Individuals of Acanthopagrus schlegelii were collected from fishing ports in Zhanjiang, China, between 2019 and 2021 ( Figure 1 ). The sample represents the population as fish were sourced from multiple regional fishing ports, ensuring spatial variability. The three-year sampling period provided temporal coverage, capturing potential changes in the population over time. After collection, the fish were immediately placed on ice and transported to the laboratory to preserve their biological integrity. The specimens were measured in the laboratory for key biological and growth parameters. Total lengths were recorded to the nearest 0.01 mm using a digital caliper. Body weight, liver weight, and gonad weight were recorded to the nearest 0.01 g using an electronic balance.

The von Bertalanffy growth function analysis using FiSAT II ( Figure 2 ) estimated an asymptotic length (L∞) of 505.05 mm, indicating the maximum potential size the species can reach. The growth coefficient (K) of 0.26 year^-1 suggests a moderate growth rate, while the theoretical age at zero length (t ₀) was calculated to be -0.296 years. The species’ estimated longevity is approximately 11.24 years, highlighting its potential lifespan under natural conditions. The growth performance index (Φ) was determined to be 4.82.

The length-converted catch curve analysis revealed a total mortality rate (Z) of 0.35 year^-¹ ( Figure 3A ). Natural mortality (M) was the predominant factor, contributing 0.33 year^-¹, while the fishing mortality rate (F) was low at 0.02 year^-¹. This resulted in an exploitation rate (E) of 0.05, indicating that 5% of total mortality was attributable to fishing activities. These mortality rates provide a quantitative assessment of the factors influencing the population of Acanthopagrus schlegelii under current conditions.

Figure 3B illustrates the length-frequency distribution alongside the probability of capture, showing that most fish caught were within the 150 mm to 350 mm range, with a peak around 290 mm. The lengths at which 25%, 50%, and 75% of the fish population were likely to be captured were estimated as L₂₅ at approximately 220 mm, L₅₀ at 290 mm, and L₇₅ at 340 mm, respectively. The capture probability reached its maximum (close to 1.0) around 370 mm, with the size at first maturity L_m estimated at 336.7 mm and 95% maturity (L₉₅ ) at 370.37 mm. These findings suggest that the gear is highly selective for fish that are near or have reached maturity.

Figure 3C shows that the population decreases steadily as fish length increases, with the highest survival rate in the smallest length group (around 91.0 mm) and fewer survivors in larger groups. Natural losses are fairly consistent but more pronounced in larger-size classes. Catch rates peak around 290 mm before dropping sharply. Fishing mortality (F), indicated by the red line, increases with length, peaking at 320 mm, then fluctuates and decreases in the largest groups. These findings underscore selective fishing pressure on mid-sized fish, with smaller and larger fish facing lower fishing mortality, highlighting the impact of the gear on the population.

The estimated growth parameters offer valuable perspectives on the population dynamics and overall health of Acanthopagrus schlegelii. The asymptotic length (L∞) of 505.05 mm indicates the species’ potential to reach a substantial size in optimal conditions. According to previous studies on this species in the Northern South China Sea ( Table 4 ), large asymptotic lengths indicate species exhibiting traits associated with healthy and robust populations, such as fast growth rates and significant contributions to stock enhancement efforts. The moderate growth coefficient (K) of 0.26 year^-¹ suggests that the species experiences a relatively slow but steady growth rate which is consistent with its estimated longevity of 11.24 years. Longevity indicates that Acanthopagrus schlegelii can sustain itself over extended periods, provided that environmental conditions remain favorable and that fishing pressure is managed appropriately. The growth performance index (Φ) of 4.82, a key indicator of the species’ growth efficiency, suggests that Acanthopagrus schlegelii is well-adapted to its environment. A high growth performance index (Φ) indicates rapid growth and the ability to reach large sizes, key factors for boosting stock productivity and faster biomass recovery. This study’s t ₀ value of -0.296 implies earlier growth to measurable sizes than in past studies ( Table 4 ). Early growth, marked by rapid size increases, is vital for population sustainability. It helps reduce predation, enhances reproductive success, and boosts resilience to environmental changes (Beverton and Holt, 1957; Lorenzen, 2000). These factors are key in stock assessment models, allowing for more accurate predictions of biomass, recruitment, and stock health (Lorenzen, 2000). Early growth and high growth performance guide management strategies, particularly size limits and fishing pressure adjustments, ensuring individuals reach reproductive size before harvest. As observed in this study, a stable growth rate and long lifespan suggest stock resilience if fishing mortality remains within sustainable limits (Chang et al., 2022).

This study’s growth parameters align with regional findings but show variations ( Table 4 ) typical among sparids, often due to differences in habitat quality and fishing pressures (Law and Sadovy de Mitcheson, 2018). For example, populations in Hongkong report lower asymptotic lengths (403–437 mm) and growth coefficients (K = 0.22–0.28 year^-¹), while a study in China observed a higher L∞ (543 mm) with a lower K (0.15 year^-¹). Higher K values and greater longevity suggest reduced fishing pressure. Phylogenetic analyses reveal genetic diversity within the Sparidae family, providing insights into the evolutionary adaptations of the species (Wu et al., 2018). These findings underscore the importance of early growth traits and high growth performance indices in distinguishing Acanthopagrus schlegelii for its adaptive strategies and contributions to stock recovery. Furthermore, they emphasize the need for sustainable fishing practices and environmental conservation to ensure the resilience and long-term sustainability of Acanthopagrus schlegelii populations.

