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Article

The Status of the Early-Stage Fish Resources and Hydrologic Influencing Conditions in the Guiping Section of the Xunjiang River

by
Huifeng Li
1,2,3,
Weitao Chen
1,2,3,
Dapeng Wang
4,5,
Xiaoyu Lin
1,2,3,
Li Yu
1,2,3,
Chengdong He
1,2,3,
Jie Li
1,2,3,* and
Yuefei Li
1,2,3,*
1
Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Guangzhou 510380, China
2
Key Laboratory of Aquatic Animal Immune Technology of Guangdong Province, Guangzhou 510380, China
3
Guangzhou Scientific Observing and Experimental Station of National Fisheries Resources and Environment, Guangzhou 510380, China
4
Guangxi Academy of Fishery Sciences, Nanning 530021, China
5
Engineering Research Center of Hongshui River Rare Fish Conservation, Guangxi Zhuang Autonomous Region, Nanning 530021, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(13), 5930; https://doi.org/10.3390/su17135930
Submission received: 13 May 2025 / Revised: 22 June 2025 / Accepted: 25 June 2025 / Published: 27 June 2025
(This article belongs to the Special Issue Methods and Practices for the Sustainable Management of Lake and River Hydrological Systems)
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Abstract

To investigate the species composition, reproductive dynamics, and hydrological drivers of fish resources in the early stage in the Guiping section of the Xunjiang River, we conducted a two-year survey (2022–2023) downstream of the Datengxia Dam. A total of 22,464 fish eggs and larvae were collected, representing 6 orders, 17 families, and 67 species, with Cyprinidae (58.2%) as the dominant family. Dominant species included Squaliobarbus curriculus, Gobiidae, Hemiculter leucisculus, and Culter, exhibiting significant interannual variation in abundance. The breeding season peaked from May to September, accounting for 94.6% of annual recruitment. Hydrological conditions strongly influenced reproductive output: the multiple flood pulse periods in 2022 (peak discharge: 29,000 m3/s) yielded 34.997 billion eggs and larvae, whereas reduced flows in 2023 (peak discharge: 12,200 m3/s) led to a 75.4% decline (8.620 billion). Redundancy analysis (RDA) revealed that discharge, water temperature, natural hydrological data, and dissolved oxygen were the primary environmental drivers, explaining 46.11% of variability in larval abundance (p < 0.001). Notably, the proportion of important economic fish, “four major Chinese carps”, plummeted from 4.9% (2022) to less than 0.1% (2023), indicating spawning ground function degradation. Our results demonstrate that flood pulses are essential for sustaining fish recruitment, particularly for pelagic spawning riverine fish like the four major Chinese carps. Their proportion plummeted to less than 0.1% in 2023, highlighting the urgent need for eco-hydrological management in the Xunjiang River.

1. Introduction

Ichthyoplankton, including fish eggs and larvae, represent the initial life stage of fish [1]. Research on ichthyoplankton dates to the late 19th century, initially focusing on species identification, sampling techniques, and spawning habits. The International Council for the Exploration of the Sea (ICES) has long emphasized the importance of ichthyoplankton surveys for identifying major economic fish species’ spawning grounds [2]. Early studies, such as Blaxter’s 1963 work on Clupea pallasi embryonic development [3], highlighted the correlation between temperature and development rate. Subsequent research by Buckley [4], Miller [5], and Kendall [6] further advanced our understanding of fish resources. Since then, numerous countries and regions have conducted extensive ichthyoplankton research to explore the ecological habits, growth patterns, and reproductive ecology of various fish species. For example, Brazil has documented over 120 freshwater fish larvae species between 1977 and 2018 [7]. In China, research has primarily focused on the Yangtze River and a few other rivers. Wang’s large-scale surveys in the Yangtze River basin [8] and Liang et al.’s description of early development in 49 Xijiang River fish species [9] are notable contributions. Academician Cao’s publication on the early life stages of Yangtze River fish species provides detailed morphological data and identification characteristics [1]. The advent of online platforms like LarvalBase has revolutionized data sharing and collaboration in ichthyoplankton research. In the Xijiang River, studies have gained attention in the past 20 years, revealing diel variations in larval fish density and species composition [10], as well as characterizing community diversity and population dynamics [11].
Investigating fish eggs and larvae is crucial for understanding fish reproductive patterns, identifying spawning grounds, and determining breeding timelines. The abundance and composition of ichthyoplankton directly influence population dynamics and recruitment success [12,13,14]. However, anthropogenic disturbances, particularly large-scale dam construction, have severely impacted early-stage fish resources. For instance, the Three Gorges Dam on the Yangtze River has significantly disrupted the reproduction of major Chinese carp species due to altered hydrological regimes [15,16]. Historical records show that during the 1960s, the middle Yangtze supported 25 spawning grounds for these carp species, accounting for 75% of the river’s fish eggs and larvae [17]. Post-impoundment monitoring revealed a 75% reduction in reproductive output and a 20-day delay in spawning events [18,19]. Similar impacts are observed in the Pearl River Basin, where the Guiping section of the Xunjiang River is a critical spawning and nursery habitat for the four major Chinese carps [20,21,22]. The Datengxia Hydropower Station’s impoundment led to a 12.3% flow reduction downstream, causing larval abundance to plummet by 69.4% [23], mirroring the Yangtze River’s post-dam spawning reduction [24,25].
In the study area, human activities, especially the construction and operation of the Datengxia Water Conservancy Hub, have significantly altered natural hydrological processes. Since 2020, the downstream flow has been reduced, with a maximum discharge of 29,000 m3/s in 2022 and 12,200 m3/s in 2023 [26]. Climate change has also affected regional precipitation patterns, impacting river discharge and water temperature. These changes likely have profound effects on fish reproduction and early resource supplementation [27,28,29,30,31]. To address conservation concerns during the Datengxia Dam’s initial impoundment phase, we conducted a comprehensive survey of fish eggs and larvae in the Guiping section. This study aims to quantify current spawning output and dynamics, analyze reproductive scale and temporal changes using historical data, and examine the impact of hydrological factors on dominant fish species’ reproduction. The data collected provide essential baseline information for future ecological scheduling and conservation strategies in the Datengxia Reservoir and the broader Pearl River Basin.

