Effects of Water Temperature, Light Intensities and Photoperiod on the Survival and Growth of Juvenile Schizothorax irregularis and Diptychus maculates

Abstract

An experimental ecological method was used to study the effects of water temperature, photoperiod, and light intensity on the survival, feeding, and growth of juvenile Schizothorax irregularis and Diptychus maculates. The Box–Benhnken experiment was designed to predict the optimal environmental conditions for juvenile growth. With the maximum specific growth rate at 15 °C and a photoperiod of LD16:8, the results demonstrated that the juvenile S. irregularis had a survival rate of over 85% in water temperatures ranging from 5 to 25 °C. A daily light duration of 15.86 h and a light intensity of 1166.28 lx, with the water temperature maintained at 10.45 °C, allowed the juvenile S. irregularis fish to attain the optimal circumstances for growth and survival. At water temperatures below 25 °C, the juvenile D. maculates exhibited maximum specific growth rates at 10 °C and LD16:8 light period. Additionally, as the light intensity reached 1000 lx, the juvenile fish grew better. Furthermore, the juvenile D. maculates fish achieved theoretically optimal survival and growth circumstances when the water temperature was maintained at 10.87 °C with a light period of 15.0.5 h per day and a light intensity of 1474.68x. The results showed that both fish species may be raised in captivity in highland regions, but precise control over water temperature is required.

1. Introduction

China has a large number of species of Schizothorax, accounting for about 80% of the world population. These fish are mainly distributed in China’s Qinghai-Tibet Plateau waters [1]. Schizothorax irregularis and Diptychus maculates belong to the class Actinopterygii, order Cypriniformes, family Cyprinidae, and subfamily Schizothoracinae. The population of these two species within the Schizothoracinae subfamily has been declining sharply in recent years due to interference from external factors, such as cutting down trees and building [2]. At the beginning of the 21st century, the results of a comprehensive survey of the native fish resources in Xinjiang within the Tarim River Basin showed that there were less than 40 Schizothorax irregularis and less than 300 Diptychus maculates [3]. In 2019, a survey and research on the fish resources in the Weigan River Basin of Xinjiang found that there were less than 20 Schizothorax irregularis and less than 400 Diptychus maculates [4]. So large-scale artificial breeding of Schizothoracinae has become a priority task to protect them.

Schizothorax irregularis is classified as Class II protected aquatic animal by the Xinjiang Uygur Autonomous Region, and Diptychus maculates was listed as a national second-class protected wild animal in 2021. Both are among the most representative indigenous fishes in Xinjiang and one of the rare fishes endemic to China [5,6]. S. irregularis and D. maculates are mainly distributed in the Tarim River system [7]. In recent years, due to the limitations of its conditions, such as slow growth, late sexual maturity, low fertility, or high requirements for spawning sites and living conditions, coupled with the constraints of human activities and the natural environment, such as the construction of reservoirs and reclamation of wasteland, S. irregularis and D. maculates have been severely persecuted, resulting in a sharp decline in germplasm resources.

Now, the distribution of fish in the basins of the Hotan, Tarim, and Kashgar rivers is extremely low. Recently, the large-scale breeding of S. irregularis and D. maculates has been greatly expanded in southern Xinjiang, though the appropriate eco-conditions for breeding are still unknown [5]. Water temperature, as an important ecological factor, affects the feeding, growth, and digestion of fish by controlling their metabolic and protein synthesis rates. The optimal temperature ensures the most efficient physiological and biochemical processes in fish [8].

When the water temperature is lower than the optimal range, metabolic activity, growth, and survival rates decrease, while a temperature higher than the optimal increases the metabolic rate of fish, resulting in slower growth and lower survival rates [9]. The optimal range is species-specific. For instance, the optimum temperature for the juvenile marbled flounder Pseudopleuronectes yokohamae is 20 °C, where it shows the highest growth performance [10]. For the European plaice Pleuronectes platessa and European flounder Platichthys flesus the optimal water temperature is 18–20 °C, at which they reach their maximum growth rates [11]. The optimal temperature range for the survival and growth of Schizothorax oconnori is 15.15–17.24 °C [12] (Appendix A 

Table A1. Comparison of the survival temperatures of different species.

