CCAMLR

Four separate acoustic surveys of Antarctic krill (Euphausia superba) were conducted around South Georgia in the 2001/2002 season: one in November 2001 (early); two during January 2002 (middle), and one in May 2002 (late). The surveys were the second in a five-year series of observations designed to complement and extend an existing time series of summer surveys maintained by the British Antarctic Survey regularly since 1996. Krill density in November was low (5 g m-2), higher in both the two surveys in January (46g m-2 and 72 g m-2 ) and had decreased to 12 g m-2 by May. Our repeated surveys at South Georgia have revealed a similar pattern of change to that observed in 2000/2001 and highlight the importance of understanding the relative contributions of physical and biological processes to krill population dynamics in the region.

During austral summer (January 21st to February 5th) 2000 an oceanographic cruise, devoted to study two krill species (Euphausia superba and Euphausia crystallorophias), was carried out in the Ross Sea area. Activities included acoustic, fishery and physical measurements. More than 2370 nautical miles were acoustically sampled to determine the euphausiid biomass; during the echosurvey, every 6 hours a haul and a CTD cast (or a XBT launch) were performed, and a XBT was launched between consecutive hauling stations. This allowed to collect 34 CTD stations and 73 temperature profiles (XBT), so identifying main water masses and oceanographic features. Acoustic data were processed in order to distinguish between the two species. Euphausia superba and Euphausia crystallorophias swarms were recognized, and average length estimated, by means of the three-frequency method, based on the fluid sphere model. Net samples were considered the ground truth data, being compared to the acoustic estimates of krill species and size. Characteristics (dimensions, volume, weight, krill mean length) for each krill swarm were determined, and the krill average biomass per squared nautical miles was computed along the ship route track. Adopted methodologies for cruise execution and for acoustic data post-processing allowed to obtain for the first time a detailed description of the krill distribution in the Ross Sea area related to oceanographic characteristics. Horizontal distribution of krill average biomass is showed separately for each species, and associated to thermohaline properties. Highest krill density biomass areas are closely investigated, and vertical sections with krill swarms weight and sea temperature are reported. Results indicate that the Euphausia superba detected biomass was about one order of magnitude greater than Euphausia crystallorophias one. The first species was almost exclusively present in the northern area, interesting only a limited portion of the continental shelf, while the second one dominated the southern area, starting from the Ross Ice Shelf region until the zone close to the shelf break, with some presence in the open ocean region too. In the proper shelf break area, Euphausia crystallorophias was practically absent, while Euphausia superba was relatively abundant. The two species had a very limited, but with relatively high biomasses, overlapping area in the northernmost part of the Joides basin. Few other minor overlapping areas were detected. Swarms of both species were mostly found in the surface water layer (Euphausia crystallorophias being located at quite deeper depths, often close to the seasonal thermocline zone) and they appeared to prefer cold waters, avoiding the warm Modified Circumpolar Waters and the warmest portion of the surface waters.

In this paper an acoustic method for identifying two euphausiid species and estimating their length is described. The approach is in fact an outgrowth from both the fluid sphere and Bayes rule methodologies.
This paper explores applications of the multi-frequency method using data from three expeditions to the Ross Sea (1980-90; 1997-98 and 1999-2000), where the environmental conditions, the sampled areas, the instrumental and the sampling strategies varied.
First, on the basis of the echo-integrations, made simultaneously either at two or at three frequency, and of the results of net samplings, the thresholds and the decision criteria to recognize the two species are established.
Next the acoustic estimates of euphausiid lengths, derived from the fluid sphere model, are compared with lengths collected from net samplings.
Finally, the developed criteria and algorithms are effectively applied to estimate E. superba biomass found in the area of the Ross Sea investigated in the December 1997 and in January/February 2000. The results are compared with those obtained from the standard method.

We present Maximum Entropy (MaxEnt) reconstructions of krill distribution and estimates of mean krill density within two survey boxes of dimensions 80 km x 100 km to the north east and north west of South Georgia. The reconstructions are generated from acoustic line transect survey data gathered in the boxes in austral summers 1996 – 2000. Krill densities had previously been determined at approximately 0.5 km intervals along transect for each of the ten 80 km transects in each box, providing about 1600 krill density estimates per box. The MaxEnt technique uses a Bayesian approach to infer the most probable krill density for each of the 32000 0.5 km x 0.5 km cells in each survey box, taking explicit account of the spatial relationship between densities in the observed data. Despite some very large interannual and regional (east box cf west box) differences in mean krill density, the MaxEnt approach seems to work well, providing plausible maps of distribution. The technique also yields mean krill densities for which the confidence limits are often narrower than for estimates based upon more conventional (Jolly and Hampton, 1990) analyses.

