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Cutter, G. R., Renfree, J. S., Cox, M. J., Brierley, A. S., and Demer, D. A. 2009. Modelling three-dimensional directivity of sound scattering by Antarctic krill: progress towards biomass estimation using multibeam sonar. – ICES Journal of Marine Science, 66: 1245–1251.Target strength (TS) estimation is a principal source of uncertainty in acoustic surveys of Antarctic krill (Euphausia superba). Although TS is strongly dependent on krill orientation, there is a paucity of information in this regard. This paper considers the potential for narrow-bandwidth, multibeam-echosounder (MBE) data to be used for estimating the orientations of krill beneath survey vessels. First, software was developed to predict MBE measurements of the directivity patterns of acoustic scattering from individual or aggregated krill in any orientation. Based on the distorted-wave, Born approximation model (DWBA), scattering intensities are predicted vs. MBE angles for specified distributions of krill orientations (pitch, roll, and yaw angles) and swarm densities. Results indicate that certain distributions of orientations, perhaps indicative of particular behaviour, should be apparent from the sonar data. The model results are compared with measurements on krill made using a 200-kHz MBE deployed from a small craft off Cape Shirreff, Livingston Island, Antarctica, in summer 2006. The stochastic DWBA model is then invoked to explain disparities between the model predictions and MBE measurements. [ABSTRACT FROM PUBLISHER]

We used 22 years of seasonally and spatially consistent monitoring data to explore marine predator–prey numerical response relationships. Specifically, we tested whether indices of offspring performance (e.g. offspring mass near the time of their independence and/or growth rate) from three Antarctic krill-dependent predators, showed positive relationships with estimates of krill density, determined using fishery-independent acoustic surveys undertaken towards the middle of the predator breeding season. Results showed that indices of predator reproductive performance had little relationship with krill density. In most years, average krill densities were higher on-shelf than off-shelf, potentially providing ecological buffering for predators provisioning offspring. Interestingly, positive response relationships were evident between predator offspring mass and the spatial distribution of krill, measured using indices that represent levels of inequality (patchiness) in krill distribution. These relationships were strongest using indices that reflected the off-shelf krill spatial distribution. We found that krill density and predator offspring mass were also both negatively influenced by sea surface temperature and the Southern Annular Mode, indicating that the environment exerts strong control over ecosystem processes. Finally, we consider the relevance of our results to the ecological framework used by managers responsible for setting catch limits for the regional fishery for krill. [ABSTRACT FROM AUTHOR]

Antarctic krill is a key prey species for many vertebrate and invertebrate predators in the Southern Ocean; it is also an abundant fishery resource in the Scotia Sea and southern Drake Passage. Here, we identify environmental correlates of krill distribution utilizing acoustic data collected during an extensive international survey in January 2000. Separate models (at scales of 10-80 nautical miles) were derived for the full study area and for each of four subregions: northern and southern shelf waters, the seasonally ice-covered open ocean, and the generally ice-free open ocean. Krill distribution was strongly correlated with bathymetry; densities were higher over island shelves and shelf breaks and decreased with increasing distance offshore. Low krill densities occurred in areas of low chlorophyll concentration and high geostrophic velocity. Krill distribution was also related to sea level anomaly but relationships were not consistent between subregions. The models explained a maximum of 44% of the observed deviance in krill density, but did not reliably identify areas of high krill density in the open ocean, and explained a small proportion of the deviance (16%) in offshore areas covered seasonally by sea ice, probably because of the strong, residual influence of retreated ice. The commercial krill fishery is currently concentrated in shelf areas, where high densities of krill are most predictable. As krill are not predictable in the open ocean, the fishery is likely to remain principally a near-shore operation, and should be managed accordingly. [ABSTRACT FROM AUTHOR]

