Analysis of the Spawning Response Characteristics of Four Major Chinese Carps to Eco-Hydrological Processes in the Three Gorges Reservoir

Abstract

The middle reaches of the Yangtze River are a crucial breeding habitat for four major Chinese carps. The ecohydrological characteristics of their spawning grounds are crucial factors influencing spawning for these species: black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix), and bighead carp (Aristichthys nobilis). To investigate the impact of ecohydrological processes within the Three Gorges Reservoir on spawning, this study focused on the spawning grounds of the four major Chinese carps in the Yichang–Yidu section of the Yangtze River. By identifying key ecohydrological indicators and leveraging hydrological and spawning monitoring data from 2013 to 2024, the response characteristics of the four major Chinese carps’ spawning to these hydrological processes were analyzed. The results showed that the key ecohydrological indicators currently stimulating spawning for the four major Chinese carps are the fish-perceived daily flow rate increase, the fish-perceived cumulative flow rate increase, and the daily flow rate increase. These three indicators are significantly positively correlated with the scale of spawning for the four major Chinese carps. The thresholds for spawning at least 20% of the annual spawning total are: a perceived daily flow increase (P_da) of 4.52–36.05%; a perceived cumulative flow increase (P_cu) of 36.15–180.23%; and a daily flow increase (Q_av) of 588–2825 m³/s. The optimal water temperature for the reproduction of the four major Chinese carps is 21–23 °C. Overall, since the Three Gorges Reservoir’s normal operation, the frequency and scale of spawning for the four major Chinese carps have been highest during periods of rising water. It is recommended that, within the corresponding thresholds, ecological operation be conducted twice a year, once in mid-June and once in early July or late June. Daily flow increases can be controlled within the range of 588–2000 m³/s. This study analyzed the correlation between eco-hydrological indicators and the stimulation of spawning of the four major Chinese carps, providing optimized flow ranges and habitat conditions for ecological operation, which is conducive to promoting the reproduction and spawning of the four major Chinese carps in the Yichang–Yidu spawning grounds in the middle reaches of the Yangtze River.

1. Introduction

The Yangtze River is the longest river in China, and 46,000 reservoirs of various types have been built in its basin, with a total storage capacity of 250 billion cubic meters [1]. The Three Gorges–Gezhouba cascade reservoirs in the upper reaches of the Yangtze River are a major water conservancy project in its basin, bringing about comprehensive benefits in many aspects. The black carp (Mylopharyngodon piceus), the grass carp (Ctenopharyngodon idella), the silver carp (Hypophthalmichthys molitrix), and the big-head carp (Aristichthys nobilis) are collectively known as the four major Chinese carps. They are important economic fish species in the Yangtze River Basin [2] and are also the core economic species of China’s freshwater fisheries. They are typical drifting spawning fish [3]. The four major Chinese carps belong to the class Osteichthyes, order Cypriniformes, and family Cyprinidae, and share similar ecological characteristics. Their reproductive cycle is mainly affected by temperature. During winter, the ovaries of these fish are in stage II, at which point the sexual maturity coefficient is low. By the end of winter, the ovaries have developed to stage III or early stage IV. In spring, as the water temperature rises above 15 °C, the ovaries of these fish rapidly develop into stage IV, at which point the sexual maturity coefficient can reach 5% to 10%. When the water temperature reaches above 20 °C, the gonads are fully mature, and spawning occurs. The spawning behavior of the four major Chinese carps is highly dependent on changes in the hydrological environment. Changes in hydrological factors will have a profound impact on the suitability of the spawning grounds of the four major Chinese carps [4,5]. Their eggs are denser than water. After being laid, the egg membrane absorbs water and swells, allowing the eggs to float and drift in the water under the influence of the current. Early-stage larvae continue to drift downstream until they develop strong upstream swimming abilities, at which point they can enter the wider, gentler sections of the lower reaches of the river. Especially in lakes connected to the Yangtze River, the larvae need to drift hundreds of kilometers from egg laying to gain upstream swimming ability. After maturing, the parent fish migrate upstream during the breeding season to find suitable spawning grounds and conditions to complete spawning. All four major Chinese carp species exhibit migratory habits, also known as semi-migratory fish. As typical drifting egg-laying fish, the four major Chinese carps not only have environmental requirements before spawning but also require suitable current speed, water temperature, and other environmental factors after spawning to ensure the survival of the eggs and larvae [6]. However, since the Three Gorges Reservoir was put into operation in 2003, the hydrological conditions of natural rivers have changed significantly due to the operation of the reservoir [7], resulting in a significant impact on Yichang and its downstream river sections. Before the main flood season (mid-July), the Three Gorges Reservoir’s operation of small and medium-sized floods weakened the natural flood peak process in the middle and lower reaches of the Yangtze River, weakening the hydrological stimulation required for the spawning and reproduction of the four major Chinese carps. After the dam impounded water, the natural ecosystem around the reservoir underwent tremendous changes. The hydrological conditions of the downstream river section showed significant differences from those under natural conditions, including the “flattening” phenomenon in the downstream flow process and the varying degrees of decline in the downstream river level. As a result, the resource volume of the four major Chinese carps in the Yangtze River continued to decline during the initial operation of the Three Gorges Reservoir. In 2009, the number of eggs and fry in the Jianli section of the river dropped to a historical low of only 42 million, while the number of eggs and fry in this area had reached 40.3 billion in the 1960s. The scale of spawning has shrunk significantly, and the genetic safety of the four major Chinese carps has been seriously threatened, which has had an adverse impact on the river ecosystem.

