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Frigate tuna Auxis thazard fishery is considered a potential marine resource in the open South China Sea (SCS). However, little is known about the spatial and temporal distribution of its habitat, and how this may respond to environmental changes. Using fish survey and remote sensing data from 2015–2019, we applied generalized additive models to identify relationships between environmental factors and the distribution of A. thazard in the SCS. To examine seasonal patterns in the habitat of A. thazard in the SCS, we generated a habitat suitability index model using environmental factors screened by generalized additive models. Results showed that A. thazard migrates from south to north in the SCS, and its suitable habitat is patchily distributed. Significant environmental factors affecting the habitat of A. thazard distribution were different in different seasons; we demonstrated A. thazard to be sensitive to Chl-a in spring (optimum 0.155, optimal range ~0.1252–0.1840), and in summer to be sensitive to SST (optimum 30.405, optimal range ~29.789–31.021) and SSH (optimum 0.741, optimal range ~0.618–0.864). Suitable habitat in spring occurs mainly in northeastern areas, while in summer it occurs mainly around the southeastern Nansha Islands. Compared with that in spring, the area of suitable habitat increases in summer, as does the habitat suitability index of the entire sea area. These results improve our understanding of environmental factors that affect the distribution of A. thazard habitat in the SCS, and provide a scientific basis for the development and management of A. thazard resources. [ABSTRACT FROM AUTHOR]

With the progression of overfishing along the coast, oceanic fisheries in the South China Sea have attracted increasing attention from bordering countries. Fishing with lights has developed rapidly over the past decade. In this study, we analysed the trend in fishing over the spring fishing season (March–April) from 2012 to 2020 in the open-SCS fishing zone based on nightly satellite. The results indicated that the number of fishing boats detected by satellites were apparently influenced by the phases of the moon. Using data from moonless nights, we estimated that the number of fishing boats increased from ∼400 to ∼2000 over the past decade. These fishing craft in the open-SCS could be classified into large falling net vessels with bright lights and tuna fishing boats with dim lights. The nightly images of large falling net vessels were studied further using records from a typical commercial fishing vessel and, on this basis, we established an algorithm to extract data for this type of craft, whose numbers were estimated to have increased from ∼20 to ∼200 over the past decade. Using this algorithm, we were able to trace the development of these fleets and map out their distribution patterns in the open-SCS. [ABSTRACT FROM AUTHOR]

Distribution patterns of lit fisheries in the South China Sea (SCS) were investigated using seasonally averaged nighttime light remote sensing data from 2016. Light-fishing activities clearly showed spatial and seasonal patterns and a newly developed "hotspot" with the presence of many intensive lights observed in the open ocean during spring. These satellite observations are consistent with seasonal changes in the fishing grounds of a Chinese large-sized, falling-net fishing fleet based on catch monitoring records for a typical commercial fishing vessel and Vessel Monitoring Systems (VMS) data from this fleet. Catch monitoring records show that this type of vessel operates both on the northern shelf and in the oceanic waters, with apparent seasonal fishing patterns in the SCS. Dominant catches on the shelf include Carangids, Trichiurus japonicus , and Loligo spp. , while Sthenoteuthis oualaniensis and Auxis spp. overwhelm catches in the oceanic waters. Furthermore, we compared VMS and Day Night Band (DNB) data to study fishing behaviors of this type of vessel, including match rates between VMS and nightly satellite images and the DNB radiance characteristics of the vessels. This study is valuable for delineating squid fishing grounds in the open SCS and provides an important basis for differentiating light fishing vessels of China from those of other countries, which may be useful for detecting illegal, unreported, and unregulated (IUU) fishing activities. Findings of this study may also shed light on mitigating the emerging threat of light pollution to the local ecosystem of the SCS. Unlabelled Image • Nightly satellite observed a newly developed fishing ground in the open SCS. • Behaviors of the fishing fleet were studied jointly using the DNB and VMS data. • VMS-reported fishing vessels matched well (67.9%) with VIIRS boat detection. • Viewing zenith angle is a major factor influencing the DNB radiance of a vessel. [ABSTRACT FROM AUTHOR]