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Understanding the impact of climate change on the distribution of marine species is of vital importance for the sustainable development of marine fisheries. In this study, an ensemble species distribution model is employed to predict the potential distribution of Japanese jack mackerel (Trachurus japonicus) in the Northern South China Sea (NSCS) under current conditions and three future climate scenarios (IPCC Representative Concentration Pathways RCP126, RCP245, RCP585). The model incorporates vital environmental variables including sea surface salinity (SSS), sea surface height above geoid (SSH), sea surface temperature (SST), ocean mixed layer thickness (MLD), and distance from land (DFL). Notably, SST and SSH are found to be the most influential factors in defining the habitat of T. japonicus. Model predictions reveal that the current suitable habitats for T. japonicus are primarily located in the offshore area to the west of the Pearl River estuary, especially in waters with depths of less than 100 m in the NSCS. The Beibu Gulf emerges as a prominent habitat for this species. However, projections indicate a significant reduction in areas with a high probability of T. japonicus occurrence, particularly in scenarios with high emissions. This reduction in suitable habitats is primarily observed in the NSCS at depths ranging from 40 to 100 m, while the northwestern coastal regions of the NSCS are projected to witness an increase in suitable habitats. The rate of habitats contraction is expected to surpass that of expansion, with an estimated 16.93 % to 29.27 % decrease in suitable habitats by 2091–2100.These findings offer fundamental insights into how climate change impacts the distribution of T. japonicus in the NSCS and provide valuable guidance for future resource development and utilization.

A detailed understanding of the impact of climate variability on fish life histories provides a fundamental basis for the ecosystem approach to fisheries management. In this study, we evaluated the relationship between fish life-history trait parameters and climate periods, using survey data from 2006−2020 for the Beibu Gulf of the northwestern South China Sea, a fishing ground with high commercial and ecological value. We show that climate events can significantly alter the structure of the life history for major commercial fishes, with more diverse life-history strategies during warm events and more concentrated mortality during cold events. Additionally, we detected special relationships between climate events and the life-history trait composite index (LTCI) in Psenopsis anomala and Trichiurus lepturus. It is possible that other indicators would be more appropriate than variability in Oceanic Niño Index (ONI), such as variation in fishing effort or the environmental resistance of fish. Anthropogenic disturbance and biological features may also help explain the magnitude of population variability. We found that changes in life-history traits of only a few commercial fishes (Decapterus maruadsi, Pennahia macrocephalus, and Upeneus sulphureus) were related to the shift in climate periods; these fishes were generally characterized by faster growth and higher mortality rates in the normal and El Niño periods, with the opposite trends in La Niña periods. However, the emergence of climate events has led to a clear complementarity of life-history strategies among some fish in the same ecological niche (same genus and/or feeding group), which may be explained by a balance between habitat conditions and fishing pressures. This study of fish life-history strategies under climate anomalies provides key insights into important attributes for managers to consider when implementing relevant measures to promote fisheries sustainability in the subtropical bay.

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.

Largehead hairtail Trichiurus japonicus is a major commercial fish species in the Beibu Gulf of the northwestern South China Sea. Despite much effort to protect the fishery resource, the current stock of T. japonicus is overexploited. As the impacts of climate change unfold globally, seasonal changes in the distribution of largehead hairtail in the Beibu Gulf have not yet been clarified. Maximum entropy model based on mixed layer depth and salinity were projected onto seasonal habitat changes of T. japonicus in the Beibu Gulf under a current scenario and three different Representative Concentration Pathways (126, 370, 585) to evaluate geographic distribution changes under the different climate-change scenarios. The current geographic distribution results showed variation with seasonality, as the wintering population shifts toward the northeast. Under each of three SSP scenarios, there is higher risk to habitat suitability in the 2090s as compared with that in the 2050s. The disadvantage to T. japonicus distribution is greatest in winter under each of the three climate change scenarios, both in the short- and long-term. Potential suitable habitat distributions have a minor range extension in Representative Concentration Pathway 370–2050 winter, but in the rest of the scenes and years they contract to south of the Beibu Gulf. The overall results indicate that seasonal differences in suitable habitat should be considered to ensure effective planning of future management strategies for T. japonicus.

Marine ecosystems are a primary conservation concern because of the separate and synergistic effects of overfishing and climate change on their productivity and biodiversity. For the purpose of implementing ecosystem-based fisheries management, it is essential to understand how fish stocks respond to climate change in marine ecosystems, especially those that have been overexploited. The Beibu Gulf in the northwestern part of the South China Sea has long been one of China’s major fishing grounds. After the collapse of the demersal ecosystem in 1990 s, small pelagic fishes, including Japanese jack mackerel (Trachurus japonicus) and Japanese scad (Decapterus maruadsi), have occupied a more-important ecological niche in this gulf. However, insufficient research on how small pelagic fish respond to climate change creates great uncertainty for implementing the current management measures. This study used fish data of trawl surveys and environmental data to investigate possible mechanisms leading to dramatic fluctuations in small pelagic fish populations and their possible effects on the Beibu Gulf marine ecosystem. Abnormal blooms of small pelagic fish, especially T. japonicus and D. maruadsi, occurred following four La Niña events (in 2007/2008, 2010/2011, 2011/2012, and 2020/2021). Moreover, the dominant fish species shifted observably and regularly, Shannon–Wiener diversity and Pielou’s evenness decreased significantly in the northeastern coastal waters of the gulf, and the spatial pattern of the fish community was reorganized in this ecosystem. The small pelagic fish blooms in summers probably resulted from a combination of climate change, overfishing, and a summer fishing moratorium, which thereby caused a series of responses in the marine ecosystem. Fluctuations in the small pelagic fish populations after La Niña events seemed more dramatic after the collapse of demersal fish stocks. Therefore, we emphasize the need for implementing ecosystem-based, preventive, and adaptive fisheries management.