In order to reduce the impact of reservoir operation on natural flood peaks, the Three Gorges Reservoir has been implementing ecological operation experiments every year since 2011 [8] to restore the drifting fish egg population, mainly composed of the four major Chinese carps species, through changes in flow rate, and to maintain the health and stability of the Yangtze River ecosystem. The determination of early ecological operation targets mainly relies on hydrological analysis methods, and operation plans are formulated by studying the hydrological characteristics of spawning of the four major Chinese carps in natural environments. Currently, both China and other countries are conducting research on restoring fish populations and improving habitats through eco-hydrological methods. Liu et al. [9] analyzed the correlation between the vorticity changes in the spawning grounds of the middle reaches of the Yangtze River and the spawning activities of the four major Chinese carps during the Three Gorges ecological operation period, and obtained the corresponding relationship between the vorticity distribution and the spawning locations of the four major Chinese carps. Stuart, I. G. et al. [10] studied the hydrological intermittency of the Darling-Barka River (LDR) in Australia caused by water withdrawal, drought and climate change. Combining the “river restart” flow after the catastrophic fish kill in 2018–2019 (increased to 3484 ML/day within 7 days) and broodstock telemetry data in 2016, they confirmed that continuous rapids habitat, stable water level (cumulative drop during spawning period < 0.3 m) and four-season base flow are the key to the recovery of Murray cod (Maccullochella peelii). Carpenter-Bundhoo L. et al. [11] used telemetry technology to analyze and evaluate how the release of ecological flows affects the migration patterns, habitat utilization and survival strategies of specific fish populations, and specifically analyzed the migration trajectories, activity ranges and habitat selection of fish after the release of ecological flows. Rytwinski, T. et al. [12] used a systematic map to sort out the evidence of the impact of flow changes on fish productivity in temperate regions, covering freshwater and estuarine fish (including resident, migratory and invasive species), focusing on flow changes caused by human (dams, hydropower stations, water withdrawal, etc.) and natural factors (climate change, floods, droughts, etc.), and measured fish productivity using 12 indicators such as biomass, abundance, growth, reproduction, and recruitment. At present, research on the relationship between fish spawning and hydrological factors has made certain progress. Zhang Di et al. [13] constructed an evaluation system for the ecological operation effect of reservoirs, calculated the weight coefficients of various indicators, and evaluated the ecological operation effect of each year. They found that the optimal range was 2–4 days of flooding, 14,000–16,200 m³/s of initial flow at the flooding section, 20,000–32,300 m³/s of peak flow at the flooding section, 1925–2825 m³/s of daily flow growth rate, and 21–23.8 °C of water temperature. King et al. [14] collected resource data, conducted a correlation analysis between hydrological indicators and spawning volume, and obtained eco-hydrological indicators that were more significant in promoting fish reproduction. Based on the eco-hydrological characteristics of the four major Chinese carps during their breeding period before the Three Gorges Reservoir was filled, Guo et al. [15] proposed that the hydrological requirements for the reproduction of the four major Chinese carps in the Yichang River section should be a daily flow rate increase of 910–2208 m³/(s·d), a daily water level increase of 0.41–0.74 m/d, and a water level increase duration of 3–8 days.

In recent years, as the Three Gorges ecological operation experiment has improved, the spawning volume of the four major Chinese carps has also continued to increase with the progress of ecological operation. However, new problems still exist in the operation process. During more than ten water management operations carried out at the Three Gorges Reservoir, some operations failed to stimulate fish spawning, resulting in the failure to reach the peak of egg and fry production during the operation period [16,17]. Under the operation of the Three Gorges Reservoir, how does the spawning behavior of the four major Chinese carps respond to the hydrological situation? How do the response characteristics change compared with the natural situation? Are the key hydrological indicators that stimulate spawning different? These are a series of new problems currently facing the ecological operation practice of the Three Gorges Reservoir, and are also the prerequisite for adaptive management and optimization adjustment of ecological operation in the future [18,19,20]. In summary, this study uses the Yichang–Yidu spawning ground, the largest in the middle reaches of the Yangtze River and the one with the strongest ecological operation response, as the research area. Based on the hydrological data from 2013 to 2024 (excluding 2016 and 2021) and the biological monitoring results of the spawning of the four major Chinese carps, we conduct an in-depth analysis of the natural reproduction status of the four major Chinese carps and the response characteristics of the eco-hydrological indicators. The objectives of this study are: (1) to identify the eco-hydrological indicators that effectively stimulate the spawning of the four major Chinese carps; (2) to evaluate the relationship between each eco-hydrological indicator and the amount of spawning of the four major Chinese carps; (3) to identify the threshold range of effective eco-hydrological indicators within the Three Gorges Reservoir. Based on these results, the implementation effect of the joint ecological operation experiment is evaluated, which can provide data support for the formulation and optimization of the Three Gorges–Gezhouba joint ecological operation experiment plan.

2. Research Area and Methods

2.1. Overview of the Study Area

The spawning grounds of the four major Chinese carps are mainly distributed in the middle reaches of the Yangtze River, with a total of more than 30 sites in history. According to surveys, the spawning scale of the spawning grounds in the middle reaches of the Yangtze River accounts for more than 70.4% of the total spawning volume of the Yangtze River [21]. Spawning scale refers to the number and distribution range of eggs (or fry) produced by fish groups during their reproductive activities. It reflects the reproductive intensity and resource status of fish populations. According to statistics from the past decade, the Yichang spawning ground is currently the spawning ground that is most directly affected by ecological operation and is also the largest and most stable spawning ground in the middle reaches of the Yangtze River [22]. The spawning ground is about 60 km downstream of the Three Gorges Reservoir and about 20 km downstream of the Gezhouba Dam, and its response to ecological operation is the most significant [23,24]. Since 2013, the China Water Resources and Hydropower Research Institute and the Yangtze River Fisheries Research Institute of the Chinese Academy of Fishery Sciences have jointly monitored the hydrological, hydrodynamic conditions and fish breeding activities in the Yichang–Yidu section of the river during the spawning season of the four major Chinese carps every year. So far, only in 2016 and 2021, no corresponding data has been obtained. In this study area, the concentrated spawning site of the four major Chinese carps is located upstream of the sampling section, in the river section between Yidu and Yunchi. The hydrological and hydrodynamic monitoring area is located in the section from Yanshou Dam to Yunchi River (Figure 1), and the fish reproduction activity monitoring section is set at the mouth of the Qingjiang River in Yidu City (30°17′48.06″ N, 111°36′40.04″ E) (Figure 1). Monitoring results show that the hydrological and topographical characteristics of the Yichang–Yidu section of the river create relatively suitable conditions for spawning of the four major Chinese carps. The hydrodynamic characteristics generated by the interaction between its hydrological conditions and topography have a certain representative role in the suitability of the spawning ground.

2.2. Ecological Operation Implementation Process

From 2013 to 2024 (excluding 2016 and 2021), the Three Gorges–Gezhouba Dam has carried out a total of 16 ecological operation experiments. Since this research team did not monitor the flow field and fish spawning behavior in the study area in 2016 and 2021, no data were obtained for these two years. Monitoring data show that it was carried out once per year on average in 2013, 2014, 2019 and 2020, and twice per year on average in the remaining years. According to statistics, the ecological operation time is mainly concentrated in mid-to-late May and mid-to-late June. It started in early May in 2013, and the second ecological operation in 2023 continued to early July. The outflow from the Gezhouba Reservoir ranges from 5700 to 31,380 m³/s, the water level continues to rise for 3 to 9 days, the daily flow rate increase is 508 to 3178 m³/s, and the daily water level increase is 0.2 to 1.3 m. The specific implementation time and hydrological process are shown in

Table 1. Spawning behavior of the four major Chinese carps and selected eco-hydrological indicators for reservoir operation.

