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Improving prediction of ecological responses to climate variability requires understanding how local fish population dynamics are impacted by climate events. The present study was conducted in the Beibu Gulf of the northwestern South China Sea where the fisheries are characterized by high ecological and commercial value. We evaluated the relationship between major commercial fish population dynamics (abundance and distribution) and climate periods, using survey data from 2006–2020. The analysis using random forest and GAM models show that climate events are not the best predictors for the variations of fish abundance, because abundance of most fish stocks decreases significantly with the year, and the increasing fishing pressure over time can better explain the overall downward trend in fishery stocks. However, environmental variables that correlate significantly with interannual variation in ONI may impact fish abundance in short terms. Our research suggests that climate events leading to higher surface seawater salinity in winter favors pelagic fishes by improving habitat availability, and higher near-surface chlorophyll-α concentration during La Niña events provides better food condition for overwintering fish. In addition, there is no clear evidence that climatic events have a significant impact on gravity center of fish distribution, whereas climate change has caused most fishes to move to cooler coastal waters in the north.

IntroductionBeibu Gulf is a semi-enclosed bay with important ecological and economic value of the northwestern South China Sea (SCS). It proved to be a stressed ecosystem and therefore clearly vulnerable to further external disturbances.MethodsData from 26 fishery bottom trawl surveys in the Beibu Gulf from 2006 to 2018 are analyzed to reveal changes in the fish abundance, dominant species composition, diversity, mean trophic level (MTL), and fishing-in-balance (FiB) index.ResultsFish abundance decreased significantly (p< 0.05) with increased anthropogenic disturbance (e.g., fishing intensity and land-derived pollutants), with catch per unit effort (CPUE) decreasing from 40.69 kg·h−1 in 2006 to 15.84 kg·h−1 in 2018. Dominant species composition has changed dramatically from 2006 to 2018, with fish communities shifting from demersal to pelagic species, and from large and high-trophic-level species to small and low-trophic-level species. Meanwhile, Margalef’s richness and Shannon–Wiener diversity indexes trend downward. The MTL declines from 3.82 in 2006 to 3.71 in 2018 (at 0.08 trophic level per decade), and decreasing with the decrease of the proportion of high-trophic fish and demersal fish in total catch (p< 0.05). A FiB index tends to be less than 0, this index declines with decreasing MTL and fish abundance (p< 0.05).DiscussionOverall, fish stocks in the Beibu Gulf continue to decline and are currently overexploited. Multiple external disturbances, such as fishing (including overfishing, dominance of trawl fishery, and ‘skipper effect’), habitat disturbance, pollution, and temperature changes, may have contributed to significant changes in fish communities and trophic structure in the Beibu Gulf.

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.

Skipjack tuna (Katsuwonus pelamis) is a highly migratory species of significant value in global marine fisheries. The South China Sea (SCS) is the largest marginal sea in the northwestern Pacific Ocean, and many tuna and tuna-like species occur within it. Although a recent Western and Central Pacific Fisheries Commission report identified skipjack tuna in the western and central Pacific to be sustainably fished, the exploitation status of skipjack tuna in the SCS is still unclear, due to limited data. We apply two data-poor methods, length-based Bayesian biomass estimation (LBB) and length-based spawning potential ratio (LBSPR), to assess the status of skipjack tuna stock in the SCS. We use electronic length frequency analysis to estimate von Bertalanffy growth parameters (asymptotic length Linf and growth coefficient K) to serve as priors for LBB and LBSPR estimation. Estimates are calculated for SCS skipjack tuna Linf (68.3 cm), K (0.325 year−1), natural mortality (0.49 year−1) and mean fork length at 50% sexual maturity (36.7 cm). LBB analysis reveals the estimated relative stock size (0.29) indicates that the SCS skipjack tuna stock maybe heavily overfished. LBSPR analysis reveals the estimated spawning potential ratio for this fishery to be 3%, which is significantly below the limit reference point of 20%. Accordingly, for the SCS skipjack tuna stock we identify a need to reduce fishing mortality by controlling fishing effort and increasing catchable size.

