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Highlights: What are the main findings? The commercial small pelagic fishes (Decapterus maruadsi and Trachurus japonicus) in the Beibu Gulf were still miniaturizing. Fisheries management, characterized by reduced fishing efforts, cannot completely restore population structure in a short period. What are the suggestions for the future fisheries development in the Beibu Gulf? Continuing to maintain low fishing mortality and increasing the catchable length should be the key ways to achieve fishery resource conservation and recovery. Simple Summary: Decapterus maruadsi and Trachurus japonicus, as the main commercial small pelagic fish in the coastal fisheries of China, have been facing the threat of population decline due to overfishing. In this work, we assessed the population status of two commercial small pelagic fish stocks in the Beibu Gulf (in the South China Sea) over the past 15 years (2006–2020). The analysis results show that the commercial small pelagic fishes in the Beibu Gulf were still miniaturizing, and fisheries management characterized by reduced fishing efforts cannot completely restore population structure in a short period. Continuing to maintain low fishing mortality and increasing the catchable length should be the key ways to achieve fishery resource conservation and recovery. Our findings will provide a key scientific basis for future improvements in offshore fisheries management. 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. [ABSTRACT FROM AUTHOR]

Highlights: What are the main findings? The abundance of most of fish stocks in the Beibu Gulf continued to decline over the last 15 years. What is the implication of the main finding? Environmental variations caused by climate events can episodically enhance the abundance of certain fish stocks. Warming may be the reason of northward shifts in distribution of most fishes. 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. [ABSTRACT FROM AUTHOR]

The seasonal variation in phyla within the phytoplankton community is evident in the Beibu Gulf region, with Bacillariophyta being the most diverse group, accounting for 55% of genera in spring and slightly increasing to 57.9% in autumn. In contrast, Pyrrophyta exhibits a higher number of genera and taxa in spring at 43.3%, but this percentage declines to 40% in autumn. During both seasons, the phylum Cyanophyta had a limited occurrence, making up only 1.7% and 2.1% of the total species counts, but in the spring season, it emerged as the dominant phylum with a high cell density of 95.4%, surpassing other phyla such as Aphanocapsa elachista , Guinardia flaccida, Chaetoceros laciniosus, Trichodesmium thiebautii, and Thalassiosira subtilis. Conversely, in the autumn season, cyanophyta constituted merely 4% of the overall cell density. The relative abundance of Bacillariophyta species (Bacteriastrum sp. > Chaetoceros affinis > Chaetoceros lorenzianus > Thalassionema nitzschioides > Chaetoceros curvisetus) displayed a distinct trend, reaching its peak cell density of 94% during autumn. The significance of this study lies in its findings that different types of phytoplankton, such as dinoflagellates and diatoms, have distinct growth patterns in the northern Beibu Gulf. It also highlights the importance of nutrient levels, particularly nitrate, nitrite, silicate, and soluble reactive phosphorus, in influencing the composition of phytoplankton communities during the spring season. Additionally, the study reveals a negative relationship between ammonium concentration and water temperature during both spring and autumn. These findings suggest that changes in nutrient availability during seasonal shifts play a critical role in shaping phytoplankton dynamics in marine ecosystems. The observed negative relationship between ammonium concentration and water temperature raises questions about the potential impacts of climate change on phytoplankton dynamics in the Beibu Gulf. As temperatures continue to rise, it is possible that the nutrient-rich conditions in the spring season may be altered, leading to changes in the composition and abundance of phytoplankton populations. This could potentially have cascading effects on the overall ecosystem. Therefore, studying the interplay between temperature and nutrient availability is essential for accurately assessing the future health and productivity of the Beibu Gulf's phytoplankton communities. [Display omitted] • Bacillariophyta is the most diverse phylum in both spring (55%) and autumn (57.9%). • Seasonal fluctuations affect Cyanophyta cell densities (95%) flourishing in the spring. • Bacillariophyta capitalizes in autumn through elevated NO 3 and NO 2 levels. • Salinity, SRP, and SiO 3 were also correlated with phytoplankton community dynamics. [ABSTRACT FROM AUTHOR]

Abstract: 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 (<30-m isobath) and offshore closures (common fishing zone) can drastically reduce fishing effort (between 20% and 30% reduction from 1999 levels), achieving much to avert the collapse of the fishery sector, especially for large-sized, high-value species. The magnitude of the biomass and the catches would obviously increase with simulation time. In a 20-year simulation, the total catches of all fishing gears would be doubled in the inshore closure simulation compared with that of offshore closure simulation with biomass recovery. The results suggest that, for purposes of fishery management in the Gulf, the inshore area within the 30-m isobath should be considered as ‘no-take’ MPAs; this may be an effective management tactic to conserve the ecosystem and to stop the decline in fisheries resources. Considering the complexity of ecosystem-based fishery management, an extension of the current work will incorporate the costs associated with restoration and monitoring efforts as well. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]