Year	Frequency	Implementation Time (d)	Outflow (m3/s)	Duration of Water Rise (d)	Daily Flow Growth [m3/(s·d)]	Daily Water Level Increase (m/d)
2013	first	5.7–5.14	5700—15,300	8	1200	0.45
2014	first	6.4–6.6	11,800—18,400	3	2200	0.63
2015	first	6.7–6.10	7030—19,740	4	3178	1.30
	second	6.25–7.2	15,740—31,380	8	1955	0.60
2017	first	5.20–5.25	14,980—18,030	6	508	0.20
	second	6.5–6.10	12,080—19,850	6	1295	0.48
2018	first	5.19–5.25	16,900—25,310	7	1201	0.37
	second	6.17–6.25	13,450—19,650	9	689	0.25
2019	first	5.25–5.31	16,550—20,340	7	541	0.29
2020	first	5.23–5.28	8800—12,800	6	667	0.31
2022	first	6.3–6.8	15,100—21,500	6	1066	0.43
	second	6.23–6.28	19,500—28,000	6	1416	0.47
2023	first	5.28–6.3	8500—18,800	7	1471	0.58
	second	7.2–7.6	10,700—19,000	5	1660	0.64
2024	first	5.19–5.23	10,900—16,900	5	1200	0.36
	second	6.16–6.21	11,800—20,100	6	1383	0.61

.

2.3. Sampling Method and Data Statistics

Since 2013, the China Water Resources and Hydropower Research Institute and the Yangtze River Fisheries Research Institute of the Chinese Academy of Fishery Sciences have conducted joint monitoring of eco-hydrology, hydrodynamic elements and early fish resources in the spawning grounds of the four major Chinese carps every year, especially during the period of ecological operation in the Three Gorges Reservoir. The sampling section was located in Yidu County, Yichang City (Figure 1). Regarding the method of collecting fish eggs, three fish egg collection points were set up on both sides of the section and in the middle of the river. Each collection point was located at the surface, middle and bottom layers of the river (the distances from the surface, middle and bottom layers to the water surface were 20%, 50% and 80% of the water depth, respectively). The collection net is a conical net with a mesh area of 0.19 m² and is made of nylon sieve with a mesh size of 0.776 mm. Fish eggs were collected using a cone net, with each collection lasting 10 min. Parameters such as the net outlet flow velocity were also recorded. During the off-season, fish eggs were collected once in the morning and once in the afternoon. During peak spawning season, fish eggs were collected day and night, with a 2 h interval. Sampling point coordinates were recorded using a GPS 12XLC (Garmin Ltd., Olathe, KS, USA), and net outlet flow velocity was measured using an LS45A current meter (Linyi Zhengheng Chemical Glass Instrument Co., Ltd., Linyi, China).

The eco-hydrological factors used in this study are based on the daily flow series at the Yichang hydrological station. The data were obtained from the Hubei Provincial Bureau of Hydrology and Water Resources (https://slt.hubei.gov.cn/ (accessed on 20 May 2025)). The ecological operation period was determined according to the ecological operation order issued by the Yangtze River Water Conservancy Commission of the Ministry of Water Resources. The calculation of the spawning scale of the four major Chinese carps in this study refers to the calculation method of Yi et al. [25]. It is mainly based on the number of fish eggs caught on the day and the number of fish eggs flowing through the section where the river flow is calculated on the day, and then adding them up day by day to get the number of eggs laid in each river flood season, and then calculate the spawning scale of the four major Chinese carps in the entire spawning period. The specific calculation formula is as follows:

$$C={\displaystyle \frac{\overline{D}}{d}}$$

(1)

$$M={\displaystyle \frac{Q}{q}}\times m\times C$$

(2)

In the formula, $C$ is the fish egg quantity coefficient; $\overline{D}$ is the average density of fish eggs at each collection point on the cross section; $d$ is the egg density at a fixed collection point; $Q$ is the river runoff at the sampling section, m³/s; $M$ is the number of fish eggs collected in one section, grains; m is the number of fish eggs sampled at a fixed point on the cross section, grains; and $q$ is the river water flow through the network, m³/s.

The number of fish eggs $M^{\prime }$ flowing through the section during the non-collection time of each day is calculated using the interpolation method, namely:

$$M^{\prime }={\displaystyle \frac{t^{\prime }}{2}}\left({\displaystyle \frac{M_1}{t_1}}+{\displaystyle \frac{M_2}{t_2}}\right)$$

(3)

In the formula: $M_1$ and $M_2$ is the number of fish eggs collected twice, particles; $t_1$ and $t_2$ is the duration of the two collections, s; and $t^{\prime }$ is the interval time between the two collections, s.

The total runoff $M_t$ of fish eggs passing through the survey section in one day is the sum of the number of fish eggs during the collection period and the non-collection period, that is:

$$M_t=M+M^{\prime }$$

(4)

2.4. Selection of Eco-Hydrological Indicators

The hydrological regime of natural rivers contains periodic elements, including rhythmic changes in flow and water level, and the frequency, timing, duration, and rate of change in hydrological pulses [26,27,28]. These factors not only shape the physical habitat of rivers, but also serve as key environmental signals, regulating the material circulation and biological community evolution of ecosystems. For fish, their reproductive behavior is highly dependent on changes in hydrological and hydrologic conditions. Many studies at home and abroad have shown that the natural reproductive behavior of the four major Chinese carps is closely related to hydrological indicators such as the initial flow of flood pulses, peak flow, duration of continuous water rise, total flow increase, and daily flow increase [29,30]. This study also selected the above five indicators as the analysis of eco-hydrological conditions and fish spawning responses.

In addition, from the perspective of biological behavior, many organisms’ changes in perception of the external environment follow the Weber–Fechner law [31]. This law explains that under moderate physical stimulation within the range of normal physiological activities, organisms can perceive that the changes in their own physical quantities have certain regularities with the original physical quantities. It can be expressed as:

$${\displaystyle \frac{∆I}{I}}=W_c$$

(5)

In the formula, $∆I$ represents the current increment of the physical quantity, $\mathrm{I}$ represents the original physical quantity, and $W_c$ represents the Weber rate, that is, the amount of stimulation felt by the organism. For the judgment of biological response, only when the difference between the current physical stimulus $∆I$ and the original physical intensity reaches a certain intensity, which ${\mathrm{W}}_{\mathrm{c}}$ exceeds a certain threshold, will the organism perceive the stimulus and make a behavioral response [32].