With the rapid modernization of methods for monitoring fishing vessels, different ways of obtaining data on large-size light falling-net vessels have developed. In particular, the vessel monitoring system (VMS) may be used to track fishing activities in the South China Sea (SCS). In this study, we used VMS data to show the characterize and differences in the information transmitted when monitoring large-size falling-net fishing vessels, as well as their advantages and disadvantages. We investigated the spatial distribution of large-size light falling-net fisheries in the SCS using Beidou VMS data from 2017 to 2020. We also compared trajectories of fishing vessels from various provinces. According to these data, vessels operate in the Nansha Islands and the waters of the Pearl River Estuary, Beibu Gulf, Zhongsha, and Xisha Islands, and there is a clear seasonal pattern of fishing in the SCS. There are the differences of vessels in terms of distribution areas and ports of entry and exit. Additionally, we found the operation area of the large-size falling-net fishing vessels divided into four main fishing zones. The analysis compares the number of days of operation in the different fishing areas as well as seasonal variations. Our study shows that the operating area of large-size light falling-net vessels has extended southwards and that the number of days for which fishing vessels operate has increased compared with the last decade. Using the Beidou VMS to study the large-size light falling-net vessels in the SCS can provide more detailed scientific reference data for the SCS fisheries.

The Beibu Gulf in the northwestern South China Sea is one of the world’s most productive fishing grounds, yet its biotic resources appear to be in rapid decline because of overexploitation. Assessments of the health status of the fisheries’ resources in the gulf provide a foundation for their conservation and management. As fish accounted for 84% to 97% of the total catch in the Beibu Gulf in the period 1962–2017, a Fish-based Index of Biotic Integrity (F-IBI) was developed for the Beibu Gulf, and data from otter trawl surveys during the period 1962–2017 were used to measure variations in the fish community. The assessment revealed a generally downward trend in total fish catch density (kg/km2), catch density of 12 traditional commercial demersal fish species, and percentage of nektonic-feeding species, but an upward trend for the percentage of fishes with a planktivorous or detritus diet, percentage of pelagic species, and percentage of the dominant species. The dominant species varied greatly over the 50 year period and showed a tendency towards small-sized species. The synthetic F-IBI variable showed a downward trend and has indicated a ‘fair’ state since 1998. The decline in the F-IBI over the last three decades suggests that anthropogenic disturbances, especially overfishing, have had a serious impact on the fish community of the Beibu Gulf. The F-IBI is currently at risk of becoming ‘poor.’ Consequently, we suggest that rigid and enforceable fishery management measures should be taken by both China and Vietnam to prevent further deterioration of the fisheries’ resources in the gulf.

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.

The Beibu Gulf is one of the most important fishing grounds in the South China Sea (SCS), and the fisheries resources in this area are exploited by both China and Vietnam. In recent decades, some indications of overfishing have appeared, including declining catch rates, frequently changing catch composition, and shrinking body sizes in main commercial fish species. Due to limited data availability, only a small subset of exploited fish stocks in this area has been assessed. Here, we applied two length-based methods, electronic length frequency analysis (ELEFAN) and length-based Bayesian biomass estimation (LBB), to stock assessment of nine exploited fish species in the Beibu Gulf. There were total 53, 652 length records of 30 target stocks used in this study during the survey period from 1960 to 2015. The results showed that the two length-based methods presented different ability in estimating exploitation rate (E), and the estimated E ranged from 0.34 to 0.87 using ELEFAN method while ranged from 0.26 to 0.86 using LBB method. The prior information from ELEFAN method was effective for LBB method, as there were significant differences in 66.7% of the 30 target stocks in estimated Linf, and 93.3% in estimated B/BMSY, using LBB method with and without prior information. The estimated Lc/Lc_opt and B/BMSY of LBB method suggest a pressing situation for the fisheries in the Beibu Gulf, as 86.7% of the 30 target stocks had been suffering from growth overfishing (Lc/Lc_opt < 1), and 83.3% had been overexploited or fully exploited (B/BMSY ≤ 1.2). In addition, we suggest using both ELEFAN and LBB methods to fit length-frequency data of data-poor fish stocks because they are complementary in estimating management reference points. We also emphasize collaboration mechanism should be established by China and Vietnam for the sustainability and recovery of fishery resources in the Beibu Gulf.

