Hydroacoustic assessments of spatial-temporal distribution and biomass of fishery resources in the central South China Sea

Four acoustic investigations on fishery resources were conducted in 12N to 15N, 111E to 117E in the central South China Sea by Nanfeng shipborne dual-frequency split beam scientific echosounder system ( Simrad EK60, 38, 120 kHz, Norway) during March to April (spring), November to December (autumn) 2014 and January to February ( winter) 2015, as well as by portable split beam scientific echosounder( Simrad EY60, 70 kHz, Norway) in July to August (summer) 2014. According to the measurement on fish target strength in situ and with use of echo integration method, we obtained the seasonal variation feature of spatial distribution of fishery resources abundance density in the survey sea area. Furthermore, the species compositions were analyzed and the amount of fishery resources was estimated based on light falling-net sampling data in summer. Eighteen kinds of fish and two kinds of cephalopods are captured in summer and Sthenoteuthis oualaniensis was the main captured species. Acoustic assessment shows that the biomass was 2.36106t, accounting for 73.19% of the total assessing biomass. The single target strength was unimodal distribution in the survey sea area and mainly accumulated from -56.5 to -41.5 dB. The individuals whose target strengths were less than -71.5 dB entirely distributed in 100 m shallow water layer, and those whose target strength over -20.5 dB completely distributed under 100 m water layer in the survey sea area.