The Norwegian fishing vessels involved in krill fisheries are also used to carry out abundance surveys around the South Orkney Islands. The involved companies have accepted running acoustic logging also during periods of commercial fishing. In this paper we give an overview of the properties of these data and demonstrate potential applications for example to extract information on changes in vertical and horizontal distribution patterns over the season.
The data are scrutinized using standard CCAMLR routines for separating krill backscatters from other organisms. The South Orkney fishing grounds are limited compared to the area covered by the assessment survey. The area has remained spatially coherent from year to year during the recent 6 years with some moderate variations, as also is the effort distribution over time in a single season. Vertical distributions inside and outside the fishing area change over the day and night and during the seasons. Diel migration, with krill being distributed close to surface at night and at deeper water at day time, stands out in the fishing area while patterns of distribution and migration is more unclear outside this area. The diel migration in the fishing area is reduced from summer to winter, and krill are generally deeper in May compared to February. The average acoustic backscattering is very variable over time during the season as measured onboard a fishing vessel, but there is no obvious trend indicating declining krill density until May in the study year.