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.

With the development of satellite technology, vessel monitoring has entered a new era. In this study, through a comparison with better-recorded fishing logs, we analyzed the performance of vessel monitoring system (VMS) and nightly satellite data in monitoring fishing activities of a large-sized falling net vessel mainly targeting the purpleback flying squid (Sthenoteuthis oualaniensis) in the open South China Sea. Logbook data indicate that the studied vessel operated fishing for 57 days in the 3-month fishing season, performing a total number of 474 fishing hauls with ∼233.7 tons of catches. Our analysis indicated that with VMS data only, fishing state can hardly be distinguished from the drifting one, while even under the influence of clouds, the nightly satellite can identify most (98 %) of the fishing operations that were emitting bright lights. In addition, catch rates recorded by fishing logs significantly correlated with the numbers of lighted lamps when the nightly satellite passed over, which implies that the radiance may even be used as a proxy for catch. Although the radiance emitting from lighted lamps onboard a fishing vessel and detected by a nightly satellite is influenced by a number of factors, such as cloud conditions and satellite viewing zenith angle, jointly use of VMS and nightly satellite data can be an effective approach in monitoring and managing this type of light fishing vessels.

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.

China’s 13th Five-Year Plan, launched in March 2016, provides a sound policy platform for the protection of marine ecosystems and the restoration of capture fisheries within China’s exclusive economic zone. What distinguishes China among many other countries striving for marine fisheries reform is its size—accounting for almost one-fifth of global catch volume—and the unique cultural context of its economic and resource management. In this paper, we trace the history of Chinese government priorities, policies, and outcomes related to marine fisheries since the 1978 Economic Reform, and examine how the current leadership’s agenda for “ecological civilization” could successfully transform marine resource management in the coming years. We show how China, like many other countries, has experienced a decline in the average trophic level of its capture fisheries during the past few decades, and how its policy design, implementation, and enforcement have influenced the status of its wild fish stocks. To reverse the trend in declining fish stocks, the government is introducing a series of new programs for sustainable fisheries and aquaculture, with greater traceability and accountability in marine resource management and area controls on coastal development. As impressive as these new plans are on paper, we conclude that serious institutional reforms will be needed to achieve a true paradigm shift in marine fisheries management in China. In particular, we recommend new institutions for science-based fisheries management, secure fishing access, policy consistency across provinces, educational programs for fisheries managers, and increasing public access to scientific data.

Based on the biological data of purpleback flying squid (Sthenoteuthis oualaniensis) collected by light falling-net in the southern South China Sea (SCS) during September to October 2012 and March to April 2013, growth and mortality of ‘Medium’ and ‘Dwarf’ forms of squid are derived using the Powell-Wetherall, ELEFAN methods and length-converted catch curves (FiSAT package). Given a lack of commercial exploitation, we assume total mortality to be due entirely to natural mortality. We estimate these squid have fast growth, with growth coefficients (k) ranging from 1.42 to 2.39, and high natural mortality (M), with estimates ranging from 1.61 to 2.92. To sustainably exploit these squid stocks, yield per recruitment based on growth and natural mortality was determined using the Beverton-Holt dynamic pool model. We demonstrate squid stocks could sustain high fishing mortality and low ages at first capture, with an optimal fishing mortality >3.0, with the optimal age at first capture increased to 0.4–0.6 years when fishing mortality approached optimal levels. On the basis of our analyses and estimates of stock biomass, we believe considerable potential exists to expand the squid fishery into the open SCS, relieving fishing pressure on coastal waters.