2. Materials and Methods

2.1. Sampling Methods

From March 2022 to December 2023, a sampling transect was established at Jiangkou Town, Guiping City (110.178711° E, 23.469114° N), approximately 40 km downstream of the Datengxia Dam (Figure 1). The survey followed standard methodologies for assessing fish resources in the early stage. Sampling was conducted using passive drift nets, in accordance with “Technical specification for drifting fish eggs and larvae sampling in river” [32]. The drift net featured a rectangular mouth (1.5 m wide × 1 m high, opening area: 1.5 m2) and a 6 m-long net body with a mesh size of 500 µm. The rear end of the drift net was connected to a collection box (80 cm × 40 cm × 40 cm, mesh size: 300 µm). During deployment, the net mouth was secured against the current, ensuring full submersion and perpendicular alignment to water flow. Sampling was performed every other day from January to December, with three 1-h sampling periods per day (06:00–07:00, 12:00–13:00, and 18:00–19:00). Concurrently, environmental factors such as hydrological discharge, dissolved oxygen, water temperature, total ammonia nitrogen, conductivity, and natural hydrological date were recorded.

2.2. Identification of Fish Eggs and Larvae Species

Ethanol-fixed samples were used for molecular identification, whereas formaldehyde-fixed samples were used for morphological analysis [33]. Fish eggs were identified through molecular sequencing, while larvae were classified based on morphological features. Morphological identification relied on the monograph Early Life Stages of Fish in the Yangtze River [34] and laboratory-accumulated morphological data. For molecular identification, a comprehensive database was created by integrating the laboratory’s existing 12S rRNA gene database with sequences from the studied river sections [35]. Sequencing was performed using universal mitochondrial 12S rRNA gene primers, yielding fragments approximately 300 base pairs in length. Operational Taxonomic Units (OTUs) were defined from the mixed sequencing data (each sample containing approximately 500,000 sequences). Sequences with a similarity greater than 97% and an e-value less than 10−5 were retained to ensure accurate species identification [15,36].

2.3. Data Processing and Analysis

2.3.1. Calculation of Fish Density and Abundance

The density of fish resources in the early stage was standardized as the number of fish eggs or larvae (ind./100 m3) filtered through the passive net mouth per 100 m3 of water. Daily average density was derived from the arithmetic mean of the three daily sampling sessions. Monthly mean density was calculated by averaging the daily density for the respective month [13,37]. The total abundance of eggs/larvae passing through the sampling transect during each collection period was computed using the following formula:
N t = m × Q S × V
where: Nt = Total number of eggs/larvae passing through the transect during sampling (ind.); m = Number of eggs/larvae collected during sampling (ind.); Q = Average river discharge during sampling (m3/s); S = Net mouth area (m2); V = Mean water velocity through the net mouth (m/s).
The number of eggs and larvae collected each day was used to calculate the average number of eggs and larvae passing through the sampling section per hour on the sampling day. The average number of fish larvae passing through the sampling section per hour during the sampling period was multiplied by 24 h to obtain the total number of fish larvae for the day. The average daily total of fish larvae for each month was calculated by taking the average of the daily totals and multiplying by the number of days in that month. The annual total of fish larvae was obtained by summing the monthly totals for the 12 months.

