Spatio-Temporal Distribution of Decapterus maruadsi and Its Relationship with Environmental Factors in the Northwestern South China Sea

The marine environment has distinct seasonal characteristics in the northwestern South China Sea, under the influence of monsoons, tides, wind stress, and coastal runoff. Decapterus maruadsi is an economically important pelagic fish along the southeast coast and is highly sensitive to the external environment. It is mainly distributed along the coast of the Beibu Gulf and in the waters of western and eastern Guangdong, with abundant resources. To understand the environmental driving mechanisms of D. maruadsi in the northwestern South China Sea, the in-situ fishery data and marine environmental remote sensing data in the northwestern South China Sea from 2012 to 2018 were used to analyze the spatio-temporal distribution of the seasonal average catch per unit effort (CPUE) of D. maruadsi using a generalized additive model. The results showed that the CPUE of D. maruadsi was related to the longitude, sea surface temperature (SST), chlorophyll a (Chl-a) concentration, depth of water, sea surface salinity (SSS), mean direction of total swell, mean direction of wind waves, and mean period of wind waves. The mean direction of wind waves contributed the most to CPUE, followed by the mean direction of the total swell and SST. D. maruadsi occurred mainly in a small area, from longitude 110.5°~114°E, SST 26~30℃, Chl-a 0.2~1.0 mg/m3, depth of water < 120 m, SSS 33.4~33.8, mean direction of total swell 70°~120°, 150°~175°, mean direction of wind waves 50°~75°, 120°~135°, 175°~190° and mean period of wind waves 3.0~4.5 s. Moreover, the SST with high CPUE changed significantly withinseasons, contrary to Chl-a. The CPUE of D. maruadsi had obvious seasonal characteristics, with the highest value of 0.848 kg/(kW·d) in summer and the lowest value of 0.087 kg/(kW·d) in winter. The abnormal increase in CPUE in 2016 may have been due to the impact of a super strong El Niño event in 2015–2016. The months in which the sea surface temperature anomaly (SSTA) exceeding 2.5℃ lasted from October 2015 to January 2016, and the spawning period of D. maruadsi is mainly winter and spring, which explains why the CPUE of D. maruadsi began to increase in the winter of 2015. Thus, the super strong El Niño had a positive impact on the replenishment of D. maruadsi in 2016. In this study, the effects of the wind waves and total swell were included in the model for the first time, and we found that wind waves and swells in a certain direction were conducive to the aggregation of D. maruadsi. This may have been due to the south and southwest winds and swells associated with upwelling activity in the northern South China Sea. The changes in the marine environment of the South China Sea are closely related to the ocean dynamics caused by monsoons. To further understand the relationship between the spatio-temporal distribution of D. maruadsi and the marine environment, and to provide a scientific basis for the conservation and adaptive management of D. maruadsi in the northern South China Sea, we can consider adding more representative ocean dynamic factors (such as sea surface and bottom velocities, mixed layer depths, and vertical velocities) to the model, to characterize different dynamic processes and to study the changes in abundance of this fish.