The Influence of Different Feeding Time Management on the Growth and Stress Response of the African Catfish Clarias gariepinuns (Burchel, 1822) Under Farming Conditions

Abstract

In this study, the growth and welfare of the African catfish (Clarias gariepinus, Burchell 1822) were investigated under industrial farming conditions. For this purpose, the growing success (cm, g) and typical stress related parameters (glucose-, lactate-, cortisol-concentrations, growth hormone, HSI-liver index) were investigated on the African catfish (102–841 g) in relation to an external stressor (working light and noise) and different feeding regimes (day, night, and day and night feeding) over 83 days. As no significant effects were found among the experimental feeding treatments in relation to the growth performance and investigated stress parameters, the time of feeding seems to have less impact to the production success and stress reactions as suggested before. Regarding our results, the effect of feeding conditioning could have played a strong factor likewise the ageing process of the reared fish species which is known to be rather photophobic. Therefore, the factor of conditioning and its influence to the time shift in feeding regimes and the impact of noise and light stressors during feeding should be investigated separately in future experiments to obtain further results in this context and clarify the validity of the best feeding conditions for African catfish.

1. Introduction

Today, fish of many different species are reared in aquaculture facilities and raised for food production. The farmed fish passes through several stages of production, beginning at an age of several months as fry or fingerlings up to its slaughter weight and its end of the production under various forms of standardized husbandry conditions. In this process, the cultured fish are exposed to the usual industrial routines under various environmental conditions. These conditions range from varying water parameters, intensive illumination, increased noise levels in the production rooms and in the holding tanks to temporal changes in their feeding. This can affect not only the stress tolerance of the fish and therefore their welfare status, but also their feed intake, growth, and the final biomass of a production batch. In order to optimize the production success and improve the welfare of the fish, the husbandry conditions should be adapted to the biological preferences of the fish species introduced. The African catfish Clarias gariepinus (Burchell, 1822), introduced to Europe in the early 1960s, has an excellent feed conversion rate FCR (1.3) [1,2] among many other farmed fish species for food production and can be reared under the most adverse environmental conditions [3]. Due to its simplicity of husbandry, resilience to many environmental parameters and robust body shape, this farmed fish species is increasingly being bred for commercial food production in European countries [4,5]. Unlike many diurnal fish species that feed primarily during daylight hours, C. gariepinus, as a nocturnal species, typically feeds at dawn to night in the wild on its diet of small fish, amphibians and other food leftovers [6,7,8]. Their vision is well adapted to twilight and color perception [9]. However, the predatory catfish do not only have to rely on their eyes to hunt prey, but can also rely on their olfactory sense, their barbels, and also on their electrical perception to detect prey [10]. Research on this topic has already shown that there is also a dependency in the food intake of nocturnal species on the time of day [11]. Since it is known that the day/night rhythm of the fish is also controlled by the hormone balance, the light (intensity/duration) can influence the hormone level of the animals and thus cause an adapted behavior of the animals [12,13]. Therefore, it may be beneficial to feed catfish in aquaculture systems during the evening and nighttime hours. Initial studies on the rearing of African catfish fingerlings have already shown dependencies of mortality rates and growth on different light conditions [14]. Here, the light sensitivity of C. gariepinus was described as rather photophobic, resulting in a farming recommendation under shaded conditions for this species [14,15,16]. Furthermore, additional research showed dependencies on selected color spectra and intensities (yellow/low brightness) that were well suited for the developmental phase of C. gariepinus (larvae) and supported an improved habitus of the fish at this age [17,18]. Various studies on the influence of the illumination regime have already shown that illuminance, color choice or duration of illumination all have a certain influence on the behavior, mortality, or habitus of African catfish [15,17,19]. However, the parameter of duration in illumination probably seems to have a greater effect than, for example, the choice of color [19,20]. As the stocking density of African catfish should undergo changes with increasing age [21,22,23], the influence of light could also have a different significance in mature than in juvenile fish populations (light sensitivity) and should therefore also be investigated more closely under cultured conditions. Besides the influence of light conditions on the feeding behavior of fish, noise pollution (industrial noise machines, work noise, facility noise technology) can also play a significant role in feeding behavior and ultimately on biomass growth [24,25,26,27,28]. The noise sensitivity of African catfish and related species has already been investigated in several studies and has been classified as sensitive [29,30]. The conspecific catfish species Lophiobagrus cyclurus demonstrated increased sensitivity to noise levels of 81.6 dB (re 1 µPa at 1 kHz), suggesting vulnerability to prevalent forms of noise pollution [31,32,33]. Noise also has a greater potential to be transmitted into the enclosure through external vibrations, particularly in RAS facilities, where pump connecting pipes pass through the enclosure and its outer walls, allowing vibrations from outside to propagate inside [34,35]. Despite the potentially high noise levels in industrial RAS (recirculating aquaculture system) facilities, the sensitivity of catfish to such noise has not yet been examined. As this could be a critical factor in the welfare of hearing-sensitive fish species, it warrants thorough investigation. Several studies focusing on industrially relevant fish species such as Oreochromis spp. [36], Salmo salar [37], and Cyprinus carpio among others [38], have explored the pronounced sensitivity of these species to light or acoustic stimuli. Given that the majority of these investigations reported beneficial outcomes, the use of light and sound emissions may also represent a promising tool within conditioning programs for African catfish (Clarias gariepinus). These stimuli should therefore be considered potential tools for optimizing husbandry conditions in aquaculture through targeted manipulation. Moreover, various experiments have demonstrated the influence of different environmental conditions in industrial aquaculture facilities due to light and noise on the biological stress response of fish and thus on the expression of cortisol, glucose, and other stress-related parameters [15,16,39,40,41]. Therefore, this correlation should also be investigated in relation to regular feeding routines and their associated stress impact for nocturnal feeders. In order to investigate and verify the combined influence of illumination periods, typical industrial noise pollution and recommended feeding times on the fish stress level and production success, essential stress parameters such as glucose, lactate, cortisol, and growth hormone should be tested within a combined experiment design. In the current study, fish welfare-related stress was assessed by measuring glucose, lactate, and cortisol concentrations in combination with a time-specific feeding regime for the African catfish (Clarias gariepinus) under industrial farming conditions. These conditions refer to large-scale RAS facilities with high stocking densities and substantial biomass, where fish are frequently exposed to environmental stressors such as noise from radios, staff activity, and other disturbances typical of enclosed working environments. This experiment attempts to validate the most effective feeding time for this popular aquaculture species (C. gariepinus) by evaluating production performance in relation to the physiological stress responses associated with different feeding routines.

