Leif Asbjørn Vøllestad

Knowledge of the three‐dimensional behavior and activity patterns of mesopelagic fishes is crucial for understanding their role in ecological and biogeochemical processes in the ocean. We here show that the use of submerged stationary echosounders enables detailed analysis of three‐dimensional swimming behavior of small mesopelagic fishes in situ. We explore a 4‐month continuous dataset containing more than 100,000 nocturnal tracks of juvenile Maurolicus muelleri , obtained from a surface‐facing, moored echosounder in a Norwegian fjord. We extend previous observations of vertical nocturnal swimming and step‐wise (stop‐and‐go) dusk and dawn migrations with the corresponding horizontal and turning behavior. In the middle of the night, the M. muelleri mainly drifted with the currents. Horizontal swimming speeds were then usually below 0.5 body length s −1 but increased substantially during the step‐wise dusk and dawn vertical migration. Most vertical relocations were followed by a change in horizontal direction. Our observations are in line with hypotheses of growth maximization in the juvenile fish, and of reducing detection and encounters with predators.