Diel vertical migration of the krill Meganyctiphanes norvegica in relation to physical environment, food and predators

Acoustic scattering layers of Meganyctiphanes norvegica in the Oslofjord, Norway, typically stayed below similar to 70 m during the day. They remained below prevailing fish schools, though varying numbers of deep-living fish occurred among the krill. The estimated light intensity at the top of the krill layer varied between 4.6 x 10(-6) and 2.3 x 10(-4) mu mol m(-2) s(-1). M, norvegica followed isolumes during ascent at dusk, but upward migration was mainly arrested at similar to 10-30 m and the krill were scattered throughout the water column at night. The minimum depth was not related to seasonal light levels, chlorophyll concentration or the pycnocline. Fish schools dispersed at sunset, and fish invaded the krill layer at night. Nocturnal avoidance of the surface water may thus have been an antipredator response. Herbivorous feeding was strongly related to abundance of algae (chl a levels). Feeding on phytoplankton increased at night, but daytime stomach pigment content was high during spring, when sedimenting phytoplankton appeared to represent a deep food source. Feeding on copepods was evident both day and night, with no consistent relation between copepod remains in the stomachs and integrated abundance of copepods in the water column. M. norvegica carried out diel vertical migrations irrespective of fluctuations in distribution and abundance of food resources.