Skip to main content

    Development in maturity stage composition and vertical distribution in an Antarctic krill (Euphausia superba) hotspot

    Request Meeting Document
    Document Number:
    B.A. Krafft, G. Skaret and T. Knutsen (Norway)
    Submitted By:
    Mr Doug Cooper (CCAMLR Secretariat)

    Antarctic krill (Euphausia superba) aggregate in various ways depending on a range of factors, from biological (e.g. antipredatory behavior, social interactions and food availability) to physical forcing (e.g. advection and retention). Some areas, often associated with topographical features such as shelf edges and canyons, are considered as hotspots of high importance for regional pelagic ecosystems and also for the krill fishing industry. Despite the importance of such krill hotspots, their dynamics over time with respect to spatial distribution and demographical composition of krill including spawning state is not well described or understood. In the present study, a krill fishing vessel during regular operation was used as a platform for frequent collection of biological data to investigate krill demographic composition and the continuous acquisition of acoustic data for studying their distribution within a 75 km strip on the shelf edge northwest of the South Orkney Islands. The study was carried out towards the end of the reproductive season, during 29 January to 1 March 2009, and during the study, the occurrence of all the sub adult male stages (MIIA1, MIIA2 and MIIA3) decreased. Concurrently, the occurrence of both male adult stages (MIIIA and MIIIB) increased, but at a lower rate. Overall, there was also a gradual decrease from more than 0.8 to less than 0.3 in the proportion of males relative to females. The decrease in subadult males indicates segregation according to demographical composition, and the change in male to female ratio that there might be a flux of krill to/from the hotspot location. There was generally a clear diel vertical migration pattern with deeper and vertically more compact swarms during daytime than night-time. However, there was days with very small differences in distribution between day and night and in general also a huge day-to-day variability in daytime depth distribution. The fishing pattern reflected the fluctuating vertical distribution. It is essential for management purposes to understand the mechanisms behind the distribution and demography patterns described here, and also to obtain quantitative data on krill flux and retention potential, especially if the fishing activity expands in future.