Acoustic blind zone when using hull mounted transducers during traditional krill biomass surveys

Krill perform diurnal vertical movements (DVM) depending on a range of factors, but the general perception is that at least in areas and time of season where krill are exposed to a high degree of predation from smaller visual predators, they tend to stay higher in the water column during night hours. During traditional acoustic ship surveys aimed at 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, a non-linear acoustic range is excluded during the acoustic data processing due to the narrow acoustic beam angle (depending on transducer type but normally 7°). In total, 15 m from the surface is excluded during the calculation of biomass. The current CCAMLR protocol recommends acoustic daytime sampling to avoid issues of krill migrating too close to the surface to be acoustic detected. Ideally this could work but in practice, to reduce expensive logistical survey cost, acoustic data are usually collected day and night. To assess diurnal as well as seasonal krill DVM patterns, acoustic moorings have been anchored to the seabed, with an upward looking echosounder covering the annual cycle over the years 2014 to 2024 near what is regarded to be a hotspot for krill near the South Orkney Island. Large temporal variations in the data were found within the blind zone. However, we see a large potential for such observations to be used as a correction factor in combination with traditional acoustic data collected for biomass estimations. The data might also contribute to determine optimal timing of traditional surveys, when the aim is to include as substantial proportion of the available biomass in an area as possible. Examples will be illustrated to open a discussion around these topics for aiming potential further future focus on how to use such data.