2. Materials and Methods

2.1. Experimental Design

The experimental trial was conducted over a period of 83 days, from January to April 2018, in the enclosed recirculating aquaculture system (RAS) facility located within the ‘Fish Glass House’ at the University of Rostock. The experimental facility consists of nine equal fish tanks, a sediment separator, a UV light bypass, and a trickling biofilter unit. Dimensions of the used culture tanks were 1.84 × 0.95 m with a processed water volume of 1.22 m³ per tank each which resulted in a total process water volume of 15.13 m³ of the total facility including the filter volumes. To validate the feeding and growth of the African catfish, three different feeding routines as treatment groups (day feeding, night feeding, day and night feeding) were examined in a complete randomized block design (CRB) during the duration of the experiment. This setup was designed to replicate commercial RAS systems commonly used in northern Germany, allowing for optimal comparability of the system configuration and the data obtained. In addition, an artificial stress regime was simulated for all test groups through the application of work lighting (598.6 lx ± 3.6) and radio music (79.8 dB m²) daily from 9 am to 6 pm. Overhead lights and radio speakers were installed above designated tanks. Subsequently, light intensity and sound volume were measured and standardized at the water surface level to ensure consistent exposure conditions across all tanks. Fish welfare indicators as values of serum glucose, lactate, plasma cortisol and growth hormone, and the hepatosomatic index and weight of the liver from 33 fish of a treatment group were determined on day 60.

