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

Abstract

A population of the northern knll Meganyctiphanes norvegica was sampled between 24 July and 3 August 1998 in the Alkor Deep, Kattegat, in order to investigate the influence of sexual and developmental factors on the vertical distribution of adult krill. Depth-discrete samples of knll were taken with a 1 m' hlOCNESS net at the cardinal tirnes of day (midnight, midday, sunnse and sunset). Specimens were immediately measured and categorised for moulting and spawning Status. Further samples were preserved for detailed analysis in the home laboratory. Results showed that the population concentrated in the deep (80 to 100 m) during day-time but segregated vertically during nighttime. Moulting occurred in the deep during night-time, away from the main part of the non-moulting population, which was located between 80 and 50 m. Spawning females were most evident in the uppermost depth interval (30 to 5 m). Yloulting at night in the deepest layers may b e viewed a s a mechanism to avoid cannibalism whilst in a vulnerable condition. Spawning in the warm upper layers accelerates reproductive processes and may also reduce the depth to which the eggs sink before hatching into nauplii.