2.3.2. Determination of Spawning Ground Location

The location of the spawning ground was determined by estimating the development duration of fish eggs based on their developmental stage and water temperature, and then calculating the drifting distance of the eggs using the average water flow velocity in the surveyed river section in accordance with “Stock assessment for drifting fish eggs and larvae in river”. The formula used for the calculation is as follows:
L = V × T
where L represents the drifting distance of the fish eggs, in meters (m); V represents the average water flow velocity in the surveyed river section, in meters per second (m/s); T represents the time experienced during the development of the fish eggs, in seconds (s).

2.3.3. Analysis Methods for Fish Breeding Season and Hydrological Factors

To examine the relationship between daily abundance and environmental factors, data on eggs and larvae (collected during the breeding period from May to October), concurrent hydrological variables (water temperature, dissolved oxygen, ammonia nitrogen, natural hydrological data, turbidity, and discharge) were analyzed [17,30]. Detrended Correspondence Analysis (DCA) was first performed before redundancy analysis, and the length of gradient (LGA) of each axis was less than 3. Therefore, the redundancy analysis (RDA) model was used in this study to analyze the relationship between daily recruitment of larvae and environmental factors. To meet the prerequisites for analysis, the following data processing methods were employed: the response variable data (larval fish abundance) was Hellinger-transformed; the explanatory variable data (environmental factors) used the log10( ) method, transformed to approximate a normal distribution more closely. The significance of the model was tested through 999 random permutations to evaluate the overall explanatory ability of the RDA model.

3. Results

3.1. Species Composition

During the 2022–2023 survey period, we collected a total of 22,464 fish eggs and larvae at Jiangkou Town in the Guiping section (Supplementary Material Table S1). The 2022 samples comprised 11,520 individuals (1151 eggs and 10,369 larvae), while 2023 yielded 10,944 individuals (41 eggs and 10,903 larvae). Through integrated analysis of 60 ethanol-preserved samples (33 from 2022, 27 from 2023) using both high-throughput sequencing and morphological identification, we identified 52 species annually, with a cumulative total of 67 species across both years. These species represented 6 orders, 17 families, and 54 genera. Cyprinidae constituted the most diverse family (39 species, 58.2%), followed by Gobiidae (6 species), Cichlidae (4 species), Bagridae and Cobitidae (3 species each), and 12 other families (Engraulidae, Synbranchidae, Mastacembelidae, Hemiramphidae, Siluridae, Loricariidae, Sisoridae, Osphronemidae, Channidae, Eleotridae, Percichthyidae, and Anabantidae), each represented by a single species.

3.2. Dominant Species

3.2.1. Interannual Variation in Dominant Species Composition

The dominant species composition showed significant interannual variation (Figure 2). In 2022, Squaliobarbus curriculus dominated the recruitment community (40.1%), followed by Gobiidae (12.0%). By 2023, Gobiidae increased dramatically to 66.0% while Squaliobarbus curriculus declined to 13.9%. Other notable species included Oreochromis, Hypophthalmichthys molitrix, and Cyprinus carpio. Particularly striking was the decline of the “four major Chinese carps” from 4.9% in 2022 to less than 0.1% in 2023. These variations reflect a fundamental transition in community structure from multi-species coexistence in 2022 to single-taxon dominance in 2023.

3.2.2. Seasonal Recruitment Patterns

The reproductive activities of fish are primarily concentrated from May to September. During this period, the fish account for 94.6% of the annual total, highlighting this period as a crucial time for sustaining fish population resources. As shown in Figure 3, the analysis of the recruitment numbers of the main dominant fish species reveals that Squaliobarbus curriculus, Gobiidae, and Hemiculter leucisculus are the primary contributors to the breeding season’s composition of the eggs and larvae. Specifically, Squaliobarbus curriculus exhibited a significant resource advantage in 2023, with proportions reaching 88.96% in August and 85.04% in June. Gobiidae demonstrated an absolute dominance in June 2022 (83.52%) and further increased to 90.68% in September 2023. In contrast, the recruitment of major economic fish species such as Hypophthalmichthys molitrix was extremely low, with only a brief appearance in May 2023 (49.29%), while Cirrhinus molitorella and Megalobrama hoffmanni accounted for less than 1%.