Seafood is sometimes wasted due to the growth of psychrotolerant microbes which secrete proteases and break down proteins. Stenotrophomonas maltophilia FF11, isolated from frozen Antarctic krill, grows at a wide range of temperatures and secretes more proteases at low temperatures. According to zymogram analysis, two kinds of proteases were produced from this strain. A major protease was produced largely at 15 °C, but not at 37 °C. The temperature-dependent secreted protease was purified to homogeneity. Its molecular mass was determined at 37.4 kDa and its amino acid sequence was also obtained. This protease is a member of the subtilase group according to the NCBI blast analysis. The enzyme was highly stable at high salt concentration (4 M). Interestingly, its activity increased about 1.6-fold under high salt condition. The enzyme remains active and stable in different organic solvents (50 %, v/v) such as dimethylsulfoxide, dimethyl formamide, dioxane and acetone. These properties may provide potential applications in quality control for sea foods, in protein degradation at high salt concentration, in biocatalysis and biotransformation within non-aqueous media, such as detergent and transesterification. [ABSTRACT FROM AUTHOR]

For the first time the entire sequence of the mating behaviour of Antarctic krill (Euphausia superba) in the wild is captured on underwater video. This footage also provides evidence that mating can take place near the seafloor at depths of 400–700 m. This observation challenges the generally accepted concept of the pelagic lifestyle of krill. The mating behaviour observed most closely resembles the mating behaviour reported for a decapod shrimp (Penaeus). The implications of the new observation are also discussed. [ABSTRACT FROM AUTHOR]

Antarctic krill is a key species in the Antarctic food web, an important prey item for marine predators and a commercial fishery resource. Although single-beam echo-sounders are commonly used to survey the species, multi-beam echo-sounders may be more efficient because they sample a larger volume of water. However, multi-beam echo-sounders may miss animals because they involve lower energy densities. We adapt distance sampling theory to deal with this and to estimate krill density and biomass from a multi-beam echo-sounder survey. The method provides a general means for estimating density and biomass from multi-beam echo-sounder data. [ABSTRACT FROM AUTHOR]

Hill, S. L., Trathan, P. N., and Agnew, D. J. 2009. The risk to fishery performance associated with spatially resolved management of Antarctic krill (Euphausia superba) harvesting. – ICES Journal of Marine Science, 66: 2148–2154.The ecosystem approach to fisheries attempts to define objectives for target species, the wider ecosystem, and critically, the fishery itself. Proposals for implementing the approach often include spatial restrictions on harvesting, so it is important to understand how these will affect fishery performance. One metric of potential performance is the probability of encountering exploitable densities of a target species at the scale of fishing operations. The probability of encountering exploitable densities of Antarctic krill, Euphausia superba, at the scale of 1 nautical mile during an acoustic survey was predicted by bathymetry and the mean krill density at the larger scale at which the fishery is managed. This suggests that the risk to fishery performance will increase if management actions relocate the fishery into deeper water. The results also suggest that ecosystem models resolved to the spatial scale of management units could usefully predict effects at the scale of fishing operations. However, correct parameterization of these models will require better characterization of threshold densities for efficient exploitation. Finally, the distribution of catch and fishing effort over an entire fishing season reflected the distribution of krill density observed during the survey. [ABSTRACT FROM PUBLISHER]

Sound-speed and density contrasts (h and g, respectively), two important acoustic material properties, of live zooplankton were measured off the western Antarctic Peninsula during a Southern Ocean GLOBEC cruise conducted from 9 April to 21 May 2002. The work included in situ sound-speed contrast and shipboard density-contrast measurements. The temperature and pressure (depth) dependence of the sound-speed contrast of Euphausia superba and E. crystallorophias as well as that of some other zooplankton species were investigated. The size range of E. superba used in the measurements varied from about 20mm to 57mm, with mean length of 36.7mm and standard deviation of 9.8mm, which covered life stages from juvenile to adult. For E. superba, there was no statistically significant depth dependence, but there was a moderate dependence of sound-speed and density contrasts on the size of the animals. The measured sound-speed contrast varied between 1.018 and 1.044, with mean value 1.0279 and standard deviation 0.0084, while the measured density contrast varied between 1.007 and 1.036, with mean value 1.0241 and standard deviation 0.0082. For E. crystallorophias and Calanus there was a measurable depth dependence in sound-speed contrast. The in situ sound-speed contrasts for E. crystallorophias were 1.025±0.004 to 1.029±0.009. For Calanus, they were variable, with one set giving a value of 0.949±0.001 and the other giving 1.013±0.002. Shipboard measurements of other taxa/species also showed substantial variation in g and h. In general, values of g ranged from 0.9402 to 1.051 and h ranged from 0.949 to 1.096. The variation of the material properties is related to species, type, size, stage, and in some cases depth of occurrence. The uncertainty of the estimates of zooplankton biomass attributable to these variations in g and h can be quite large (more than 100 fold). Improvements in making biological inferences from acoustic data depend strongly on increased information about the material properties of zooplankton and the biological causes for their variation, as well as a knowledge of the species composition and abundance. [Copyright &y& Elsevier]

