Vertical distribution and feeding patterns in fish foraging on the krill Meganyctiphanes norvegica

Fish and krill were studied at a 120 m deep site in the Oslofjord, Norway. Herring (Clupea harengus), whiting (Merlangius merlangus), and Norway pout (Trisopterus esmarkii) were foraging on krill (Euphausiacea, Meganyctiphanes norvegica) during both day and night. During daytime, herring and whiting were foraging in the upper and middle part of the krill assemblage, while the deep-living, and often benthopelagic Norway pout was approaching the krill from below. Krill and fish ascended and fish schools dispersed at dusk. At night, herring and whiting were feeding near the surface, with the shallowest distribution suggested for herring. Norway pout foraged in midwater. Krill antipredator behaviour comprised diel vertical migration and instantaneous escape reactions, and the krill also appeared to actively seek out strata with low acoustic recordings of fish. Fish accumulated beneath the research vessel when the ship was anchored at a fixed location during acoustic studies, apparently resulting in artificially high local fish abundances. Since we suggest that krill respond to the presence of fish, such high fish abundance may bias studies of interactions between the fish predators and their krill prey.