2.2. Fish Stocking and Feeding

A total of 765 African catfish (Clarias gariepinus) were used in the experiment. The fish were obtained from the Dutch company Fleuren & Nooijen and divided into three experimental groups, each consisting of three tanks with 85 fish per tank. The respective mean weights of the groups were 107.13 ± 30.29 g (day), 101.46 ± 23.88 g (night) and 102.81 ± 25.07 g (day and night). Accordingly, the initial stocking density was approximately 8.28 kg/m³, increasing to 69.29 kg/m³ by the end of the experiment. The experimental groups were fed at different intervals, so that the day group was fed six times every 2 h during the day (7:00 to 17:00), the night group six times every 2 h at night (19:00 to 5:00) and the day/night group was fed continuously six times every 4 h (1:00 to 21:00). All tanks were equipped with automatic feeders following a standard feeding protocol of coppens feed table for ME-4.5 Meerval Top. The feed used had the following composition: Crude protein (44.0%); crude fat (14.0%); crude fiber (1.4%); crude ash (8.5%); calcium (2.0%); sodium (0.5%); phosphorus (1.2%); E672 Vitamin A (5000 U.I/Kg); E671 Vitamin D3 (750 U.I/Kg); iron (42.0 mg/Kg); iodine (2.1 mg/Kg); copper (5.0 mg/Kg); manganese (16.0 mg/Kg); zinc (100.0 mg/Kg); E324 Etoxyquin (50.0 mg/Kg) (Skretting, Fontaine-lès-Vervins, Frankreich, ME-4.5 Meerval Top, 4.5 mm).

2.3. Sampling

During the experimental period of 83 days, 28 fish were sampled at the beginning, the middle, and the end of the experiment (all four weeks) from each of the three experimental tanks within each experimental group. For all sampled fish, weight and length measurements were recorded. Additionally, blood and liver samples were collected from five fish at each sampling point to analyze liver- and stress-related parameters. Blood samples were collected from the caudal vein of each fish to analyze stress-associated parameters. Glucose and lactate levels were measured immediately during blood sampling using portable devices: glucose concentration was determined with the Accu-Chek Aviva (Roche, Mannheim, Germany), and lactate concentration with the Accutrend Plus (Roche, Mannheim, Germany). To quantify cortisol and growth hormone levels, blood plasma was later analyzed. For this purpose, blood was collected in EDTA-coated tubes and stored at 4 °C until centrifugation. Samples were centrifuged at 1500× g for 10 min using a Hettich Universal 320 R centrifuge. The plasma was then analyzed using enzyme-linked immunosorbent assay (ELISA) kits. Cortisol levels were determined with the CSB-E08487f ELISA kit, and growth hormone concentrations with the CSB-E12121Fh ELISA kit (Cusabio, Houston, TX, USA). Absorbance values were measured using an iMark Microplate Reader photometer.

Test animals designated for sampling were anesthetized in ice water prior to each sample collection (15 min), in accordance with animal welfare regulations, and were only processed further after complete anesthesia was confirmed. Typical growth factors such as the feed ratio SGR, the mortality rate, the condition factor, the mass and length gain, the fillet yield, the feeding rate and the hepatosomatic index were as well calculated from the collected data.

The specific growth rate (SGR %/day):

$${\displaystyle \frac{\left(\ln \left(W_1\right)-\ln \left(W_2\right)\right)\ast 100}{(t_1-t_0)}}=\mathrm{S}\mathrm{G}\mathrm{R}\left({\displaystyle \frac{\%\mathrm{B}\mathrm{w}}{\mathrm{d}}}\right)$$

(1)

• W₁ = End biomass;
• W₂ = Initial biomass;
• t₁ = Duration in days;
• t₀ = Duration in days.

The feed conversion ratio (FCR):

$${\displaystyle \frac{TFI}{{(W}_1-W_0)}}=\mathrm{F}\mathrm{C}\mathrm{R}$$

(2)

• TFI = Total feed intake;
• W₁ = Final fish weight (g);
• W₀ = Initial fish weight (g).

Hepatosomatic index:

$${\displaystyle \frac{{(W}_L\ast W_B)}{100}}=\mathrm{H}\mathrm{P}\mathrm{I}\left(\%\right)$$

(3)

• W_L = Liver weight;
• W_B = Bodyweight.

Mortality:

$${\displaystyle \frac{n_{dead}}{n_0}}\ast 100=Mo(\%)$$

(4)

• N₀ = Initial fish number;
• N_dead = Number of dead fish.

Growth in length:

$${\mathit{L}}_1-{\mathit{L}}_0=\mathit{G}\mathit{r}\mathit{o}\mathit{w}\mathit{t}\mathit{h}\left(\mathbf{c}\mathbf{m}\right)$$

(5)

• L₀ = Initial fish number;
• L₁ = Number of dead fish.

