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A prevailing, controversial hypothesis is that fishing pressure has played a greater role than climatic and environmental drivers, in changing fish species succession and biomass fluctuation in the South China Sea (SCS). Based on otter trawl survey data from 1959 to 2010 in the outer Beibu Gulf (OBG), northern SCS, large seasonal and interannual variation is reported for fish species composition, the proportional abundances of dominant taxa, and fish biomass. Generalized additive models are developed to quantify relationships between fish biomass and the external factors of fishing pressure and climate change. Fishing pressure proved to be the main driver of sharp declines in demersal fish stocks, with high-value species being replaced by low-value ones over time. Abrupt decreases in fish biomass during the years of 1993 and 1998 correspond to El Nino events, with climate change possibly the main driver of proportional representation of pelagic species in fisheries trawl catch. The need to differentiate impacts of fishing and environmental drivers on fish species with different life history strategies is stressed to better understand fish community dynamics.

Based on length frequency data of miter squid (Uroteuthis chinensis) collected in the northeastern South China Sea in 1975–1977, 1997–1999, and 2018–2019, asymptotic length, optimal length at first capture, relative mortality, and relative biomass of the stock were estimated using length-based Bayesian biomass estimation (LBB). The LBB-estimated asymptotic length for 2018–2019 was smaller. Optimal lengths at first capture for the later far exceeded average lengths in catches because of a major increase in fishing intensity. Between 1975 and 1977, relative total mortality (Z/K) was low, but it increased in the latter two periods, while relative natural mortality (M/K) showed a downward trend. Relative biomasses (B/B0 and B/Bmsy) indicated that the stock was close to unexploited between 1975 and 1977, but they declined to the levels of 6% and 4% in the later periods, which correspond to growth in fishing horsepower. Indeed, by 2018, fishing horsepower increased by nearly four times the optimal level. The analysis suggests that the stock of miter squid has been overfished since the mid-1980s and is now under heavy fishing pressure. To recover the stock, it is imperative to reduce fishing intensity and enforce size-at-first-capture regulations.

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.

A combination of traditional fish sampling methods (hand-line and gill net) and modern hydroacoustic techniques were used to study fish community structure, distribution, and diel movements of fish on Zhubi Reef to enhance understanding of the ecosystem. We collected 126 individuals from 29 species, 20 genera, 17 families, three orders, and two classes using traditional gear. Perciforms were the dominant group in terms of species richness and biomass. The acoustic data indicated that very small (target strength [TS], dB) -55 dB). The number of single-target detections was significantly higher during the night than during the day (P

Knowledge of target strength (TS) is essential for accurate assessment of fish abundance using acoustic methods. The in situ TS (dB) of purpleback flying squid Sthenoteuthis oualaniensis in the South China Sea was measured with 70 and 120 kHz split-beam echosounders. Biological samples were collected using a light falling-net with a valid working depth of 50 m. Acoustic data sets in which S. oualaniensis contributed more than 90% of the total catch by number were selected to analyse in situ TS and determine relationships between TS and squid mantle length (ML). In situ TS increased with squid density when density was greater than a cut-off point, indicating a bias from accepting multiple targets at high squid density. At 70 kHz the TS to ML of squid was: TS70 (kHz) = 19.13 log ML - 77.84 (n = 9, R-2 = 0.8782, ML: 7.5-25 cm); at 120 kHz the TS to ML of squid was: TS120 (kHz) = 26.68 log ML - 88.00 (n = 9, R-2 = 0.8521, ML: 7.5-25 cm). This study provides new data for improving the acoustic abundance estimation of squid S. oualaniensis, and may be a useful reference for TS estimation of other squid species. (C) 2015 Elsevier B.V. All rights reserved.

