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.

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) inPsenopsis anomalaandTrichiurus 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, andUpeneus 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.

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 lengthL_infand growth coefficientK) to serve as priors for LBB and LBSPR estimation. Estimates are calculated for SCS skipjack tunaL_inf(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.

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 (L_c) and high fishing mortality (F) (compared to F_0.1), 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.

Overfishing and climate change have led to the depletion of fishery resources in the offshore South China Sea. The purpleback flying squid (Sthenoteuthis oualaniensis) has emerged as a promising alternative due to its ecological and economic value. However, information on its preferred habitat conditions remains scarce. This study integrates geostatistical and fisheries oceanographic approaches to explore optimal spatial–temporal scales for habitat modeling and to assess habitat changes under warming scenarios. Utilizing fishery data from 2013 to 2017, environmental variables including SST, sea surface temperature anomaly (SSTA), and chlorophyll-a concentration (CHL) were analyzed. Fishing effort data revealed significant seasonal differences, with the highest vessel numbers in summer and the lowest in autumn. Among the six modeling schemes, the combination of 0.5° × 0.5° spatial resolution and seasonal temporal resolution yielded the highest HSI model accuracy (84.02%). Optimal environmental ranges varied by season. Simulations of SST deviations (±0.2 °C, ±0.5 °C, and ±1 °C) showed that extreme warming or cooling could eliminate suitable habitats. These findings highlight the vulnerability of squid habitats to thermal shifts and support adaptive fishery strategies in the South China Sea.

Fishing grounds of the purpleback flying squid (Sthenoteuthis oualaniensis) in the South China Sea are commonly related to chlorophyll-a (Chl-a), yet the temporal structure of this relationship is rarely tested explicitly. We examined whether fishing-ground formation responds to concurrent or preceding Chl-a conditions using monthly vessel monitoring system (VMS) records from 2014–2018 (50 valid months) aggregated to a 0.5° grid in the southern South China Sea, together with Copernicus Marine SST and 4 km gap-free Chl-a products. Four Chl-a windows (lag 0, lag 1, lag 2, and mean lag 1–2) were compared for mean log10 Chl-a and for the Chl-a spatial gradient using within-month Spearman correlations, centroid distances, Jaccard overlap, response curves, and cross-validated generalised additive models. The analyses showed a consistent separation between productivity level and productivity structure. Mean Chl-a associations remained small and did not produce a stable spatial lag, whereas fishing-ground centroids were substantially closer to high-gradient cells (median 166–181 km) than to high-Chl-a cells (280–295 km). The largest seasonal gradient correlation occurred in spring at lag 2 (ρ = 0.114), and median gradient overlap at the principal threshold was highest at lag 2 (0.034). Annual cross-validation, however, showed no consistent lag-2 advantage. Chl-a gradients therefore help characterise fishing-ground position, while the antecedent association depends on season and spatial metric rather than indicating a generally optimal two-month lag. This pattern is compatible with delayed food-web transmission and prey-field reorganisation, while its causal interpretation is considered in the Discussion.