Licence to Krill: Diel Vertical Migration and the Predator-Prey Interaction between Meganyctiphanes norvegica and Fish in the Oslofjord

The pelagic zone is the largest ecosystem on Earth. The inhabitants of this ecosystem are not provided with any substances to hide behind. Diel vertical migration (DVM) is believed to be a trade-off between substantial feeding and minimal predation pressure. In the northern Atlantic, Meganyctiphanes norvegica is a key species in the marine food web, being important food for higher trophic levels such as fish, birds, and marine mammals. M. norvegica aggregations are common in Norwegian fjords, representing accessible ocean model systems in a sheltered environment. This study's aim is to address M. norvegicas DVM in relation to the predator-prey interaction with fish in a shallow site, identifying when and where the fish will attack the krill and the krill’s antipredator response. For this purpose, an upward-facing echosounder was placed at the seafloor in the Oslofjord, making continuous recordings of the krill and fish movements during this period. Hydrographical and light measurements were done, together with different types of net hauls performed to sample the target organisms. The organisms, including their stomach content, were analysed to investigate if they fed on krill. The acoustic recordings showed DVM performance by the krill. During the daytime, the krill were attacked by fish schools from above, showing clear avoidance responses towards the fish. The krill layer concentrated near the seafloor during September and October, where they became vulnerable to predation by deep-living predators. The krill migrated towards the surface in the afternoon, followed by midnight sinking. At night, the fish interacted with the krill near the surface. Groups of fish were present throughout some nights and gathered as schools again in the morning. Single fish were also interacting with the krill both day and night as well. In the mornings, the fish descended after the krill. The distribution of the krill seemed to be determined by the light intensities penetrating the water, both day and night. The daytime distribution was shallowest during the period with the highest light extinctions. During full moon events, the nocturnal distribution was deeper and vice versa. In lighter periods, the krill descend seemed to be blocked by the topography at the site. This is beneficial for the seafloor-related fish, which could visually forage more efficiently on the light-exposed krill. Krill is an important prey for fish in the Oslofjord.