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

Daya Bay, is one of the largest and most important semi-closed bays along the southern coast of China. Due to the favorable geomorphological and climatic conditions, this bay has become an important conservation zone of aquatic germplasm resources in South China Sea. To characterize the trophic structure, ecosystem properties and keystone species, a food-web model for Daya Bay has been developed by the means of a mass-balance approach using the Ecopath with Ecosim software. The mean trophic transfer efficiency for the entire ecosystem as a whole is 10.9% while the trophic level II is 5.1%. The primary- and secondary-producers, including phytoplankton, zooplankton and micro-zoobenthos demonstrated the important overall impacts on the rest of the groups based on mixed trophic impact (MIT) analysis and are classified as the keystone groups. The analysis of ecosystem attributes indicated that ecosystem of Daya Bay can be categorized as an immature one and/or is in the degraded stage. A comparison of this model with other coastal ecosystems, including Kuosheng Bay, Tongoy Bay, Beibu Gulf and Cadiz Gulf, underpinned that the ecosystem of Daye Bay is an obviously stressed system and is more vulnerable to the external disturbance. In general, our study indicates that a holistic approach is needed to minimize the impacts of anthropogenic activities to ensure the sustainability of the ecosystem in the future.

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.

Forcing factors and mechanisms underlying multidecadal variability in the production of the world’s major fish stocks are one of the great mysteries of the oceans. The Japanese and California sardine are species that exhibit the regime shifts. It is shown in the present work that during two periods of frequent Asian dust events over the last 100 years, sardines on opposite sides of the Pacific Ocean only flourished under a dust-active regime. The earlier such regime that peaked in the 1930s was strong, and it brought synchronous changes in the two species that were linked to the frequency of Asian dust events. However, there is an apparent mismatch in the rise and fall of abundance between the two species in the current dust-active regime. The massive increase in Japanese sardine stock in the 1970s was related to high levels of ocean precipitation and strong winter mixing, whereas the stock collapse since 1988 has been attributed to diminished winter mixing. High levels of ocean precipitation in the western North Pacific effectively cause wet deposition of Asian dust and enhance Japanese sardine stock, whereas it reduces dust flux that can be transported to the eastern North Pacific, delaying the increase of California sardine stock. Analysis further indicates that productivity of Japanese sardine stock is jointly controlled by wet deposition of Asian dust and winter mixing, which supplies macronutrients from depth. California sardine productivity is inversely related to precipitation in the western North Pacific and is positively affected by precipitation off western North America. This indicates that Asian dust influx dominates productivity of the species because of iron-limited ocean productivity in the California sardine ranges. The analysis suggests that dust regime shifts influence shifts in sardine productivity regimes and that iron input from Asian dust during trans-Pacific transport is directly responsible. It appears that in addition to enhancing phytoplankton growth and carbon sinks, atmospheric iron deposition can regulate the production of living ocean resources, which is an extension of Martin’s iron hypothesis.

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.

The tenets of ecosystem approach to fisheries (EAF) now occupy centre stage in our efforts to maintain the sustainability of fisheries and rebuild marine ecosystems. The paper discusses how an EAF can be adopted for data limited fisheries and uses the northern South China Sea fishery as an example to demonstrate the selection and use of indicators for determining the states of the fishery and its relevant ecosystem to provide advice on management. Implementing EAF management requires indicators and models that address the impact of fishing across entire ecological communities and determine management actions to be taken to achieve the preset objectives according to decision rules. Data limited fisheries necessarily have limited resources for data collection and scientific studies and therefore could not support complex models. To overcome the data limitation and absence of modelling support, simple indicators have to be used to assess the current state and monitor changes of the fishery and its ecosystem. Such indicators should: (1) be observable and understandable by all stakeholders, (2) be based on easily obtainable and reliable data, (3) adequately reflect the condition of the resource, and (4) have associated reference values and responsive management measures. The paper also reviews the recent development and use of indicators and harvest strategies in both conventional and EAF management to better understand the differences and to shed light on the challenges of EAF, in particular for data limited fisheries.

