Estimates on Age, Growth, Sex Composition, and Mortality of Silurus lanzhouensis (Chen, 1977) in the Upper Yellow River, China

Abstract

Silurus lanzhouensis is a large-sized commercial fish species endemic to the mid-upper reaches of the Yellow River, serving as a “Card of Aquatic Biological Species in the Yellow River”. However, due to factors such as overfishing and habitat changes, it has been listed as an endangered species. In order to protect and restore its wild resources, we conducted a detailed study for the first time from 2022 to 2024 on the age composition, growth characteristics, mortality rate, and current status of resource exploitation of the S. lanzhouensis population in the upper Yellow River. A total of 489 individuals underwent measurements for their total length (L) and body weight (W), with age determination conducted using lapillus otoliths. The collected samples exhibited a spectrum of total lengths spanning from 2.5 to 63.7 cm, body weights ranging from 0.11 to 1974.15 g, and ages ranging from 1 to 6 years. Further analysis of the length–weight relationships unveiled an allometric growth index (b) of 2.9562 for the overall samples, indicating an isometric growth pattern. Additionally, insights into the growth characteristics of S. lanzhouensis were provided by the von Bertalanffy growth function, revealing an asymptotic total length (L_∞) of 119.30 cm and a growth coefficient (K) of 0.1278 yr⁻¹. The growth characteristic index (φ) totaled 3.2598, suggesting a moderate growth rate in comparison to other Silurus species. The total mortality rate (Z) recorded for the population of S. lanzhouensis was found to be 0.5296 yr⁻¹. Through the application of three distinct methodologies on exhaustive samples, the average natural mortality rate (M) was calculated to be 0.3105 yr⁻¹. Consequently, the fishing mortality rate (F) for the entire sample set was determined to be 0.2191 yr⁻¹, leading to an exploitation rate (E) of 0.4137. Based on the survey findings, it is evident that the population of S. lanzhouensis has not been subjected to over-exploitation, attributable to ongoing stock enhancement. These results also provide foundational data for the protection and restoration of S. lanzhouensis in the upper Yellow River.

1. Introduction

The investigation of biological characteristics, including age, growth, mortality, and resource exploitation, is critical for understanding fish growth patterns, population dynamics, and fluctuations in resources, thereby providing essential data and a scientific foundation for fisheries resource management [1,2,3]. In recent decades, the ecological environment of the Yellow River Basin has increasingly deteriorated due to anthropogenic factors such as water pollution, climate change, overfishing, and hydraulic projects [4,5,6]. Fish habitats and the “Three Habitats and One Channel” concept have been compromised to varying degrees, resulting in a significant decline and even depletion of fish population resources, particularly among endemic species [7,8]. Consequently, conducting research on the fundamental biological traits of fish in the Yellow River Basin is vital for the timely monitoring of fish resource status.

Silurus lanzhouensis, classified within the order Siluriformes, family Siluridae, and genus Silurus, is primarily located in the mid-upper reaches of the Yellow River, notably in Gansu Province, the Ningxia Hui Autonomous Region, and the Inner Mongolia Autonomous Region of China [4,9,10]. This species is a large-sized, carnivorous fish of considerable economic importance endemic to these regions of the Yellow River [11]. Renowned for its tender meat, which contains few bones, high yield, and rich protein content, S. lanzhouensis is colloquially esteemed as the “Ginseng of the Yellow River” and has been designated as a “Card of Aquatic Biological Species in the Yellow River” [12,13,14]. In recent decades, the wild populations of this species have progressively declined due to habitat loss, fragmentation, water pollution, and human activities, resulting in its classification as an “Endangered Species” on the Red List of China [15]. Since 2007, National Aquatic Germplasm Conservation Areas for S. lanzhouensis have been established in sections of the mid-upper Yellow River across Inner Mongolia, Ningxia, and Gansu [16]. Subsequent efforts have aimed to enhance and reintroduce S. lanzhouensis in these river segments to protect and restore the wild populations of the species.