3.2.3. Temporal Patterns of Recruitment Peaks

From a temporal perspective, the recruitment peaks of different dominant fish species exhibit distinct patterns. Squaliobarbus curriculus formed a continuous peak from June to August 2023, with proportions ranging from 59.34% to 88.96%. Gobiidae, on the other hand, had two distinct peaks in June 2022 (83.52%) and September 2023 (90.68%). The recruitment of Hemiculter leucisculus was relatively stable, contributing significantly in June 2022 (1.7%) and June 2023 (10.04%). Notably, the non-native species Oreochromis niloticus formed recruitment groups in June 2022 and from May to July 2023, with proportions ranging from 0.83% to 5.72%, indicating potential ecological competition pressure. Overall, the recruitment proportion of all dominant species remained above 80% during the breeding season, underscoring their pivotal role in maintaining the recruitment of fish resources in the Xunjiang River.

3.2.4. Linkage to Hydrological Conditions

These structural changes are closely linked to hydrological conditions. The significant flood process from the end of May to mid-July 2022, with a maximum discharge of 29,000 m3/s, corresponded to a rich community composition. In contrast, 2023 lacked major flood peaks, with only slight rises in water levels in late June and late August, reaching a maximum discharge of 12,200 m3/s. The insufficient hydrodynamic conditions led to a sharp decline in the recruitment of drifting fish, resulting in a community dominated by Gobiidae. This pattern reflects the profound impact of hydrological changes (such as the impoundment of the Datengxia Reservoir) on fish reproductive strategies, where the more adaptable Gobiidae and Squaliobarbus curriculus gradually replaced traditional economic fish species to become the dominant groups.

3.3. Hydrological Dynamics and Ichthyoplankton Recruitment Response

3.3.1. Hydrological Conditions and Fish Recruitment in 2022

In 2022, the Guiping section of the Xunjiang River experienced one significant flood event, which occurred from the end of May to late July. During this period, there were multiple flood peaks, with the maximum discharge recorded on 14th June, when the discharge at the Guiping Dahuangjiangkou Hydrological Station reached 29,000 m3/s. From late May to early July, the runoff volume remained above 10,000 m3/s. The dynamics of fish resources in the early stage in 2022 are shown in Figure 4a. As can be seen from the figure, eggs and larvae were present from March to December, with the peak period occurring from mid-to-late July to early September. The density of eggs and larvae was relatively low during other periods. The highest peak density occurred on 1st September, with a value of 770.4 ind./100 m3. The proportion of the total number of fish eggs and larvae in each month to the total annual supplementation population is shown in Figure 4. June had the highest proportion at 29.7%, followed by July at 25.1%, and September at 22.0%. The combined proportion for June to September accounted for 88.9% of the total annual egg and larval supplementation population. The remaining months together accounted for only 11.1% of the total annual supplementation population.

3.3.2. Hydrological Conditions and Fish Recruitment in 2023

In 2023, the Guiping section of the Xunjiang River had no significant flood pulses. There were only two minor rises in water discharge, occurring in late June and from late August to mid-September. The maximum discharge was recorded on 26th June, when the discharge at the Guiping Dahuangjiangkou Hydrological Station reached 12,200 m3/s, which was only 42.1% of the maximum discharge in 2022. The dynamics of fish resources in the early stage in 2023 are shown in Figure 4b. As can be seen from the figure, the fish eggs and larvae were concentrated from late April to late November, with a relatively low density of fish eggs and larvae before mid-April and after December. The highest peak density occurred on 27th June, with a value of 114.24 ind./100 m3.

3.3.3. Monthly Distribution of Fish Eggs and Larvae

The proportion of the total number of fish eggs and larvae in each month to the total annual supplementation population is shown in Figure 5. June had the highest proportion at 65.9%, followed by August at 15.3%, and September at 10.0%. The combined proportion for June to September accounted for 94.3% of the total annual egg and larval supplementation population. The remaining months together accounted for only 5.7% of the total annual supplementation population. Fish eggs were mainly concentrated from March to September, with 6–9 months accounting for 92.2% of the total annual egg abundance. March, April, and May accounted for 0.9%, 4.3%, and 2.6% of the total annual egg abundance, respectively.