Growth in weight:

$${\mathit{W}}_0-{\mathit{W}}_1=\mathit{G}\mathit{r}\mathit{o}\mathit{w}\mathit{t}\mathit{h}\left(\mathbf{g}\right)$$

(6)

• W₀ = Initial fish number;
• W₁ = Number of dead fish.

Condition factor:

$$\left({\mathit{W}}_{\mathit{B}}\mathbf{\ast }{\mathit{L}}_{\mathit{B}}\right)=\mathit{K}$$

(7)

• W_B = Weight body;
• L_B = Length body.

2.4. Water Analysis

Daily, physicochemical water parameters, including temperature, oxygen saturation, electrical conductivity (EC), and redox potential, were measured between 8:00 and 10:00 a.m. prior to feeding, using a portable multiparameter meter (HQ40D, Hach Lange, Berlin, Germany). In addition, concentrations of common chemical water parameters, including ammonium (NH₄⁺), nitrite (NO₂⁻), nitrate (NO₃⁻), iron (Fe²⁺), potassium (K⁺), magnesium (Mg²⁺), calcium (Ca²⁺), sulfate (SO₄²⁻), and phosphate (PO₄³⁻), were determined twice weekly from the recirculating water. Samples were collected on designated days, cooled, and stored for subsequent photometric analysis using an automated photometer (Gallery™, Thermo Fisher Scientific, Waltham, MA, USA).

2.5. Disturbances and Welfare Indicators

In order to include the additional influence of daily work routines of commercial aquaculture facilities on the feeding behavior in relation to the feeding times, a stress regime (light and noise) was added via a time control. For the implementation of the stress regime, 3W LED lights were positioned above the tanks and their illuminance was set to 598.6 lx ± 3.6 and subsequently checked using a lux meter (type: LM-200, “Eurolite”, Langenau, Germany). The simulation of the noise level was realized by placing several radios at the same distance from the fish tanks. The sound level of 79.8 dB m² was checked and adjusted for each fish tank using a measuring device (LEQ sound meter PCE-353). Further, relevant welfare indicators (glucose, lactate, cortisol, growth hormones, and the HIS-Index of the liver weight) for the determination of the stress load were then collected once during the sampling. For this purpose, the blood serum taken from the caudal vein of the fish was used to determine the glucose and lactate values. The glucose value was measured using the Accu-Check Aviva measuring device and the lactate value was measured using the Accutrend Plus measuring device, both from (Roche Diagnostics Deutschland GmbH, Mannheim, Germany). For the measurement of cortisol and growth hormone concentrations in blood plasma, a blood sample was transferred to EDTA BD vaccutainer after initial collection and kept refrigerated until centrifugation (4 °C, 1500× g, 10 min; Universal 320 R, Hettich, Tuttlingen, Germany). The cortisol and growth hormone concentrations were then determined using an enzyme-linked immunosorbent assay (ELISA) using the respective test kits (Fish Cortisol ELISA Kit CSB-E08487f, Cusabio; GH ELISA Kit CSB-E12121Fh, Cusabio). The resulting absorbance values were then analyzed using a microplate reader (iMark Microplate Absorbance Reader, Bio-RAD, Feldkirchen, Germany).

2.6. Statistic

Generated data sets were summarized for the growth measurements for each treatment group (d/night/day and night) and over the entire duration according to their treatment for statistical evaluation. The measurement data of the animal welfare indicators were summarized only according to their treatment affiliation. All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS), version 27.0 (IBM Corp., 2020, Armonk, NY, USA) and the selected significance level of p < 0.05. The normal distribution and variance homogeneity of the data were checked using the Shapiro–Wilk and Levene’s test. Normally distributed data sets were evaluated using a single-factor analysis of variance (ANOVA) with a significance level of p < 0.05. If the data were not normally distributed, a non-parametric Kruskal–Wallis test was used. If the analysis of variance indicated homogeneous values, a Tukey–HSD post-hoc test was used for further evaluation. If the data was not homogeneously distributed, a Dunnett T3 post-hoc test was used instead. The standard curve for determining the cortisol and growth hormone concentrations was carried out with the CurveExpert, version 1.4 (Hyarms Development, 2020) program. The vapor pressure model was used for evaluation.

3. Results

3.1. Water Parameters

Mean values of the recorded water parameters from the used aquaculture recirculation system were summarized in

Table 1. Total water parameter mean values during 83 days of C. gariepinus feeding experiments. DO as oxygen saturation. EC as electrical conductivity.