Regarding the Weber–Fechner law, Goodwin et al. [33] extended this formula and applied it to the study of fish’s response to hydraulic characteristics. They also proposed two hydrological indices to describe the daily and cumulative stimulations felt by fish, the daily flow rate increase perceived by fish called $P_{da}$ (%) and the cumulative flow rate increase perceived by fish called $P_{cu}$ (%), and so the specific calculation method can be expressed as:

$$P_{da}={\displaystyle \frac{Q_{av}}{Q_{min}}}$$

(6)

$$P_{cu}={\displaystyle \frac{Q_t}{Q_{min}}}$$

(7)

In the formula, $Q_{av}$ is the daily flow increase (m³/s), $Q_t$ is the total flow increase during the flooding process (m³/s), and $Q_{min}$ is the initial flow of the flood pulse (m³/s). The daily flow increase (Q_av) is calculated as (peak flow—initial flow of flood pulse)/duration of flooding during a flood pulse; and the total flow increase (Q_t) is calculated as the difference between the peak flow and the initial flow of flood pulse during a flood pulse.

Fish respond to external stimuli by emitting signals [34]. Only when these differences exceed a certain threshold can they detect changes in characteristic quantities. Spawning behavior is also a physiological response in fish. To investigate the correlation between fish spawning behavior and flood pulse stimulation, this study applied 10 years of measured data to perform computational processing and combined it with new data to obtain more accurate results. In summary, this paper selected seven key eco-hydrological indicators that affect the spawning of the four major Chinese carps, see

Table 2. Ecohydrological indicators that potentially affect four major Chinese carps spawning.

Hydrological Indicators	Flood Pulse Initial Discharge	Peak Flow	Duration of Flood	Daily Flow Growth	Total Flow Growth	Fish Sense Daily Flow Increases	Fish Feel Cumulative Flow Increase
Variable name Unit	$Q_{min}$ m3/s	$Q_{max}$ m3/s	$T_{dur}$ d	$Q_{av}$ m3/s	$Q_t$ m3/s	$P_{da}$ %	$P_{cu}$ %

for details.

2.5. Selection of Correlation Analysis Methods

This study used Pearson correlation analysis to explore the response of fish spawning to eco-hydrological indicators during flooding.

In this paper, Microsoft Excel 2016 was used to organize and compile basic data, IBM SPSS Statistics 27 software was selected to calculate the correlation analysis of eco-hydrological indicators on the annual spawning ratio of the four major Chinese carps, Origin 2018 was used to draw data result graphs.

3. Results Analysis

3.1. Interannual Variation in Spawning of the Four Major Chinese Carps

Since the Three Gorges Reservoir began ecological operation, the response of the four major Chinese carps to spawn during rising water levels has reached its largest scale. According to early resource survey data provided by the Yangtze River Fisheries Research Institute, during the monitoring period from 2013 to 2024 (excluding 2016 and 2021), the spawning scale of the four major Chinese carps in the Yichang–Yidu spawning grounds downstream of the Three Gorges Reservoir showed an increasing trend year by year (Figure 2). Among them, the monitored spawning volume in 2024 reached 19.2624 billion, the highest since the implementation of ecological operation, an increase of 93.35% compared with 2023. This shows that after many practices of ecological operation of the Three Gorges Reservoir, the resource volume of the four major Chinese carps has been restored quite intuitively.

3.2. Impact of Hydrological Processes on Spawning Scale

Due to the large fluctuations in the number of broodstock of the four major carp species participating in reproduction each year, there are significant inter-annual differences in the scale of spawning. To facilitate comparison of the stimulating effects of various hydrological processes on spawning in different years, this study standardized the daily spawning volume by the ratio of daily fish eggs to the total annual spawning volume of the spawning grounds (i.e., the daily number of fish eggs divided by the total spawning volume of the four major Chinese carps in the spawning grounds that year). Based on this, a relationship diagram between the spawning of the four major Chinese carps and long-term hydrological processes was drawn (Figure 3). It can be seen intuitively from the figure that: during the rising water process, the spawning frequency and scale of the four major Chinese carps are the most intensive; the spawning frequency and scale are second only to those in the high-flow-maintenance stage; while in the low-flow-maintenance stage and during the water receding process, the spawning frequency and scale are relatively low. Among them, due to the influence of ecological operation, there were two large-scale spawning events during the water receding period.

Statistical results show that during the statistical year and statistical period, there were 230 days of flooding, of which 101 days triggered spawning of the four major Chinese carps. The proportion of spawning during these days to the annual spawning scale ranged from 0 to 48.31%, and the total spawning scale during flooding accounted for as high as 31.45% to 99.26% of the annual spawning scale. The high flow period lasted for 95 days, triggering spawning of the four major Chinese carps for 39 days. The proportion of spawning during these periods to the annual spawning scale ranged from 0.1% to 12.31%. The total spawning scale during high flow periods accounted for 0.31–24.99% of the annual spawning scale. The low-flow period lasted 227 days, triggering spawning of the four major Chinese carps for 35 days. The proportion of spawning during these periods to the annual spawning ranged from 0 to 34.64%. The proportion of spawning during low-flow periods to the annual spawning ranged from 0 to 37.63%. There was no low-flow period in 2013. The water retreat process lasted a total of 151 days, triggering spawning of the four major Chinese carps for 30 days. The scale of spawning in all previous times accounted for 0.04–45.45% of the annual spawning scale, and the total scale of spawning during the water retreat process in all years accounted for 0–54.51% of the total spawning scale of the year.

Further analysis revealed that the four major Chinese carps’ effective spawning behavior during long-term hydrological processes is mostly concentrated during flooding and subsequent high-flow periods. However, under natural conditions, their spawning activity often occurs during flooding, suggesting a difference in this characteristic. The reason is that natural flood processes are characterized by steep rises and falls, with short flood peaks and few long periods of high-flow maintenance. However, due to the operation of the Three Gorges Reservoir, the flood peaks of the downstream hydrological process have been flattened, and the peak shape has tended to be gentle, which often leads to a high-flow-maintenance process for many days after the water rises. The fact that the four major Chinese carps continue to spawn during the high-flow-maintenance stage after water rise also reflects their ability to adapt to controlled hydrological processes from another perspective.

3.3. Effects of Eco-Hydrological Characteristics During Flooding on Spawning of Four Major Chinese Carps

3.3.1. Analysis of the Response Relationship Between Spawning Behavior and Eco-Hydrological Indicators During Flooding

According to the results in Figure 3, since the normal operation of the Three Gorges Reservoir, the four major Chinese carps have spawned the most frequently and on the largest scale during flood season. To further analyze the response characteristics of the spawning behavior of the four major Chinese carps to eco-hydrological indicators during flooding from 2013 to 2024, this paper will collect daily flow data of the flooding process that effectively stimulates the spawning of the four major Chinese carps (i.e., more than 20% of the annual spawning rate) and their standardized spawning response data. Applying the above eco-hydrological indicators, we calculate and quantify the hydrological characteristic indicators during the flooding process, and analyze the response relationship between the characteristic indicators and the spawning of the four major Chinese carps.

Table 3. Table of main eco-hydrological indicators and spawning scale of the four major Chinese carps.