Mass-balance models (Ecopath) of the ecosystem before and after collapse (1959–1961 and 1997–1999) of fish stocks were developed with Ecopath software to compare the differences in ecosystem structure, functioning and ecosystem properties of the Beibu Gulf. The model includes 20 functional groups consisting of commercial important fish groups and other ecologically important groups in the ecosystem such as zooplankton, phytoplankton, and detritus. Results indicated that biomass and catches of the system have changed drastically between the 1960s and 1990s, especially for the high trophic levels (TL). The biomass of level V in the early 1960s was 32 times higher than that of the late 1990s, however, the biomass of level I and II in the 1990s was higher than the 1960s. Despite the higher catches in the 1990s, fishing was ecologically less expensive during the 1990s than 1960s due to small fish catches were large. Mean transfer efficiency decreased from for 10.2% in the 1960s to 9.1% in the 1990s periods. According to the summary statistics, the parameters of net system production (NPS) and total primary production to total respiration ratio were increased from 1.013 in the 1960s to 2.184 in the 1990s, however, the connectance index (CI), system omnivore index, Finn’s cycling index and mean path length decreased from the 1960s to the 1990s. The overhead (O) was higher in the 1990s model while the ascendancy (A) decreased nearly 10% in the 1960s. The ‘Keystoneness’ result indicate that zooplankton was identified as keystone species in 1960s, however, the elasmobranches was keystone species in the late 1990s. The average trophic level of the fishery decreased from 3.32 in the 1960s to 2.98 in the 1990s, and exhibits classic symptoms of “fishing down the food web”. All the indices of the system attributes suggests that the Beibu Gulf ecosystem in 1960s was found to be more mature than in the 1990s due to the collapse of demersal ecosystem, and the ecosystem changed from being dominated by long-lived, high trophic level groundfish dominated system toward a system with small-size and low-value species over fifty years.

To understand the responses of fish production to both fishing pressure and climate variability in the northern South China Sea (NSCS), catch time series are partitioned into interannual trends and variations. These are separately related to growth in fishing effort and physical variables. Catches of the fishery underwent dome-shaped trends corresponding to a monotonic growth in fishing effort, first in the inshore and then in the offshore demersal fishes, while total catch and catches of the low-trophic species showed a rising trend until 1990, and eventually experienced a decline under high fishing pressure. After accounting for the trends from the growth in fishing pressure, variations in catches were lag correlated with land precipitation, monsoon wind speeds, and a proposed index of tropical cyclone influence. The linkage suggests that river runoff, monsoon circulation and tropical cyclone impacts are the physical forcing factors dominating the catch variations, and the effects are largely through controlling the nutrient supply for biological production. Runoff provides nutrients to the inshore waters, while monsoons and tropical cyclones control the distribution and availability of nutrients. The winter monsoon increases primary production and hence fish production by offshore diffusion of the nitrogen-rich riverine water masses and by enhancing vertical mixing. In contrast, the summer monsoon reduces availability of nutrients for biological production by confining the distribution of the riverine water masses and strengthening water column stratification. Tropical cyclones are a strong forcing factor that has a positive effect on production of fishes. Tropical cyclones mobilize nutrient elements and enhance biological production by increasing water circulation, wind mixing and upwelling, and by inducing heavy rainfall and territory runoff. Based on the suggested physical forcing factors and their controlling mechanisms, the analysis predicts that increasing anthropogenic nutrient inputs and future climate changes are likely to result in an increase in fish production in the NSCS; this may be associated with even larger interannual and interdecadal fluctuations in fish production.

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.

