Krill Density Research Articles

Variation in the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands during the 1999/2000 austral summer

Variation in the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands during the 1999/2000 austral summer

Krill demography and large-scale distribution in the southwest Atlantic during January/February 2000

Abstract This paper summarizes the results of krill demographic studies from the Commission for the Conservation of Antarctic Marine Living Resources 2000 Survey—a large-scale krill survey across the Scotia Sea conducted in January/February 2000. Standard Rectangular Midwater Trawl net tows were carried out at midnight and midday stations between 20° and 70°W south of the Polar Front. The overall mean numerical density and biomass of krill estimated from nets (247 krill per 1000 m3 and 18.7 g m−2, respectively) were similar to lower values reported previously for the Southwest Atlantic, and comparable with high values for other regions of the Southern Ocean. Krill varied in mean size across the survey area, with cluster analyses showing three distinct groups of length-frequency distribution. Small juvenile and immature krill occurred east of the South Orkney Islands. Adult krill 50 mm were mostly restricted to the west of the South Orkney Islands. Maturity stage composition indicated that peak spawning occurred before early February. Distribution of the spawning stock showed two hotspots, the first between the South Shetland and South Orkney Islands, and the second around the South Sandwich Islands. Reproductive krill were largely absent in the central Scotia Sea and around South Georgia. Krill larvae concentrations occurred slightly to the east of the spawning stock. Mean density of larvae in the western Scotia Sea was 2044 m−2, but only scattered aggregations of larvae were recorded east of 36°W (2 m−2). Recruitment indices for 1- and 2-year-old krill were low in the western part of the survey area, the outflow from the Bellingshausen Sea/Antarctic Peninsula region ( R 1 = 0.0 , R 2 = 0.11 ) indicating spawning failure and/or poor recruitment for several years. In the eastern part of the survey area, mostly the outflow of the Weddell Sea, recruitment indices were high and above the long-term average ( R 1 = 0.60 , R 2 = 0.72 ), suggesting a population with constant and successful reproduction, recruitment and mortality. The distribution and structure of the krill population did not show any clear relationship to the position of the major oceanic fronts in the Scotia Sea.

Variation in the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands during the 1999/2000 austral summer

Abstract Vessels from Japan, Peru, and the USA conducted four sequential surveys designed to estimate the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands between late December 1999 and early March 2000. The surveys were conducted during the same austral summer as the CCAMLR 2000 Survey in the Scotia Sea (Watkins et al., Deep-Sea Research, II, this issue [doi: 10.1016/j.dsr2.2004.06.010]), and the data were analyzed in a similar manner. Biomass densities were not significantly different between the surveys and averaged 49 g m−2. Maps of krill biomass indicate three areas of consistently high density: one near the eastern end of Elephant Island, one mid-way between Elephant Island and King George Island, and one near Cape Shirreff on the north side of Livingston Island. The areas of highest krill density appeared to move closer to the shelf break as the season progressed. This apparent movement was accompanied by a change in the demographic structure of the population, with smaller krill absent and a larger proportion of sexually mature animals present in late summer.

Biomass of Antarctic krill in the Scotia Sea in January/February 2000 and its use in revising an estimate of precautionary yield

Abstract In January and February 2000, a collaborative survey designed to assess the biomass of Antarctic krill across the Scotia Sea was conducted aboard research vessels from Japan, Russia, the UK and the USA using active acoustic and net sampling. Survey design, sampling protocols, and data analysis procedures are described. Mean krill density across the survey area was estimated to be 21.4 g m −2 , and total biomass was estimated to be 44.3 million tonnes (CV 11.4%). This biomass estimate leads to a revised estimate of precautionary yield for krill in the Scotia Sea of 4 million tonnes. However, before the fishery can be permitted to expand to this level, it will be necessary to establish mechanisms to avoid concentration of fishing effort, particularly near colonies of land-breeding krill predators, and to consider the effects of krill immigrating into the region from multiple sources.

