Behavioural differences in relation to pycnoclines during vertical migration of the euphausiids Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars)

An observation on differences in the vertical migration pattern between Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars) caused by the presence of a thermocline is reported. Generally, M.norvegica did not migrate through a thermocline at 50-60 m depth while T.raschii did. This observation is discussed in the context of how the combination of differences in vertical migration and advection over sill depth may affect the abundance of fjord stocks of these two euphausiid species.