The Gulf of Beibu, recognized as one of the traditional fishing grounds, is a center of rich biodiversity in the northern South China Sea. Based on the Beibu Gulf ecosystem constructed by the Ecopath and Ecosim model (in the late 1990s), we used Ecospace to evaluate the existing fishery management system and assess the potential of marine protected areas (MPAs) in the Beibu Gulf over a short (5-year), medium (10-year), and long-term (20-year) scenario. The results suggest that the current trawl closure and the midsummer moratorium system used in the Gulf fishery management approach appear to offer minimal benefits for stock recovery because of the high implementation and administrative costs; also, the biomass of valuable groups would decrease drastically with simulation time, and the large predator groups, such as the large demersal and pelagic fishes, would even be reduced to depletion in the long-term (20-year) simulation. Simulations of MPAs indicate that outcomes beneficial to all are possible but not guaranteed. Both ‘no-take’ MPAs, inshore closures (

Using the Ecopath with Ecosim software, a trophic structure model of the Beibu Gulf was constructed to explore the energy flows and provide a snapshot of the ecosystem operations. Input data were mainly from the trawl survey data collected from October 1998 to September 1999 and related literatures. The impacts of various fishing pressure on the biomass were examined by simulation at different fishing mortality rates. The model consists of 20 functional groups (boxes), each representing organisms with a similar role in the food web, and only covers the major trophic flows in the Beibu Gulf ecosystem. It was found that the food web of the Beibu Gulf was dominated by the primary producers path, and phytoplankton was the primary producer mostly used as a food source. The fractional trophic levels ranged from 1.0 to 4.02, and the marine mammals occupied the highest trophic level. Using network analysis, the ecosystem network was mapped into a linear food chain, and six discrete trophic levels were found with a mean transfer efficiency of 11.2%. The Finn cycling index was 9.73%. The path length was 1.821. The omnivory index was 0.197. The ecosystem had some degree of instability due to exploitation and other human activities, according to Odum's theory of ecosystem development. A 10-year simulation was performed for each fishery scenario. The fishing mortality rate was found to have a strong impact on the biomass. By keeping the fishing mortality rate at the current level for all fishing sectors, scenario 1 had a drastic decrease in the large fish groups. The biomass of the small and medium pelagic fish would increase to some extent. The biomass of the small and low trophic level species, jellyfish, prawns and benthic crustaceans would be stable. The total biomass of the fishery resources would have a 10% decrease from the current biomass after 10 years. In contrast, the reduced fishing mortality rate induced the recovery of biomass (scenarios 2-4). In scenario 2, the biomass of the large demersal fish and the large pelagic fish would increase to over 16 times and 10 times, respectively, of their current level. In scenario 4, the biomass of the large pelagic fish would increase to over 3 times of its current level. The total biomass of the fish groups, especially the high trophic level groups, would become significantly higher after 10 years, which illustrates the contribution on biomass recovery by relaxing the fishing pressure.

The Beibu Gulf is a semi-enclosed sea surrounded by land territories of China, Vietnam and China's Hainan Island. Historically, the Beibu Gulf supported various commercial, recreational, and artisanal fisheries. Many fisheries are now depleted or had experienced substantial decline in productivity. In this paper, we developed a mass balance model using Ecopath with Ecosim (EwE), to evaluate how the ecosystem may respond to changes in fisheries activities over a period of 20 years. Input data were mainly from the information collected in trawl surveys from October 1998 to September 1999. Four fishery management scenarios, which maximized three independent (fishery profits, employment, and ecosystem) and the combination of the above three objectives were simulated with different vulnerability settings. Results suggested that socioeconomically driven policy caused the ecosystem to be vulnerable whereas maximized ecological stability and the compromise scenarios were generally consistent with different vulnerability settings. To maximize social and economical criteria, the ecosystem structure was shifted to a simplified state where the high trophic level species became depleted and the low trophic level species gained dominance. Therefore, the optimal state (social strategy) had the lowest trophic level, 2.78. When an ecological stability criterion was considered, the model predicted effort should decrease more than 70% for all fishing sectors. A trade-off analysis indicated that ‘Big compromise’ strategy would be optimal to balance fishery and conservation. These results indicate that developing multispecies harvesting strategies is a complex task, and goal functions may be conflicting, while initial model conditions can affect the results.

