Variation of heat flux and its effect on the reproduction of four major Chinese carp

In recent years, river flow and water temperature (WT) have been impacted by global warming and the construction of hydraulic engineering structures (Guo et al. 2022a, 2022b, 2022c; Yang & Xing 2022; Yin et al. 2022), and the river heat flux (HF) situation has undergone a significant transformation. Changes in HF and their ecological and environmental impacts have been a hot topic of interest for many scholars (Lammers et al. 2007; Yang et al. 2021). HF is the flow of heat energy or the quantity of heat that passes through a unit area per unit time (Tananaev et al. 2019). HF is mathematically defined as a function of flow and WT, which are two important water environment parameters, and since both directly (or indirectly through a combination) reflect the physical properties and thermal conditions of the water environment, HF helps to characterize the response of rivers to climate change. Changes in river flow and WT can limit the health of river ecosystems by affecting their metabolism and productivity, destroying habitat conditions in fish habitats (P. Zhang et al. 2019; H. Zhang et al. 2019; W. H. Zhang et al. 2019; C. Zhang et al. 2021; J. Zhang et al. 2021; P. Zhang et al. 2021), and interfering with the energy exchange, growth rate, and reproductive cycle of aquatic communities (Wang et al. 2014; P. Zhang et al. 2019; H. Zhang et al. 2019; W. H. Zhang et al. 2019). Consequently, it is essential to analyze the patterns of temperature and flow variation in river water and to quantitatively comprehend the patterns of HF characteristics.

With the development and construction of large reservoirs, many methods and models have been developed by scholars for the study of river flow and water temperature variations. For example, the Mann-Kendall trend test is often used to examine trends in hydro meteorological long time series, which has the advantage of being independent of outliers and does not need to satisfy a specific probability distribution (Jiang et al. 2020). However, this method is affected by the autocorrelation of data itself and needs improvement. Periodic change detection methods are mainly various forms of wavelet analysis, which can detect the periodicity of meteorological and hydrological elements (Xu et al. 2013; Durocher et al. 2016). Morlet complex wavelets have better localization in both time and frequency domains, and complex wavelets have more advantages than waves in real form in applications (Briciu 2014). For the evaluation of the river hydrological situation, Richter et al. (1996) initially constructed a system of indicators of hydrological alteration (IHA) to describe the alteration of river hydrological status; in 1997, they also introduced the range of variability approach (RVA) to quantitatively describe the impact of water conservancy construction on rivers, which provides a basis and management for river ecological management.

Numerous scholars are currently studying river heat fluxes. Yang et al. (2021) determined the seasonal cycles of flow, WT, and HF in the Yukon and Mackenzie River basins and discovered that similar seasonal cycles of flow and WT exist in both basins, i.e., the rivers have the highest flow in June/July and the highest WT in July/August, thereby enhancing our understanding of thermal conditions and heat transfer in northern Canadian rivers. Magritsky et al. (2018) studied the flow and HF changes of the Lena River in Siberia over the last 30 years to better understand the drivers of changes in river HF conditions. He discovered that climatic factors increased lower Lena River runoff by 41.7 km³ and HF by 0.8 × 10¹⁵ kJ per year. In addition to climatic factors, human activities are a significant cause of changes in HF. The operation of the Three Gorges Reservoir (TGR), which is ‘clear and muddy’ after the construction of the Three Gorges Dam (TGD) (Jin et al. 2010), has a significant impact on the seasonal and intra-annual distribution of heat fluxes in the Yangtze River's middle and lower reaches. That is, the summer flow decreases and the annual proportion of HF decreases, whereas the winter flow rises and the annual proportion of HF rises (Li & Li 2011). Georgiadi et al. (2017) studied the HF variability of the Yenisei River, one of the largest rivers in Siberia, over the past two decades. They discovered that anthropogenic factors (primarily related to the operation of the reservoir system) caused a 12% decrease in the average long-term HF of the Yenisei River over the entire observation period. In summary, previous research on heat fluxes has primarily focused on the variation patterns of heat fluxes and the evaluation of their driving forces, whereas the impact of HF situation changes on river ecology, particularly aquatic organisms, has been relatively understudied.