Year	Time Period (d)	Qmin (m3/s)	Qmax (m3/s)	Tdur (d)	Qt (m3/s)	Qav (m3/s)	Pda (%)	Pcu (%)	Proportion of Spawning Per Year (%)
2013	6.22–6.25	10,400	21,700	4	11,300	2825	27.16	108.65	50.00
2014	6.3–6.7	11,400	18,900	5	7500	1500	13.16	65.79	25.27
	7.1–7.4	18,500	27,100	4	8600	2150	11.62	46.49	23.97
2015	6.7–6.11	7030	19,700	5	12,670	2534	36.05	180.23	39.98
	6.22–6.27	16,200	26,850	6	10,650	1775	10.96	65.74	25.56
2017	6.5–6.10	12,080	19,850	6	7770	1295	10.72	64.32	34.09
	6.16–6.19	16,810	23,920	4	7110	1778	10.57	42.3	23.85
	6.25–6.29	19,740	27,020	5	7280	1456	7.38	36.88	21.86
2018	5.19–5.25	15,960	25,310	7	9350	1336	8.37	58.58	34.75
	6.18–6.25	11,780	19,650	8	7870	984	8.35	66.81	20.61
2019	5.28–6.1	16,550	20,280	5	3730	746	4.51	22.54	23.19
	6.10–6.13	14,950	17,760	4	2810	703	4.7	18.8	21.63
2020	6.1–6.8	13,000	17,700	8	4700	588	4.52	36.15	29.53
	6.21–6.30	21,200	37,400	10	16,200	1620	7.6	76.4	27.64
2022	5.26–5.31	18,900	21,900	6	3000	500	2.64	15.87	24.66
2023	5.28–6.3	8760	18,800	7	10,040	1434	16.37	114.61	27.15
2024	7.7–7.13	19,100	34,000	7	14,900	2129	11.15	78.01	21.79

lists the eco-hydrological indicators corresponding to the flooding processes that effectively stimulated the spawning of the four major Chinese carps during the monitoring period of 2013–2024 (excluding 2016 and 2021) and the cumulative spawning scale data of the four major Chinese carps corresponding to each flooding process. From 2013 to 2024 (excluding 2016 and 2021), a total of 17 peak periods of effective spawning of the four major Chinese carps were monitored in the Yidu monitoring section, and the spawning accounted for 20.61–50.00% of the annual spawning scale of the spawning ground. Based on the eco-hydrological indicators established above, the results show that the initial flow is 7030–21,200 m³/s, the peak flow is 17,700–37,400 m³/s, the duration of the rising water process is 4–10 days, the total flow increase is 2810–16,200 m³/s, the daily flow increase is 500–2825 m³/s, the daily flow increase felt by fish is 2.64–36.05%, and the cumulative flow increase felt by fish is 15.87–150.23%.

3.3.2. Identification of Key Eco-Hydrological Indicators Affecting Spawning of Four Major Chinese Carps

Table 4. Correlation between eco-hydrological indicators and the annual proportion of spawning during the flood season in the Yichang River section.

Hydrological Indicators	Flood Pulse Initial Discharge	Peak Flow	Duration of Flood	Daily Flow Growth	Total Flow Growth	Fish Sense Daily Flow Increases	Fish Feel Cumulative Flow Increase
Pearson correlation coefficient	−0.534 *	−0.168	−0.125	0.325	0.547 *	0.718 **	0.611 **

Note: * indicates significant correlation at the 0.05 level; ** indicates significant correlation at the 0.01 level.

calculates the correlation between various eco-hydrological indicators during previous floods and the scale of fish spawning during peak periods caused by floods (standardized processing). The results show that among the seven independent variables, the three eco-hydrological indicators (daily flow increase, daily flow increase perceived by fish, and cumulative flow increase perceived by fish) have a significant positive correlation with the spawning scale of the four major Chinese carps, with Pearson correlation coefficients of 0.547, 0.718, and 0.611, respectively. The initial flow rate showed a significant negative correlation with the spawning scale of the four major Chinese carps, with a Pearson correlation coefficient of −0.534; the peak flow and duration of flood were not significantly correlated with the spawning scale of the four major Chinese carps, with the Pearson correlation coefficients being −0.168 and −0.125, respectively. The total flow increase has a weak correlation with the spawning scale of the four major Chinese carps, with a Pearson correlation coefficient of 0.325. Among them, the two indicators of initial flow and daily flow growth were significantly correlated with the spawning scale of the four major Chinese carps at the 0.05 level; the two indicators, the daily flow rate increase perceived by fish and the cumulative flow rate increase perceived by fish, were significantly correlated with the spawning scale of the four major Chinese carps at the 0.01 level.

Historical documentation indicates that, prior to the impoundment of the Three Gorges Reservoir, prolonged flooding and peak discharges were closely associated with the spawning behavior of the four major Chinese carps [29,35]. However, according to the Pearson correlation analysis presented in this paper, since the Three Gorges Reservoir began normal operation, flooding duration and peak discharge have shown no significant correlation with the spawning scale of the four major Chinese carps (Table 4), indicating that these two hydrological factors no longer significantly influence the spawning behavior of the four major Chinese carps. This reflects two potential changes: First, the hydrological conditions required for spawning of the four major Chinese carps have become controllable, and the Three Gorges Reservoir has developed a strong hydrological operation capacity. During the four major Chinese carps’ breeding season (May to June), especially before the main flood season, peak discharges and other indicators are influenced by the operation of small and medium-sized floods, losing their natural hydrological characteristics and becoming more controlled by power grid peak operation. Second, with the gradual maturity of the Three Gorges Reservoir’s operation model, particularly the multi-day artificial flood peak operation implemented during the breeding season, hydrological factors such as flooding duration are no longer limiting the spawning of the four major Chinese carps. Under the current operation mode of the Three Gorges Reservoir, the response of the four major Chinese carps to hydrological stimuli is more reflected in the perception of daily changes in flow and cumulative changes in flow.

Previous studies have shown that if the spawning scale of the four major Chinese carps during rising water accounts for more than 20% of the annual spawning scale, the process can be regarded as a hydrological process that can effectively stimulate the spawning of the four major Chinese carps. Based on this, this paper statistically analyzes the range of key eco-hydrological indicators of these flooding processes to provide a reference for determining the thresholds of key hydrological indicators that stimulate the four major Chinese carps spawning. The threshold ranges corresponding to these key indicators are fish perceived daily flow increase (P_da) 2.64–36.05%, the average is 11.52%; fish perceived cumulative flow increase (P_cu) 15.87–180.23%, the average is 64.6%; daily flow increase (Q_av) 500 m³/s–2825 m³/s, the average value is 1491.35 m³/s; and initial flow (Q_min) 7030 m³/s–21,200 m³/s, the average value is 14,884.71 m³/s.

