Stock Assessment of the Commercial Small Pelagic Fishes in the Beibu Gulf, the South China Sea, 2006-2020.

Long-term variations in population structure, growth, mortality, exploitation rate, and recruitment pattern of two major commercial small pelagic fishes (CSPFs) (Decapterus maruadsi and Trachurus japonicus) are reported based on bottom trawl survey data collected during 2006-2020 in the Beibu Gulf, South China Sea. All individuals collected during each sampling quarter over a period of 15 years were subjected to laboratory-based analysis. In this study, the stock of D. maruadsi and T. japonicus inhabiting the Beibu Gulf was assessed using length-based methods (bootstrapped electronic length frequency analysis (ELEFAN)) to complete stock assessment in different fishery management periods (the division of fisheries management periods was based on China's input and output in the South China Sea offshore fisheries over 15 years, specifically divided into period I (2006-2010), period II (2011-2015), and period III (2016-2020)). The results showed that the mean body length, dominant body size, and estimated asymptotic length of two CSPFs decreased, whereas their growth coefficient decreased, indicating miniaturization and slower growth, respectively. Estimated exploitation rates and catching body length for two CSPFs indicated that both stocks in the Beibu Gulf were overexploited in period I and moderately exploited after 2011. These stocks were taking a good turn in status in period III, with the exploitation rate much lower than the initial period and reversing the downward trend in catching body length. Furthermore, the variations in the spawning season of the two CSPF stocks and their barely satisfactory expected yield indicated the complexity of the current fishery management in the Beibu Gulf. These results suggest that management measures to reduce fishing pressure may have a positive influence on the biological characteristics of those CSPFs in the Beibu Gulf; however, the stock structure already affected by overfishing will be a huge challenge for the conservation and restoration of fisheries resources in the future. Given that the current stocks of D. maruadsi and T. japonicus in the Beibu Gulf still have low first-capture body length (Lc) and high fishing mortality (F) (compared to F0.1), we identify a need to refine population structure by controlling fishing efforts and increasing catchable size, and more consideration should be given to the local fishery resource status in fisheries management.

Social-ecological vulnerability and risk of China's marine capture fisheries to climate change.

Climate change is a new disrupter to global fisheries systems and their governance frameworks. It poses a pressing management challenge, particularly in China, which is renowned as the world's largest fishing country and seafood producer. As climate change continues to intensify in the region and climate awareness grows within the country's national policy, the need to understand China's fisheries' resilience to the escalating climate crisis becomes paramount. In this study, we conduct an interdisciplinary analysis to assess the vulnerability and risk of China's marine capture fisheries in response to climate change. This study employs a spatially explicit, indicator-based approach with a coupled social-ecological framework, focusing on 67 species and 11 coastal regions. By integrating diverse sets of climatic, ecological, economic, societal, and governance indicators and information, we elucidate the factors that could hinder climate adaptation, including a limited understanding of fish early life stages, uncertainty in seafood production, unequal allocation and accessibility of resources, and inadequate consideration of inclusive governance and adaptive management. Our results show that species, which have managed to survive the stress of overfishing, demonstrate a remarkable ability to adapt to climate change. However, collapsing stocks such as large yellow croaker face a high risk due to the synergistic effects of inherent biological traits and external management interventions. We emphasize the imperative to build institutional, scientific, and social capacity to support fisheries adaptation. The scientific insights provided by this study can inform fisheries management decisions and promote the operationalization of climate-resilient fisheries in China and other regions.

Opportunity for marine fisheries reform in China.

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.

[Risk assessment of coastal ecosystem in Beibu Gulf, Guangxi of South China].

Based on the marine ecological investigation in the coastal area of Beibu Gulf, Guangxi in September 2009, a GIS-based evaluation was conducted on the present status of ecological environment quality, including seawater quality, nutrient level, biomass, primary productivity, biodiversity, and ecological buffer capacity, in the area in autumn, and the integrated ecological risk index (ERI) was adopted to assess the risk of the coastal ecosystem in the Gulf. In September 2009, the study area had a better ecological environment quality. Most of the risk indicators were at medium or lower level, and the total area was overall at low ecological risk level. The ERI showed that there was an obvious spatial heterogeneity in the distribution of the ecological risk. The nearer to the harbors, the higher the risk was.

