Estimates on age, growth, and mortality of Leuciscus chuanchicus (Kessler 1876) in the Ningxia section of the upper reaches of the Yellow River, China

Sampling

The study was conducted in July 2022, March 2023, May 2023, and September 2023 using standardized net cages (length: 15 m, width: 40 cm, height: 40 cm) and gill nets (mesh sizes: 1, 2, 3 and 4 cm). A total of 472 specimens of L. chuanchicus were systematically captured in the Ningxia section of the upper Yellow River (Table 1). The fish samples were immediately subjected to routine biological measurements under fresh conditions including precise recordings of their total length (TL) up to an accuracy of ±0.01 cm and body weight (W) up to an accuracy of ±0.01 g. Subsequently, gender identification was performed through morphological examination based on gonadal morphology. Lapillus otoliths were carefully extracted from the inner ear sac of each specimen using tweezers followed by removal of surface connective tissue before being placed into numbered centrifuge tubes containing a solution consisting of ethanol at a concentration of 95% for preservation. Water temperature during the survey was measured using a portable water quality analyzer manufactured by HACH (Loveland, CO, USA). All sampling procedures strictly adhered to the guidelines provided by Heilongjiang River Fisheries Research Institute of CAFS for Laboratory Animal Welfare and Ethical Review.

Otolith processing and aging

The otolith, an essential material for fish age determination, can effectively and accurately reflect the biological characteristics of fish (Lowerrebarbieri, Chittenden & Jones, 1994). To prepare the samples, lapilli were mounted on glass slides using transparent colorless nail polish. Subsequently, they were hand-ground with wet sandpaper (1,500 and 2,000 grit) and polished with alumina paste (3 μm) until the core and most annuli became visible under a microscope. The age of L. chuanchicus was determined by observing the lapillus annuli. Furthermore, blind examination following Li et al.’s (2016) method was employed to identify the age of each otolith.

Growth characteristics

The length–weight relationship was fitted using the power function model:

$$\mathrm{W}=\mathrm{a}{\mathrm{L}}^{\mathrm{b}}$$

In the equation, W is the body weight (g), L is the total length (cm); a is the condition factor for growth; and b is the growth index. When $\mathrm{b}=3$, it indicates isometric growth, while $\mathrm{b}\ne 3$ suggests allometry (Cazorla & Sidorkewicj, 2008). The t-test was employed to examine whether the power exponential b value significantly deviated from the anticipated isometric growth three (Ricker, 1975). The statistical analyses were carried out using the Microsoft Excel 2019 and R 4.31.

The growth conditions are described using Von Bertalanffy (1938) growth equation, with the following equation:

$${\mathrm{L}}_{\mathrm{t}}={\mathrm{L}}_{\mathrm{\infty }}[1-{\mathrm{e}}^{-\mathrm{K}(\mathrm{t}-{\mathrm{t}}_0)}]$$

In this equation, t is age, L_t is total length at age t, L_∞ is asymptotic total length, K is the growth coefficient, and t₀ represents the assumed theoretical starting age of growth. When the rate of body weight gain reaches its maximum or when the acceleration of body weight gain approaches zero, it signifies a critical inflection point in the fish’s growth trajectory. The maintenance of fish resources’ sustainability heavily relies on this crucial parameter. The age at which this pivotal moment occurs can be determined by employing the following formula (Zhan, Lou & Zhong, 1986):

$${\mathrm{t}}_{\mathrm{p}}={\displaystyle \frac{\mathrm{l}\mathrm{n}\mathrm{b}}{\mathrm{k}}+{\mathrm{t}}_0}$$

The growth characteristic index (φ) was calculated using the formula:

$$\phi =\mathrm{l}\mathrm{g}(\mathrm{K})+2\mathrm{l}\mathrm{g}({\mathrm{L}}_{\mathrm{\infty }})$$

The parameters are all derived from the estimation of the growth equation (Munro & Pauly, 1983). Moreover, the residual sum of squares (ARSS) was employed to statistically compare the fitted growth curves between genders (Chen, Jackson & Harvey, 1992).

Mortality estimation

By employing Pauly’s methodology, a linear correlation was established between the natural logarithm of the sample size within each age group and their corresponding ages, resulting in an equation of the form ‘y = n + mx’. In this equation, the absolute value of parameter m represents the total instantaneous mortality rate (Z) (Beverton & Holt, 1957; Ricker, 1975).