To better understand how fish communities respond to environmental changes under extreme climate events, we examine changes in fish communities in Beibu Gulf during strong El Niño and La Niña events. Strong La Niña and El Niño events affect the composition, abundance, and distribution of fish communities in Beibu Gulf. Fish community distribution and composition change before and after La Niña and El Niño events, and dominant species within them change with stable fishing intensity. The abundance and distribution of small pelagic fish such as Japanese jack mackerel (Trachurus japonicus) and Japanese scad (Decapterus maruadsi) are the most affected. Using a generalized additive model (GAM), we explore relationships between the abundance of T. japonicus and D. maruadsi and a suite of environmental variables. The GAM results revealed that sea surface salinity and sea surface temperature best explain changes in catch per unit effort of these two species during a La Niña event; depth, sea surface temperature, and mixed layer depth during an El Niño event. The results obtained in this study will offer support for implementing more-accurate, scientific fisheries management measures.

In the world, more than 80% of the fisheries by numbers and about half of the catches have not been formally analyzed and evaluated due to limited data. It has led to the fast growth of data-poor evaluation methods. There have been various studies carried out on the comparative performance of data-poor and data-moderate methods in evaluating fishery exploitation status. However, most studies to date have focused on coastal fish stocks with simple data sources. It is important to pay attention to high sea fisheries because they are exploited by multiple countries, fishing gears and data may be divrsified and inconsistent. Furthermore, a comparison of the performance of catch-based, length-based, and abundance-based methods to estimate fishery status is needed. This study is the first attempt to apply catch-based, length-based, and abundance-based data-poor methods to stock assessment for an oceanic tuna fishery and to compare the performance with a data-moderate model. Results showed that the three data-poor methods with various types of data did not produce an entirely consistent stock status of the southern Atlantic albacore (Thunnus alalunga) fishery in 2005, as the estimated B2005/BMSY ranged from 0.688 to 1.3 and F2005/FMSY ranged from 0.708 to 1.6. The Monte Carlo Catch maximum sustainable yield model (CMSY) produced a similar time series of B/BMSY and F/FMSY and stock status (recovering) to the Bayesian state-space Schaefer model (BSM). The abundance-based method (AMSY) gave the most conservative condition (overfished) of this fishery. Sensitivity analysis showed the results of the length-based Bayesian biomass estimation method (LBB) are sensitive to Linf settings, and the results with higher Linf were similar to those of other models. However, the LBB results with setting Linf at lower levels produced more optimistic conditions (healthy). Our results highlight that attention should be paid to the settings of model parameter priors and different trends implied in various types of data when using these data-poor methods.

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.

The objective of this research is to explore the relationships among various multidimensional factor groups and the density of fishery resources of ecosystems in offshore waters and to expand the application of deep machine learning algorithm in this field. Based on XGBoost and random forest algorithms, we first conducted regulatory importance ranking analysis on the time factor, space factor, acoustic technology factor, abiotic factor, and acoustic density of offshore fishery resources in the South China Sea. Based on these analyses, data slicing is carried out for multiple factors and acoustic density, and the relationship between multidimensional factor group and the density of marine living resources in the ecosystem of offshore waters is elaborately compared and analyzed. Importance ranking shows that the concentration of active silicate at 20 m depth, water depth, moon phase perfection, and the number of pulses per unit distance (Ping) are the first-order factors with a cumulative contribution rate of 50%. The comparative analysis shows that there are some complex relationships between the multidimensional factor group and the density of marine biological resources. Within a certain range, one factor strengthens the influence of another factor. When Si20 is in the range of 0–0.1, and the moon-phase perfection is in the range of 0.3–1, both Si20 and moon-phase perfection strengthened the positive influence of water depth on the density of fishery biological resources.

Accurate information on the temporal and spatial distribution of fish stocks is necessary for informed management and conservation of fisheries resources. We report on the temporal and spatial variation in biomass and density of a hitherto little-studied species of commercial importance, the threadfin porgy Evynnis cardinalis, it was listed as endangered (EN) in a recent International Union for Conservation of Nature (IUCN) red list, on the northern South China Sea continental shelf, using data collected during fisheries surveys from 2014 to 2015. Seasonal variation in the frequency of occurrence of E. cardinalis in trawl samples on this shelf was highest during summer (68.57%) and lowest during winter (31.43%). Average biomass was highest in autumn (33.05 kg·km−2) and density was highest in spring (936 ind·km−2). The highest annual average biomass (41.6 kg·km−2) and density (1021 ind·km−2) occurred off Shantou, and the lowest occurred off Zhanjiang (7.2 kg·km−2 and 165 ind·km−2, respectively); seasonal patterns in density are more similar in waters off Yangjiang and Shantou than elsewhere. The highest annual average biomass occurred at 40 m depth (56.9 kg·km−2), and the highest annual average density (1084 ind·km−2) occurred at 30 m depth. Density of E. cardinalis varied with depth, with lower densities occurring in shallow and deeper waters, and higher densities at intermediate depths.

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.