The above results show that since the operation of the Three Gorges Reservoir, the key hydrological indicators that effectively stimulate the spawning of the four major Chinese carps are the daily flow increase, the daily flow increase felt by fish, and the cumulative flow increase felt by fish. As the spawning volume of the four major Chinese carps increases year by year, fish spawning will also change due to indicators of various hydrological factors. According to the Weber–Fechner law, the greater the difference between the stimulation caused by flow changes during rising water and the stimulation to which fish have adapted, the stronger the fish’s perception of the eco-hydrological factors caused by rising water will be, thereby stimulating spawning. The above analysis results show that fish spawning is most sensitive to diurnal changes in flow, followed by cumulative changes in flow.

3.4. Impact of Ecological Operation of the Three Gorges Reservoir on the Spawning of Four Major Chinese Carps

3.4.1. Stimulating Effect of Ecological Operation on Spawning of Four Major Chinese Carps

After the dam was built, the Three Gorges Reservoir evened out its discharge, resulting in a flattened surface during the rising water flow, which negatively impacted the spawning of the four major Chinese carps. Since 2011, the Three Gorges Reservoir has conducted ecological operation experiments to stimulate the spawning of the four major Chinese carps from May to July each year. As shown in Section 2.2, the Three Gorges Reservoir implemented 16 ecological operations between 2013 and 2024. However, the effects of these operations on the spawning of the four major Chinese carps varied, and not every operation was able to effectively stimulate the spawning of the four major Chinese carps. Figure 4 shows the proportion of runoff of eggs of the four major Chinese carps during the monitoring period of 2013–2024. There was no spawning activity of the four major Chinese carps during the ecological operation period in 2013. The spawning runoff accounted for only 2.7% during the ecological operation period in 2019, and the spawning runoff accounted for only 0.4% during the ecological operation period in 2020. It can be seen that the ecological operation in some sessions failed to effectively promote the spawning of the four major Chinese carps, or there was a certain lag between the peak spawning period and the water rise time.

To further analyze the stimulating effects of artificial flooding caused by the ecological operation of the Three Gorges Reservoir on the spawning of the four major Chinese carps, this paper plots the relationship between the discharge at the Yidu spawning ground in the Yichang River section and the spawning proportion of the four major Chinese carps during the monitoring period from 2013 to 2024, with the ecological operation period marked with a dotted line, as shown in Figure 5. As can be seen from the figure, the first ecological operation in 2013, 2017, 2020, 2022, and 2024 did not cause the spawning peak of the four major Chinese carps; the ecological operation in 2014 resulted in the first spawning peak that year, with the spawning scale accounting for 25.27% of the total spawning volume that year; during the two ecological operation periods in 2015, the second and third spawning peaks of the year were, respectively, triggered, with the spawning scale accounting for 39.98% and 25.56% of the total spawning volume of the year; the second ecological operation in 2017 caused the first spawning peak of the year, with the spawning scale accounting for 34.09% of the total spawning volume that year; the two ecological adjustments in 2018 caused the second and third spawning peaks of the year, respectively, with the spawning scale accounting for 34.75% and 20.61% of the total spawning volume that year; the ecological operation in 2019 caused the first spawning peak of the year, with the spawning scale accounting for 23.19% of the total spawning volume that year; the second ecological operation in 2022 caused the second spawning peak of the year, and the spawning scale accounted for 38.9% of the spawning amount of the year; during the two ecological operation periods in 2023, the first and second spawning peaks of the year were, respectively, triggered, with the spawning scale accounting for 24.54% and 45.45% of the spawning volume of the year; the second ecological operation in 2024 caused the first spawning peak of that year, and the spawning scale accounted for 28.8% of the spawning volume that year.

It is worth noting that the end date of the ecological operation in 2019 was 31 May, while the first peak spawning date of that year occurred on 1 June; similarly, the end date of the second ecological operation in 2023 is 6 July, and the second spawning peak occurs on 10 July. Therefore, this study believes that there is a certain lag between the water rise behavior generated during the ecological operation and fish spawning. In addition, the ecological operation in 2013, 2017 and 2020 did not stimulate the spawning of the four major Chinese carps, indicating that water temperature also plays a decisive role in the spawning of the four major Chinese carps.

It can be seen that except for the ecological operation in 2013 and 2020, which failed to cause the peak of spawning of the four major Chinese carps, the implementation of ecological operation in other years caused large-scale spawning of the four major Chinese carps. The spawning scale of the four major Chinese carps during the ecological operation period accounted for as high as 23.19%~69.99% of the annual spawning scale, which shows that the artificial flood process created by ecological operation has a good promoting effect on the spawning and reproduction of the four major Chinese carps.

3.4.2. Response of Spawning of the Four Major Chinese Carps to Water Temperature

The replenishment of early fish resources not only depends on their own reproductive activities, but is also closely related to the external water environment, among which water temperature is a key factor affecting fish reproduction. According to the monitoring data from 2013 to 2024, the water temperature corresponding to the spawning of the four major Chinese carps was monitored to be between 18.5–24.4 °C during the 10 years, and the average water temperature in each year was 21.2–22.9 °C, respectively. Monitoring data show that the average water temperature range during the period of effective stimulation of the four major Chinese carps spawning (spawning scale of more than 20%) is 20.3–24.4 °C, and the average water temperature is 22.1 °C, which meets the spawning requirements of Yangtze River fish. According to Figure 6, the spawning distribution areas with dense distribution are mostly between 21–23 °C. Therefore, this study believes that the most suitable water temperature for the reproduction of the four major Chinese carps in the Yidu spawning ground of the Yichang section of the middle reaches of the Yangtze River is 21–23 °C.