Familia	Species	Optimal Survival Temperature	Optimal Growth Temperature
Cyprinidae	Schizothorax biddulphi [24]	5–25 °C	15 °C
	Schizothorax lissolabiatus [22]	7–26 °C	-
	Schizothorax prenanti [23]	5–27 °C	-
	Schizothorax oconnori [12]	15.15–17.24 °C	-
Bothidae	Pseudopleuronectes yokohamae [11]	-	20 °C
Clupeidae	Sillago robusta [20]	25–28 °C	-
Mugilidae	Chelon labrosus [21]	-	22 °C

).

Photoperiod is also one of the most important environmental factors affecting the biological activity of most fish, and an appropriate increment in photoperiod has been shown to improve their growth and survival rate [13]. For example, the specific growth rate of Scophthalmus maximus is the highest under the condition of 8 h of illumination [14]. The growth performance of Oncorhynchus mykiss under 16 h of illumination is significantly higher than that under 12 h of illumination [15]. In the actual aquaculture process, photoperiod is not the only factor that must be considered; the light’s intensity is equally important for fish growth [16]. Proper light intensity makes easier for fish to find and feed on prey. In aquaculture, increasing the light intensity has been shown to be effective in enhancing the survival and growth rates, such as reported for Scophthalmus maximus [14] and Takifugu rubripes [17].

This study aims to explore the effects of different combinations of water temperature, photoperiod, and light intensity on the survival and growth of juvenile of both species. By using response surface methodology (RSM), precise model equations for growth and survival will be constructed. Furthermore, the optimal parameter combinations of water temperature, photoperiod, and light intensity that enable the juvenile Schizothorax irregularis and Diptychus maculates to achieve the best growth and survival in the artificial breeding environment will be determined, providing scientific and reliable environmental conditions data for the large-scale artificial breeding of these two rare fish species.

2. Materials and Methods

2.1. General System Design

The juvenile S. irregularis (an initial body length of 4.45–4.63 cm and a body mass of 0.93–0.97 g) and D. maculates (an initial body length of 4.41–4.61 cm and a body mass of 0.92–0.97 g) were collected from the Karaberi Fish Breeding and Release Station of Xinjiang Uygur Autonomous Region. The test water was the groundwater from the Karaberi Fish Propagation Station, which was treated by a biological filtration tower. The principle of the biological filtration tower is mainly based on the biofilm method, the metabolic action of microorganisms, and the physical filtration function. Through the synergistic effect of multiple mechanisms, this treatment method maintains the water quality. Meanwhile, an HZ-060 type aeration and oxygenation machine was used to supply oxygen. The holding water had a temperature of 16.5 ± 0.5 °C, a pH of 8.45 ± 0.11, dissolved oxygen of 3.5 ± 0.2 mgL⁻¹, and salinity of 1.7 ± 0.1‰. The fish were kept in captivity in a transparent glass tank (60 cm × 35 cm × 45 cm) and were fed three times a day according to body mass 3.0–5.0% satiated. The feces were sucked out before feeding. The residual bait was sucked out 30 min after feeding, dried, and weighed, and the feeding amount was adjusted. During the experiment, the tanks were checked daily for mortality. Water exchanges were used to replace the siphoned water and to maintain water quality levels.

2.2. Experiment 1: Water Temperature

The first group of tests was set up with six temperature gradients: 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, and 30 °C, with three parallel groups of 30 juveniles each. The water temperature was regulated using a WEIPRO MX300 IC temperature controller with a fluctuation of ±0.5 °C during the test period of 80 d. The photoperiod of each test group was the natural photoperiod (LD12:12). We used 36 w fluorescent lamps to improve the light, and the light intensity changed with the alteration of the natural light day and night, with the highest light intensity peaking at 1000 lx.

2.3. Experiment 2: Photoperiod

In the second experiment, a total of 15 glass tanks with five light cycles, LD8:16, LD12:12, LD16:8, LD24:0, and LD0:24 were set up with three parallel groups, with 30 juveniles in each group. Each group was illuminated by a 36 W fluorescent lamp, and black plastic sheeting was used to shade the glass tanks for 80 d. We used 36 w fluorescent lamps to improve the light, and the light intensity changed with the alternation of natural light day and night, with the highest light intensity peaking at 1000 lx. The water temperature of each test group was controlled at the local groundwater temperature (2.0 ± 0.5) °C.