A comparison of net and acoustic estimates of krill density in the Scotia Sea during the CCAMLR 2000 Survey

A multi-ship, multi-national survey to assess the acoustic biomass of Antarctic krill across the Scotia Sea was undertaken in January and February 2000. In addition, a total of 135 Rectangular Midwater Trawls (RMT8) were undertaken to determine the structure of the krill population and to validate acoustic target detection techniques. This paper reports a comparison of the density estimates derived from net and acoustic sampling for a range of spatial scales; from individual net hauls to regional estimates of krill biomass. The different sources of error and the different characteristics of the density estimates from net and acoustic techniques are also defined. Direct quantitative comparisons of net and acoustic densities are shown to be inappropriate at both the small scale (i.e. individual net tows; the typical net sampling unit for census surveys) and the large scale (i.e. regional surveys). Therefore, a direct comparison of density estimates from net and acoustic surveys is not practicable for retrospective analysis of krill abundance. However, the results of net and acoustic surveys do appear comparable in terms of trends in krill distribution at the large scale. Therefore, the combined use of net and acoustic data can be useful in the analysis of interannual trends in the variability of krill distribution at the regional level. At the local level, data from trawl surveys using comparable nets can be used to examine interannual variability in krill distribution as there has been little change in the methodology used for net surveys over the last 25 years.

A comparison of net and acoustic estimates of krill density in the Scotia Sea during the CCAMLR 2000 Survey

A comparison of net and acoustic estimates of krill density in the Scotia Sea during the CCAMLR 2000 Survey

Oceanographic variability and changes in Antarctic krill (Euphausia superba) abundance at South Georgia

AbstractOceanographic data collected to the north of South Georgia were examined for three consecutive summers (1996/97, 1997/98, 1998/99). The results show the existence of a shelf break front during each period. The most reliable means of defining the front was the potential density anomaly at the near‐surface potential temperature minimum. In each year, off‐shelf waters were separated from on‐shelf waters by water with a potential density anomaly between 27.22 and 27.29 kg m−3. During 1997/98, the near‐surface potential temperature minimum was much colder and much shallower than in other years and was consistent with waters originating from much further south than South Georgia; these differences were further evident at a single deep off‐shelf station. The oceanographic changes during 1997/98 were consistent with a mesoscale or large‐scale movement of the southern Antarctic Circumpolar Current front.Acoustically determined densities of Antarctic krill, Euphausia superba, at South Georgia showed consistent patterns between years. Densities were substantially higher over the shelf compared with off‐shelf, with the highest densities at the shelf edge; densities were also higher to the east of the island. During 1997/98, acoustic densities of krill were substantially higher than in other years. The coincidence of the elevated acoustic density and the cooler oceanographic conditions was explored. When data from all years were combined and analysed by Generalized Additive Model, an inverse relationship between acoustic density and temperature was apparent. Historical data were also examined and it was noted that the only other occurrence of such a high estimate of krill density at South Georgia, was when oceanographic conditions were also colder.

Three-dimensional acoustic mapping and simulation of krill distribution in the Saguenay—St. Lawrence Marine Park whale feeding ground

Geostatistical conditional simulations are used to get a family of non-smoothed three-dimensional (3D) images of the St. Lawrence krill aggregation from echointegration data at 38 and 120 kHz on a systematic grid of transects. These maps respect the inherent patchiness of krill. Their ensemble gives an histogram of the krill density estimates at any point of the 3D grid, not only the mean density as in kriging. The spatially consistent simulations are conditional to match the observations and their histogram and variogram. The 3D problem is by-passed by transposing to a short series of 2D ones, which simplifies the modelling of the autocorrelation function and has the additional advantage of reducing by a factor of 5 the number of points to simulate. The spatially structured krill density profile is summarised by the first few factors of a principal component analysis, where the variables are the volume backscattering strength at 120 kHz for the integrated vertical bins along the profiles and the observations are the different profiles. The principal component scores are simulated over a 2D-grid and then used to reconstruct the full 3D-image. The method adequately reproduces the histogram, variogram and mean profile of krill density, and cross-validations replicate the observations. It is as precise as an alternative kriging approach to estimate the mean density, but has the additional advantages of requiring less computing time for our particular application. These conditional simulations enable estimating the probability density function of the krill density at any point, an essential information for predator-prey interactions and other ecosystem studies.

A Bayesian maximum entropy reconstruction of stock distribution and inference of stock density from line-transect acoustic-survey data

AbstractWe 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.