3.4. The Location and Scale of Spawning Ground

In 2022, the total number of eggs and larvae passing through the Guiping section was 34.997 billion individuals, including 1.19 billion fish eggs. Analysis of the developmental stages of the collected fish eggs indicated that 92.1% of the eggs were in the late gastrula to somite appearance stages, with a development duration of approximately 8–12 h. Based on the average flow velocity of the river, the spawning grounds were estimated to be concentrated 24–36 km upstream of the sampling point, mainly in the section from the Dongta spawning ground to downstream of the Datengxia Dam. The spawning scale was 1.09 billion eggs.
In 2023, the total number of eggs and larvae passing through the Guiping section was 8.62 billion individuals, including 0.15 billion fish eggs. Analysis of the developmental stages of the collected fish eggs showed that 86% of the eggs were in the otic vesicle to muscle effect stages, with a development duration of approximately 18–22 h. Based on the average flow velocity of the river, the spawning grounds were estimated to be concentrated 54–80 km upstream of the sampling point, mainly in the Qianjiang River section upstream of the Datengxia Dam. The spawning scale was 0.13 billion eggs, with only approximately 2 million eggs in the river section downstream of the Datengxia Dam.

3.5. Relationship Between Fish Resources and Hydrological Environment

Redundancy analysis revealed a highly significant correlation between the density of fish resources and environmental factors (p < 0.001). Specifically, water temperature (p < 0.001), dissolved oxygen (p < 0.001), discharge (p < 0.001), and natural hydrological data (p < 0.001) were identified as the primary environmental factors influencing fish density in this region. The first canonical axis accounted for 87.81% of the variance, while the second canonical axis contributed 7.93%. The total fish density, as well as the density of Oreochromis and Hemiculter leucisculus, exhibited positive correlations with discharge and ammonia nitrogen but negative correlations with pH. In contrast, the densities of Gobiidae and Squaliobarbus curriculus were positively correlated with water temperature and natural hydrological data but negatively correlated with dissolved oxygen and electrical conductivity (Figure 6).
The RDA model was validated through 999 permutations, with an overall explanatory power of 46.11% and a significance level of p < 0.001, indicating a highly significant model fit. A hierarchical partitioning analysis of the environmental factors revealed that discharge had the greatest relative contribution to the recruitment of fish resources, accounting for 79.29%. The relative importance of the remaining environmental factors, in descending order, was as follows: dissolved oxygen (5.96%), natural hydrological data (5.28%), ammonia nitrogen (4.52%), and water temperature (4.19%). Hierarchical partitioning analysis further confirmed the dominant role of discharge in the supplementation of fish resources in the early stage, a result consistent with Zhou et al. [38,39,40], indicating that hydrological processes are the core factors regulating fish reproduction.

4. Discussion

4.1. The Dynamics of Fish Eggs and Larvae

The Guiping section of the Xunjiang River is one of the most important fish breeding areas in the Pearl River Basin. Understanding the spatial-temporal distribution patterns of fish eggs and larvae in this area is of great significance for comprehending the ecological functions of fisheries in this region. The survey results from 2022 to 2023 showing that the fish resources in the early stage were present throughout the year, but the supplementation density exhibited seasonal differences, mainly concentrated from May to September, with the highest density occurring in June and July. The peak density in 2022 was 770.4 ind./100 m3, while in 2023 it was 114.24 ind./100 m3. This is closely related to the biological and environmental factors in the Guiping section, which is in the subtropical climate zone with abundant summer rainfall, suitable water temperatures, and high levels of reproductive hormones [41]. The breeding season also varies due to interannual hydrological fluctuations. In 2022, a typical flood event occurred, while in 2023, the flood event was weaker, resulting in a significant decrease in the total amount of fish resources in the early stage.
In terms of dominant species, the proportion of Gobiidae increased from 12.0% in 2022 to 66.0% in 2023. In contrast, Squaliobarbus curriculus decreased from 40.1% in 2022 to 13.9% in 2023. This change may be due to differences in hydrological conditions, as well as changes in the parent fish population or degradation of spawning ground functions [42,43,44]. Overall, the dominant species that frequently appeared in both years, such as Squaliobarbus curriculus, Gobiidae, Hemiculter leucisculus, and Culter, indicate that these groups have a stable breeding base in the Xunjiang system and are important species for maintaining population stability. At the same time, fish with different reproductive strategies show heterogeneity in different time periods. For example, fish that lay drifting eggs are concentrated during the rising water period, while those that lay sticky and sinking eggs often spawn before or after the rising water period.
Our study found that the proportion of the four major Chinese carps dropped to 4.9% in 2022 and further declined to less than 0.1% in 2023, indicating severe challenges of insufficient replenishment and population endangerment for these economically valuable species. The main causes include the altered hydrological rhythms and reduced flood pulses due to the construction and operation of the Datengxia Water Conservancy Hub, degradation of spawning grounds by human activities, and intensive fishing that decreased adult populations [45,46]. To address these issues, we recommend the following conservation measures: (i) Implement eco-hydrological scheduling to mimic natural flood rhythms and restore favorable spawning conditions; (ii) protect and restore critical spawning grounds, such as the Dongta spawning ground, by establishing no-fishing zones and reducing human disturbances; (iii) conduct stock enhancement programs by releasing hatchery-reared juveniles to augment wild populations; and (iv) continuously monitor fish populations and their habitats to assess the effectiveness of conservation measures and further investigate their reproductive ecology. These actions will help restore the populations of the four major Chinese carps and provide a scientific basis for ecological conservation.