Currently, research on S. lanzhouensis predominantly emphasizes morphology [17], conservation genetics [14,18,19], comparative genomics [20], and genetic breeding [13,21]. However, there is a noticeable absence of reports concerning the wild population structure and resource exploitation of S. lanzhouensis. Moreover, plateau aquatic ecosystems, characterized by their simplistic structure, susceptibility to disturbances, environmental fragility, and low productivity, pose significant challenges to the recovery of fish resources once disrupted [22]. In light of the multifaceted impacts stemming from overfishing, environmental changes, and stock enhancement, it is imperative to investigate whether these pressures induce alterations in the phenotypic traits, such as biological indices, of S. lanzhouensis populations. Consequently, this article presents a comprehensive study on the age, growth, mortality, and resource exploitation of S. lanzhouensis, based on fisheries resource surveys conducted along the main stream of the Yellow River, spanning from Longyangxia to Shizuishan between 2022 and 2024. The aims of this study are (1) to delineate the characteristics of the population structure; (2) to clarify the mortality traits and current status of the wild populations; and (3) to provide foundational data for the protection and restoration of S. lanzhouensis in the upper Yellow River.

2. Materials and Methods

2.1. Study Area and Sample Collection

Between July and August 2022; as well as in March, May, and September 2023; and in May and July 2024, fish samples were collected from the upper Yellow River (Figure 1) using three types of fishing gear: cage nets, fixed combined gillnets, and triple-layer drift gillnets. The selection of the appropriate fishing method at each survey site was predicated on the specific environmental characteristics. The specifications for the cage nets were as follows: 15 m in length, 40 cm in width, and 40 cm in height. The fixed combined gillnets measured 30 m in length and either 2 m or 3 m in height, with mesh sizes ranging from 1 to 12 cm. The triple-layer drift gillnets measured 100 m in length and 2 m in height, with mesh openings spanning from 5 to 12 cm. A total of 489 specimens of S. lanzhouensis (Figure 2) were collected, comprising 177 samples in 2022, 110 samples in 2023, and 202 samples in 2024. The fish samples, extracted from the fishing nets in their natural habitat, experienced mortality due to oxygen depletion and water deprivation shortly after removal, as no anesthetic was employed. The biometric data, including total length (L, measured to the nearest 0.1 cm) and body weight (W, measured to the nearest 0.01 g), were recorded while the samples remained in a fresh condition. Gonads were dissected for sex determination. The lapillus otoliths were extracted, placed in petri dishes containing anhydrous ethanol for thorough cleaning to eliminate any epithelial remnants, and subsequently air-dried. They were then placed in 1.5 mL centrifuge tubes and transported to the laboratory for further processing. During the survey, water temperature was measured using a portable water quality analyzer (HACH, Loveland, CO, USA).

2.2. Age Estimation

The lapillus otoliths were embedded and affixed to a slide using transparent nail polish, with their convex surfaces oriented upward. The otoliths were then polished sequentially with water sandpaper, ranging from 800 to 3000 grit, followed by additional polishing with polishing paper. Throughout this process, the otoliths were periodically examined under a microscope to observe their annuli. Once the core of the otolith was made distinctly visible, acetone was employed to dissolve the nail polish, allowing for the otolith to be realigned and re-embedded. The same sanding and polishing procedure was repeated until both the central and peripheral annuli of the otoliths were distinctly discernible.

2.3. The Relationship Between Total Length and Body Weight

The allometric relationship between total length (L) and body weight (W) was modeled using the power function $W=a L^b$, where a is the intercept and b is the slope of the log-transformed relation [23]. A statistical assessment utilizing the t-test, with a significance threshold set at 0.05, was conducted to ascertain deviations in the estimated value of b from the theoretical isometric growth value of “3” [24]. The formula for the t-test is as follows: $t={\displaystyle \frac{\mathrm{S}\mathrm{D}\left(L\right)}{\mathrm{S}\mathrm{D}\left(W\right)}}\times {\displaystyle \frac{\left|b-3\right|}{\sqrt{1-r^2}}}\times \sqrt{n-2}$. Herein, SD (L) and SD (W) stand for the standard deviations of the natural logarithms of the total length and body weight, respectively. Additionally, n represents the sample size, and r represents the correlation coefficient. This method provides insight into whether growth is isometric or allometric.

