Diel vertical migration behaviour of the Northern krill (Meganyctiphanes norvegica Sars)

The prototype of Meganyctiphanes norvegica diel vertical migration (DVM) behaviour comprises ascent around dusk, feeding near the surface at night, and descent at dawn, explained as a trade-off between feeding and predator avoidance in an environment where both food and risk of predation is highest near surface. Light is the proximate cue, and daytime distribution is deeper in clear waters and sunny weather and nocturnal distributions deeper in moonlight. However, both internal state and external factors further affect and modify the diel migration pattern. While Meganyctiphanes migrates in synchrony to the surface at sunset, part of the population may descend soon after the ascent with individuals re-entering upper layers throughout the night. This has been explained with hungry individuals being prone to take larger risks and hence stay shallower, while satiated individuals seek shelter at depth. Females migrate closer to the surface than males of equivalent size, possibly due to their greater demand for energy to fuel egg production. Freshly moulted M. norvegica remain at depth throughout the diel cycle. This has been related to the fact that that krill do not feed during moulting, to reduced swimming capacity, and as a mechanism to avoid cannibalism whilst in a vulnerable condition. In some locations large parts of the population remain at depth at night. Such behaviour may incur access to demersal food sources, provide avoidance of predators, or can be a means to avoid horizontal transport to adjacent, unfavourable areas. Environmental gradients can arrest migrations of M. norvegica, yet the effect of physics is not always distinguished from associated biological properties, like subsurface maxima of phytoplankton located at pycnocline boundaries. Deeper nocturnal distribution when predators were abundant has been reported, and krill may adjust their distribution upwards when exposed to deep-living predators. Instantaneous escape to approaching predators is a common component of the anti-predator repertoire of Meganyctiphanes. Occasionally reported schooling behaviour that overrides normal DVM behaviour may serve anti-predation purposes, as well as being related to reproduction. M. norvegica can remain within confined areas, often defined by the bottom topography, even when exposed to strong currents. Behaviourally mediated retention may be accomplished by vertical migration in depth-stratified flows, but evidence for active use of DVM for the purpose of retention is so far circumstantial among M. norvegica. In several instances, large aggregations of krill that repeatedly occur in the same location appear to be accidental consequences of krill vertical migration behaviour interacting with the mean circulation and bottom topography, rather than representing active retention behaviour.