| Summary: | The euphausiid Meganyctiphanes norvegica performs diel vertical migrations (DVMs), staying in deeper waters during the day and ascending to the shallows at night. This behaviour is interpreted as a trade-off between food availability and visual predator avoidance. Separation between day and night is a broad simplification, as nocturnal light may vary by orders of magnitude. The aim of this study was to research the behavioural differences in DVM by M. norvegica during the dark winter nights and brighter summer nights, along with the effect of moonlight during midwinter. Predator avoidance in these settings were assessed. An upwards facing 200 kHz echosounder was mounted at a 100 m deep location in the inner Oslofjord, Norway. Recordings were done during full and new moon periods from winter to summer in 2021. Trawls were used to identify and measure the organisms in the scattering layer. Hydrographical and light measurements were made to supplement the acoustic data. The individuals caught were mainly M. norvegica, the glass shrimp Pasiphaea sp. accounting for ~10% of the catch at most. The length and weight of M. norvegica increased significantly with depth, suggesting that larger individuals stayed deeper to avoid visual predators. Hydrographical measurements did not seem to affect the distribution of M. norvegica. M. norvegica performed DVM throughout the study period, ascending and descending in relation to the timing of sunset and sunrise. The population remained at ~40-100 m during the day, depending on season and weather. At night, M. norvegica were generally spread throughout the water column, possibly displaying asynchronous migrations. The nocturnal illumination seemed to influence the distribution of M. norvegica, causing a deeper distribution by midnight sinking when more light was available on a clear, moonlit night in winter compared to a cloudy night in spring. Yet, M. norvegica appeared to migrate to the upper waters during the brighter summer nights. These findings suggest that small ...
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