| dc.description.abstract | Abstract
The abundance and distribution of Antarctic krill is poorly known for some parts of the Southern Ocean. Recent, and anticipated enhanced interest in krill fisheries highlights the need for precise and reliable data to secure the population through international agreements.
The acoustic survey AKES 1 (Acoustic Krill Estimation Survey) made by IMR (Institute for Marine Research, Norway) was conducted in the Southern Ocean between South Georgia and Bouvet Island, during January and February 2008, as a part of the International Polar Year 2007/2008. Waters around South Georgia, the Open Sea region between South Georgia and Bouvet, and the area close to Bouvet Island were investigated. There is limited knowledge on krill distribution in the two last areas.
The abundance of the Antarctic krill, Euphausia superba, was studied using a 38 kHz and 120 kHz Simrad EK60 echo sounder, and by sampling with a “krill trawl”. Acoustic characteristics of krill, together with trawl catches were used to identify the acoustic targets. Antarctic krill was the prevailing organism in the water column. Sampling of environmental data comprised continuous measurements of surface temperature, salinity and fluorescence along the cruise track, as well as vertical profiles (CTD) at selected stations.
The acoustic logging was made continuously throughout the entire survey. In the subsequent analysis, data for the depth interval 0-200m were allocated to three regions; South Georgia, Open Sea and Bouvet Island. Krill abundance, vertical and horizontal distribution, school structures and variations between night and day were analyzed.
Krill aggregations had larger size and density close to South Georgia and Bouvet Island, compared with the Open Sea. At South Georgia aggregations were over all larger than in the two other areas, and this was the region with the highest krill abundance per surface area. The largest single aggregations of krill were found at Bouvet Island, though few in numbers. The Open Sea area hold few and small aggregations compared to the volume sampled, even in cold waters south of the Polar Front. The krill were more or less absent close to, and north of the Polar Front.
The higher abundance of krill close to the two islands coincided with intermediate chlorophyll a levels (chl a), known to be one of the factors contributing to preferable krill habitats.
Krill occurred in the densest aggregations and displayed the most marked Diel Vertical Migrations (DVM) in waters around South Georgia, which I ascribe to the highest predation pressure from land-based predators in the region.
At South Georgia the Antarctic krill aggregated in a dense belt close to the surface (though sometimes with a vertical extension of 60-70 m) during night and became assemblaged into even denser swarms of varying size at 50-100m during day.
In the Open Sea south of the Polar Front, krill often occurred in a continuous low-density belt in the upper 60m of the water column during night. At daytime krill formed small aggregations with a peak around 40-60m depth, with densities higher than what could be found in the diffuse belt at night.
At Bouvet Island krill aggregated close to the surface in dense compact swarms during night and at daytime krill formed both small swarms with a low density and some large aggregations between 20-80m depth.
Our findings on regional differences in DVM are not known from previous studies. Predation pressure, together with growth potential and food concentrations (chl a), seem to be the three factors with the largest influences on krill distribution in this study. | eng |