Vertical migration behaviour in the northern krill Meganyctiphanes norvegica is influenced by moult and reproductive processes

A population of the northern krill Meganyctiphanes norvegica was sampled be tween 24 July and 3 August 1998 in the Alkor Deep, Kattegat, in order to in vestigate the influence of sexual and developmental factors on the vertical distribution of adult krill. Depth-discrete samples of krill were taken wi th a 1 m(2) MOCNESS net at the cardinal times of day (midnight, midday, sun rise and sunset). Specimens were immediately measured and categorised for m oulting and spawning status. Further samples were preserved for detailed an alysis in the home laboratory. Results showed that the population concentra ted in the deep (80 to 100 m) during day-time but segregated vertically dur ing nighttime. Moulting occurred in the deep during night-time, away from t he main part of the non-moulting population, which was located between 80 a nd 50 m. Spawning females were most evident in the up permost depth interva l (30 to 5 m). Moulting at night in the deepest layers may be viewed as a m echanism to avoid cannibalism whilst in a vulnerable condition. Spawning in the warm upper layers accelerates reproductive processes and may also redu ce the depth to which the eggs sink before hatching into nauplii.