Josefin Titelman

The behavior of the mesopelagic fish Benthosema glaciale was studied at 60°N in mid‐summer. We hypothesized that diel vertical migration (DVM) is constrained by short and dusk nights (surface illumination > 10 −2 μ mol m −2 s −1 ) and that individuals are active at depth during the long summer days. Submerged echosounders provided high‐resolution data throughout the water column. During the day, a part of the population ascended toward the increasing daylight. Short vertical relocations were followed by minutes of vertical inactivity. Swimming included horizontal turns and loops associated with the vertical steps. Normal DVM was initiated ~ 4 h before sunset and reflected independent individual decisions. The fish initially ascended stepwise but switched to mostly straight upwards swimming attaining 3–4 cm s −1 . Their vertical speed was faster than the slow ascent of isolumes and even the deepest living fish potentially could reach upper layers shortly after sunset. However, many individuals aborted their ascent and returned to depth before the darkest time of the night, while others returned downward closer to sunrise. The daytime swimming and individual variability in diel migration behavior have implications for encounters with prey and predators in the twilight zone and the biological carbon pump. A principal conclusion is that mesopelagic fishes can modify their behavior and migration patterns to suit a wide range of changing conditions.

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.