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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]

Abstract: 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. [Copyright &y& Elsevier]

[Display omitted] • Higher fish TD α , FD α and PD α were detected in summer in the nSCS. • TD β , FD β and PD β of fish community significantly increased in winter. • Species replacement predominantly drove fish beta diversity in the nSCS. • Water environmental factors mainly influenced TD β , FD β , PD β in summer. Comprehending fish diversity through multiple dimensions is a crucial aspect in advancing our understanding of fish populations and promoting sustainable fisheries as well as diversity conservation. Traditional research on fish diversity has been limited to non-genetic methods. The advent of environmental DNA (eDNA) technology enables simultaneous examination of both non-genetic and genetic aspects of fish diversity. In this study, we evaluated the ability of eDNA to capture multidimensional fish diversity in the northern South China Sea, with emphasis on phylogenetic diversity. The effect of environmental change and geographical location on multidimensional fish diversity across different seasons was also investigated. The analysis of species accumulation curves revealed that the observed species richness approached an asymptote, achieving 94.7% and 89.5% of the predicted values during the summer and winter, respectively. Alpha diversity of taxonomic (species, 225 vs. species, 102), functional (FRic, 0.51 vs. FRic, 0.19), and phylogenetic (PD, 26.27 vs. PD, 15.81) dimensions were found to be higher in summer compared to winter, with environmental factors significantly influencing these diversities across seasons. Regarding the three dimensions of beta diversity, replacement made a greater relative contribution to total taxonomic, functional, and phylogenetic beta diversity than richness difference. These effects were primarily influenced by water environment factors (e.g., NO 2 −, SiO 3 2−, surface temperature) during the summer and geographical factors (e.g., longitude and latitude) during the winter. This study provides evidence supporting the feasibility of utilizing eDNA techniques to uncover fish diversity in multiple dimensions, with a specific emphasis on its intrinsic genetic characteristics, which hold substantial value for the protection of diversity and the scientific administration of fishery resources. [ABSTRACT FROM AUTHOR]