4.2. The Promotive Effect of Flood Pulses on Fish Reproduction and Interannual Differences

Flood pulses are key hydrological phenomena that regulate fish reproduction and resource replenishment, directly influencing the density of eggs and larvae and the structure of dominant groups [39,47]. Our study, spanning an entire year, revealed distinct seasonal patterns in flood events, with the majority occurring from May to July. This period coincides with the peak reproductive activity of fish in the Guiping section of the Xunjiang River.
In 2022, the Guiping section experienced a typical flood process from the end of May to July, characterized by multiple flood peaks and a maximum discharge of 29,000 m3/s. These flood pulses triggered synchronous spawning among fish, resulting in a large number of drifting fish eggs and larvae. The period from June to September accounted for 88.9% of the annual total recruitment, indicating that flood events significantly enhanced fish reproduction. In contrast, 2023 saw only two moderate rises in water levels, with a maximum discharge of 12,200 m3/s. The reduced frequency and magnitude of flood events led to a significant decline in fish recruitment, with the annual peak density decreasing by about 85%. The total resource amount dropped from 34.997 billion to 8.62 billion individuals. This stark difference highlights the critical role of flood pulses in fish reproduction. Our analysis also revealed that the scale of fish reproduction is more closely related to the occurrence and magnitude of flood events than to mere changes in water flow volume. Flood pulses not only expand spawning space and provide more habitats but also improve water quality and promote synchronous spawning and migration of parent fish. A stable flood peak period is conducive to higher embryo hatching success rates and larval survival rates [28,48,49]. When hydrological processes are insufficient, as in 2023, even suitable conditions fail to trigger large-scale reproductive responses.
Moreover, the interannual shift in spawning ground locations reflects the impact of flood events. In 2022, spawning grounds were located 24–36 km upstream of the sampling point, while in 2023, they shifted to 54–80 km upstream, indicating a decline in the spawning function of the river section downstream of the Datengxia Dam. This shift underscores the importance of flood pulses in maintaining spawning grounds and fish reproduction. In conclusion, flood events play a decisive role in fish reproduction, and their seasonal patterns and magnitude significantly influence fish recruitment. Future research should further explore the relationship between flood events and fish reproduction to inform effective eco-hydrological management strategies.