The mortality rates offer further insights into the current status of the Acanthopagrus schlegelii population ( Figure 3A ). The results highlight the dominance of natural mortality (M) over fishing mortality (F), suggesting that natural factors play a critical role in shaping population dynamics. With a minimal contribution of fishing to total mortality, the exploitation rate remains low, underscoring the relatively low impact of current fishing practices. These findings suggest that the population is stable and that the existing fishing pressure is sustainable. However, careful monitoring is required to ensure that any future increases in fishing activity do not compromise this stability. Sustainable management practices should continue to prioritize maintaining low fishing pressure to support the long-term health of the stock (Melnychuk et al., 2021).

The observed dominance of natural mortality in this population is likely driven by a combination of ecological factors, including habitat degradation, water quality fluctuations, predation, and disease. Environmental stressors, such as seasonal changes in temperature, low dissolved oxygen, and habitat degradation, may exacerbate stress on juveniles and sub-adults, leading to higher mortality rates. These drivers are likely to have a more significant impact on population dynamics than fishing mortality, particularly under conditions of low exploitation. For example, seasonal temperature fluctuations can affect metabolic rates, while hypoxic events may lead to localized die-offs, especially in nursery habitats. In addition, predation pressure, both from natural predators and the increased vulnerability of stressed individuals, can further elevate natural mortality rates. Furthermore, disease outbreaks can cause significant population losses, particularly when fish are already physiologically stressed by environmental factors. These ecological drivers likely regulate the population more effectively than fishing pressure alone. Continued research into these factors is essential for identifying key stressors and informing targeted management strategies that can mitigate their effects. By addressing these challenges such as habitat quality and predation pressures, alongside maintaining low fishing pressure, we can enhance the long-term resilience and sustainability of Acanthopagrus schlegelii populations.

The analysis of capture probability and selectivity, as illustrated in Figures 3B–D , reveals important insights into the interaction between the species and the fishing gear used. The results suggest that the gear is selective for individuals approaching maturity, which is encouraging for sustainable fisheries management (Froese, 2004). The estimated lengths at which fish were likely to be captured indicate that the fishing gear targets fish that have already reached a significant size, thereby reducing the chance of catching smaller, immature individuals. Moreover, the close match between L₅₀ and the peak capture probability suggests that current fishing practices are sustainable. By targeting fish that are near or have already reached maturity, the gear helps ensure that individuals have the opportunity to reproduce before being caught (Froese, 2004). This is vital for preserving the population’s reproductive potential and ensuring long-term stock viability. Moreover, the findings suggest that any changes in gear selectivity, such as increasing mesh size, could further enhance the sustainability of the fishery by allowing more fish to reach maturity before being caught (Gatti et al., 2019).

The Yield Per Recruit (YPR) analysis findings call for careful control of fishing efforts in the Acanthopagrus schlegelii fishery. It shows that yield increases with exploitation, peaking at E_max = 0.421, after which it declines Figure 3D . Exceeding this rate risks overfishing, making it essential to keep exploitation below E_max for sustainability. Two important reference points have been identified in the analysis. The first, E _0.5 = 0.278, indicates the level at which the biomass per recruit is reduced to 50% of its unfished level, reflecting the effects of moderate exploitation on the population. The second, E _0.1 = 0.355, represents a more conservative threshold that aims to balance yield with sustainability, helping to minimize the risk of overexploitation. These thresholds provide vital guidance for maintaining healthy fish stocks (Beverton and Holt, 1957).

The growth patterns of Acanthopagrus schlegelii, as assessed through the length-weight relationship (LWR), highlight notable differences between sexes and seasons. Females demonstrate positive allometric growth, which may indicate higher energy allocation towards growth and reproductive activities, particularly during the summer. In contrast, males exhibit nearly isometric growth, suggesting a more balanced energy allocation between growth and other physiological processes ( Figure 4 ). This difference between sexes may reflect different energy allocation strategies, with females potentially investing more in reproductive tissues, while males maintain a more balanced growth pattern throughout the year (Ladisa et al., 2021; Rljnsdorp and Ibelings, 1989).

The condition factor (K) further highlights these seasonal and sex-based differences. Females were found to be in better condition during the summer than in the winter, likely due to the increased energy demands of reproduction. On the other hand, males consistently exhibited higher condition factors in both seasons, suggesting that males maintain a generally better physical condition year-round. The statistical significance of these differences, confirmed by ANOVA and t-tests, underscores the biological relevance of these findings. The seasonal variations in K-values may be related to environmental factors such as food availability and water temperature, which can influence the energy reserves and overall health of the fish (Morato et al., 2001).