2.4. Estimation of Growth Equation Parameters

The von Bertalanffy growth function was employed to elucidate the growth dynamics of S. lanzhouensis. The growth in length over time is described by the equation $L_t=L_{\infty }\left[1-e^{-K \left(t-t_0\right)}\right]$, where L_t indicates the total length of the fish at age t (cm), L_∞ is the asymptotic total length (cm), K is the growth coefficient (yr⁻¹), t represents the age of the specimen (yr), and t₀ is the hypothetical age at which the fish’s length approaches zero (yr) [25]. Additionally, the growth characteristic index (φ) was computed using the formula $\phi =\lg K+2 \lg L_{\infty }$, integrating parameters K and L_∞ derived from the von Bertalanffy growth function. Furthermore, to analyze differences in growth trajectories between genders, the adjusted residual sum of squares (ARSS) method was utilized, as described in Chen et al. [26].

2.5. Total Mortality, Fishing Mortality and Exploitation Rate

An age-based catch curve analysis, as delineated in Pauly et al. [27], was employed to evaluate the total mortality rate (Z). Catch curves were constructed by plotting the natural logarithm of the counts of fish within each age class against their respective ages. Only those age classes that were fully recruited to the fishing gear were included in the estimation of Z. The computation involved fitting a linear regression model, “$y=mx+n$”, to the descending limb of the catch curve, where the absolute value of the regression slope (m) signifies Z [28].

The estimation of the natural mortality rate (M) was approached through three distinct methodologies: (1) Pauly’s length-based empirical formula $\ln M=-0.0066-0.279\ln L_{\infty }+0.6543\ln K+0.4634\ln T$ [29] was utilized, integrating variables such as T (the annual habitat temperature, °C), L_∞ (asymptotic total length), and K (growth coefficient) from the von Bertalanffy growth function; (2) Ralston’s age-based method [30] used a regression model, $M=0.0189+2.06K$, where K also represents the growth coefficient in the von Bertalanffy growth function; (3) the technique used by Zhan et al. [31] derived mortality via $M=-0.0021+2.5912/t_m$, where t_m represents the maximally observed age in years.

For the fishing mortality rate (F), it was calculated by subtracting the natural mortality rate (M) from the total mortality rate (Z), thereby yielding the equation $F=Z-M$. Furthermore, the population exploitation rate (E) was determined by the ratio of the fishing mortality rate to the total mortality rate, expressed as $E=F/Z$. These calculations are instrumental in assessing the extent of fishing impacts relative to overall mortality, with the exploitation rate illustrating the proportion of mortality attributable to fishing activities, as presented in Gray et al. [32].

2.6. Statistical Analyses

Statistical analyses were conducted utilizing Microsoft Excel 2017 (Microsoft, Redmond, WA, USA). The Mann–Whitney U test was performed using SPSS Statistics 22.0 (SPSS for Windows, IBM, Armonk, NY, USA) to explore the differences in total length and body weight among S. lanzhouensis of various genders. Furthermore, the chi-square (χ²) analysis in SPSS Statistics 22.0 was employed to confirm significant deviations in the sex ratio of the species under study from the expected 1:1 ratio. The simulation of the von Bertalanffy growth function was carried out with the ggfishplots package in R version 4.3.1 (https://www.r-project.org/, accessed on 15 October 2024). The graphical representations were generated using Microsoft Excel 2017 and GraphPad Prism 8.0 (Graphpad Software, San Diego, CA, USA).

3. Results

3.1. Population Structure

During the survey period, a total of 489 specimens of S. lanzhouensis were collected, comprising 192 females, 150 males, and 147 individuals of indeterminate sex. The sex ratio was reported as 1.28:1 (females to males). There were significant differences between the number of females and males (p < 0.05), and the sex distribution closely approximates a 1:1 ratio (χ² = 5.158, p = 0.023). Female specimens exhibited a total length ranging from 13.8 cm to 63.7 cm, with a mean of 34.83 ± 12.07 cm, while their body weight ranged from 19.17 g to 1974.15 g, with a mean weight of 373.61 ± 390.67 g. Male specimens displayed a total length ranging from 14.5 cm to 63.3 cm, with a mean of 31.16 ± 11.35 cm, and a weight variation from 17.88 g to 1887.51 g, averaging 284.72 ± 382.75 g. Individuals of unknown sex had a total length between 2.5 cm and 15.8 cm, with an average of 8.80 ± 3.15 cm, and their body weight varied from 0.11 g to 22.77 g, averaging 6.32 ± 6.04 g. Across the sampled population of 489 individuals, the mean total length was 25.88 ± 15.08 cm, and the mean body weight was 235.93 ± 358.53 g. The Mann–Whitney U test indicated a significant difference in total length between females and males (Z = −3.060, p < 0.05), with females displaying significantly greater lengths. Similarly, the body weight of females was significantly higher than that of males (Z = −3.113, p < 0.05). Regarding the distribution range, approximately 81.59% of S. lanzhouensis specimens had a total length of less than 40.0 cm (Figure 3a), while approximately 87.32% of them weighed less than 500 g (Figure 3b).