The middle stages of the Yangtze River are an important flow portion that connects the upper and lower reaches of the river, and several big water conservation projects have been constructed on the main stream. Simultaneously, there are key fish breeding areas, such as the ‘four major Chinese carp’ in the Yichang River's middle reaches, which is exceptionally rich in fish resources (Shaoping et al. 2005). The Three Gorges Project is a massive hydropower project that has significantly altered the hydrological and hydrothermal conditions in the Yangtze River's middle reaches, affecting the suitability of fish habitat (Dang et al. 2018). According to the current research, hydrological variables such as WT and flow are significant drivers of fish spawning and reproduction. Appropriate WT is required for the development of fish gonads and is a crucial factor in ensuring the hatching of fish eggs and embryos. Fish survival and reproduction require adequate river flow, a decent habitat environment, and sufficient nutrients (Chen et al. 2014; C. Zhang et al. 2021; J. Zhang et al. 2021; P. Zhang et al. 2021).

Prior studies on the ecological environment of the Yangtze River basin focused on the analysis of water heat potential and hydrological conditions and their effects on fish reproduction, but few researchers have examined the changes in heat fluxes in the Yangtze River and their impacts on fish reproduction (Guo et al. 2021; C. Zhang et al. 2021; J. Zhang et al. 2021; P. Zhang et al. 2021). Therefore, our study proposes a new approach to explore the HF variation characteristics from a new perspective of the river HF situation. The main objective of this study is to apply various methods and models to characterize and quantify the evolution of heat fluxes in rivers and to investigate the effects of heat fluxes on fish spawning and reproduction. To achieve this goal, this study: (1) analyzes the flow and WT variation characteristics of the entire upstream of the Yangtze River and the degree of change of hydrological indicators under the influence of the TGD construction using the Mann-Kendall test method, wavelet analysis, and IHA-RVA model; (2) calculates heat fluxes and analyzes the evolution of heat fluxes; and (3) uses correlation analysis and Sturge's rule to explore the relationship between spawning and reproduction and HF changes in the Yichang River section of the ‘four major Chinese carp’. This is important to further develop the nature evaluation system, maintain the ecological health of rivers and the biosecurity of river aquatic organisms, and reduce the unfavorable ecological environment.

The Yangtze River is teeming with diverse species. According to incomplete figures, there are two species of freshwater whales, 424 kinds of fish, more than 1,200 types of phytoplankton, and 753 species of zooplankton. There are 1,008 species of benthic animals and about 1,000 species of aquatic higher plants. There are wild animals under critical national protection such as sturgeon, white sturgeon, and Chinese sturgeon, as well as crucial economic fish such as ‘four major Chinese carp’ (FMCC) (Chen et al. 2020). This paper examines FMCC, also known as Mylopharyngodon piceus, Ctenopharyngodon idellus, Hypophthalmichthys molitrix, and Aristichthys nobilis, which is a significant component of the Yangtze River system's fish resources. The main reasons for choosing the FMCC as representative fish are: (1) The FMCC are important economic fish in the Yangtze River, and they are also good examples of drifting egg-producing fish and migratory fish in rivers and lakes. Fish in the lower parts of the river tend to reproduce in the same ways (Zhang et al. 2022); (2) The breeding of the FMCC has relatively strict requirements for the hydrological situation of the river (Bai et al. 2013); (3) The monitoring data of the FMCC in the Yangtze River are the most abundant, with the exception of the Chinese sturgeon. The natural reproduction data of the FMCC species before the TGR impoundment were obtained from the ecological and environmental monitoring bulletin of the Yangtze River Three Gorges Project and related literature, and the natural reproduction data of the FMCC species after the Three Gorges impoundment were obtained from the in situ monitoring data of the Jianli section (Chen 2013; B. Li et al. 2021a; Z. D. Li et al., 2021).

The Mann-Kendall (M-K) trend test is a common method for studying trends in meteorological and hydrological data on long time scales. It can effectively distinguish whether there are significant trends in meteorological and hydrological processes (Jiang et al. 2020). It has exceptional applicability to the analysis of non-normally distributed hydrometeorological data since the data does not need to conform to a particular distribution and is not affected by a few outliers. Please refer to the corresponding references for the calculation process (Guo et al. 2022a, 2022b, 2022c).

