Individual daytime swimming of mesopelagic fishes in the world's warmest twilight zone

We assessed the activity and swimming patterns of mesopelagic fishes in the Red Sea using bottom-moored, upward-facing echosounders deployed at 555 and 700 m depth. The vertically migrating mesopelagic scattering layer descended close to the bottom during daytime. This permitted assessment of behavior at mesopelagic depths by applying acoustic target tracking for individuals traversing the acoustic beam. Swimming activity did not fit the notion of torpid behavior in the daytime habitat. The fishes were moving continuously, with a prevailing downward direction before noon and upward after, though individuals were swimming in both directions at all times. They moreover were swimming horizontally at estimated speeds of ∼2.1 cm s−1, suggesting ∼0.5–1 body length s−1, intermittently turning. High activity at high temperatures suggests high respiration at depth, considered a key element for the active carbon pump.