The LWR and K analyses provide a crucial understanding of the growth and health of Acanthopagrus schlegelii, indicating potential seasonal energy allocation shifts between males and females. Investigating the mechanisms behind these shifts could offer deeper insight into their effects on this species’ population dynamics.

The GSI and HSI values provide valuable insights into the reproductive and nutritional status of Acanthopagrus schlegelii across seasons ( Table 2 ). The significant increase in GSI during winter (mean GSI = 4.49%) compared to summer (mean GSI = 0.226%) suggests that the winter season is a critical period for reproductive activity ( Figure 5A ) (Zhang H. et al., 2022). This seasonal variation in GSI indicates the species’ spawning cycle, where energy is allocated towards gonad development in preparation for spawning. Temperature seems to affect sexual activity and breeding in fish (Davies and Bromage, 2002; Kawai et al., 2022). The higher GSI in winter aligns with the observed increase in HSI during the same period (mean HSI = 1.34% in winter versus 0.89% in summer), suggesting that the liver, which stores energy reserves, plays a crucial role in supporting reproductive processes during the colder months ( Figure 5B ) (Abderrazik et al., 2016; Caballero-Huertas et al., 2022). The seasonal increase in GSI observed in Acanthopagrus schlegelii, peaking during winter, aligns with the reproductive cycle of Acanthopagrus latus, which also spawns during cooler seasons (Tang et al., 2023). This pattern suggests an adaptive energy allocation strategy, where both species prioritize gonad development during peak reproductive periods under optimal environmental conditions, likely to maximize offspring survival. However, a study on Acanthopagrus schlegelii from Japan reports a spawning period from April to June, indicating an extended spawning season as a response to varying ecological pressures (Yamashita et al., 2015). These findings highlight the importance of region-specific reproductive management strategies tailored to the unique spawning cycles of each species.

The significant seasonal differences in GSI and HSI, as confirmed by t-tests, Wilcoxon rank-sum tests, and ANOVA, highlight the adaptive strategies employed by Acanthopagrus schlegelii and other sparids to optimize reproductive success and energy utilization. The moderate Pearson correlation coefficient between GSI and HSI ( Table 3 ) further reflects the energy trade-offs that Acanthopagrus schlegelii must balance between growth, reproduction, and survival, a challenge common across regional sparids. The cluster analysis presented in Figure 5C supports these findings by grouping the fish into distinct clusters based on their GSI and HSI values. These clusters illustrate the variability in reproductive and nutritional investments among individual fish, underscoring the species’ ability to adapt to varying environmental conditions, similar to its sparid relatives (Barrett et al., 2015; Sokołowska and Kulczykowska, 2006).

The findings of this study carry important implications for managing Acanthopagrus schlegelii and its broader ecosystem.

Additionally, implementing size-based harvest restrictions, such as minimum size limits, would help prevent the capture of immature fish that have not yet reproduced. This would allow more individuals to contribute to the reproduction output of the population, thus supporting long-term population growth. Size-based restrictions are particularly important in ensuring that fish are allowed to grow to their full reproductive potential before being harvested.

Together, seasonal fishing moratoriums and size-based harvest restrictions would provide a dual strategy to sustain reproductive output while maintaining a balanced population structure. These measures are crucial for managing Acanthopagrus schlegelii populations and ensuring the sustainability of regional fisheries.

The growth and reproductive traits of Acanthopagrus schlegelii are crucial considerations for fisheries management. Variations in growth rates and reproductive timing, especially in comparison to other sparid species, underscore the need for adaptive management strategies. Flexible size limits and seasonal closures that reflect these differences are essential for ensuring the sustainability of regional fisheries and protecting reproductive stocks.

Over the past few years, a noticeable quantity of black sea bream, especially juvenile fish of at least 5 cm in length, have been released into the Zhanjiang Sea region as part of a stock enhancement initiative. This is a key strategy for rebuilding the population, which had been under historical pressure. Despite the adoption of new fishing methods by both fishermen and anglers, the intensity of traditional fishing operations, such as trawling and gill netting, has remained stable. This is primarily due to the successful implementation of enhanced fisheries protection measures in Zhanjiang Port, which have kept overall fishing pressure relatively low.

The low exploitation rate and fishing mortality observed in our study support the conclusion that fishing pressure, in terms of fishing mortality, is not a primary driver of the fluctuations in the black sea bream population. Instead, natural mortality is the dominant factor influencing population dynamics. This finding underscores the need for ongoing stock enhancement efforts and continued habitat protection, rather than solely reducing fishing activity.

Moreover, the successful recovery of the black sea bream population in certain areas (Lauden et al., 2024) is likely a result of the combined effect of stock enhancement and controlled fishing. Measures such as seasonal closures and restrictions on fishing gear have proven effective in reducing fishing pressure during critical life stages, such as spawning seasons, and facilitating the population to regenerate. The enforcement of these management measures is essential for ensuring the long-term sustainability of the fishery. We emphasize that maintaining and strengthening these protections, especially during key reproductive periods is vital for the continued recovery and stability of the black sea bream population in Zhanjiang’s coastal waters.