In the early 1980s, Morlet proposed a wavelet analysis with a time-frequency multi-resolution function that improved the study of time series issues and could reveal the various change cycles concealed in the time series. This wavelet analysis can also qualitatively estimate the future development trend of the system and fully reflect the changing pattern of different time scales (Briciu 2014).

Among them, n denotes the number of indicators, and D_o values between 0 and 33% are defined as unchanged or low-level changes (L), 33%–67% as moderate changes (M), and 67%–100% as high changes (H).

The amount of heat storage in a body of water is determined by the numerous heat exchanges between the water storage body and the surrounding media (atmosphere, soil at the bottom of the reservoir, inflow, outflow, etc.). The heat entering the water body mainly includes the heat input by the inflow, the solar short-wave radiation heat, the atmospheric long-wave radiation heat, the heat released by the condensation of water vapor, and the heat input by the direct precipitation. The heat loss of the water body includes the heat loss caused by the outflow output heat, the water surface reflection, the water surface long-wave radiation, the heat consumption of the water surface evaporation, and so on (Song & Jing 2015).

From 1956 to 2019, the WT of Yichang Station generally showed an upward trend, and the trend rate of WT rise was 0.2 °C/10 years. The average WT before the Three Gorges impoundment (before 2003) was 18 °C, and the WT in 1956–1970 and 1981–1990 was lower than the average, and the WT was low. Since the early 1990s, the WT has generally shown an upward trend, and the annual WT has been higher than 18 °C except in 1996. The M-K method was further used to test the trend of annual average WT and the results showed that the WT test statistic was 6.5, indicating that the annual average WT was increasing; it passed the 99% significance test indicating that the annual average WT was increasing significantly. We conclude that the continuous rise of WT at Yichang Station is mainly due to the influence of climate warming and the regulation and storage of the TGR.

The Morlet wavelet analysis method, which is commonly used in the hydrometeorological analysis, is selected to analyse the annual average flow and the periodic evolution of WT at the Yichang hydrological station of the Yangtze River. MATLAB software is used to process the data and draw the wavelet contour map.

As shown in Figure 3(b), the evolution of WT has multi-time scale features, and the periodic change of WT is completely different from the flow. The evolution of WT at Yichang Station consists primarily of three scales of change cycles: 19–23 years, 9–13 years, and 5–8 years. The curve is not closed over the 30–32-year period, indicating that the periodic variation law has not been reached. The 19–23-year cyclic oscillation is the most significant of these three scales of cyclic variation, and on this large time scale, the WT undergoes 10 cycles of low-to-high alternation. the analysis of the period between 9 and 13 years reveals that the WT has experienced 14 cycles of low to high temperatures. During the entirety of the analysis period, the periodic changes of the above-mentioned scales are extremely stable and global; The WT alternated and fluctuated more frequently over a small-scale period of 5–8 years. On this small scale, it is separated primarily into two stages. Before 1980, the cycle change was not obvious, and after 1980, the cycle change was stable.

According to Figure 4(b), the first group of monthly median WT was high variation in seven indicators and low to medium variation in five indicators, with an overall variation of 72% (high variation). In the second group, there were five indicators of high variation in yearly extreme WT, four indicators of moderate variation, and two indicators of low variation, for a total variation of 63% (near to high variation). The time of occurrence of the minimum WT in the third group was high variation and the time of occurrence of the maximum WT was moderate variation, with an overall variation of 56% (moderate variation). The fourth group of high WT pulses had a moderate variation in frequency and duration, and the low WT pulses had a low variation in frequency and duration, with an overall variation of 40% (moderate variation). The fifth group had a moderate variation in both temperature increase and temperature decrease rates, a low variation in the number of reversals, and an overall variation of 37% (low variation). The overall degree of variation of the 32 indicators in the five groups is 62% (near to high variation), indicating a significant change in WT at Yichang Station around 2003.

The seasonal heat fluxes from 1983 to 2019 were divided into two periods: 1983–2002 and 2003–2019 (based on the operation time of the TGD), and the HF model was used to process the data and plot the seasonal HF changes at Yichang Station.

