Mid-summer fish behavior in a high-latitude twilight zone

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 μmol 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.