Parameters	Temp.	DO	EC	Redox	pH
	°C	mg/L	µS/cm2	mV
Mean Value	27.02	7.76	1834.28	149.62	5.78
SE ±	0.06	0.05	33.22	1.59	0.07
Parameters	NH4+	NO3−	NO2−	PO43−	SO42−
	mg/L	mg/L	mg/L	mg/L	mg/L
Mean Value	12.26	527.04	0.21	39.78	136.54
SE ±	1.30	27.13	0.05	3.16	3.67
Parameters	K+	Ca2+	Mg2+	Fe2+
	mg/L	mg/L	mg/L	mg/L
Mean Value	39.89	177.16	39.36	0.05
SE ±	4.05	4.77	1.25	0.00

. Water temperature and oxygen saturation were attempted to be kept constant throughout the experiment, but in the case of temperature the values fluctuated slightly as a result of technical difficulties. The measured EC values ranged from 1082.0 to 2660.0 µS/cm and reached their highest peak at the end of the experiment. The pH values determined were stable for the duration of the experiment and did not fall below 4.2 pH. For all chemical water parameters, ammonia (NH₄⁺), nitrate (NO₃⁻), and potassium (K⁺) fluctuated considerably over the entire measurement period. In the case of NH₄⁺, the values were influenced by feeding and bacterial activity in the filter system, while in the case of NO₃⁻ the concentration increased over time (227.37 to 912.93 mg/L).

3.2. Growth and Mortality

Determined growing parameters (growth, length, SGR) of the analyzed catfishes showed no significant effects between the day, night and day and night treatment groups p > 0.05. Also, additional parameters like FCR and filet weight did not show to affected by the feeding treatments (

Table 2. Mean performance values of 765 African catfish (Clarias gariepinus) subjected to different feeding time treatments, as determined at the end of the 83-day experiment. Values are presented as means ± standard deviation (SD).

Parameters	Treatments
	Day	Night	Day/Night
Mean feed input per tank (kg)	39.33 ± 0.30	39.32 ± 0.19	38.99 ± 0.16
Initial weight (g)	107.13 ± 30.29	101.46 ± 23.88	102.80 ± 25.07
Final weight (g)	796.47 ± 298.74	808.16 ± 283.41	841.12 ± 285.93
Initial length (cm)	24.91 ± 2.24	24.54 ± 2.01	24.77 ± 2.15
Final length (cm)	45.40 ± 4.64	45.06 ± 5.04	45.56 ± 5.88
Filet weight (g)	305.10 ± 87.12	287.86 ± 96.28	319.40 ± 98.32
Last day growth (g)	688.07 ± 8.50	657.07 ± 35.66	688.31 ± 20.91
Last day growth (cm)	20.50 ± 4.65	20.52 ± 5.24	21.09 ± 6.08
FCR	0.81 ± 0.01	0.85 ± 0.04	0.84 ± 0.03
SGR (%)	2.42 ± 0.01	2.42 ± 0.06	2.46 ± 0.03
Condition factor	0.82 ± 0.07	0.88 ± 0.05	0.85 ± 0.01
Mortality (%)	14.51 ± 1.80	14.51 ± 0.68	17.65 ± 1.18

). In terms of performance metrics, fish from all treatment groups showed considerable variability in final body size, especially in weight, which was reflected in the consistently high standard deviations across treatments. Nevertheless, the condition factors of the unsorted fish of all treatment groups did not differ significantly from each other and therefore suggest a similar growth. Fish mortality ranged from 14.51 ± 1.80% to 17.65 ± 1.18% and showed no significant effect to the applied feeding treatment.

Most indicating fish performance indicators were visualized after a 83 days experimental period for growth (Figure 1 and Figure 2) and for the condition factor (Figure 2) of male and female catfish in the individual treatment groups. For growth, male and female catfish final weights showed less differences among the treatments and between the sexes. Some extreme values were seen from male African catfish of the day feeding treatment group but were also seen in the treatment group of day and night feeding. Female catfish seemed to be lighter in mean weight in direct comparison to the respective male conspecific but were less influenced by the set feeding treatments.

Additional regression diagram was calculated for the growth of the three treatment groups at four different sampling time points in the course of the experiment. Linear regressions were quite similar in all three feeding groups. The highest, but not statistically significant, growth was achieved by the day and night feeding group, followed by the treatment group with night feeding and the last group with day feeding (Figure 2).