Spring and winter heat fluxes grow annually, whereas HF in autumn declines annually. The primary cause is a combination of the effects of anthropogenic activities and climate change (anthropogenic activities are dominant). Affected by climate change, the temperature, rainfall, and evaporation have changed to varying degrees, and the corresponding river WT and flow have also changed. The TGR's effect of ‘stagnant cold and stagnant heat’ causes the river's WT to increase in the spring and autumn and decrease in the summer and autumn; The operation of the three-reservoir system has altered the flow distribution law in the middle and lower reaches of the Yangtze River throughout the year: the flow during the flood season decreases as a percentage of the annual flow, while the flow during the dry season increases as a percentage of the annual flow.

According to the statistical analysis of HF data, the monthly average HF of Yichang Station in the past 40 years changed significantly from March to December. The HF showed an upward trend from March to July, from 2.4 × 10¹⁰MJ to 2.6 × 10¹¹MJ, while the HF showed a downward trend from August to December, from 2.3 × 10¹¹MJ to 3.2 × 10¹⁰MJ. The maximum HF occurs in July, when the flow is at its peak and the WT is higher.

Our analysis of the seasonal changes of HF and the relationship among flow, WT and HF found that there is a similar change cycle between HF and flow rate and WT, and the change of flow rate and WT control the change of HF. These findings are also similar to those of Yang et al. (2021). Our analysis revealed that HF is also one of the most important factors affecting the spawning output of FMCC. Fish spawning and reproduction is a complex process that is influenced by numerous environmental factors, each of which has distinct effects on fish reproduction activities. For instance, lower or higher WTs can promote the sexual maturity of fish, and fish spawning must occur within a specific WT range (Chen & Li 2015); turbulent hydrodynamic conditions significantly accelerate fish gonadal development (P. Zhang et al. 2019; H. Zhang et al. 2019; W. H. Zhang et al. 2019); when fish reproduce, sufficient dissolved oxygen content in water is a key environmental factor for their choice of spawning grounds (Yang 2020). In general, a single environmental factor cannot determine fish spawning and reproduction activities, and spawning and reproduction are frequently the results of the combined effect of multiple environmental factors. WT, as well as the process of flow and flooding, are widely thought to be the major factors determining fish spawning and reproduction (Fang et al. 2021). In this study, the variability of flow and WT in the middle reaches of the Yangtze River is quantitatively analyzed from three aspects (interannual variability, cyclical variability, and degree of hydrological variability). It was found that the changes in flow and WT at Yichang Station in the middle reaches of Yangtze River after Three Gorges impoundment were 64% and 62%, respectively, which shows that the construction of reservoirs has a significant impact on hydrological elements and the changes in hydrological elements directly affect the stability of river ecology.

The variation in WT is a significant factor influencing the spawning and breeding season of FMCC. When the WT reaches the spawning and breeding requirements, the flow and water up process is a crucial hydrological factor in determining the amount of reproduction during the subsequent spawning and breeding of FMCC. From Figure 4, we can see that due to the construction and operation of the TGR, the storage effect of the reservoir reduces the runoff below the dam and the flood peak is reduced, resulting in different degrees of reduction in high flow hydrological indicators such as rise rate and duration of high flow pulse. Once the ecological flow demand of domestic fish is not met, the scale of spawning will be greatly reduced. According to studies, the abundance of the fry of the FMCC is positively correlated with the water rise rate, and the duration of the flood pulse, duration of flood fluctuations, and flow size are all positively correlated with domestic fish reproductive output (He et al. 2021; Hu et al. 2022). Previous studies on the impact of the ecological operation of the TGR on the early resources of the FMCC show that the process of rising water effectively stimulates the natural breeding activities of the FMCC. Maintaining the daily growth rate of the outflow rate above 2,000 m³/s·d and the duration of water rise at 4 d can stimulate the concentrated reproduction of FMCC (B. Li et al. 2021a; Z. D. Li et al. 2021).

