult stage also had a significant influence on swimming, with moulting/post-moult individuals having significantly lower swimming capacities, making them unlikely to keep pace with intermoult counterparts. Both the rate and the strength of the pleopod beat were significantly lower in satiated compared to unsatiated individuals, which may contribute to the dispersion of swarms during feeding.