We present a maximum entropy (MaxEnt) method for inferring stock density and mapping 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 distribution of Antarctic krill throughout areas 100 x 80 km. Survey data were integrated at 0.5 km intervals along ten 80 km transects, giving approximately 1600 observed data. We inferred krill density for all 32000 0.5 x 0.5 km cells in the area. The method is computationally demanding but appears to work well, even in cases when the distribution of density is highly skewed. The MaxEnt technique has proved powerful for reconstruction of quantitative images from incomplete and noisy physical data (e.g. radio telescope data) and we suggest that it could be of benefit to the fisheries acoustic community, increasing the accuracy of acoustic estimates of stock density and generating superior maps of stock distribution.

Penguin populations are potentially sensitive indicators of change in the Antarctic and sub-Antarctic and are used extensively to monitor changes in the marine ecosystem. Aerial photographic surveys may provide the most robust method for estimating breeding population size, particularly for large colonies, for colonies in areas of complex terrain, or for colonies where ground access is difficult. Obtaining population estimates from aerial photographs is laborious and time consuming, and is usually carried out manually by counting individual birds on highly magnified prints. Here we present the results of a computer based image analysis of digitally scanned colour aerial photographs of macaroni penguin colonies on Bird Island, South Georgia. We compare the results with conventional ground counts for the study colonies, highlighting various assumptions that contribute to differences in the population estimates. The software analysis method provides population estimates that are considered to be more reliable than the associated ground counts. The technique potentially provides a reliable method for carrying out large-scale wildlife management surveys.

While constrained by the requirement to feed their young, land-based marine predators act as central-placed foragers with movements between their breeding colony and their food resource. In a system where the distribution and abundance of prey is highly variable, foraging behaviour must be highly adaptable. In this paper we consider the behaviour of seal and seabird krill-dependent predators foraging to provision their young and to feed themselves. We examine data from a variety of species to determine whether foraging is more intense close to the colony, or whether it is more widely distributed within the wider potential foraging range.
During the breeding season, parents are able to forage for increasingly long periods and over much greater areas as their offspring develop. Their potential foraging range increases as the breeding season advances, potentially affecting local levels of predation. We look to determine whether foraging areas are different at different times of the breeding season; we also look to see whether differences are apparent between years.
We use this information about where and when marine predators forage to identify candidate ‘small-scale management units’, these are areas where potential resource competition could exist between krill-dependent predators and the commercial fishery for Antarctic krill, Euphausia superba. The criteria used to identify these units are described as an example of how they could be used in other locations where the fishery operates.

Net samples data from the 15th Italian Antarctic Oceanographic Cruise (Jan-Feb 2000) were analysed to obtain a general picture of the summer distribution pattern, abundance and demography of krill in the western Ross Sea (Antarctica). A midwater sampler-trawl (Hamburg Plankton Net) was used to collect zooplankton and fish larvae. Mean relative biomass of Antarctic krill Euphausia superba, in the area north of Continental Shelf, was 9.3 g/1000m3 of filtered water with a mean density of 10.9 individuals per 1000m3. Ice krill Euphausia crystallorophias replaced the Antarctic krill in dominance in the High Antarctic Zone (south of 74°), with a mean relative biomass of 3.0 g/1000m3 and mean density of 19.1 ind/1000m3. The present data have demonstrated that in the Ross Sea during the summer period, the two species of euphausiid inhabited different areas. Oceanographic data indicate that both euphausiid species were found in surface waters, Euphausia crystallorophias in proper Ross Sea water, and Euphausia superba in Antarctic Surface Water. The catch data of Euphausia superba were characterized by the complete absence of larval stages, scarce occurrence of juveniles and composed primarily of large adult stages, whereas the overall length frequency distribution of Euphausia crystallorophias was characterized by a first mode of juvenile individuals and a second mode consisting of sub-adults and adults.