Studies have shown that the secondary factors affecting fish growth and reproduction are dissolved oxygen, pH, and geology, among others. Different fish growth and breeding have their critical oxygen concentration. Fish will stop growing and even die when the dissolved oxygen content of the water body is below the critical oxygen concentration (Franklin 2014). Fish generally prefer slightly alkaline waters (Warren et al. 2010). The quality of the water environment affects the gonadal development of fish, and high levels of organic pollutants in the waters can inhibit the gonadal growth of fish (Tiwari et al. 2017). Different fish species have different requirements for spawning ground substrate (Bai et al. 2022). In sturgeon and white salmon, fertilized eggs are sticky, and the substrate of their spawning grounds is generally boulder-like. The fertilized eggs of Oncorhynchus keta Walbaum are non-cohesive and subject to water currents, and their fertilized eggs are mostly in the sand scintillation layer beneath the stones. FMCC require a drifting water environment to ensure that their fertilized eggs can float and hatch along their journey (George et al. 2017). In summary, existing research has a more detailed understanding of the environmental factors affecting the spawning and reproduction of FMCC. However, few scholars have suggested the relationship between river heat flux and the reproductive activity of fish such as FMCC. The analysis of this paper found that there was a significant positive correlation between fry runoff and heat flux of FMCC, and further calculated the appropriate range of heat flux during spawning of FMCC.

Therefore, to better protect the habitat of economic fish and increase the number of fish resources, it is necessary to develop operational rules for WT compensation during the operation of the TGR, such as raising the WT from the early spawning period to expand the spawning window, and based on ensuring flood control safety, high flow free discharge should be carried out during the main flood season or before the artificial flood peak in spring to appropriately increase the high flow value. Meanwhile, this study further quantifies the suitable HF range during the spawning period of the FMCC, which provides a scientific basis for the development of a comprehensive ecological scheduling plan for the Three Gorges Project.

This study examines and quantifies heat fluxes in the middle reaches of the Yangtze River. Based on a careful statistical analysis of existing flow and WT data, we determined flow, WT, and HF patterns and characteristics in the middle reaches of the Yangtze River and explored the relationship between HF and fish spawning volume. Meanwhile, this study still has some limitations that can be addressed in future studies. First, in this study, there was limited access to fry runoff data, and there were some deviations in the range of spawning-adapted heat fluxes for the FMCC. Second, this study focused on analyzing the relationship between spawning output and heat flux in FMCC and lacked analysis of the effects of heat flux changes on reproductive activities such as gonadal development and spawning cycle in FMCC. The results are as follows:

• (1)

  The annual average flow rate at Yichang Station has decreased over the past 60 years, but the yearly average WT has increased. The analysis of the flow and WT cycle shows that there are three scale change cycles in the flow and WT processes of Yichang Station. Among these, the periodic changes of flow on a time scale of 25–31 years and the periodical variations in WT on a time scale of 19–23 years are the most apparent. The overall hydrological alteration of flow and WT was 64% and 62%, respectively, both close to high alteration.

• (2)

  After the Three Gorges impoundment, the HF increased significantly in spring and winter; the overall HF in autumn showed a decline with a significant downward trend, while the overall HF in summer showed little change; compared with before and after the impoundment of the Three Gorges, the change in HF is the largest in spring, followed by winter, and not much affected in summer and autumn.

• (3)

  Similar cyclical cycles exist for HF, flow, and WT, rising from March to July and falling from August to December. The relationship between HF, flow, and WT exhibits hysteresis; the maximum HF and flow peaked in July, whereas the maximum WT peaked in August, one month later than the peak HF and flow.

• (4)

  HF is one of the key factors affecting fish spawning and reproduction. Before the impoundment of the Three Gorges and following the ecological operation of the TGR, the heat flow was considerably positively linked with the reproduction of the FMCC, and the correlation coefficient was higher than 0.7. Based on the spawning time heat flow data and the Sturges rule, we determined that the spawning adaption HF of the FMCC was 8.2 ∼ 10.4 × 10¹⁰MJ.

W.G.: funding acquisition; project administration; resources; investigation; supervision;

W.C.: conceptualization; data curation; formal analysis; investigation; methodology; resources; software; validation; visualization; writing – original draft, writing – review & editing

N.H.: investigation; formal analysis; methodology; validation; visualization

H.W.: funding acquisition; project administration

This study was supported by the National Natural Science Fund of China (51779094); the 2016 Henan University Science and Technology Innovation Talent Support Plan (16HASTIT024); the Guizhou Provincial Water Resources Department 2020 Water Conservancy Science and Technology Project (KT202008); Basic Research Project of Key Scientific Research Projects of Colleges and Universities of Henan Province (23ZX012).

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.