Determination of age is of fundamental importance in the fields of fish biology and population management studies (de Santana et al., 2020; Nazir and Khan, 2020). The age data thus obtained can be used to calculate growth rates, mortality and productivity of fish populations (Campana, 2001; Ma et al., 2011). Annuli are usually formed in hard structures, e.g., scales, otoliths, fin rays and vertebrae, reflecting seasonal growth patterns, particularly in temperate waters (Nazir and Khan, 2020; Phelps et al., 2011; Ma et al., 2011). In the context of age identification for this species, although scales are a conventionally employed method, they are less precise in distinguishing older individuals. Otoliths, or ear bones, have been identified as a potential alternative, with studies indicating the presence of clear annual rings. The development of a method for age identification through the comparison of different materials is a future research direction. Field surveys have revealed that Nansi Lake has been significantly impacted by anthropogenic activities, leading to miniaturization of fish. Individuals of O. potamophila in the river are typically small in size, with large specimens being rare. Among the hard structures used to determine ages, otoliths are recommended in most of the investigated species and researches with most distinguishable annuli (Campana and Thorrold, 2001; Ferri and Brzica, 2022). In order to use any hard structure to determine age, it is critical to identify the annuli in the structure and validate its annual periodicity before any applications (Nazir and Khan, 2020).

The dark sleeper, Odontobutis potamophila (Günther, 1861), is a valuable aquaculture species due to its high-quality meat and market demand. It is commonly found in floodplain lakes across eastern China (Zhao et al., 2017; Wang et al., 2017). It is usually a by-catch species of shrimp trap and occurs in the harvest mainly during the spawning period from March to June. The O. potamophila needs a specific nest to lay eggs during the spawning period, with males guarding the nest and the fertilized eggs until the eggs hatched (Hao, 1960). Populations of this species have declined sharply in the last several decades in most of the lakes due to multiple factors, specifically overfishing and habitat degradation (Tang et al., 2015; Zhang et al., 2018).

However, the paucity of basic biological information regarding this species is a matter of concern. A significant body of research has recently been dedicated to O. potamophila, with studies focusing on reproduction, husbandry, and larval rearing (Liu et al., 2008). Phylogenetic analysis of the mitochondrial 12S rRNA sequence was conducted to ascertain the taxonomic composition of Chinese odontobutis. The analysis revealed the presence of four species: O. potamophila, Odontobutis sinensis, Odontobutis haifengensis and Odontobutis yaluensis. To date, no research has been conducted on the age determination of this species, which consequently hinders further biological study.

Previously, the maximum age of this species in Taihu Lake and Chaohu Lake was determined through the analysis of otolith size, with a maximum age of four years old confirmed (Li et al., 2007; Sun et al., 1996). Meanwhile, the Odontobutis sinensis in Honghu Lake was confirmed to be in the age range of 1-3 years old by using the scale (Xiang et al., 2021). Nevertheless, the maximum age measurements for this species in Nansi Lake have yet to be documented. The experimental results demonstrated that the age inferred based on otolith mass was largely consistent with the actual age, confirming that otolith mass is an accurate and rapid method of age identification (Han et al., 2022). The significant positive correlation between otolith weight and fish age suggests that otoliths can be used as a valid indicator for estimating fish age, especially in age groups where there is no overlap between otolith weight and body length. This method can provide a high level of accuracy. Nevertheless, it is important to note that the accuracy of this method may be compromised in circumstances where there is overlap between the parameters under consideration. Consequently, further validation and refinement of the analysis is required to ensure the optimal performance of this technique (Britton and Blackburn, 2014; Khan et al., 2018).

In this study, Nansi Lake is a unique study site because it is a key node of the South-to-North Water Diversion East Route Project, which has important ecological protection and water resource allocation roles. Therefore, we firstly analyzed the sectioned sagittal otoliths to distinguish any potential marks of annuli in Nansi Lake,. We then validated the annual periodicity of the marks, and used the validated annuli to determine age structure and to calculate growth patterns mortality of the population. The findings of this study, which concern the age identification of the O. potamophila and population parameters, may shed light on the growth patterns and population dynamics of O. potamophila. They may also provide critical data for assessing the reproductive potential and structural stability of the population. Furthermore, they may be essential for developing rational fishing strategies and for assessing the health of fishery resources.

A total of 591 individuals were analyzed of otoliths, of which 294 males (SL: 54.18-145.58 mm; BW: 3.75-88.52g) and 297 females (SL: 55.97-135.96 mm; BW: 4.34-85.32g) could be identified ( Table 1 ). The mean SL was not statistically different between males (92.21 ± 17.73mm) and females (91.03 ± 14.9 mm; p=0.926>0.05).