Calculated condition factor values showed less differences between the feeding treatments and indicate a similar and equal growing in size and weight of the fish (Figure 3). While a higher value of the condition factor indicates a well grown fish in length and weight, a smaller value indicates a stocky and thick habitus instead.

3.3. Welfare Indicators

Measured welfare indicators revealed no significant effects between the measured stress reaction and the set feeding treatments (

Table 3. Stress-related welfare indicators of African catfish (Clarias gariepinus) after an 83-day period under different feeding time treatments and exposure to an external stressor.

Parameters	Treatments
	Day ± SD	Night ± SD	Day/Night ± SD
Glucose (mmol/L)	3.76 ± 0.39	3.63 ± 0.21	3.70 ± 0.19
Cortisol (ng/mL)	32.87 ± 1.77	33.26 ± 1.30	33.63 ± 2.48
Lactate (mmol/L)	2.60 ± 1.43	2.52 ± 1.39	2.52 ± 0.83
Growth hormone (pg/mL)	715.60 ± 61.63	747.63 ± 47.15	721.85 ± 55.34
HSI-Index	1.31 ± 0.34	1.26 ± 0.33	1.29 ± 0.25
Liver weight (g)	11.29 ± 5.92	10.46 ± 5.38	11.89 ± 5.09

). All determined indicators (glucose, cortisol, lactate, growth hormone, his, and liver weight) showed quite similar values among the set treatments of day, night, and day and night feeding without any significant effect or trend.

4. Discussion

The growth of the African Catfish was investigated under industrial farming conditions against the influence of different feeding times (day, night, day and night). For this purpose, an experiment was performed to simulate the typical routine working environment by illuminating it with industrial lights and noise from 9:00 to 18:00. Since no data sets for the growth of African catfish under such conditions have been published yet, the collected data enables us to determine the fish performance under industrial conditions for the first time. Thus, this study is providing new datasets that take into account the feeding behavior and stress sensitivity of the catfish species, which is described as rather photophobic, under commercial husbandry.

Analyzed physical and chemical water parameters did not reveal any significant effect to the set feeding treatments and stressor as the amount of the feed were also similar among the treatments. Typically, fluctuations of NH₄⁺ and NO₂⁻ were observed during the experiment as the biofilter activity was reacting to the feeding of the fish per tank and are therefore expectable observations [42]. All determined physico-chemical water parameters were found in acceptable ranges for the African catfish [43,44]. To the end of the experiment the quality parameters of the circulating water are clearly unaffected by the set feeding routines.