[Evaluation of sustainable utilization of fishery resources in Guangdong Province].

Sustainable utilization of fishery resources is the key for the sustainable development of fisheries, and its evaluation is a critical topic in the management of fisheries. Based on the theories of sustainable utilization, an index system suitable for the evaluation of the sustainable utilization of fishery resources in Guangdong Province was established, and a hierarchy and comprehensive grey evaluation model was adopted to evaluate this sustainable utilization in 1978-2007. In the study period, the utilization level of fishery resources in this province had an overall decrease, being the lowest in 1998, only 37.3% of that in 1978; but in the nearest 10 years, this utilization level had an increasing trend, with that in 2007 being 55.1% of 1978. The results of comprehensive evaluation were consistent with the observed facts, indicating that the model was feasible to evaluate the sustainable utilization of regional fishery resources.

[Variations of fish species diversity, faunal assemblage, and abundances in Daya Bay in 1980-2007].

Based on the 2004-2005 otter trawl survey data and the 1980-2007 relevant historical records, this paper analyzed the variations of fish species composition, faunal assemblage, diversity indices, dominant species, and abundance in Daya Bay. In the 2004-2005 trawl survey, a total of 107 fish species were recorded, belonging to 50 families and 13 orders, among which, meso-demersal fish were predominant, with 48 species recorded, and followed by pelagic and demersal fishes, with 37 and 21 species, respectively. The fishes in the Bay belonged to tropical and subtropical fauna, with the dominance of warm water fishes (97 species) and warm-temperate water fishes (10 species). The diversity index was the highest in summer (3.82), followed by in winter (3.37) and autumn (3.00), and the lowest in spring (2.40). The seasonal variation of Pielou evenness index mimicked that of diversity index. In 1980-2007, the characteristics of fish community in the Bay changed obviously. The species number reduced from 157 species in the 1980s to 110 species in the 1990s and to 107 species in 2004-2005, and the dominant species shifted from the high-value fishes such as hairtail and pomfret in the 1980s to low-value fishes such as sardine fish, anchovy, and juvenile porgy. A non-linear regression model composed of inter-annual trend and seasonal cycle was used to simulate the changes of fish stock density in 1980-1999 and 1990-2007, and the results indicated that in the two periods, the fish stock density in the Bay all showed a decreasing trend, but the decrement was larger in 1990-2007 than in 1980-1999. The seasonal variation of the stock density in 1980-1999 was relatively small, with an amplitude being 0.099, while that in 1990-2007 was relatively larger, with the amplitude being 0.420, illustrating that the fish abundance in the Bay had a larger seasonal fluctuation in 1990-2007.

[Effects of fishing on the marine ecosystem of Beibu Gulf].

By using Ecopath with Ecosim 5.1 software, the Ecosim model of Beibu Gulf marine ecosystem in 1959-1960 was constructed, which included about 20 functional groups such as fishery, marine mammals, sea-birds, sharks, pelagic fishes, demersal fishes, and benthic crustaceans, etc. Through the comparison with the investigation data in 1997-1999, the effects of fishing on the structure and function of Beibu Gulf marine ecosystem were analyzed. The results indicated that with the increasing fishing pressure in past forty years, the ecosystem structure and function shifted drastically, with the biomass of long-lived, high trophic level and piscivorous fishes declined while short-lived and small fishes and benthic invertebrates dominated gradually. The biomass of piscivorous species in 1999 was only 6% of that in 1960, while cephalopods increased 2.7 times or more. The trophic level of the catch declined from 3.2 in 1960 to 2.98 in 1999, which fitted the rule of "fishing down the food web" and suggested that the present exploitation patterns were unsustainable. Based on the data of the 1990s, the changes of the ecosystem under decreasing fishing pressure were predicted. This study validated the feasibility of Ecosim model in predicting the effects of fishing pressure on marine ecosystem.

[Mass-balance ecopath model of Belbu Gulf ecosystem].

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.

[Estimation of growth and mortality parameters of Argyrosomus argentatus in northern South China Sea].

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.

[Fisheries biology of Nemipterus bathybius and its optimum catchable size in northern South China Sea].

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.