4.3. Multivariate Analysis of the Impact of Environmental Factors on the Density of the Fish Resources in Early Stage

The present study employed RDA to investigate the relationship between fish resources in the early stage and environmental factors. The results indicated a highly significant correlation between fish density and discharge, water temperature, dissolved oxygen, and natural hydrological data (p < 0.001), which were identified as the key environmental factors influencing fish density. The RDA analysis revealed that discharge had the greatest relative contribution to fish resources, accounting for 79.29%, while the relative contributions of dissolved oxygen, natural hydrological data, and ammonia nitrogen were 5.96%, 5.28%, and 4.52%, respectively. Moreover, the total density of fish eggs and larvae, as well as the density of Oreochromis and Hemiculter leucisculus, were positively correlated with discharge and ammonia nitrogen but negatively correlated with pH. In contrast, the density of Gobiidae and Squaliobarbus curriculus was positively correlated with water temperature and natural days but negatively correlated with dissolved oxygen and electrical conductivity. These findings suggest that density responds differently to various environmental factors, with discharge and water temperature being the core driving factors.
From an ecological perspective, the significant impact of discharge on the density of fish eggs and larvae is likely related to fish spawning behavior. High flow conditions typically enhance dissolved oxygen levels in the water and provide more suitable habitats for fish eggs and larvae, thereby promoting the recruitment of fish resources. Additionally, the natural hydrological data may reflect the synchronization of fish reproduction and historical hydrological rhythms (e.g., seasonal rainfall triggers spawning), and its contribution (5.28%) indicates that maintaining the natural hydrological cycle is essential for ecological regulation. Related research [15,17] shows that water temperature plays a crucial role in fish reproduction and development. Suitable water temperatures can promote gonadal development in fish, accelerate the hatching speed of eggs, and increase the survival rate of larvae. However, excessively high water temperatures may lead to reduced dissolved oxygen levels in the water, which can negatively impact the density of fish eggs and larvae. Moreover, the availability of dissolved oxygen directly affects the physiological metabolism and survival of fish larvae. The negative correlation between dissolved oxygen and the density of fish eggs and larvae for some fish species in this study may reflect potential inhibitory effects under high concentrations, possibly related to oxidative stress or changes in ecological niches.
It is important to note that the RDA model in this study did not incorporate biological factors (e.g., competition and predation) or underwater structural factors (e.g., riverbed morphology and obstacles) [50,51,52,53]. These non-hydrological factors may also play significant roles in the dynamics of the density of fish resources in the early stage. For example, riverbed morphology and obstacles can provide habitats and protection for fish eggs and larvae, while predation pressure can directly affect the efficiency of early resource replenishment. Future studies should consider integrating these factors into the model to gain a more comprehensive understanding of the ecological response mechanisms of the recruitment of fish resources.
Consistent with findings from similar studies in other regions, our results confirm that discharge, water temperature, dissolved oxygen, and water quality indicators are key environmental variables influencing the recruitment of fish resources. As part of a subtropical river ecosystem, the Guiping section of the Xunjiang River shares commonalities in fish reproductive ecology with other large rivers [15,17,29,37,54] but also exhibits greater sensitivity to human-induced disturbances. Therefore, establishing long-term monitoring of environmental factors and developing multivariate ecological models are essential for the conservation of fishery resources and ecological restoration.
Compared with other large river ecosystems, the early-stage fish resources exhibit certain similarities in ecological changes. For instance, similar to the Yangtze River basin [44,55], fish reproduction in the Xunjiang River is significantly influenced by hydrological conditions, with flood pulses playing a crucial role in fish reproduction and resource supplementation. However, the fish community structure in the Xunjiang River shows higher diversity but a declining proportion of economically valuable species. This ecological change trend is consistent with the monitoring results of other middle reaches of the Xijiang River, such as the Fengkai section [46], indicating that fish resources in the Pearl River Basin are undergoing an ecological shift characterized by stable diversity but weakened economic value. Moreover, compared with studies in countries like Brazil [56], research on early fish resources in the Xunjiang River still needs to be further deepened in terms of species identification and ecological response mechanisms. Future studies should strengthen comparative research with other river basins to better understand the universality and specificity of river ecosystems under global change.

5. Conclusions

This study systematically analyzed the species composition, dominant species dynamics, hydrological responses, and relationships with environmental factors of the recruitment of fish resources in the Guiping section of the Xunjiang River from 2022 to 2023. The results show that fish eggs and larvae in this section are diverse, with significant interannual variations, especially in the structure of dominant species and the total amount of resources, which exhibit distinct hydrological driving characteristics. The flood event in 2022 significantly promoted the recruitment of fish resources, while the insufficient flood event in 2023 led to a sharp decrease in the total amount of resources and an obvious upstream shift in spawning grounds. Water temperature, discharge, and dissolved oxygen are the main factors affecting the density of fish eggs and larvae. Meanwhile, the proportion of the four major Chinese carps has significantly decreased, indicating that the degradation of spawning ground functions in this area has begun to restrict important economic species.
Overall, the Guiping section of the Xunjiang River still has important breeding ecological value in the Xijiang River Basin. It is necessary to carry out multidimensional protection combining eco-hydrological scheduling, habitat restoration, resource enhancement, and release. However, we acknowledge that our study has limitations, particularly regarding the sampling design. Due to resource and time constraints, we conducted our research at a single sampling site, which is known for its ecological significance in fish spawning and larval development. While this site allowed us to capture seasonal and short-term variations in fish reproductive activities through long-term monitoring, we recognize that the data may not fully represent the broader ecological dynamics of the entire river section. We suggest that future studies consider sampling at multiple locations to enhance data representativeness and reliability. Additionally, given the interannual variability in hydrological conditions and fish recruitment, a longer study duration would provide a more comprehensive understanding. We recommend that future research further deepen the study of the response mechanisms between early life stages of fish and their habitats, construct multi-factor ecological regulation models, and promote the sustainable management and restorative governance of fishery resources in the Xijiang River Basin.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17135930/s1, Table S1: Species composition of eggs and larvae in the Guiping section in 2022 and 2023.