2.4. Experiment 3: Light Intensity

In the third experiment, a total of five light intensities were set at 2000 lx, 1000 lx, 500 lx, 100 lx, and 10 lx, each in a group of three parallels with 30 juvenile fish in it. A total of 15 glass water tanks were evenly arranged into five columns. Four 36 W fluorescent lights from the upper outside wall of the first column of the glass tank moved the spacing of each column of the glass tank so that the light intensity shifted to a light gradient of 10 to 2000 lx. The light period was kept natural (LD12:12) for the test period of 80 d. The water temperature of each test group was controlled at the local groundwater temperature 2.0 ± 0.5 °C.

2.5. Experiment 4: Combined Effects of Water Temperature, Photoperiod, and Light Intensity

Following the results of the single-factor test, the temperature of the experimental breeding water for juvenile S. irregularis was reduced to 5~20 °C, the light time per day was reduced to 12~20 h, and the light intensity was reduced to 500~2000 lx. For juvenile D. maculates, the temperature of the experimental breeding water was reduced to 5~15 °C, the light time per day was reduced to 12~20 h, and the light intensity was reduced to 500~2000 lx. Three factors (water temperature, photoperiod, light intensity) and three levels (survival rate, specific growth rate) were used in a Box–Benhnken experimental design. The levels of three factors were coded as −1, 0, and 1. A total of 17 test sites were used, each repeated three times, with 30 juvenile fish per site and a test period of 80 days.

2.6. Data Collection

Throughout the experiment, feeding rates were maintained by adjusting food based on daily mortality and weekly growth. Before weighing initial and final wet masses separately to the closest 0.01 g, the juvenile fish were gently wiped with paper towels to remove excess water. The parameters measured included initial length (L_i), final length (L_f), initial wet mass (M_wi), final wet mass (M_wf), survival rate (SR), and specific growth rate (SGR). The mean SGR of juveniles in each tank was calculated by SGR% day⁻¹ = [(ln M_wf − ln M_wi)/t] × 100, where M_wf = final wet mass, M_wi = initial wet mass, and t = number of days. Mean value ± standard error (mean ± SE) was used to express the data.

A one-way analysis of variance (ANOVA) and Duncan’s multiple range test were applied to determine the difference among the different treatments using SPSS 26.0 (SPSS Inc., Chicago, IL, USA). Differences were considered significant at a probability level of p < 0.05. Origin 2021 (Hampton, MA, USA) was employed to plot the data.

3. Results

3.1. Experiment 1: Water Temperature, Photoperiod, and Light Intensity

The results demonstrated that water’s temperature and photoperiod affected the growth of juveniles during a period of 80 days.

At the end of the experiment, the survival rate of the juvenile S. irregularis appeared the same for the test groups with water temperatures of 5 °C and 15 °C, whereas the survival rates of other temperature test groups differed significantly (p < 0.05) (Figure 1a). The specific growth rate of S. irregularis reached its maximum at 15 °C. There were significant differences among all of the groups (p < 0.05) (Figure 2).

At the end of the experiment, the survival rate of D. maculates was highest at a water temperature of 5 °C. However, once the water temperature exceeded 20 °C, the D. maculates began to die in large numbers. There were significant differences in the survival rates among the six experimental groups kept in different water temperatures (p < 0.05) (Figure 1d). The specific growth rate of D. maculates reached its maximum at 10 °C. There were significant differences among all groups (p < 0.05) (Figure 3).

3.2. Experiment 2: Photoperiod

The juvenile S. irregularis and D. maculates grew smoothly at a photoperiod of LD 16:8, and at the end of the experiment, the survival rate and specific growth rate were the highest values among all experimental groups (Figure 1b,e, Figure 2 and Figure 3).

When both species are exposed to light for 0 or 24 h a day, conditions are not advantageous for the fishes’ survival. Especially under the condition LD 0:24, the survival rate is 0. Therefore, the analysis of the specific growth rates of the two species was not conducted, as seen in Figure 2 and Figure 3.

The survival rate and the specific growth rate of each experimental group varied significantly (p < 0.05) (Figure 1b,e, Figure 2 and Figure 3).

3.3. Experiment 3: Light Intensity

The results revealed that juvenile growth was affected by light intensity for an 80-day period (Figure 1c,f, Figure 2 and Figure 3).