An investigation of avoidance by Antarctic krill of RRS James Clark Ross using the Autosub-2 autonomous underwater vehicle

The autonomous underwater vehicle (AUV) Autosub-2 was deployed on eight missions ahead of RRS James Clark Ross in the northern Weddell Sea and in the Bransfield Strait, Southern Ocean, to assess avoidance of the research vessel by Antarctic krill Euphausia superba. The AUV was equipped with the same type of scientific echosounder as the research vessel (Simrad EK500 operating at 38 and 120 kHz) and measured the density of krill along transect acoustically (g m −2 wet mass) prior to the ship’s arrival. We hypothesised that if krill avoided the ship, perhaps in response to radiated noise, then the ship should detect less krill than the AUV which is known to have much lower noise levels than the ship. We were unable to detect any significant difference between the density of krill detected by the ship or the AUV, either at the transect level or at finer scales within transects. We conclude, therefore, that avoidance by krill of RRS James Clark Ross will not significantly bias acoustic estimates of krill abundance by this vessel.

Demography of krill in the Elephant Island area during summer 2001 and its significance for stock recruitment

A net sampling survey was carried out for krill in a standard station grid around Elephant Island during 27 January to 4 February 2001. In comparison with recent years the station grid was extended south, where a large proportion of small size classes, one-year-old juvenile krill was found. Results show a spatial separation of the juvenile krill and the spawning stock, Krill density was significantly higher than during the past years (229 krill 1000 m−3 or 13.0 g m−2). The proportional recruitment index for the entire survey area for the 1999/2000 year class was R1 = 0.573, which is among the highest values recorded during the past 20 years. The maturation index (based on the proportion of gravid stages) was G = 0.99, indicating an early initiation of the spawning season. The results indicate a turning point after a succession of years with poor recruitment success and low stock biomass. This is thought to be the first step for a successful spawning event and a later potential recruitment success of the 2000/01 year-class. The spatial extent of the station grid is discussed in the light of a representative coverage of the stock and the estimated recruitment index.

Antarctic krill under sea ice: elevated abundance in a narrow band just south of ice edge.

We surveyed Antarctic krill (Euphausia superba) under sea ice using the autonomous underwater vehicle Autosub-2. Krill were concentrated within a band under ice between 1 and 13 kilometers south of the ice edge. Within this band, krill densities were fivefold greater than that of open water. The under-ice environment has long been considered an important habitat for krill, but sampling difficulties have previously prevented direct observations under ice over the scale necessary for robust krill density estimation. Autosub-2 enabled us to make continuous high-resolution measurements of krill density under ice reaching 27 kilometers beyond the ice edge.

Distribution of foraging by female Antarctic fur seals

The study examined the distribution of critical habitat for foraging by female Antarctic fur seals breeding at the island of South Georgia. Bathymetric features of the continental shelf around the island of South Georgia were an important indicator for the localisation of foraging. This pattern was consistent among years of different prey availability. Lactating females were constrained to forage mainly within 100 km of the location at which the offspring was being raised. When this constraint was removed at the end of lactation, females foraged to much greater ranges and dispersed to specific regions of the continental shelf east of Patagonia (>1000 km) and to the northern edge of the Antarctic pack ice (500 km). The empirical distribution of foraging during the breeding season was used to develop a function that described the foraging distribution for the whole breeding population of females. The result was consistent with past observations from ship-based surveys and it allowed estimation of the spatial impact of breeding female fur seals on krill at South Georgia. This suggested that, in extreme cases and assuming that krill influx is limited, female fur seals could eat most of the krill present in some regions where they forage intensively. However, mean consumption was about one-tenth of the mean density of krill.

Multi-modal surveys of fin whales in the Sea of Cortez, Mexico

A population of fin whales (Balaenoptera physalus), resident to the Gulf of California, Mexico, was studied over two seasons using an integrated approach. Systematic vessel-based visual survey and photo-ID efforts were conducted every 5–7 days to independently estimate the number and distribution of whales within a 10×30 mi2 study area. Some whales were tagged with time-depth-recorders. Sets of 5–6 distributed autonomous seafloor acoustic recorders, operating continuously during each season’s research period, were used to detect, locate, and track vocalizing whales. A 16-element towed array tracked individual vocal whales in real-time concurrently with visual observations, allowing biopsy samples of known vocal animals. Active acoustics was used to collect data on the density and distribution of krill so as to place measured variation in whale numbers, distribution, and behavior within an ecological context. The primary whale activity was feeding. Whale feeding patterns and survey distribution followed prey distribution. Vocal whale distribution followed dial feeding patterns and prey distribution. All biopsied vocal animals were males. Numbers of whales estimated by vessel survey, photo-ID, and passive acoustics were correlated. Results suggest that under certain conditions, vocal activity is a reliable measure of distribution and relative abundance. [Work supported by ONR.]