There were inner margin of the translucent zone in the plane of the whole sagittal otoliths ( Figure 2 ). The edge type was translucent during the months August-November. In December, the otolith margin of some fish was opaque, and then this proportion increased to its maximum in March of the following year, and afterwards decreased during March and May. In June, 100% of the otolith edges were translucent. Thus, the shift in growth pattern from translucent to opaque occurred in March and May ( Figure 3 ). Additionally, estimated age ranged from 0 to 3 years, i.e. 4 age classes. Individuals of age 0 and 1 accounted for 85.11% of the harvest ( Figure 4 ).

The RSS showed no significant difference in SL-at-age relationships between males and females (F < F _{0.05, 3, 2} = 19.1643, p > 0.05, Table 2 ). We therefore combined data from both sexes to fit growth function. The LGF had the lowest AICc and Δ_i ( Table 3 ), thus, was the best function fitting the SL-at-age relationship S L t = 172.94 / ( 1 + e − 0.46 ( t − 0.79 ) ) (n=591, R ² = 0.622; Figure 5 ). The GPI ( φ ′ ) of the population was 4.14.

The slope (b-values) of the SL-to-BW regressions for both males and females did not differ from the value of 3 (Male: t = 1.13<t _0.05, p>0.05; Female: t = 1.10<t _0.05, p>0.05). The LWRs were also not significantly different between males and females (ANCOVA, F = 3.382, p = 0.066). The LWR for pooled sexes was B W = 0.00003 S L 3.00 (n = 591, R ² = 0.949) ( Figure 6 ).

The total mortality rates (Z) obtained via the age structure catch curve was 1.69/year (n=591, R² = 0.97; Figure 7 ). The natural mortality (M) estimated was 0.53/year, the fishing mortality rate (F) was 1.16/year, and the exploitation rate (E) was 0.69/year.

This is the first study to provide age-verified information on the sagittal otoliths, growth, and mortality of O. potamophila from Nansi Lake. Annuli in otoliths of fish are typically identified by successive opaque/translucent growth zones (Yoneda et al., 2007; Uehara et al., 2020), and such growth zones were also observed in our study. The otolith edge-type analyses revealed an annual pattern of opaque zone deposition. The opaque zone deposition in otoliths occurred from December 2017 to May 2018 appears to be associated with increased water temperatures in the study area. The translucent zone was laid down when water temperatures increased, whereas the decrease in water temperature was accompanied by the laying down of the opaque zone. This temperature-dependent seasonal growth pattern in otoliths has been observed in many fish species, including Coilia mystus (He et al., 2008), Pelates octolineatus (Veale et al., 2015), Siniperca kneri (Zhao et al., 2018), and Pseudorhombus jenynsii (Coulson et al., 2020). The results of the present study show that the transition from the opaque zone to the translucent zone occurs in March and May ( Figure 3 ), at the during the spawning season, similar to that previously reported for this species in Taihu Lake (Sun and Guo, 1996).

Information about the age structure and growth parameters of the fish population would be used for fish stock assessment (Hollyman, 2017).These parameters can be utilized to modify fishing strategies, thereby promoting ecosystem restoration and regeneration. This can be achieved by reducing fishing effort, implementing selective fishing, establishing marine protected areas and rebuilding depleted stocks, among other measures. The aforementioned measures are intended to reduce pressures on marine ecosystems, conserve biodiversity and achieve the long-term sustainability of fisheries resources. Otolith analysis demonstrated that the maximum age of this species caught in Nansi Lake was 3 years in both sexes ( Figure 4 ), indicating that this species is short-lived. The maximum age that we estimated is likely to be close to the maximum age (4-year in Taihu Lake and Chaohu Lake respectively) for this species. Individuals of age-0 and -1 accounted for 85.11% of the harvest, suggesting that younger fish dominated Nansi Lake population ( Figure 4 ). Given the short lifespan of the O. potamophila, which is similar to that of Neogobius melanostomus, a highly successful invasive species in North America and Europe (Kornis et al., 2017). it is intriguing to consider the potential parallels in sexual maturation and reproduction. However, given the absence of direct examination of maturity-at-age and measurement of gonad development in the O. potamophila, it remains unfeasible to make conclusive inferences regarding its reproductive capacity. Consequently, future research should concentrate on these aspects to facilitate a more comprehensive understanding of the reproductive patterns of the O. potamophila. This would facilitate a more informed comparison with N. melanostomus and could provide valuable insights into the ecological and evolutionary strategies of the O. potamophila (Cerwenka et al., 2023). This study shows that small dataset variations significantly affect the selection of the best-fit growth model. The ARSS showed no significant difference in the SL-at-age relationship between males and females ( Table 2 ). Data from both sexes were then combined to fit growth functions and subsequently, the Logistic GF was concluded to be the best model to describe the growth pattern of Nansi Lake population based on the AICc and Δ_i selection. Smaller values of AICc and ΔI are more appropriate, and larger R2 (Huang et al., 2022). This study is also the first to estimate growth parameters for this species worldwide. The Logistic GF parameters obtained from the pooled dataset ( S L ∞ =172.94 mm, K=0.46 year^-1, t₀ = 0.79 year, φ ′ =4.14) were much larger than the values reported by Zhu et al. (1999) of the same genus O. potamophila in Bao’an Lake, China (Female: S L ∞ =152.90 mm, K=0.41 year^-1, t₀ = 0.16 year, φ ′ =1.41; Male: S L ∞ =167.10 mm, K=0.37 year^-1, t₀ = 0.11 year, φ ′ =1.36).