The final biomass of a raised fish species in a cultivating experiment usually provides reliable information about the farming success and food conversion of the fish. Consequently, the final biomasses were also calculated in the present experiment. However, no significant differences in the final biomass or growth were detected among the experimental groups in the experiment. In terms of numerical values, fish of the day and night feeding treatment showed the highest final weight, biomass, and growth in mass after 4 weeks of feeding followed by the fish of the night treatment and at last day treatment. Nevertheless, the determined performance values and additional parameters of weight, length, growth, FCR, SGR, K-Factor, and mortality did not show any trend or seemed to be affected by the feeding treatments and additional stressor at the end of the experiment. Therefore, the feeding at day seems to have the same impact to the growth of the fish as the feeding at night time. This contradicts the research of [14,45], who claimed better growth and less mortality of African catfish at a shadowed to night shift photo regime. Even though [45] had shown some positive effects of feeding under shaded to dark conditions for nocturnal fish species (C. gariepinus) on growth and conversely [19] had described better growth conditions for C. gariepinus under bright light conditions, none of the observations previously made in the mentioned studies above could be confirmed by the results of the current study. The observed tendency of light avoidance could not be confirmed in this study, as no significant differences were found in growth parameters across the different feeding time treatments, particularly between the day and night feeding groups, nor in mortality rates. This suggests that the calculated effect size and the sample size employed in this trial may have been insufficient to detect statistically significant effects. As the calculated performance values of FCR, SGR, and final growth also did not differ statistically from each other, it could be questionable that the fish had been adopted to the feeding times by conditioning and overcome their habit of photophobic behavior in this context. In a study by [11], the feed acceptance of African catfish (Clarias gariepinus) was assessed across different times of the day, revealing a significantly higher acceptance during nighttime periods. Moreover, certain individuals developed consistent feeding patterns during the early morning and late evening hours, indicating an initial influence of temporal feed conditioning aligned with the daily light–dark cycle. Complementary findings were reported by [9,46], who examined the effects of conditioning on both diurnal and nocturnal fish species, including Gadus morhua and Clarias gariepinus. Their results emphasized the potential of conditioning to alter natural behavioral rhythms. This method is hypothesized to exert a significant effect and may offer practical benefits to aquaculture practitioners in terms of feeding efficiency and fish harvesting. Conditioning in nocturnal fish, as well as stress levels at the respective feeding times, should be further investigated regardless of the time of day, age group, and working rhythm in aquaculture facilities in order to obtain further results in this context and clarify the validity of the best feeding conditions for African catfish. Within the context of this study, concentrations of established physiological welfare indicators—including glucose, lactate, cortisol, and growth hormone—were quantified, as these parameters are recognized for their sensitivity to stress responses elicited by various abiotic and biotic stressors, such as elevated stocking density, handling procedures (e.g., netting), and increased light intensity [15,47,48,49]. Since no significant effects were found among all analyzed indicators of the treatment groups, the set feeding regimes did not seem to have an impact on the fish’s stress level. Although ref. [45] had indicated a possible influence of feeding times on resistance to stress responses in fish, and ref. [50] reported lower lactate concentrations in fish fed at night, these effects could not be detected in the results of the current study, as cortisol and lactate levels were stable under the feeding regimes analyzed. Further, ref. [40] had measured cortisol and glucose concentrations of African catfish in an aquatic experiment under light conditions ranging from 410 to 1060 lux. At the end of the experiment from [40], cortisol values averaging 14.7 ng/mL and 12.7 ng/mL, respectively, and glucose concentrations averaging 3.2 mmol/L were determined. In the current experiment, cortisol and glucose concentrations of 33.25 ng/mL ± 0.38 and 3.70 mmol/L ± 0.07 were measured, which, compared to the study by [40], could indicate a stress reaction during the day feeding and additional active artificial stressors. Additionally, we recognized a relative increased number of bite marks on the skin of the fish which were fed only at day time. As aggressive behavior was often associated with stress reactions before [15,16,45], especially by photophobic species, the agonistic behavior of African catfish of the current study, which got fed only at day, may therefore have been influenced by their feeding treatment. Unfortunately, this was not investigated any further by this study and should be observed in future studies. The importance of environmental conditions on the expression of hormones such as growth hormones has already been emphasized by [51]. Thus, according to [52,53], growth hormones can have a direct influence on the behavior of fish in terms of aggressiveness, appetite or foraging behavior. In the case of Oncorhynchus mykiss, the relation to the feeding regime and the measured concentration of growth hormones was already demonstrated by [54]. Unfortunately, such overlapping effects could not be observed in the current study, as the results of the groups studied were insignificant.

In animal health studies, the liver is known for its detoxifying role in metabolism, but also for its energy storage capacity (e.g., glycogen). The HIS-Index can be used to evaluate the storage capacity of a fish liver and is also dependent by environmental factors. Low environmental conditions are often related to a low HIS-Index as the energy capacity is then modest [22,55]. Optimal environmental conditions can therefore allow a greater capacity to the liver. Ref. [55] showed an HIS-Index of 1.05 to 1.46% in C. gariepinus in feeding experiments using different types of diets. In a toxicity experiment with a plant extract from Lepidagathis aloecuroides by [22], a lower HIS-Index of 0.6 and 0.83% was determined. However, the HIS-Index for this study was calculated at 1.29% ± 0.03, which places it between those of the other two studies. Nevertheless, no difference in the HIS-Index was found between the test groups with the selected sample size.

5. Conclusions

The parameters examined in the current experiment—mortality, growth, stress indicators, liver weight—all remained unaffected by the feeding treatment groups used and the stressor added at daytime (light and noise). Thus, the feeding for the African catfish during the day seems to achieve the same growth success as night feeding. Although no significant results were found between the feeding groups of nocturnal catfish in terms of growth and stress associated parameters, the effect of conditioning may have been essential in this experiment. The potential of feed conditioning for Clarias gariepinus, which exhibits a notable growth rate and a feed conversion ratio, represents a promising avenue for further research.