The results showed significant differences in the survival rates and the specific growth rates of the juvenile S. irregularis and D. maculates in all of the experiments of light intensities (p < 0.05).

The suitable light intensity for the juvenile S. irregularis is 500–2000 lx, and its survival rate is the highest when the light intensity is 500 lx. The suitable light intensity for the juvenile D. maculates is 1000–2000 lx, and its survival rate is the highest when the light intensity is 2000 lx.

When the light intensity was 1000 lx, the highest specific growth rates were achieved by both fish species (Figure 2 and Figure 3).

3.4. Experiment 4: Combined Effects of Water Temperature, Photoperiod, and Light Intensity

The experimental data for the survival rate and specific growth rate of Schizothorax labrosus and Diptychus maculates, along with the experimental design used in the response surface methodology study, are provided in Appendix B (

Table A2. Central composite circumscribed design used in response surface method studies and experimental value.

Group	Coded Value			SR (%)	SGR (% Day−1)
	W (°C)	I (h)	L (lx)
Schizothorax irregularis
1	0(12.5)	0(16)	0(1250)	92.22	1.73
2	1(20)	0(16)	1(2000)	82.22	1.64
3	0(12.5)	1(20)	1(2000)	85.56	1.61
4	0(12.5)	1(20)	−1(500)	88.89	1.61
5	1(20)	−1(12)	0(1250)	81.11	1.67
6	0(12.5)	0(16)	0(1250)	92.22	1.73
7	−1(5)	1(20)	0(1250)	87.78	1.67
8	−1(5)	0(16)	1(2000)	90.00	1.63
9	−1(5)	−1(12)	0(1250)	88.89	1.63
10	1(20)	0(16)	−1(500)	81.11	1.64
11	0(12.5)	−1(12)	1(2000)	88.89	1.62
12	1(20)	1(20)	0(1250)	78.89	1.62
13	0(12.5)	0(16)	0(1250)	92.22	1.73
14	0(12.5)	0(16)	0(1250)	92.22	1.73
15	0(12.5)	0(16)	0(1250)	92.22	1.73
16	0(12.5)	−1(12)	−1(500)	88.89	1.64
17	−1(5)	0(16)	−1(500)	88.89	1.68
Diptychus maculates
1	0(10)	0(16)	0(1250)	92.22	1.74
2	1(15)	−1(12)	0(1250)	88.89	1.62
3	1(15)	1(20)	0(1250)	78.89	0.78
4	1(15)	0(16)	−1(500)	50.00	1.12
5	0(10)	0(16)	0(1250)	91.11	1.74
6	0(10)	−1(12)	−1(500)	40.00	1.02
7	1(15)	0(16)	1(2000)	93.33	1.72
8	−1(5)	−1(12)	0(1250)	85.56	1.43
9	0(10)	0(16)	0(1250)	91.11	1.73
10	−1(5)	0(16)	1(2000)	92.22	1.64
11	0(10)	0(16)	0(1250)	90.00	1.73
12	−1(5)	0(16)	−1(500)	47.78	1.68
13	0(10)	−1(12)	1(2000)	90.00	1.58
14	−1(5)	1(20)	0(1250)	82.22	0.75
15	0(10)	1(20)	1(2000)	84.44	0.81
16	0(10)	1(20)	−1(500)	3.33	0.69
17	0(10)	0(16)	0(1250)	90.00	1.74

Note: W, I, and L represent the water temperature, illumination time, and light intensity; SR and SGR represent survival rate and specific growth rate.

).

The results of our statistical analysis are presented in

Table 1. Regression coefficients, standard errors, and 95% confidence intervals for the predicted model of survival rate (SR).