Author Contributions

Conceptualization, W.C., X.L., L.Y., J.L. and Y.L.; Data curation, W.C., X.L., L.Y., J.L. and C.H.; Formal analysis, H.L. and W.C.; Funding acquisition, W.C., L.Y. and Y.L.; Investigation, X.L. and J.L.; Methodology, H.L., W.C., L.Y. and J.L.; Project administration, D.W., L.Y. and Y.L.; D.W., W.C., X.L. and L.Y.; Software, H.L., D.W., L.Y. and J.L.; Supervision, D.W., L.Y. and Y.L.; Validation, H.L. and W.C.; Visualization, D.W., X.L. and L.Y.; Writing—original draft, H.L.; Writing—review and editing, X.L. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Pearl River Fisheries Resources Investigation and Evaluation Group Project (2023TD10), and the Pearl River Fishery Resources and Habitat Investigation Project funded by the Ministry of Agriculture and Rural Affairs; Guangzhou Municipal Science and Technology Program Project (Grant No. 2025A04J3579): Remote Sensing Identification and Suitability Assessment of Key Fish Habitats—A Case Study of Spawning Grounds of Megalobrama hoffmanni in the Xijiang River, Guangdong Province.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article or Supplementary Material.

Conflicts of Interest

No conflict of interest exists in the submission of this manuscript, and the manuscript is approved by all authors for publication.

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Figure 1. Sampling station in the Guiping section of the Xunjiang River in the Pearl River Basin. (The content in the red box is a thumbnail of the geographic location map outside).
Figure 1. Sampling station in the Guiping section of the Xunjiang River in the Pearl River Basin. (The content in the red box is a thumbnail of the geographic location map outside).
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Figure 2. The population structure of fish eggs and larvae in the Guiping River Section of Xunjiang River from 2022 to 2023.
Figure 2. The population structure of fish eggs and larvae in the Guiping River Section of Xunjiang River from 2022 to 2023.
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Figure 3. Abundance in percentage of fish eggs and larvae population structure of dominant species at the Guiping section in the breeding period in 2022 and 2023.
Figure 3. Abundance in percentage of fish eggs and larvae population structure of dominant species at the Guiping section in the breeding period in 2022 and 2023.
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Figure 4. Discharge and density of fish resources in the early stage in the Guiping section in 2022 (a) and 2023 (b).
Figure 4. Discharge and density of fish resources in the early stage in the Guiping section in 2022 (a) and 2023 (b).
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Figure 5. Monthly proportion of fish eggs and larvae in the Guiping section of Xunjiang River from 2022 to 2023.
Figure 5. Monthly proportion of fish eggs and larvae in the Guiping section of Xunjiang River from 2022 to 2023.
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Figure 6. The impact of environmental factors at the Guiping section on larval fish recruitment during 2022–2023 (Abbreviations in the figure: D represents hydrological discharge, DO represents dissolved oxygen, WT represents water temperature, TN represents total ammonia nitrogen, CD represents conductivity, NHD represents natural hydrological data, and PH represents degree of acidity or alkalinity).
Figure 6. The impact of environmental factors at the Guiping section on larval fish recruitment during 2022–2023 (Abbreviations in the figure: D represents hydrological discharge, DO represents dissolved oxygen, WT represents water temperature, TN represents total ammonia nitrogen, CD represents conductivity, NHD represents natural hydrological data, and PH represents degree of acidity or alkalinity).
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MDPI and ACS Style

Li, H.; Chen, W.; Wang, D.; Lin, X.; Yu, L.; He, C.; Li, J.; Li, Y. The Status of the Early-Stage Fish Resources and Hydrologic Influencing Conditions in the Guiping Section of the Xunjiang River. Sustainability 2025, 17, 5930. https://doi.org/10.3390/su17135930

AMA Style

Li H, Chen W, Wang D, Lin X, Yu L, He C, Li J, Li Y. The Status of the Early-Stage Fish Resources and Hydrologic Influencing Conditions in the Guiping Section of the Xunjiang River. Sustainability. 2025; 17(13):5930. https://doi.org/10.3390/su17135930

Chicago/Turabian Style

Li, Huifeng, Weitao Chen, Dapeng Wang, Xiaoyu Lin, Li Yu, Chengdong He, Jie Li, and Yuefei Li. 2025. "The Status of the Early-Stage Fish Resources and Hydrologic Influencing Conditions in the Guiping Section of the Xunjiang River" Sustainability 17, no. 13: 5930. https://doi.org/10.3390/su17135930

APA Style

Li, H., Chen, W., Wang, D., Lin, X., Yu, L., He, C., Li, J., & Li, Y. (2025). The Status of the Early-Stage Fish Resources and Hydrologic Influencing Conditions in the Guiping Section of the Xunjiang River. Sustainability, 17(13), 5930. https://doi.org/10.3390/su17135930

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