The instantaneous rate of natural mortality (M) is estimated more accurately by evaluating it through three empirical equations:

• (1) Based on length (Pauly, 1980): $\mathrm{l}\mathrm{n}\mathrm{M}=-0.0066-0.279\mathrm{l}\mathrm{n}{\mathrm{L}}_{\mathrm{\infty }}+0.6543\mathrm{l}\mathrm{n}\mathrm{K}+0.4634\mathrm{l}\mathrm{n}\mathrm{T}$,

• (2) Based on age (Zhan, Lou & Zhong, 1986; Ralston, 1987): $\mathrm{M}=0.0189+2.06\mathrm{K}$; $\mathrm{M}=-0.0021+{\displaystyle \frac{2.5912}{{\mathrm{t}}_{\mathrm{m}}}}$.

The variable T represents the annual mean water temperature of this specific river section, while L_∞ and K are estimated parameters for the Von Bertalanffy growth equation. Additionally, t_m denotes the maximum age observed in the captured samples.

The instantaneous rate of natural mortality (M) refers to the relative death rate per unit time within a fish population resulting from natural factors such as predation, disease, and aging. Conversely, the instantaneous rate of fishing mortality (F) represents the relative death rate caused by fishing activities. These two components together constitute the total instantaneous mortality (Z), which can be mathematically expressed as: $\mathrm{Z}=\mathrm{M}+\mathrm{F}$ (Sainsbury & Sainsbury, 1982).

The population exploitation rate (E) can be determined by dividing the fishing mortality (F) by the total instantaneous mortality (Z), as expressed in the following equation: $\mathrm{E}=\mathrm{F}/\mathrm{Z}$ (Ricker, 1975).

Population structure

Among 472 L. chuanchicus samples, 240 were males and 232 were females, and the female-to-male ratio was 0.96:1 (Table 2). The total length distribution for females ranged from 4.60 to 37.45 cm, with body weight ranging from 0.68 to 552.43 g; while for males, the total length distribution ranged from 4.52 to 31.40 cm, with body weight ranging from 0.82 to 436.18 g. The majority of individuals exhibited total length concentrated between the range of 5 and 30 cm, accounting for approximately 91.94% of the population. The body weight distribution primarily ranges from 0.68 to 140 g, constituting approximately 75.00% of the total samples (Fig. 1). In a sample of 472 individuals, the mean total length was 17.96 ± 8.69 cm and the mean body weight was 97.17 ± 106.67 g.

Age structure

The otolith was used for aging of L. chuanchicus (Fig. 2). Through the application of alumina paste to polish the otolith and subsequent observation under an optical microscope, a discernible pattern consisting of alternating dark and bright regions can be observ (Fig. 2). The growth rings were analyzed through the observation of otolith grinding slices. The results revealed that L. chuanchicus individuals ranged from 1 to 7 years old (Fig. 1). Among them, age-1, age-2, and age-3 individuals were the most abundant, making up 80.73% of the total samples. Additionally, the proportion of age 4, age 5 and age 6 individuals was relatively low. Among them, there is even only one age 7 individuals.

Growth characteristics

The relationship between total length and body weight of L. chuanchicus was determined through regression analysis, yielding the equation: $\mathrm{W}=0.0052{\mathrm{L}}^{3.19}({\mathrm{R}}^2=0.9934)$ (Fig. 3). Notably, the b value for females was 3.24, while for males was 3.13. After conducting a t-test, the analysis revealed a significantly higher value of the b coefficient (3.19) in the relationship equation compared to 3 (t-test, $\mathrm{t}=2.97>{\mathrm{t}}_{(0.05,472)}$), indicates that the growth of L. chuanchicus adheres to positive allometry.