An initial examination of relationships between the distribution of whales and Antarctic krill Euphausia superba at South Georgia

The distribution of whales and krill in two survey boxes north of South Georgia was examined by comparing sightings and underway acoustic data collected as part of a multi-disciplinary research cruise carried out during January/February 1998. A total of 222 cetaceans of 10 species was recorded with the southern right whale (Eubalaena glacialis) and humpback whale (Megaptera novaeangliae) the two most frequent. The largest aggregation of cetaceans (21 southern right whales, 16 fin whales (Balaenoptera physalus), 4 sei whales (B. borealis), 1 humpback whale and 8 hourglass dolphins (Lagenorhynchus cruciger) occurred close to the largest single aggregation of krill. The level of association between baleen whales and krill was examined at a number of spatial scales. There was a positive relationship between whale abundance and mean krill density at the largest spatial scale examined (803100km). At progressively smaller scales the relationship weakened, due mainly to the increased frequency of areas of high krill density where whales were not recorded. In particular, whales were absent from inshore areas (up to 300m depth) that had higher mean krill densities compared with areas where whales were recorded. To thoroughly compare krill and whale distribution, particularly at smaller scales, will require information on krill swarm structure and density, as well as more information on the behaviour and feeding requirements of whales. Such information may also be crucial to understanding the role of scale-dependence in potential interspecies competition among krill-feeding marine predators.

Distribution and abundance of Antarctic krill ( Euphausia superba) off East Antarctica (80–150°E) during the Austral summer of 1995/1996

A hydroacoustic survey was conducted in the waters off East Antarctica (CCAMLR Division 58.4.1) during January to March 1996 to estimate the biomass ( B 0) of Antarctic krill ( Euphausia superba). The krill biomass in the area surveyed (872,500 km 2) was estimated to be 4.83 million tonnes with a CV of 17%. Dense aggregations of krill, although encountered infrequently, dominated the biomass estimate. At least 97% of these aggregations were less than 200 m in transected length, and the largest was 1 100 m. Large clusters of aggregations (>1 km in transected length) were encountered on 4 of the 18 transects, and these dominated the krill biomass encountered during the survey. Krill were more abundant, with a broader latitudinal distribution, in the west of the survey area (80–115°E) than in the east (115–150°E). The absence of krill and the presence of warmer oceanic waters characterized the northeastern sector of the surveyed area. Krill aggregations were most frequently encountered in the shelf break region where the summer ice edge, 1000 m isobath, and the Antarctic Slope Front (ASF) (associated with the cooler coastal and warmer oceanic waters) coincided. The majority of krill aggregations were found in the top 100 m of the water column, and when deeper (>100 m) aggregations occurred they were usually coincident with aggregations in the top 100 m. The mean krill density for the area surveyed was 5.54 g m −2, consistent with previous observations that the Indian Ocean sector is relatively impoverished when compared with krill density values for the South Atlantic. This density is at the low end of the range of values reported for surveys around South Georgia, Elephant Island, and the adjacent Prydz Bay region. However, this low average biomass results from averaging over a few krill-rich areas with large areas where krill is scarce or absent.

Population structure and condition of Antarctic krill ( Euphausia superba) off East Antarctica (80–150°E) during the Austral summer of 1995/1996

This paper summarises the results of krill demographic studies from BROKE—a large-scale survey off East Antarctica conducted in January – March, 1996. The survey progressed from 80°E in late January to 150°E in mid March. Krill varied in mean size across the survey area, and the various maturity stages showed some longitudinal and latitudinal trends. Males, early stage females, and juveniles were, on average, larger in the east of the survey area whereas gravid and spent females differed little in length across the survey area. Gravid females were only found north of the shelf break; juveniles, males and other stages of females were found throughout the latitudinal extent of the survey. Growth rate measurements resulted in comparable figures to other studies in the Indian Ocean and were greatest in February for all stages. Observed growth rates were sufficient to account for the changes in mean sizes of krill observed across the survey area. The mean density of krill revealed by nets (2.65 krill per 1000 m −3) was low compared to other areas that have been surveyed. Similarly, the overall estimates of proportional recruitment were below average when compared to the range of reported values. Krill larvae were moderately abundant in the survey area but were more frequently encountered in the east, which is probably a function of the lateness of the season when the larvae would be rising into the surface layer sampled by the net.