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.

Based on the investigation of fishery resources and eco-environment in the Beibu Gulf of northern South China Sea from October 1997 to May 1999, and with EwE software, a mass-balance ecopath model of Beibu Gulf ecosystem was constructed, which consisted of 16 functional groups (boxes) including marine mammals and seabirds, each representing the organisms with similar roles in the food web, and covered the main trophic flow in Beibu Gulf ecosystem. The food web in Beibu Gulf ecosystem was dominated by detrital path, and benthic invertebrate played a significant role in transferring energy from detritus to higher trophic levels. Phytoplankton was the primary producer, and the fractional trophic levels ranged from 1.00 to 4.04, with marine mammals occupying the highest trophic level. By using network analysis, the system network was mapped into a linear food chain, and six discrete trophic levels were found, with a mean transfer efficiency of 12.3% from detritus, and 12.2% from primary producer within the ecosystem. The biomass density of commercially utilized species estimated by the model was 8.7 t x km(-2), and the bioproduction only accounted for 1.81% of the net primary production, which indicated that the system was still in developing status and instable.

Based on the investigation data in 1997-1999 and 1964-1965, this paper studied the population structure, stock density distribution and its seasonal variation, growth and mortality, as well as the feeding behaviour of Nemipterus bathybius (Snyber) in northern South China Sea. The results indicated that the body length of the samples was 4.2-21.5 cm, and body weight was 2.9-241 g, with the length of 9.6-14.0 cm and weight of 27 -59 g as predominant. The stock density was the highest (9.92 kg km(-2)) in spring and the lowest (5.53 kg x km(-2)) in summer, and had an obvious zonal distribution. Catch rate increased with the water depth in 60-150 m water zone. Macrura, fishes and cephalopods were the main foods of Nemipterus bathybius. The growth parameters of von Bertalanffy formula estimated with ELEFAN I in FiSAT II package were Linfinity = 22.39 cm, k = 0.44 a(-1), and t0 = -0.63 a. The natural mortality calculated with Pauly's empirical equation was M = 0.94, and the total mortality estimated by length-converted catch curve was Z = 2.65. Assessment with Beverton-Holt model indicated that the optimum catchable age and body length were 1.1 a and 12 cm, respectively. The stock was over-exploited, mainly due to the capture of under-size juveniles. It was suggested that the legal body length of Nemipterus bathybiu at its first capture in northern South China Sea should be 12 cm.

In this paper, Argyrosomus argentatus in northern South China Sea was grouped into the colony in continental shelf of northern South China Sea and that in Beibu Gulf. Based on the trawl survey data during 1960s and 1990s, and by using ELLEFAN technique, the growth and mortality parameters of Argyrosomus argenrtatus in northern South China Sea were estimated. The estimated parameters for Von Bertalanffy growth equation were Linfinity = 382 mm, K = 0.42 and t0 = -0.16 for the colony in Beibu Gulf, and Linfinity = 315 mm, K = 0.35 and t0 = -0.23 for that in continental shelf of northern South China Sea. The turning point for body weight growth curve of the stock was situated at t = 2.44 in Beibu Gulf, and t = 2.87 in continental shelf. Accordingly, the instantaneous total mortality (Z), natural mortality (M) and fishing mortality (F) were 3.55, 0.93 and 2.62, and 3.12, 0.85 and 2.27, respectively. The exploitation rates in recent years were 0.74 and 0.73, and the stock was on the status of over-exploitation. According to Beverton-Holt dynamic model, the optimum fishing age of Argyrosomus argenrtatus should be larger than 1.90 and 1.95 years old, and the optimum body-length should be above 211 mm and 168 mm, respectively.

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.