By fitting the Von Bertalanffy equation, growth parameters for both female and male L. chuanchicus were estimated (Fig. 4). The growth equations for female and male are as follows:

Famale: ${\mathrm{L}}_{\mathrm{t}}=39.2\left[1-{\mathrm{e}}^{-0.452\left(\mathrm{t}-0.597\right)}\right]$

Male: ${\mathrm{L}}_{\mathrm{t}}=32.7\left[1-{\mathrm{e}}^{-0.605\left(\mathrm{t}-0.557\right)}\right]$

An ARSS test was conducted to evaluate the growth disparities between males and females (F = 0.338 < ${\mathrm{F}}_{(0.05,6,9)}$). The results revealed no statistically significant differences, the length growth equation for all samples was determined as ${\mathrm{L}}_{\mathrm{t}}=37.9\left[1-{\mathrm{e}}^{-0.462\left(\mathrm{t}-0.552\right)}\right]$. Moreover, the growth inflection point age has been determined as 3.068 $y{r}^{-1}$ using formula. Additionally, the growth characteristic index (φ) for L. chuanchicus is calculated as 2.820. Table 3 lists the age range, growth parameters and other indicators of several Leuciscus genus fishes.

Mortality and exploitation rate

Due to the limited performance of the fishing gear in the survey, there were certain constraints encountered during data collection for age 0, age 1 and age 2. Moreover, only a single sample was available for age 7. Consequently, when computing the overall instantaneous mortality rate, data from age 1, age 2, and age 7 were excluded (Fig. 5) (Beverton & Holt, 1957). The total instantaneous mortality rate (Z) was determined to be 1.1302 $y{r}^{-1}$.

The estimation of the instantaneous rate of natural mortality (M) for the entire sample is calculated using three empirical formulas (Pauly, 1980; Zhan, Lou & Zhong, 1986; Ralston, 1987), resulting in a value of 0.7167 $y{r}^{-1}$. Subsequently, employing their mathematical correlation, the corresponding instantaneous rate of fishing mortality (F) is computed as 0.4134 $y{r}^{-1}$. Finally, based on these values, an exploitation rate (E) of 0.3658 is determined. Furthermore, pertinent parameters for both females and males were separately determined as presented in Table 4.

Age and growth

Although many scholars usually use scales as Leuciscus genus fishes age identification materials (Hu et al., 2008; Qi et al., 2011), scales can be difficult to read and may have incomplete growth history due to loss of scales, leading to inaccurate age estimation (Campana, 2001; Howland et al., 2004). Otoliths serve as a reliable method for determining fish age, characterized by the formation of distinct growth rings and maintenance of a stable chemical composition throughout their development. Despite eventual growth cessation, the recorded information remains preserved without any reabsorption (Campana & Thorrold, 2001). In cyprinids, the lapillus is the largest otolith and therefore preferred over the sagitta for fish aging purposes (Rohtla et al., 2015). It should be noted that otoliths require sacrificing the fish which may pose a problem when dealing with endangered species or populations (Howland et al., 2004; Zymonas & McMahon, 2009).

The age distribution of the L. chuanchicus population in the investigated area ranges from 1 to 7 years, with a relatively lower proportion of individuals aged 5, 6, and 7. The age structure exhibits a relatively simplified pattern. The age structure of L. chuanchicus is higher compared to that of L. merzbacheri (1–4) (Guo et al., 2005; Hu et al., 2008), L. idus (1–5) (Hu et al., 2008), and L. baicalensis (1–3) (Hu et al., 2008) captured within China. It exhibits similarities with L. waleckii (1–7) (2–8) (An et al., 2008; Lu et al., 2019; Duan et al., 2022), but significantly lower than the age structure observed in Leuciscus idus (1–29) (Rohtla et al., 2015) caught in the Baltic Sea.

The b value represents the relationship between total length and body weight, which not only varies among different species but also differs among individuals of the same species or population, as well as at different developmental stages, reflecting variations in stomach fullness, overall appetite condition, and gonadal stages (Flura et al., 2015). In this study, the b value of L. chuanchicus is 3.17 in winter, whereas it is 3.18 in summer, exhibiting minimal disparity between the two seasons, the overall b value of the L. chuanchicus population is 3.19, indicating allometry. It is comparable to L. waleckii (3.17; 3.18), higher than L. baicalensis (2.84) and L. idus (2.97), but lower than L. merzbacheri (3.29). Additionally, the b value of females (3.24) surpasses that of males (3.13), suggesting a greater inclination among females to prioritize weight gain as a means to enhance reproductive investment. This adaptive strategy reflects the population’s response to its environment (Zhao et al., 2023).