Term	Coefficient	d.f.	SE	95% CI
				Low	High
Schizothorax irregularis
Intercept	92.22	1	0.4309	91.2	93.24
W	−4.03	1	0.3406	−4.83	−3.22
I	−0.7775	1	0.3406	−1.58	0.0279
L	−0.3338	1	0.3406	−1.14	0.4717
WI	−0.2775	1	0.4817	−1.42	0.8616
WL	0	1	0.4817	−1.14	1.14
IL	−1.22	1	0.4817	−2.36	−0.0834
W2	−5.33	1	0.4695	−6.44	−4.22
I2	−2.72	1	0.4695	−3.83	−1.61
L2	−1.33	1	0.4695	−2.44	−0.2223
Diptychus maculates
Intercept	90.89	1	0.6974	−1.23	2.07
W	0.4162	1	0.6974	−4.85	−1.55
I	−3.20	1	0.6974	21.96	25.26
L	23.61	1	0.9862	−4.00	0.6671
WI	−1.66	1	0.9862	−2.61	2.05
WL	−0.2775	1	0.9862	−2.05	2.61
IL	0.2775	1	0.9613	−1.33	3.22
W2	0.9460	1	0.9613	−10.22	−5.67
I2	−7.94	1	0.9613	−23.27	−18.73
L2	−21.00	1	0.6974	−1.23	2.07

Note: W, I, and L represent the water temperature, photoperiod, and light intensity. The values in the table are all coded values, and the coefficient was estimated according to the coded value.

and

Table 2. Regression coefficients, standard errors, and 95% confidence intervals for the predicted model of specific growth rate (SGR).

Term	Coefficient	d.f.	SE	95% CI
				Low	High
Schizothorax irregularis
Intercept	1.73	1	0.0028	1.72	1.74
W	−0.005	1	0.0022	−0.0102	0.0002
I	−0.0062	1	0.0022	−0.0115	−0.001
L	−0.0087	1	0.0022	−0.014	−0.0035
WI	−0.0225	1	0.0031	−0.0299	−0.0151
WL	0.0125	1	0.0031	0.0051	0.0199
IL	0.005	1	0.0031	−0.0024	0.0124
W2	−0.0275	1	0.0031	−0.0347	−0.0203
I2	−0.055	1	0.0031	−0.0622	−0.0478
L2	−0.055	1	0.0031	−0.0622	−0.0478
Diptychus maculates
Intercept	1.74	1	0.0482	1.62	1.85
W	−0.0325	1	0.0381	−0.1227	0.0577
I	−0.3312	1	0.0381	−0.4214	−0.2411
L	0.1513	1	0.0381	0.0611	0.2414
WI	−0.0400	1	0.0539	−0.1675	0.0875
WL	0.1600	1	0.0539	0.0325	0.2875
IL	−0.1025	1	0.0539	−0.2300	0.0250
W2	−0.0418	1	0.0526	−0.1660	0.0825
I2	−0.5493	1	0.0526	−0.6735	−0.4250
L2	−0.1542	1	0.0526	−0.2785	−0.0300

Note: W, I, and L represent the water temperature, photoperiod, and light intensity. The values in the table are all coded values, and the coefficient was estimated according to the coded value.

. The regression equations were determined to be as follows:

Y_{SR of S. irregularis} = 92.22 − 4.03 W − 0.7775 I − 0.3338 L − 0.2775 WI − 1.22 IL − 5.33 W² − 2.72 I² − 1.33 L²

Y_{SGR of S. irregularis} = 1.73 − 0.005 W – 0.0062 I − 0.0087 L − 0.0225 WI + 0.0125 WL + 0.005 IL − 0.0275 W² − 0.055 I² − 0.055 L²

Y_{SR of D. maculates} = 90.89 + 0.4162 W − 3.20 I + 23.61 L − 1.66 WI − 0.2775 WL + 0.2775 IL + 0.9460 W² − 7.94 I² − 21.00 L²

Y_{SGR of D. maculates} = 1.74 − 0.0325 W − 0.3312 I + 0.1513 L − 0.0400 WI + 0.1600 WL − 0.1025 IL − 0.0418 W² − 0.5493 I² − 0.1542 L²

SR and SGR are indicated as survival rate and specific growth rate, where W, I, and L represent water temperature, photoperiod, and light intensity.

The two models’ lack-of-fit test was not significant (p > 0.05) for both the juvenile S. irregularis and D. maculates. The survival and specific growth models’ coefficients of determination (R²) for juvenile S. irregularis were 0.9797 and 0.9923 with adjusted coefficients (Adj-R²) of 0.9535 and 0.9825. However, for D. maculates the survival and specific growth models’ coefficients of determination (R²) were 0.9960 and 0.9703, with adjusted coefficients (Adj-R²) of 0.9908 and 0.9321, respectively.

