The acoustic survey of fisheries resources for various seasons in the mouth of Lingshui Bay of Hainan Island

Lingshui Bay is a lagoon harbor located in the southeast of Hainan Island, and it is the focus of the marine development zone in Hainan Province, China. In recent years, with the development of coastal marine aquaculture, offshore coastal waters ecological environment has been influenced to a certain extent so that the community structure and diversity of fisheries resources have changed. How to effectively monitor the fisheries resources in species composition and quantity change has become an important problem to be solved. In order to know the background conditions of fishery resources in Lingshui Bay, Hainan Province, four acoustic investigations were conducted with portable split beam scientific echosounder (Simrad EY60, Norway) from November 2014 to January 2016 for various seasons. The structure composition of fisheries resources, number density, biomass density and spatial distribution in the survey sea area were investigated and estimated according to the use of echo integration method and trawl sampling. As a result, a total of captured nectons and benthic invertebrates were 86 species, of which the average number density and biomass density of 55 kinds of acoustic assessment species were 9.34×10⁵ ind/km² and 5.08 t/km² respectively in November 2014. The sum of captured nectons and benthic invertebrates were 114 kinds, of which the average number density and biomass density of 63 kinds of acoustic assessment species were 1.12×10⁵ ind/km²and 0.93 t/km² respectively in August 2015. A total of captured nectons and benthic invertebrates were 105 species, and the average number density and biomass density of 55 kinds of acoustic estimation species were 0.16×10⁵ ind/km²and 0.32 t/km² respectively in January 2016. In May 2015, the sum of captured nectons and benthic invertebrates were 56 species, including 34 kinds of acoustic assessment species. Fish echo was dominantly distributed in 30 m shallow depth layer in November 2014 and August 2015, and mainly focused on 10 m to 20 m layer in May 2015. In January 2016, fish echo was mainly distributed in 20 m shallow depth layer and the echo in 20 m to 30 m water layer is slightly greater than half of 0 m to 10 m layer or 10 m to 20 m layer. Single target strength in the survey area was mainly less than –58 dB of small individual fish. Single target strength tended to increase with the increasing water depth, and the individuals whose target strength were greater than –50 dB were entirely distributed under 10 m depth layer.