From acoustic scattering layers to individuals - behaviour of a mesopelagic fish at different scales

Abstract

Enormous abundances of small fish commute each night from several hundred meters depth to forage in the productive surface waters. Such so-called mesopelagic fishes are central in the food web and their daily vertical migration likely plays an essential role in carbon and nutrient cycling. Their behaviour is so far mainly described on the community level. However, the fishes’ ecological and biogeochemical role is ultimately defined by the individuals constituting the migrating communities. Therefore, learning more about the smaller behavioural scales is essential. Christiansen and colleagues analysed a 10-month high-resolution acoustic dataset to describe the behaviour of a vertically migrating fish (Maurolicus muelleri) at the individual level. By acoustically tracking individual M. muelleri by echosounders deployed at depth we revealed details of the three-dimensional swimming behaviour, which provided new insights into the species’ migration and nocturnal activity. The results further suggest that M. muelleri combine several anti-predation strategies and emphasize the diversity of behaviours in this single species. Moreover, knowledge of individual swimming patterns will help predicting the effects of behaviour on acoustic biomass estimates. Such knowledge is required in the crucial task of assessing the role of mesopelagic fish in the ocean.