Water temperature is a limiting factor affecting the spawning of the four major Chinese carps. When the water temperature is lower than 18 °C, the four major Chinese carps will stop spawning [36]. During the implementation of ecological operation of the Three Gorges Reservoir in 2013, the water temperature in the Yidu spawning ground was relatively low, so the ecological operation failed to effectively stimulate the spawning behavior of the four major Chinese carps. In addition, the construction of water conservancy projects will change the hydrological rhythm of the river section below the dam. After the Three Gorges–Gezhouba cascade reservoirs were put into operation, the low-temperature downstream discharge caused the water temperature in the downstream river to “stagnate and cool”, which delayed the maturation of the gonads of fish in the river section below the dam, and reduced the fish’s ability to reproduce, which in turn had an adverse effect on their reproduction. During the 2013 ecological operation period, the fish in the spawning grounds felt a daily flow increase (P_da) of 16.79%, the fish felt a cumulative flow increase (P_cu) of 134.3%, and the daily flow increase (Q_av) of 1096.25 m³/s. The eco-hydrological indicators all reached the threshold for stimulating spawning of the four major Chinese carps. However, the average water temperature from 7 May to 14 May 2013, was only 17.9 °C, so the fish failed to spawn. During the first ecological operation in 2017, the fish in the spawning grounds felt a daily flow increase (P_da) of 7.09%, a cumulative flow increase (P_cu) of 42.53%, and a daily flow increase (Q_av) of 897 m³/s. All eco-hydrological indicators reached the threshold for stimulating spawning of the four major Chinese carps. However, the average water temperature from May 20 to May 25 2017, was only 19.2 °C, so spawning of the domestic fish was not stimulated. During the first ecological operation in 2020, the fish in the spawning grounds felt a daily flow increase (P_da) of 7.58%, the fish felt a cumulative flow increase (P_cu) of 45.4%, and the daily flow increase (Q_av) of 667 m³/s. The eco-hydrological indicators all reached the threshold for stimulating spawning of the four major Chinese carps. However, the average water temperature from 23 May to 28 May 2020, was only 19.8 °C, so it failed to stimulate the spawning of four major Chinese carps. Based on the collection of eggs of the four major Chinese carps in the Yichang River section in this study, the water temperature for reproduction and spawning of the four major Chinese carps in the Yichang River section is mostly between 21–23 °C, which is similar to the conclusion of the suitable water temperature in the previous statistics. Therefore, this study believes that under the same ecological and hydrological factors, water temperature in the suitable range can effectively promote the reproduction and spawning of the four major Chinese carps.

4. Discussion

4.1. Optimization Suggestions for Ecological Operation

In addition to being related to eco-hydrological indicators, fish spawning is also affected by multiple external indicators, such as dissolved oxygen in water, pH value, salinity, rainfall, etc. [37,38]. However, flow rate and water temperature are the two most critical factors in stimulating spawning of the four major Chinese carps. During the fish breeding season, by artificially creating flood peaks, the four major Chinese carps can obtain the optimal hydrological conditions required for reproduction, thereby increasing their spawning scale. After 2020, with the start of a ten-year fishing ban in the Yangtze River Basin, annual monitoring of the spawning numbers of the four major Chinese carps has shown a significant increase. According to the correlation analysis in Section 3.3.2, the four major Chinese carps have a significant correlation with three eco-hydrological indicators: daily flow increase, fish-perceived daily flow increase, and fish-perceived cumulative flow increase. Their daily flow increase ranges from 500 to 2825 m³/s, the average value is 1491.35 m³/s, demonstrating a significant effect on ecological operation.

At the same time, by observing the ecological operation in 2013, the ecological operation in 2020, and the first ecological operation in 2024, the operation time occurred in May. Due to the influence of climate, the water temperature was relatively low, resulting in the ecological operation failing to effectively stimulate the spawning of the four major Chinese carps. When the water temperature exceeds 21 °C, the gonadal development of the four major Chinese carps will be completed quickly, which can effectively promote spawning. Therefore, based on the implementation of ecological operation and the analysis of this study, it is recommended that the ecological operation period be carried out after the water temperature in the middle reaches of the Yangtze River reaches 21 °C. It is recommended to carry out one session in mid-June and one session in early July or late June, completing two ecological operation sessions in one year, and the daily flow growth can be controlled within the range of 588–2000 m³/s to promote the reproduction activities of the four major Chinese carps in the Yichang section of the middle reaches of the Yangtze River to lay drifting eggs. The recommended water temperature is 21–23 °C. At the same time, the key eco-hydrological indicator thresholds such as the fish-perceived daily flow increase (P_da) of 2.64–36.05%, with an average of 11.52%, and the fish-perceived cumulative flow increase (P_cu) of 15.87–180.23%, with an average of 64.6% are used as the indicator range for evaluating the ecological operation effect.

Previous studies have used ecological operation to stimulate spawning of the four major Chinese carps, often focusing on changes in ecological flow, and adjusting the flow to create a suitable environment for spawning of the four major Chinese carps [4,16,39]. The research conclusions of this paper on flow range are consistent with those of previous studies. However, unlike previous studies, this paper analyzes data from 2013 to 2024 (excluding 2016 and 2021), covering a longer timeframe. Furthermore, the proposed ecohydrological indicators derived from the four major Chinese carps’ perception of external stimuli fill a gap in existing research. Existing research provides valuable empirical and theoretical support for ecological operation. Future research will combine this study with previous approaches to provide more practical recommendations for optimizing future operation schemes.

4.2. Comparison of Domestic and International Studies on Fish Population Restoration Based on Eco-Hydrological Indicators

Ecological reservoir operation is an important means of mitigating the adverse ecological impacts of reservoirs. It helps protect rivers from damage to fish populations caused by reservoir construction and helps restore fish populations in rivers. In China, reservoir operation based on ecological flow control has been applied to some extent to the protection of the four major carps, particularly around large-scale water conservancy projects such as the Three Gorges Reservoir. However, due to the complex hydrological environment and the diversity of water resource demands in the region, the implementation of ecological flow standards often faces challenges.

In contrast, many foreign countries have carried out relatively systematic ecohydrological research to support fish population protection. One of the more prominent studies is based on multi-indicator modeling, which proposes appropriate flow ranges for fish physiological activities. Baruch, E. M. et al. [40] analyzed 25 years (1993–2019) of fish community and functional flow data from Putah Creek, California, to assess ecological effects of the 2000 Putah Creek Agreement. Using the MARSS model, they found that when flow factors such as drought duration, autumn pulse magnitude, and rainy-season timing approached natural conditions, the quasi-extinction risk of native fish dropped by over 40%. The study recommended maintaining an annual autumn pulse flow (median ≥ 2.41 m³/s), reducing drought base flow (<0.85 m³/s), and limiting the reproductive advantage of alien species. Espinoza, T. et al. [41] used acoustic telemetry and a GLMM to study the movement of Mary River cod and Australian lungfish in Obi Obi Creek, Queensland. They found that environmental flow releases, particularly in winter and spring, effectively triggered fish movement. Dattilo, J. et al. [42] analyzed the Elk and Kiamitch Rivers and their reservoirs in the U.S. using catch curve residuals and multiple regression to study how hydrological dynamics affect freshwater drum and gizzard shad recruitment. They found that frequent high pulses enhance river–floodplain connectivity and spawning opportunities for juvenile gizzard shad, recommending reservoir releases that mimic natural spring flows to support species recruitment.

This study used linear relationships to analyze the correlation between ecohydrological indicators and the spawning of the four major Chinese carps. Although the application of ecohydrological indicators differs from international studies, both emphasize the importance of flow variation in fish conservation. International studies tend to use multi-indicator models to restore and protect a variety of fish species under varying hydrological conditions. Therefore, future research could draw on these experiences, particularly in developing precise ecoflow measurement and scheduling models, to further optimize habitat conservation strategies for the four major Chinese carps.