3.2. Age Distribution

After the lapillus otolith of S. lanzhouensis was ground, a distinct ring pattern, characterized by alternating dark and light zones, was revealed under microscopic examination. Notably, the presence of a broad dark zone adjacent to a slender bright zone was indicative of growth ring formation (Figure 4).

Table 1. Numbers of samples and total length (L) and body weight (W) in different ages of Silurus lanzhouensis in the upper Yellow River.

Sex	Age (yr)	n	L (cm)		W (g)
			Min–Max	Mean ± S.D.	Min–Max	Mean ± S.D.
Unknown	1	147	2.5–15.8	8.80 ± 3.15	0.11–22.77	6.32 ± 6.04
Female	1	5	13.8–16.0	14.86 ± 0.82	19.17–27.76	22.74 ± 3.67
	2	59	16.9–29.8	22.13 ± 3.38	28.15–170.98	67.06 ± 30.93
	3	60	25.8–42.0	34.59 ± 3.64	120.72–448.58	276.47 ± 77.65
	4	45	36.9–50.9	43.08 ± 3.46	342.97–838.5	496.37 ± 117.70
	5	16	43.2–59.8	53.81 ± 5.48	575.32–1552.08	1085.38 ± 300.81
	6	7	59.5–63.7	61.71 ± 1.42	1303.83–1974.15	1624.71 ± 225.81
Male	1	6	14.5–16.8	15.53 ± 0.95	17.88–28.65	21.69 ± 3.82
	2	70	17.3–28.4	23.24 ± 3.48	23.89–175.82	72.78 ± 27.32
	3	43	28.1–39.7	33.51 ± 3.35	139.30–423.62	263.80 ± 78.08
	4	19	34.7–47.4	42.96 ± 3.66	320.89–726.80	493.66 ± 129.23
	5	7	48.5–58.2	55.20 ± 3.57	540.61–1498.62	1126.89 ± 349.33
	6	5	60.5–63.3	62.06 ± 1.09	1719.87–1887.51	1773.42 ± 69.10
Total	1	158	2.5–16.8	9.25 ± 3.46	0.11–28.65	7.42 ± 7.15
	2	129	16.9–29.8	22.74 ± 3.47	23.89–175.82	70.17 ± 29.05
	3	103	25.8–42.0	34.14 ± 3.55	120.72–448.58	271.18 ± 77.70
	4	64	34.7–50.9	43.05 ± 3.49	320.89–838.50	495.57 ± 120.20
	5	23	43.2–59.8	54.23 ± 4.94	540.61–1552.08	1098.01 ± 308.80
	6	12	59.5–63.7	61.86 ± 1.25	1303.83–1974.15	1687.10 ± 188.31

provides a comprehensive breakdown of the number of samples, total lengths, and body weights distributed across different sexes and age groups. The age determination analysis revealed that S. lanzhouensis exhibits an age range of 1 to 6 years, encompassing both female and male specimens within this interval. Interestingly, specimens with undetermined sex were exclusively 1 year old. Among female specimens, the predominant age groups were 2 to 4 years, comprising 85.41% of the overall sample. In contrast, the most prevalent age groups for male specimens were 2 and 3 years, accounting for 75.33% of the sample. Collectively, within the analyzed cohort of 489 specimens, ages 1 to 3 years emerged as the most common, constituting 79.75% of the total sample.