A two-dimensional Lagrangian particle model was developed to examine the spatial distribution of Antarctic krill (Euphausia superba). The time-dependent location of particles, which represent krill individuals, is determined by random diffusion, foraging activity, and movement induced by the presence of neighbours. Foraging activity is based on prescribed food conditions and is such that krill swim slower and turn more frequently in areas of high food concentration. The presence or absence of neighbours either disperses krill, if the local concentrations become too dense, or coalesces krill, if concentrations become too dilute, respectively. Predation on krill is included and affects swarm characteristics by removing individuals. Sensitivity studies indicate that the rate of krill swarm formation and the total number of swarms formed are determined primarily by foraging response and nearest neighbour sensing distance. Simulations using food distributions that are representative of those encountered at boundaries, such as fronts, mesoscale eddies, or the sea ice edge, show that foraging activity can produce rapid swarm formation. Results from other krill swarm models show that attraction between individuals is the primary mechanism producing krill swarms. However, the parameterizations for krill interactions used in those models and that used in this model differ, thereby implying different biological dynamics. Thus, parameterization of the basic interactions in krill swarm models remains to be defined. [Copyright &y& Elsevier]

We present a maximum-entropy (MaxEnt) method for inferring stock density and mapping stock distribution from acoustic line-transect data. MaxEnt is founded on the bedrock of probability theory and allows the most efficient possible use of known data in the inference process. The method takes explicit account of spatial correlation in the observed data and seeks to reconstruct a distribution of density across the whole survey area that is both consistent with the observed data and for which the entropy is maximized. The method is iterative and uses the Bayesian approach of evaluating the posterior probability of a candidate solution under the constraint of the observed data to progress towards a converged solution. We apply the method to reconstruct maps of the distribution of Antarctic krill throughout areas 100 × 80 km2. Survey data were integrated at 0.5-km intervals along ten 80-km transects, giving approximately 1600 observed data points. We inferred the krill density for all 32 000 0.5 × 0.5 km2 cells in the area. The method is demanding computationally, but appears to work well even in cases when the distribution of density is highly skewed. The MaxEnt technique has proved to be powerful for reconstructing quantitative images from incomplete and noisy physical data (e.g. radio-telescope data): we suggest that it has utility in biological systems too and could be of benefit to the fisheries-acoustics community, increasing the accuracy of acoustic estimates of stock density and generating better maps of stock distribution. [Copyright &y& Elsevier]

An area of high krill (Euphausia superba Dana) density was continuously monitored with down-looking and side-looking sonars over a period of 24 h. Measurements of volume backscattering strength were used to describe the density of krill with depth and the vertical movement of krill over time. In situ measurements were made of dorsal aspect target strength (TS) and, as krill moved into the near-surface layer (0–15 m), in situ measurements were made of lateral-aspect TS. The probability density function (PDF) of TS measurements made with the down-looking transducer had a mode at approximately −73 dB. The PDF of TS measurements made with the side-looking transducer was broader with a mode at approximately −67 dB. Sampled krill had a bi-modal length distribution with a major mode at 44 mm and a minor mode at approximately 30 mm. [ABSTRACT FROM PUBLISHER]