The use of acoustic sampling to estimate the dispersion and abundance of euphausiids, with an emphasis on Antarctic krill, Euphausia superba

Acoustic sampling has been used to investigate the ecology of Antarctic krill ( Euphausia superba) and to provide information on dispersion and abundance necessary to manage their harvest. Population estimates based on multi-ship acoustic surveys have been used to set catch limits. More localized acoustic surveys have been conducted to study the response of land-breeding krill predators to local variations in their food supply. These and future surveys may result in additional controls on the fishery. In this context, the use of acoustics to survey euphausiids is reviewed and major sources of uncertainty are discussed. These issues are organized as they pertain to the two broad steps of acoustic surveys: (1) estimating the volumetric density of krill (measurement uncertainty) and (2) mapping krill distribution and estimating abundance (sampling uncertainty).

Relationships between Antarctic krill (Euphausia superba) variability and westerly fluctuations and ozone depletion in the Antarctic Peninsula area

An assessment of the environmental processes influencing variability in the recruitment and density of Antarctic krill (Euphausia superba) is important, as variability in krill stocks affects the Antarctic marine ecosystem as a whole. We have assessed variability in krill recruitment and density with hypothesized environmental factors, including strength of westerly winds (westerlies) determined from sea level pressure differences across the Drake Passage, sea ice cover, and ozone depletion. We found a significant positive correlation between krill recruitment in the Antarctic Peninsula area and the strength of westerlies during 1982–1998. Years with strong westerlies during the austral summer season resulted in high krill recruitment in 1987/1988, 1990/1991, and 1994/1995, while the years of weak westerlies resulted in low krill recruitment in 1982/1983, 1988/1989, 1992/1993, and 1996/1997. The strength of westerlies was significantly related to recruitment of 1‐year‐old krill (r = 0.57) and 2‐year‐old krill (r = 0.69) with a level of significance of 5%. In addition, the strength of westerlies also had a strong correlation with chlorophyll a (r = 0.63) and sea ice cover with a 1‐year time lag (r = 0.67). The strength of westerlies is considered to be a key environmental factor. We also found significant correlations between krill density in the Antarctic Peninsula area and the Antarctic ozone depletion parameters during 1977–1997 (e.g., total ozone in October at Faraday/Vernadsky Station of r = 0.76 with a level of significance of 1%). We suspect that ozone depletion impacts directly and/or indirectly on the variability in krill density.

Concordance of interannual fluctuations in acoustically estimated densities of Antarctic krill around South Georgia and Elephant Island: biological evidence of same-year teleconnections across the Scotia Sea

Acoustic estimates of the densities of Antarctic krill, Euphausia superba, in areas around South Georgia (SG) and Elephant Island (EI) were compared for seven austral summers between 1981 and 1997. Estimated densities of krill at SG were most often lower than at EI, although this may simply have been a function of differences in the survey and data-analysis techniques used at each site. More interestingly, the magnitudes of density and between-year gradients of density at each site were mirrored by those at the other location; for example 1991 and 1994 were years of very low krill density at both SG and EI. There was no apparent lag in changes in density between sites, and ranked between-year gradients in density at both locations were closely correlated. These pronounced similarities suggest that densities of krill at both locations are linked directly, and may be impacted by the same gross physical and biological factors (e.g. recruitment, dispersal and environmental variability) acting over the same temporal and spatial scales. The observed concordance also implies that the pelagic ecosystems at these widely separated sites (≃1500 km distant at opposite sides of the Scotia Sea) are not operating in isolation. Fluctuations in krill density were investigated with reference to cyclical variations in sea ice extent, and in air and sea-surface temperature. The resulting model suggests that the 1999/2000 austral summer will be one of low krill density.