Length-frequency data of eight commercial fish species in the Beibu Gulf (Golf of Tonkin), northern South China Sea, were collected during 2006–2007. Length-weight relationships and growth and mortality parameters were analyzed using FiSAT II software. Five species had isometric growth, two species had negative allometric growth, and one species had positive allometric growth. Overall, the exploitation rates of the eight species were lower in 2006–2007 than in 1997–1999: for four species (Saurida tumbil, Saurida undosquamis, Argyrosomus macrocephalus, and Nemipterus virgatus) it was lower in 2006–2007 than in 1997–1999, for two species (Parargyrops edita and Trichiurus haumela) it remained the same, and for the other two species (Trachurus japonicus and Decapterus maruadsi) it was higher in 2006–2007 than in 1997–1999. The exploitation rates might have declined because of the decline in fishing intensity caused by high crude oil prices. The optimum exploitation rate, estimated using Beverton-Holt dynamic pool models, indicated that although fishes in the Beibu Gulf could sustain high exploitation rates, the under-size fishes at first capture resulted in low yields. To increase the yield per recruitment, it is more effective to increase the size at first capture than to control fishing effort.

To obtain critical information on distribution and abundance of Indo-Pacific humpback dolphins (Sousa chinensis) residing in the Pearl River Estuary (PRE), systematic line-transect surveys and photo-identification studies were conducted in the eastern and western sections of the estuary in 2005–2006 and 2007–2008, respectively. Dolphins in the eastern section occurred throughout Lingding Bay, whereas their distribution in the western section extended from the mouth of Modaomen to the channel between Shangchuan and Xiachuan islands. Variations in dolphin distribution during the wet and dry seasons were evident and probably associated with movements of their prey species. Photo-identification of individuals confirmed exchange of at least some individuals between the western and eastern sections of PRE, and thus dolphins from both areas almost certainly comprise a single population. Using line-transect analysis, the total population size of the PRE humpback dolphins was estimated to be 2555 during the wet season and 2517 during the dry season. However, these should be considered preliminary as the coefficients of variation in some survey areas were high. Further studies should focus on refining these estimates and working towards understanding the western boundary of the PRE population.

Summary Sixty-nine fish species belonging to 41 genera from 29 families and seven orders were collected during 2006–2007 by otter trawl in Beibu Gulf, northern South China Sea, for analysis of their length-weight relationships (WLRs). The analysis included 19 species for which no previous WLR estimates were available in FISHBASE.

Nemipterus bathybius is a small-size demersal species,which is mainly distributed in tropical and subtropical oceanic waters.The species is mostly in habitat offshore areas(60 m) and caught commercially by hook-and-line and bottom trawl.The output of N.bathybius ranked first in the catch compositions of demersal fishes during bottom trawl survey conducted in 1964—1965.Recently,the stock density and yield of N.bathybius have changed dramatically,and the catches mainly consisted of young fish and the population structure was low-aged and miniaturization.Therefore,the aims of this research are to elu-cidate the effects of fishing activities and environment variations on its biological parameters,and provide theoretic basis for reasonable utilization and exploration.Based on the biological data obtained from bottom trawl conducted in the Beibu Gulf during 1992—1993(1992),1997—1999(1997) and 2009—2010(2009),population structure,growth and mortality parameters,as well as its annual variations of N.bathy-bius were analyzed.The results presented miniaturization and maturation at younger ages(and/or smaller sizes) of N.bathybius,as well as the growth rate increased and sexual maturity becameearlier.The average body length of N.bathybius lessened from 138.25 mm in 1992 to 112.70 mm in 1997,and 118.28 mm in 2009,respectively,the corresponding average body weight were 60.06 g,39.03 g and 48.85 g.The parameters,asymptotic length(L∞),growth coefficient(k) and zero-length age(t0) in the von Bertanaffy growth equation in 1992,1997 and 2009,were 242.2 mm,0.39 and -0.39 a;220.4 mm,0.45 and -0.35 a;235.5 mm,0.48 and -0.32 a,respectively.The inflexion age reduced from 2.19a in 1992 to 1.97a in 1997 and 1.86 a in 2009.Body length at 50% sexual maturity(L50) diminished from 128.6 mm in 1992 to 116.5 mm in 1997,and 110.5 mm in 2009.The exploitation rates(E) in 1992,1997 and 2009 were 0.57,0.62 and 0.58,respectively.It suggested that the stock has been over-exploited since the 1990s.A series of changes of population parameters of N.bathybius in the Beibu Gulf indicated that in order to sustain their populations,N.bathybius has undergone adaptive changes under high fishing pressure.