The K value is commonly employed to indicate the rate of population growth. Branstetter (1987) categorized the growth coefficient into three groups: 0.05–0.1 represents species with slow growth; 0.1–0.2 represents species with moderate growth; 0.2–0.5 represents species with rapid growth. The K value of L. chuanchicus is 0.461, higher than the documented K values (0.1990–0.3269) (An et al., 2008; Duan et al., 2022; Guo et al., 2005; Hu et al., 2008; Lu et al., 2019) for the Leuciscus genus fishes. The asymptotic total length L_∞ of females (39.2 cm) exceeds that of males (32.7 cm), while their K value is comparatively smaller. Consequently, females require more time to attain their asymptotic total length in comparison to males. This indirectly reflects the prioritization of reproductive capacity and successful reproduction by females, potentially resulting in slower growth rates. Conversely, males are inclined to allocate resources towards growth and competition, leading to accelerated growth rates (Luo et al., 2017). In addition to habitat factors, the size range of captured samples also influences the K value estimation. Therefore, it is crucial to adjust fishing gear and select appropriate fishing locations for obtaining precise parameter estimates (Campana, 2001).

The growth characteristic index (φ) can be utilized for intra-genus comparison of fish growth performance (Munro & Pauly, 1983; Pauly, 1987). The growth characteristic index of L. chuanchicus in this study is 2.820, surpassing the range observed in all known Leuciscus genus fishes (2.277–2.692) (An et al., 2008; Duan et al., 2022; Guo et al., 2005; Hu et al., 2008; Lu et al., 2019). Consequently, the L. chuanchicus is classified as a species exhibiting excellent growth performance among the Leuciscus genus fishes. The growth inflection point age (3.068 $y{r}^{-1}$) for L. chuanchicus was subsequently determined using a mathematical formula. The proportion of individuals at the growth inflection point age (3 $y{r}^{-1}$) in the total catch is only 31.77%, while individuals at age-1 and 2 constitute a larger portion of the overall population (49.15%). This finding suggests that current fishing practices may exert an adverse impact on the population recovery of L. chuanchicus (Ye et al., 2012).

The mortality and exploitation rate of L. chuanchicus

The exploitation rate (E) of fish is a crucial parameter in fisheries management (Yao et al., 2018). It is imperative to maintain the exploitation intensity for fish below 0.5, as surpassing this threshold would result in overexploitation and have detrimental effects on the sustainability of fish stocks (Hu et al., 2012; Sun et al., 2021). In this study, three empirical formulas were employed to calculate the instantaneous natural mortality rate (M). As a result, an exploitation rate (E) of 0.3658 was obtained for L. chuanchicus, which remained within the acceptable range of exploitation intensity (<0.5). The instantaneous natural mortality rate of L. chuanchicus, however, is relatively high; females exhibit a higher natural mortality rate compared to males. The final results reveal that the influence of human fishing activities on the L. chuanchicus is relatively insignificant, with a greater contribution stemming from environmental pressures. The section of the river under discussion is situated within a national aquatic germplasm reserve in China (Hu et al., 2022), where local authorities enforce stringent regulations to combat illegal fishing activities. However, it is important to note that this particular area also serves as a crucial Yellow River irrigation district, providing substantial benefits to a population of approximately 5.6 million individuals (Gao et al., 2023). In the summer, the visibility in this section of the river can be reduced to less than 1 cm, exerting a significant impact on aquatic organisms (Niu et al., 2024), where inadequate regulation has resulted in the escape of farmed species, leading to the establishment of invasive alien species (Vythalingam et al., 2022). The aforementioned factors have contributed to a relatively elevated natural mortality rate of L. chuanchicus. To address this issue, we propose the implementation of the following measures: (1) Efforts to combat illegal fishing and enforce stringent penalties should be strengthened in order to enhance law enforcement. Moreover, the promotion of public awareness and engagement in safeguarding fish resources can be achieved through educational initiatives and outreach campaigns. (2) Monitoring of Yellow River water quality should be enhanced, including measures such as reducing industrial wastewater discharge and controlling agricultural pollutants. In particular, intensified efforts should be made to invest in tributaries management with the aim of improving the water quality of tributaries that flow into the Yellow River. (3) Strengthen the management of alien species, especially in terms of preventing escape and private release control of farmed fish, promptly detect and address issues related to invasive alien species.