Vertical distribution and diel migration of mesopelagic fishes on the northern slope of the South China sea

Abstract Mesopelagic fishes are major components contributing to deep-sea scattering layers in the open ocean. They have the potential to support a substantial higher trophic level harvest, but knowledge about this possible resource is still rudimentary. Some of the mesopelagic fishes undertake diel vertical migration (DVM) over large depths by feeding in shallow water during darkness and migrating to depth to avoid visual predators during daylight. The vertical distribution and diel migration pattern of the mesopelagic fishes on the northern slope of the South China Sea were investigated during two multidisciplinary surveys carried out in October 2014 and June 2015. Based on acoustic and biological sampling, a distinct deep scattering layer (DSL) consisting mainly of mesopelagic fishes was identified in deep water (>300 m). Extensive diel vertical migration for hundreds of meters to form dense aggregations in the epipelagic zone (0–150 m) at night was observed in both seasons, but the migration was highly species dependent. While most of the Myctophiformes and the Perciformes undertook DVM, most of the Salmoniformes and Beryciformes remained in deep water both day and night. Based on paired day-night differences of acoustic density in the DSL, the migrating proportion of mesopelagic backscatter varied markedly in different locations, ranging from 28% to 63%, with a mean of 48% in two seasons. The extensive vertical migration occurred in separate groups at different periods during sunset and sunrise hours. Additional analysis showed that the center of gravity of the mesopelagic backscatters (200–1000 m) and their night distribution in the subsurface layer (10–150 m) were positively correlated with the mean temperature in respective zones. Moreover, some of the mesopelagic communities on the slope were found to undertake pronounced diel horizontal migration in a pattern of towards the shore in the evening and away from the shore after midnight. The horizontal migrant species were mainly Myctophidae, in particular Diaphus chrysorhynchus and Diaphus garmani. Simultaneous analysis on typical tidal currents indicated that the horizontal migration, over a large distance with high speed and biomass, could not be explained by passive transport due to tidal currents alone, and the active swimming of the mesopelagic fishes may also play an important role.