4.3. Limitations and Analysis of the Study

In this study, we analyzed the impact of ecohydrological characteristics during the flooding phase of the four major Chinese carps breeding season from 2013 to 2024 (excluding 2016 and 2021) on their spawning. We then used Pearson correlation to analyze the relationship between hydrological processes and spawning. However, Pearson correlation assumes a linear relationship between variables. In real ecosystems, nonlinear or complex interactions between hydrological processes and spawning can exist. Furthermore, limited sample size increases the uncertainty of this study. According to our field investigations over the years, each spawning event of the four major Chinese carps lasts for 1–8 days. After the Gezhouba Dam and the Three Gorges Dam were jointly operated from 2003 to 2014, the average number of water level rises and the average duration of water level rises during the breeding season of the four major Chinese carps in the middle reaches of the Yangtze River (May to July) dropped from 7.5 times and 4.5 days in 1981–2002 (before the Three Gorges Dam was cut off and the Gezhouba Dam was in independent operation) to 6.2 times and 4.2 days, respectively (the above data are from the hydrological data of the Yichang Hydrological Station). Therefore, if statistics are taken on a single day, regardless of whether the water level changes are caused by natural factors or affected by the regulation of large-scale water conservancy projects, the number of valid data samples related to the spawning volume of the four major Chinese carps and the eco-hydrological characteristics will not exceed 10. This small sample size problem also exists in other fields, such as pathological analysis and language model development [43,44]. Admittedly, the correlation between ecohydrological characteristics and spawning size revealed by a single survey may be random, but when multiple surveys show strong correlations between certain indicators, this correlation cannot be simply explained by random phenomena. Our years of field research have revealed a correlation between fish spawning behavior and ecohydrological preferences, implying a necessary link between fish behavior and hydrological conditions in spawning grounds. This study examined various flooding processes during the spawning season of four major Chinese carps, quantified them using multiple ecohydrological indicators, and analyzed the correlations between these indicators and spawning scale. This approach reveals more detailed ecohydrological responses to flooding during spawning, as well as indicator thresholds. While this method still requires further refinement, it already possesses considerable practical value.

Hydrological processes are only one driving factor for domestic fish spawning. Under the same hydrological conditions, not all four major Chinese carps in all river sections can feel the stimulation and trigger spawning. In other words, the local hydrodynamic environment formed in the spawning grounds during the flooding process has its own unique characteristics. At the same time, the interaction mechanism between the hydrological regime under reservoir regulation, the hydrodynamic environment of the spawning grounds, and fish behavior has not been fully elucidated. Secondly, this study only used indicators of hydrological processes and did not fully consider the influence of other potential ecological factors on fish spawning. For example, factors such as water temperature, habitat quality, and hydrodynamic conditions may also play an important role in fish spawning. Previous analyses have indicated that appropriate flow velocity and vorticity within the spawning grounds play a crucial role in the transport of fry of the four major Chinese carps [45,46]. They can increase the mixing intensity of fish sperm and eggs, thereby improving the fertilization rate of fish eggs; at the same time, they can increase the dispersion of fish eggs, expand their living space, and reduce their chances of being preyed upon. Meanwhile, changes in the topography within the spawning grounds also alter hydrodynamic factors. Over the years, the riverbed in the spawning grounds has experienced some siltation, and the riverbed topography in the study area exhibits a phenomenon of scouring followed by siltation. In the upstream area of the Yanzhi Dam, the degree of siltation in recent years is less than the degree of scouring in the early stages of the Three Gorges Project’s operation. In the middle reaches of this section, the degree of siltation is similar to the degree of scouring, while in the downstream section, the degree of siltation in recent years is greater than the degree of scouring in the early stages of the project’s operation. Therefore, future research should comprehensively consider more environmental variables and combine multi-indicator analyses. We hope to further investigate the driving responses and feedback loops between reservoir-regulated hydrological regimes, the hydrodynamic environment of spawning grounds, and fish behavior, thereby enhancing our understanding of the relationship between hydrological processes and fish spawning in ecosystems.

5. Conclusions

This paper collected and systematically analyzed the hydrological and biological data of the Yichang–Yidu spawning grounds in the middle reaches of the Yangtze River during the monitoring period from 2013 to 2024 (excluding 2016 and 2021), and used ecohydrological indicators to analyze the relationship between fish spawning and the current hydrological characteristics of the spawning grounds. The results show that:

(1)

Since the Three Gorges Reservoir began operating normally, the three indicators of fish perceived daily flow increase (P_da), fish perceived cumulative flow increase (P_cu), and flow daily increase (Q_av) have shown a significant positive correlation with the spawning scale of the four major Chinese carps; the initial flow rate (Q_min) has shown a significant negative correlation with the spawning scale of the four major Chinese carps; the total increase in flow rate (Q_t) showed a weak correlation with the spawning scale of the four major Chinese carps; and the two indicators of rising water duration (T_dur) and peak flow rate (Q_max) lost their correlation with the spawning scale of the four major Chinese carps.

(2)

Currently, the key eco-hydrological indicators that can stimulate the spawning of the four major Chinese carps are the daily flow rate increase felt by fish, the cumulative flow rate increase felt by fish, and the daily flow growth. The indicator range thresholds that can effectively stimulate the spawning of the four major Chinese carps, with the spawning scale accounting for more than 20% of the annual spawning scale of the same year, are: fish perceived daily flow increase (P_da) 2.64–36.05%, the average is 11.52%; fish perceived cumulative flow increase (P_cu) 15.87–180.23%, the average is 64.6%; and flow daily increase (Q_av) 500 m³/s–825 m³/s, the average value is 1491.35 m³/s. The most suitable water temperature for the reproduction of the four major Chinese carps is 21–23 °C.

(3)

Since the Three Gorges Reservoir has been operating normally, the four major Chinese carps have spawned most frequently and on the largest scale during rising water levels. However, there are still many ecological and hydrological indicators during rising water levels that do not fall within the above threshold range, which shows certain limitations. In order to further promote the reproduction and spawning of the four major Chinese carps and optimize the ecological scheduling effect in the future, it is recommended that the ecological scheduling period be carried out once in mid-June and once in early July or late June, completing two ecological operations a year, and the daily flow growth can be controlled within the range of 588–2000 m³/s.

This paper only evaluates the spawning response of the four major Chinese carps caused by the ecological operation of the Three Gorges Reservoir in the past 10 years. The ecological operation of the Three Gorges Reservoir has effectively stimulated the spawning of the four major Chinese carps. As for the corresponding protection measures for other fish species in the Yangtze River Basin, relevant research has not yet been carried out, but further research will be carried out in the future. Therefore, it is recommended that the eco-hydrological indicator thresholds obtained in this study be used as a reference when formulating ecological scheduling plans for the Three Gorges Reservoir in the future.