The model equations generated for growth and survival are shown with their coefficient estimates, along with significance and 95% confidence intervals (CIs).

The significance of these four models is indicated by their F-value. The likelihood of an F-value this large occurring due to noise is a mere 0.01 percent. The survival rate and specific growth rate model equations reflected the experimental results (p < 0.0001) adequately (

Table 3. Analysis of variance table for the quadratic model of the response survival rate.

Source	SS	d.f.	MS	F-Value	p-Value
Schizothorax irregularis
Model	313.02	9	34.78	37.47	<0.0001
W	129.85	1	129.85	139.89	<0.0001
I	4.84	1	4.84	5.21	0.0564
L	0.8911	1	0.8911	0.9601	0.3598
WI	0.3080	1	0.3080	0.3319	0.5826
WL	0.000 0	1	0.0000	0.0000	1.0000
IL	5.98	1	5.98	6.44	0.0388
W2	119.73	1	119.73	128.99	<0.0001
I2	31.15	1	31.15	33.56	0.0007
L2	7.48	1	7.48	8.05	0.0251
Diptychus maculates
Model	6759.00	9	751.00	193.03	<0.0001
W	1.39	1	1.39	0.3563	0.5694
I	81.73	1	81.73	21.01	0.0025
L	4459.46	1	4459.46	1146.20	<0.0001
WI	11.09	1	11.09	2.85	0.1352
WL	0.308 0	1	0.3080	0.0792	0.7866
IL	0.308 0	1	0.3080	0.0792	0.7866
W2	3.77	1	3.77	0.9685	0.3578
I2	265.7 1	1	265.71	68.30	<0.0001
L2	1857.1 1	1	1857.11	477.33	<0.0001

and

Table 4. Analysis of variance table for the quadratic model of the response specific growth rate.

Source	SS	d.f.	MS	F-Value	p-Value
Schizothorax irregularis
Model	0.0355	9	0.0039	100.54	<0.0001
W	0.0002	1	0.0002	5.09	0.0587
I	0.0003	1	0.0003	7.95	0.0258
L	0.0006	1	0.0006	15.59	0.0055
WI	0.002	1	0.002	51.55	0.0002
WL	0.0006	1	0.0006	15.91	0.0053
IL	0.0001	1	0.0001	2.55	0.1546
W2	0.0032	1	0.0032	81.05	<0.0001
I2	0.0127	1	0.0127	324.21	<0.0001
L2	0.0127	1	0.0127	324.21	<0.0001
Diptychus maculates
Model	2.66	9	0.2955	25.41	0.0002
W	0.0085	1	0.0085	0.7267	0.4222
I	0.8778	1	0.8778	75.49	<0.0001
L	0.1830	1	0.1830	15.74	0.0054
WI	0.0064	1	0.0064	0.5504	0.4823
WL	0.1024	1	0.1024	8.81	0.0209
IL	0.0420	1	0.0420	3.61	0.0990
W2	0.0073	1	0.0073	0.6312	0.4530
I2	1.27	1	1.27	109.24	<0.0001
L2	0.1002	1	0.1002	8.62	0.0219

).

The first-order term W, the second-order term W², and I² all had an extremely significant impact on the survival rate (SR) of S. irregularis (p < 0.01) (Table 3). The interaction term IL and the second-order term L² both had a significant impact on the SR of S. irregularis (p < 0.05) (Table 3). The first-order term L, the interaction terms WI and WL, and the second-order terms W², I², and L² all had an extremely significant impact on the specific growth rate (SGR) of S. irregularis (p < 0.01) (Table 4). The first-order term I had a significant impact on the SGR of S. irregularis (p < 0.05) (Table 4).

The first-order and second-order terms I and L both had an extremely significant impact on the SR of D. maculatus (p < 0.01) (Table 3). The first-order terms I and L and the second-order term I² all had an extremely significant impact on the SGR of D. maculatus (p < 0.01) (Table 4). The interaction term WL and the second-order term L² both had an extremely significant impact on the SGR of D. maculatus (p < 0.05) (Table 4).

As illustrated in Figure 4, the responses to the effects of combined water temperature and photoperiod, water temperature and light intensity, and photoperiod and light intensity all displayed elliptical projected outlines at the bottom and a mountainous form with an opening downhill.

