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 < 100 m were greatest, with Myctophidae the most abundant category. In 2014, from 350 to 650 m, day and night mean catch rates of mytophids were similar, however, in 2015, mean day catch rates were 64.1% night values. Cephalopoda are an important component of mesopelagic nekton. At escape rates of 0.95 and 0.99, daytime trawl–based biomass densities in the 350–650 m depth range in 2015 were 0.339 g m−2 and 1.693 g m−2, respectively. Mean daytime NASC from 200 to 1000 was 3163 ± 1223 m2 nmi−2 (2014) and 2503 ± 698 m2 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.