3.3. Length–Weight Relationship

The relationship between total length and body weight of S. lanzhouensis was assessed using regression analysis, resulting in the equation $W=0.0072{ L}^{2.9562}$ (R² = 0.9907) (Figure 5). Notably, the b value was 3.0449 for females and 3.0970 for males. A t-test revealed that the b coefficient (2.9562) in the regression equation was not significantly different from “3” (t-test, t = 0.4214, p = 0.3368), indicating that the growth of S. lanzhouensis follows an isometric pattern, whereby total length and body weight increase proportionally.

3.4. Growth Equation

In this investigation, the relationship between total length and age for S. lanzhouensis was characterized using the von Bertalanffy growth function. The total length growth formulas were $L_t = 120.3 \left[1-e^{-0.1267\left(t-0.371\right)}\right]$ for females and $L_t = 119.1 \left[1-e^{-0.128\left(t-0.385\right)}\right]$ for males. To assess the significance of growth disparities between genders, an analysis of residuals sum of squares (ARSS) test was executed. The findings indicated no significant difference in growth trajectories between female and male S. lanzhouensis (ARSS test, F = 0.2452, p = 1.332). Consequently, the unified length growth equation for the total sample was established as $L_t = 119.3 \left[1-e^{-0.1278\left(t-0.377\right)}\right]$, where the L_∞, K, and t₀ values were 119.3 cm, 0.1278 yr⁻¹, and 0.377 yr, respectively (Figure 6). Additionally, the growth characteristic index (φ) for S. lanzhouensis was computed to be 3.2598.

3.5. Mortality and Exploitation Rate

According to Pauly et al. [27], the Z value for S. lanzhouensis was determined to be 0.5296 yr⁻¹ (Figure 7). The average water temperature (T) in the study area was recorded as 12.5 °C. Citing Pauly [29], Ralston [30], and Zhan et al. [31], the natural mortality rates (M) for the aggregated samples were found to be 0.2195 yr⁻¹, 0.2822 yr⁻¹, and 0.4298 yr⁻¹, respectively. By averaging these values, the estimated overall natural mortality rate for the samples was calculated to be 0.3105 yr⁻¹. Subsequently, the fishing mortality (F) for the aggregate samples was computed to be 0.2191 yr⁻¹, and the exploitation rate (E) was ascertained to be 0.4137.

4. Discussion

This study conducted a comprehensive examination of the wild populations inhabiting the mainstream of the upper Yellow River. According to historical records, during the 1970s and 1980s, the largest recorded total length of S. lanzhouensis in the upper Yellow River reached 100 cm, with a corresponding weight of 4000 g [9]. However, the samples obtained in this study exhibited significantly smaller lengths and weights compared to the historical records. It is tentatively inferred that this trend towards reduced sizes may be attributed to the combined effects of prolonged overfishing pressure and habitat degradation [4,5,8]. Furthermore, this study found that the females of this species exhibited a greater overall size. This observed sexual dimorphism, where females are larger than males, is consistent with findings in other teleost species, such as Xenocypris argentea [3], Coregonus ussuriensis [23], Gymnocypris firmispinatus [33], and Lateolabrax latus [34].

The sex composition serves as a significant indicator of population structure in fish species, influencing the reproductive potential and breeding habits of the breeding population and, indirectly, the fluctuations in population numbers [35]. Generally, within a fish population, an increase in the proportion of male individuals tends to diminish reproductive potential and breeding efficiency [36]. For the S. lanzhouensis population in the upper Yellow River, the sex ratio was recorded at 1.28:1 (female to male), which was lower than Gymnodiptychus pachycheilus (1.33:1) [37] in the same waters but higher than the findings for Triplophysa scleroptera (1.04:1) [38], Leuciscus chuanchicus (0.96:1) [39], and Gobio huanghensis (1:1) [40]. Based on these findings, we hypothesize that the S. lanzhouensis population possesses a relatively high level of reproductive potential and efficiency in the upper Yellow River compared to other species in the same aquatic region.