As shown above, the model demonstrated its validity by analyzing and forecasting the impact of three environmental conditions on the growth and survival of the juvenile S. irregularis and D. maculates effectively. According to the prediction model (Figure 5), combining a temperature of 10.45 °C, illumination period of 15.86 h, and light intensity of 1166.28 lx led to a maximum survival rate of 92.96% and a specific growth rate of 1.73% day⁻¹ for growth and survival models that were optimized simultaneously.

D. maculates’s survival rate and specific growth rate reached their best values of 96.52% and 1.83% day⁻¹, respectively, when the water temperature was 10.87 °C, the light time was 15.05 h per day, and the light intensity was 1474.68 lx.

4. Discussion

4.1. Water Temperature

Within a specific temperature range, the maximum growth rate will increase as the temperature rises. However, when the temperature exceeds a certain threshold (the optimal temperature), both the feeding rate and the growth rate will decline [10,18]. For instance, the survival limit temperature of Labeo rohita is as high as 41.6 °C [19], Sillago robusta reaches the upper threshold of its growth within the temperature range of 25 °C to 28 °C [20], and Chelon labrosus grows best at 22 °C [21].

In this experiment, the water temperature range for the tested fish was relatively wide. The juvenile S. irregularis can survive normally in water with a temperature ranging from 5 °C to 25 °C, but when the water temperature rises to 30 °C, its survival rate decreases significantly. When the water temperature rises above 20 °C, D. maculates begins to die in large numbers. When the water temperature drops below 10 °C, the specific growth rate of the juvenile fish increases. When the water temperature reaches 15 °C, these fish start to lose their appetite.

This result is similar to that of Schizothorax lissolabiatus (with an optimal water temperature of 7–26 °C) [22], Schizothorax oconnori (with an optimal water temperature of 15.15–17.24 °C) [12], Schizothorax prenanti (with an optimal water temperature of 5–27 °C) [23], and Schizothorax biddulphi [24]. It may be because the fish of the genus Schizothorax have long lived in high-altitude areas with low water temperatures. In order to adapt to the environment, their bodies have developed the ability to tolerate low temperatures.

4.2. Photoperiod

In the present study, the juvenile S. irregularis and D. maculates exhibited high survival and SGR when the photoperiod was LD 16:8, but 24 h of continuous light or no light at all was detrimental to the survival of juvenile fish. These results are comparable to those of Zebrasoma flavescens (Bennett) [25] and Takifugu rubripes larvae [26]. As visual predators, juvenile fish require a specific level of light to locate their prey [13,27]. In the absence of light, the juvenile fish will be unable to capture any prey and starve to death. According to Downing and Litvak [28], certain species require a dormant phase to sustain normal growth following foraging.

Resting juvenile fish at the end of feeding activities, which allows them to absorb all the nutrients in the diet and expend less energy to eat continuously, will help them grow. This may explain the improved growth and survival of juveniles in a few hours of darkness.

Fielder [29], discovered that a prolonged photoperiod for Pagrus auratus larvae provides the juveniles more time to feed and increased energy expenditure, because of the prolonged feeding. As a result, at LD 24:0 light, the juveniles had a significantly lower SGR than those at LD 16:8 light. Likewise, Martinez-Cardenas [30] discovered that the growth and survival rate for Hippocampus abdominalis under 16 h of light were higher than under 24 h of continuous light. The findings of the present experimental study validate the previous findings as well.

4.3. Light Intensity

When light intensity was kept between 100 and 2000 lx, the juvenile S. irregularis showed a high survival rate. However, when light intensity was 10 lx, the pups’ survival rate was extremely low. It is evident that their survival was adversely affected by the reduced light. When light intensity was increased to 1000 lx, S. irregularis grew rapidly, but juvenile fish grew more slowly when the light intensity was too strong or too weak. The survival rate and growth rate of Thunnus albacares are the highest when the light intensity is between 700 and 800 lux [31]. The specific growth rate and feed conversion rate of Maccullochella peelii under a light intensity of 1200 lx are significantly better than those under a light intensity of 3600 lx [32]. It was hypothesized that the fish would prefer to feed in low light intensity and are able to adapt to a higher light intensity. In consistent with the findings of this experiment and hypotheses, the author concluded, following two months of survey, that the juvenile S. irregularis preferred to relax in direct sunlight and fed at sunrise or sunset.

