Species composition and biomass density of mesopelagic nekton of the South China Sea continental slope

Abstract Two surveys using a combination of midwater trawl and acoustics were undertaken in October 2014 and June 2015 to investigate species composition and biomass density of mesopelagic nekton over the continental slope of the South China Sea (CSSCS). In two surveys, diversity and dominant species of nektons (Chondrichthyes, Osteichthyes and Cephalopoda) showed obviously diel and depth–dependent difference. In 2014, in general species richness and diversity of nektons in the 0–100 m at night were greater than that in the 350–650 m during the day and 350–650 m at night. In Jun. 2015, species richness and diversity of nektons in the 350–650 m during the day were much greater than that in the 350–650 m at night and 0–100 m at night. Myctophidae and Diaphus are the most common fishes in the CSSCS mesopelagic zone. Jellyfishes could cause the nautical area scattering coefficient (NASC) to increase, but the effect of jellyfishes on NASC could vary over the light–night cycle and trawl depth. Catch rates of different nektonic categories varied by time of day and depth. Night–time catch rate at trawl depths −2 and 1.693 g m −2 , respectively. Mean daytime NASC from 200 to 1000 was 3163 ± 1223 m 2 nmi −2 (2014) and 2503 ± 698 m 2 nmi −2 (2015); corresponding acoustic and trawling data produced mesopelagic nekton biomass density estimates of 9.80 g m −2 (2014) and 28.45 g m −2 (2015). More fishes with swimbladders (mainly small sized lanternfishes of light weight) dominated catch in 2014 than in 2015.