Dynamics of daily vertical migration in mesopelagic fish communities across the Southern Ocean.

Mesopelagic fishes transport a vast biomass of organic material between surface and deep waters through diel vertical migration (DVM), which plays a crucial role in both food web functioning and deep-ocean carbon sequestration. However, there is considerable uncertainty about which species perform DVM and the extent of their migrations. This uncertainty is particularly pronounced in polar regions, where inferences of migratory behaviour have typically been made using spatially and temporally constrained data. Here, we use a recently compiled multi-decadal net trawl database to conduct a circumpolar-scale analysis of DVM behaviour in lanternfishes (Myctophidae), the dominant mesopelagic fishes of the Southern Ocean. Generalized additive models show that all eight of the most abundant myctophid species perform vertical migrations, but the species differ in the distance of migration. Only a proportion of each population undergoes DVM at a given time, demonstrating variable DVM behaviour among individuals. We estimate from our models that of a total biomass of 48.3 million tonnes of lanternfishes across the Southern Ocean, 14.2 million tonnes undergo daily vertical migrations. This study reveals the DVM pattern of key myctophid species in previously unattainable detail and provides a framework to better understand their critical role in the Southern Ocean ecosystem.