Age is a crucial parameter in the study of fish biology [37,41]. As fish grow, annuli marks are deposited on calcified structures, which facilitate age determination based on these annular characteristics. Commonly used materials for age determination include scales, otoliths, vertebral bones, and opercular bones [42,43]. Numerous studies have shown that otoliths are the most extensively utilized and effective means for age determination [23,40,44]. In this study, otoliths were selected as the material for age verification of S. lanzhouensis, with the characteristics of the otolith annuli distinctly visible, displaying clearly alternating dark and light bands. Subsequently, a detailed analysis of the age structure of the S. lanzhouensis population in the upper Yellow River was conducted. The results showed that the age composition of the S. lanzhouensis population was relatively straightforward and predominantly comprised age groups 1 to 6 years, with individuals aged 1 to 3 years overwhelmingly comprising 79.75% of the population. Therefore, it is deduced that the population structure of S. lanzhouensis in the upper reaches of the Yellow River currently exhibits characteristics of a trend towards a younger age structure and smaller body sizes [22].

The assessment of fishery resources necessitates the conversion of total length and body weight, requiring the determination of the relationship between total length and body weight. The recognized functional relationship is the power function, and the allometric growth index (b) of the power function is a crucial parameter indicating growth characteristics [3,24,32]. As posited by Pauly [24], the b value is indicative of growth patterns in fish; a value approximating “3” suggests isometric growth, while deviations from this value denote allometric growth. In this study, the power function exponent b value for S. lanzhouensis (2.9562) showed no significant difference from “3”, indicating isometric growth within the S. lanzhouensis populations in the upper Yellow River. Specifically, compared to other fishes within the same genus (Table 2), our findings align with those related to S. meridionalis (2.9203) inhabiting the mid-upper reaches of the Yangtze River [45]. This contrasts with the negative allometric growth (2.8602) observed in S. glanis from the Ili River in Xinjiang [46] and is distinct from the positive allometric growth exhibited by S. aristotelis in Greece’s Lake Pamvotis (3.0160 for females, 3.2130 for males) [47].

In this study, the growth coefficient (K) of S. lanzhouensis was 0.1278 yr⁻¹. Compared with other Silurus fishes (Table 2), the K value of S. lanzhouensis is similar to that of S. glanis (0.1050 yr⁻¹) from the Lower Kama Reservoir in Russia [48] but significantly lower than that of S. meridionalis (0.2019 yr⁻¹) [45] and S. aristotelis (0.3700 yr⁻¹) [47]. This indicates that variations in the b value and the K value are evident among different species within the same genus, potentially influenced by factors such as fish size, habitat environments, and inherent genetic characteristics [38,39,49]. The growth characteristic index (φ) integrates the values of L_∞ and K, exhibiting a positive correlation with the growth rate and serving as a metric for comparing the growth performance of fish within the same genus [38,39]. Currently, reports indicate that the φ value for Silurus fishes ranges broadly from 2.6837 to 3.6710. In our study, the φ value was recorded at 3.2598 (♀ and ♂), surpassing that of S. aristotelis (2.6837) [47], yet falling short of S. meridionalis (3.5393) [45] and S. glanis (in three different waters, the φ value were higher than 3.5000) [46,48,50]. In summary, the growth rate of S. lanzhouensis in the upper Yellow River is considered moderate among Silurus fishes, potentially attributable to a combination of factors such as low water temperatures, high sedimentation levels within the aquatic habitat, and restricted access to food organisms in the study area [43,51].

Table 2. Comparison of growth parameters among several Silurus fishes in different studies.

Species	Investigation Area	Sexual	Growth Parameters				Source
			b	L∞ (cm)	K (yr−1)	φ
Silurus meridionalis	Mid-upper Yangtze River, China	♀ and ♂	2.9203	130.95	0.2019	3.5393	[45]
Silurus aristotelis	Lake Pamvotis, Greece	♀	3.0160	36.12	0.3700	2.6837	[47]
		♂	3.2130
Silurus glanis	Yili River, China	♀ and ♂	2.8602	191.10	0.0870	3.5020	[46]
Silurus glanis	Lower Kama Reservoir, Russia	♀ and ♂		191.83	0.1050	3.5870	[48]
Silurus glanis	Menzelet Reservoir, Turkey	♀	3.1295	260.00	0.0640	3.6361	[50]
		♂	2.9133	303.20	0.0510	3.6710
Silurus lanzhouensis	Upper Yellow River, China	♀	3.0449	120.30	0.1267	3.2633	This study
		♂	3.0970	119.10	0.1280	3.2590
		♀ and ♂	2.9562	119.30	0.1278	3.2598

Table 2. Comparison of growth parameters among several Silurus fishes in different studies.

