Observations of krill vertical distributions: implications for correction factors and timing of traditional acoustic surveys

Krill performs diurnal vertical movements (DVM) depending on a range of factors. DVM is often documented in areas and time of season where krill are exposed to a high degree of predation from sight-based airbreathing predators. They reduce the risk of predation by foraging on phytoplankton in the upper water layers during hours of darkness. There are also examples where no systematic DVM can be detected, and there are also examples of reversed DVM. During traditional acoustic ship surveys designed for mapping krill biomass, there is an acoustic blind zone at the surface water layer, as hull mounted transducers usually are located at 5-8 m depth. Further, due to the conical shape of the acoustic beam, a non-linear acoustic range is excluded during the acoustic data processing, resulting in 15 m from the surface layer is not included during the calculation of biomass. In addition, when using the preferred 120 kHz frequency for krill, all registrations below the depth of 250 m are also not included in the biomass calculation. This is due to the range of this frequency and the likelihood of acoustic noise reducing the quality of the data. Acoustic data is usually collected day and night during a survey to reduce logistical survey cost. To assess the significance of diurnal as well as seasonal krill DVM patterns, data was analyzed from stationary acoustic moorings, with upward looking acoustic Doppler current profilers integrated with multi-frequency echosounder functions. They were deployed throughout the annual cycles of 2023 and 2024, near South Orkney Island. Large temporal variations in the data were found within the upper blind zone and below 250 m, especially during the months of January and February which coincides with the timing of the Norwegian krill survey. There is a need for combining such observations to develop correction factors for inclusion with traditional ship based acoustic data collected for biomass estimations. Examples will be provided to open the discussion on how to employ such data to complement traditional surveys.