Sexual growth dimorphism was not evident in length-weight relationships (Afrooz et al., 2014). There were no statistical differences in the LWRs between sexes and demonstrated isometric growth patterns. The b-values exhibits differences in waters from different regions (Sun and Guo, 1996; Liu, 2001; Tang et al., 2015), while the observed differences in this value may be attributed to differences in habitat, sample size used for disease, and genetic diversity (Wootton, 1998; Dong et al., 2019) ( Table 4 ).

Although our results showed no statistical difference in the mean SL between sexes, males tended to be larger than females by mRNA-seq and miRNA-seq analysis (Zhao et al., 2017; Zhang et al., 2018). This indicated some trade-off between growth and reproduction. Because male displays parental care behavior which consumes a lot of energy during the spawning period (Hao, 1960). Prior to sexual maturity, males invest more energy into somatic growth while females put energy into gonads, which induces males tended to be larger. Additionally, large individuals gain an advantage during courtship and obtain more chances to mate (Ahti et al., 2020). The observed lack of difference in size between males and females may be due to the fact that the samples were collected randomly and did not differentiate between males and females at the same time of the year. In addition, the samples were grown in the same environment, which could have affected the observations.

In this study, the total mortality (Z), natural mortality (M), fishing mortality (F) and exploitation rate (E) were evaluated for the first time in the Nansi Lakes, which is essential for the implementation of sustainable fisheries management strategies (Ogle, 2016). It was determined that the Z value of 1.69 per year was less than the value for small but high-mortality species, which may be attributed to varying fishing effort and the lake’s heterogeneous habitat (Dong et al., 2019). The M/K ratio is a pivotal metric in evaluating and overseeing the well-being of fishery resources. It plays a pivotal role in formulating fisheries management policies that are grounded in scientific principles. The M/K ratio of 1.15 was within the 1.12-2 range. The M/K ratio of 1.15 fell within the 1.12-2.0 range, indicating that the M value was reasonably well estimated (Pauly, 1980). The M value of 0.53/year was higher than the 0.2-0.3/year value, suggesting that species in the Nansi Lakes have a relatively short lifespan and relatively high natural mortality rates among species (Pauly, 1980). The fishing mortality rate (F) was found to be 1.17 per year, a figure that is considerably lower than those previously reported. This discrepancy may be attributed to the utilization of disparate fishing gears (Ye et al., 2014; Dong et al., 2019). Based on the estimates of F and Z, the exploitation rate E for the Nansi lakes was 0.69/year. Either VBGF or logistic growth function, they are used to estimate S L ∞ and K. In accordance with the rule of thumb (E > 0.5 indicates that the stock is experiencing overexploitation), the study found that the findings suggest that the stock is slightly overexploited (Clovis and Simon, 2024), emphasizing the need for stricter fisheries management policies. For example, implementing seasonal restrictions or establishing catch limits may help protect younger age classes. This study represents the inaugural examination of specific pairs of O. potamophila growth and mortality rates, which may be considered in future research, commencing with other lakes (e.g., Taihu Lake) for specific causes.

In conclusion, the results of this study indicate that opaque areas in O. potamophila otoliths form annually and that the opaque areas that form are a consequence of O. potamophila ageing. Meanwhile, our results regarding the age structure, growth, and mortality provided fundamental information for stock assessment and sustainable management strategies for this important commercial fishery resource. The relatively small maximum size, low maximum age, predominance of younger fish and relatively high natural mortality determined an opportunistic life-history trait, This opportunistic life history may lead O. potamophila to grow rapidly in the early stages of their life cycle in order to reach reproductive conditions as soon as possible, and to reproduce multiple times during their life cycle in order to increase their reproductive opportunities. Based on our conclusions that the life span of O. potamophila is concentrated in the 0-1 year age range, it is possible that O. potamophila reach sexual maturity at a relatively young age in order to start reproducing as early as possible and produce a large number of surviving offspring, suggesting a relative resilient to exploitation (Hitt et al., 2020). Furthermore, the opportunistic and equilibrium traits of O. potamophila may enhance its resilience to exploitation and adaptability to changing environments. These findings are crucial for formulating conservation policies and sustainable fishery practices, as in the case of the invasive goby Tridentiger bifasciatus in Nansi Lake (Mashiko, 1976; Qin et al., 2020). Consequently, future studies on the reproductive biology, parental care behavior and feeding habits of this species will contribute to management of fishery resources and assessing reproductive biology or parental care behaviors.