A higher light intensity was chosen by the juvenile D. maculates for survival, which was consistent with the environmental conditions of the sampling sites. The higher elevation of the river segment in Jigen Township, which has clear icy water and longer daily light hours, is in line with the results of this experiment.

The range of light intensity to which different fish species can adapt varies. Plectropomus leopardus had a higher survival rate at higher light intensities [33], whereas Lates calcarifer did not exhibit a significant difference at light intensities ranging from 5 to 400 lx [16]. The millet-seed butterflyfish had a higher feeding rate at 1314 light intensities compared to 3016 lx [34], and Paralichthys lethostigma had a higher feeding rate at lower light intensities [35]. The final body weight of Takifugu rubripes under a light intensity of 750 mW/m² is significantly greater than that under a light intensity of 50 mW/m², 250 mW/m² and 500 mW/m² [17]. To gain a further understanding of how light intensity impacts fish growth and development will require more investigation.

4.4. The Synergistic Effect of Temperature, Photoperiod, and Light Intensity

The final model derived from this experiment was able to precisely analyze and anticipate how the three environmental elements will affect the growth and survival of juvenile fish. Simultaneous optimization of the growth and survival models produced the highest values of SR and SGR for S. irregularis (92.96% and 1.73% day⁻¹) for a combination of temperature (10.45 °C), light duration (15.86 h), and light intensity (1166.28 lx). For D. maculates, the maximum values of SR and SGR were 96.52% and 1.83% day⁻¹, for a combination of temperature (10.87 °C), light duration (15.05 h), and light intensity (1474.68 lx), respectively.

Based on the findings, it was concluded that the Qinghai-Tibet Plateau in China and other areas with year-round low water temperatures and abundant light are appropriate for large-scale breeding of juvenile S. irregularis and D. maculates. The high altitude, low water temperature, clear water quality, and long daylight hours can all together be ideal for the large-scale breeding of juvenile thick-lip splittails.

This study has achieved certain results in exploring the effects of water temperature, photoperiod, and light intensity on the survival and growth of juvenile S. irregularis and D. maculates. However, it still has limitations:

(1)

Limitations of the experimental period: The research period of this study was set to 80 days, which may not be sufficient for the study of long-term growth and development processes. Fishes may respond differently to environmental factors at different growth stages. A relatively short experimental period may not be able to fully capture these changes, and key information may be overlooked, thus affecting the accurate judgment of their growth and survival laws.

(2)

Simplification of experimental conditions: In the experiment, only the main environmental factors such as water temperature, photoperiod, and light intensity were controlled. However, the actual natural environment is extremely complex and contains the interactions of multiple environmental factors, such as water flow velocity, dynamic changes in dissolved oxygen in the water, the content of various minerals and trace elements in the water, and the water’s microbial community. Since these factors were not considered in the study, there may be a deviation between the experimental results and the actual situation in the natural environment.

In the later stage, we will extend the experimental period and conduct long-term monitoring on multiple growth stages of juvenile S. irregularis and D. maculates from the juvenile stage until sexual maturity. We will record in detail the responses to changes in water temperature, photoperiod, and light intensity at different stages, including changes in growth rate, physiological indicators, and behavior patterns, and construct a more complete growth and survival law model. At the same time, we will simulate more natural and complex environmental conditions, set up multiple groups of control experiments, and explore the growth and survival conditions of the two types of juvenile fish when different environmental factors are added (such as simulating natural water flow, regulating the mineral content in the water, and introducing natural water microorganisms). We will deeply analyze the interaction mechanisms among various environmental factors to make up for the deficiencies of the previous research. This will provide a more practical theoretical basis for the artificial cultivation of S. irregularis and D. maculates and also offer more accurate references for the formulation of protection strategies in their natural environment.

5. Conclusions

The model equations were developed to predict the effect on the growth and survival of juvenile fish in relationships with the optimal conditions of water temperature, photoperiod, and light intensity. Both species showed preferential ranges for the three environmental parameters evaluated, but each species has an optimal combination of the three which provides the best growth and survival rate.

This research identifies the optimal environmental conditions for the artificial propagation of juvenile Schizothorax irregularis and Diptychus maculatus, providing significant insight for the conservation of schizothoracin germplasm resources.