Species	Investigation Area	Sexual	Growth Parameters				Source
			b	L∞ (cm)	K (yr−1)	φ
Silurus meridionalis	Mid-upper Yangtze River, China	♀ and ♂	2.9203	130.95	0.2019	3.5393	[45]
Silurus aristotelis	Lake Pamvotis, Greece	♀	3.0160	36.12	0.3700	2.6837	[47]
		♂	3.2130
Silurus glanis	Yili River, China	♀ and ♂	2.8602	191.10	0.0870	3.5020	[46]
Silurus glanis	Lower Kama Reservoir, Russia	♀ and ♂		191.83	0.1050	3.5870	[48]
Silurus glanis	Menzelet Reservoir, Turkey	♀	3.1295	260.00	0.0640	3.6361	[50]
		♂	2.9133	303.20	0.0510	3.6710
Silurus lanzhouensis	Upper Yellow River, China	♀	3.0449	120.30	0.1267	3.2633	This study
		♂	3.0970	119.10	0.1280	3.2590
		♀ and ♂	2.9562	119.30	0.1278	3.2598

The exploitation rate of fish is a crucial parameter in fishery management. According to Gulland [52], a rate of 0.5 is often regarded as a benchmark for sustainable fishing practices. In this study, we used three different methods to determine a relatively accurate range for the true value of the natural mortality rate [29,30,31]. The results of these multiple evaluation methods ensure the accuracy of the assessment, indicating that the natural mortality rate of S. lanzhouensis in the upper Yellow River was 0.3105 yr⁻¹. Our findings indicate that the resource exploitation rate of S. lanzhouensis was 0.4137, suggesting that its resource availability has not been over-exploited, as it is below the critical threshold of 0.5. Currently, the resource exploitation status of various fish species in the upper Yellow River has been investigated, revealing that the resource exploitation rates exceed 0.5 for species such as T. scleroptera [38], L. chuanchicus [39], and G. huanghensis [40], while the rate for Triplophysa pseudoscleroptera [43] remains below 0.5. Moreover, investigations in recent years have shown that stock enhancement aimed at promoting the proliferation of S. lanzhouensis has been consistently implemented in the upper Yellow River, specifically in the Lanzhou, Zhongwei, Yinchuan, and Shizuishan sections. The annual stocking totals approximately one million individuals, each with a total length exceeding 5 cm. These continuous stock enhancement efforts have effectively replenished the wild population of S. lanzhouensis in the upper Yellow River, thus preventing the over-exploitation of this species.

In addition, we found that in the Red List of Threatened Species published by the International Union for Conservation of Nature (IUCN) in 2024, S. lanzhouensis was classified as a near-threatened (NT) species [53]. Compared with its classification as an “Endangered Species” in the Red List of China in 2009 [15], its endangerment level has decreased, which is consistent with the findings of our research. It can be reasonably inferred that the continuous stock enhancement and resource protection efforts have effectively increased the population resources of S. lanzhouensis. As a result, the conservation strategies for S. lanzhouensis in the upper Yellow River focus on stocking enhancement and strict law enforcement. Specifically, the following recommendations are put forward: (1) enhancing the scale and quantity of S. lanzhouensis stock enhancement; (2) implementing a fishing ban in parallel to combat illegal fishing practices; and (3) strengthening public education campaigns to raise awareness of conservation issues. Ultimately, these initiatives are aimed at promoting the sustainable recovery of the S. lanzhouensis population and maintaining the ecological integrity of the Yellow River Basin.

5. Conclusions

This study carried out a comparative analysis of the age, growth, sex composition, mortality, and exploitation rate of S. lanzhouensis in the upper Yellow River. The findings showed that both female and male S. lanzhouensis are estimated to have a maximum age of 6 years. Through an assessment of the correlation between total length and body weight, we determined that this species follows an isometric growth pattern. Moreover, when compared with other fishes in the genus Silurus, the growth rate of S. lanzhouensis in the upper Yellow River is considered moderate. Significantly, our results suggest that the population of S. lanzhouensis has not been subjected to over-exploitation, which can be ascribed to the continuous stock enhancement efforts.