Compensatory growth of Piaractus mesopotamicus raised in net cages and subjected to a period of fasting and refeeding

Abstract

The present study evaluated the effects of feed restriction on production, blood and tissue parameters of Piaractus mesopotamicus raised in net cages. The experiment was divided into two phases. In phase I, which lasted for 30 days, 960 P. mesopotamicus (368.44 ± 155.05 g) were distributed in a completely randomised design in six net cages with two treatments and three replications each, namely: fish subjected to 15 days of feed restriction followed by 15 days of feeding (FR) and fish fed continuously (CF). In phase II, which lasted for 60 days, 600 P. mesopotamicus (780.40 ± 96.07 g) were distributed in six net tanks, with the same treatments as were used in phase I. Growth performance parameters did not differ significantly between treatments (p > 0.05) in both experimental phases. However, feed intake was significantly lower in the FR fish (p < 0.05). In phase I, higher haemoglobin and erythrocyte levels were observed in the FR fish compared to the CF fish (p < 0.05). In phase II, no significant differences (p > 0.05) were observed in blood parameters between treatments. The hepatocyte histomorphometric parameters of pacu demonstrated no significant variation between treatments (p > 0.05) across both phases. The protocol of biweekly cycles of feed restriction followed by refeeding did not harm the growth of P. mesopotamicus raised in net cages and had minimal effects on blood and hepatocyte histomorphometric parameters.

Keywords:
feeding strategy; fish farming; neotropical fish; pacu

Resumo

O presente estudo avaliou os efeitos da restrição alimentar sobre parâmetros produtivos, sanguíneos e teciduais de Piaractus mesopotamicus produzidos em tanques-rede. O experimento foi dividido em duas fases. Na fase I, com duração de 30 dias, 960 exemplares de P. mesopotamicus (368,44 ± 155,05 g) foram distribuídos em delineamento inteiramente casualizado em seis tanques-rede, com dois tratamentos e três repetições cada, sendo: peixes submetidos a 15 dias de restrição alimentar seguidos por 15 dias de alimentação (FR) e peixes alimentados continuamente (CF). Na fase II, com duração de 60 dias, 600 exemplares de P. mesopotamicus (780,40 ± 96,07 g) foram distribuídos em seis tanques-rede, com os mesmos tratamentos utilizados na fase I. Os parâmetros de desempenho zootécnico não diferiram significativamente entre os tratamentos em ambas as fases experimentais. No entanto, o consumo de ração foi menor nos peixes FR. Na fase I, valores mais elevados de hemoglobina e eritrócitos foram observados nos peixes FR em comparação aos peixes CF. Na fase II, não foram observadas diferenças significativas nos parâmetros sanguíneos entre os tratamentos. Os parâmetros histomorfométricos dos hepatócitos do pacu não apresentaram variação significativa entre os tratamentos nas duas fases. O protocolo de ciclos quinzenais de restrição alimentar seguidos de realimentação não prejudicou o crescimento de P. mesopotamicus produzidos em tanques-rede e teve efeitos mínimos sobre os parâmetros sanguíneos e histomorfométricos hepáticos.

Palavras-chave:
estratégia de alimentação; piscicultura; peixes neotropicais; pacu

1. Introduction

Feed costs represent one of the main constraints in modern aquaculture, often accounting for the largest proportion of production expenses. Therefore, strategies capable of reducing feed consumption without compromising growth performance are increasingly necessary to improve economic efficiency, enhance sustainability, and increase the competitiveness of aquaculture products. In this context, feed restriction followed by refeeding has been investigated as a potential management strategy to optimize feed utilization and reduce production costs, including reductions in feed use and labor requirements (¹).

Different species of fish can survive long periods of food restriction in their natural environment. In colder months (winter), the reproductive migration and/or pre-spawning phase may represent periods of natural deprivation of food intake (²). According to these same authors, many species of fish can fast for long periods and then fully recover after refeeding. These species are well adapted to mobilize their endogenous energy reserves, such as glycogen, lipids, and proteins in tissues such as liver, muscle, and mesenteric fat, to survive periods of food deprivation (³; ⁴; ⁵).

In this context, cycles of feed restriction followed by refeeding are justified by the species’ ability to restore their metabolic reserves and, consequently, promote an increase in growth rate (length or body mass), a process known as compensatory growth (³; ⁶; ⁷). However, when not associated with adequate refeeding periods, these cycles may cause significant effects on the physiological (hematological and biochemical) parameters of fish (⁸; ⁹). The magnitude of compensatory growth, in turn, is proportional to the intensity and duration of the feed restriction imposed prior to refeeding (¹⁰).

In aquaculture, the benefits of inducing compensatory growth for different fish species include improved feed utilization due to efficiency in retaining consumed protein, control of body fat, flexible feed management, reduced labor and waste, and, consequently, reduced environmental impacts and production costs (²; ⁵; ⁶; ¹¹; ¹²). However, the use of cycles of feed restriction and refeeding can compromise the structures of a fish's digestive tract and connected organs, such as the liver (¹³; ¹⁴). Therefore, knowledge of liver histomorphometry becomes relevant to investigating metabolic changes and abnormalities when fasting and refeeding fish.

Pacu, Piaractus mesopotamicus, is an omnivorous fish that is widely distributed in the Paraná, Paraguay and Uruguay river basins (¹⁵), and considered one of the most important species in South America (⁶). The species has been highlighted in Neotropical aquaculture in Brazil, especially in the Center-West and Southeast regions, due to its excellent meat and favorable breeding characteristics, such as hardiness and fast growth (¹⁶). It has been studied over the years and shows a good response to cycles of feed deprivation followed by refeeding (¹¹; ¹⁷). However, most studies in the literature demonstrate the effects of compensatory growth in laboratory-reared P. mesopotamicus (⁵; ¹⁸; ¹⁹).

The present study, therefore, aimed to investigate the effects of feed restriction and refeeding on compensatory growth, metabolic homeostasis and hepatic histomorphometry of P. mesopotamicus raised in net cages under real production conditions, namely a commercial fish farm.

2. Material and methods

All protocols were approved by the Ethics Committee on the Use of Animals (CEUA) of the State University of Mato Grosso do Sul (UEMS) under registration number 015/2019.

2.1 Experimental design and management

This study was divided into two experiments with the aim of evaluating the strategy of food restriction and refeeding in P. mesopotamicus at two different weight stages. The two phases of the experiment (phase I and phase II) were carried out in a completely randomized design with two treatments and three replications each, totaling six net cages with a useful volume of 4 m³ each. The experiment (both phases) took place in a 5-ha reservoir in the municipality of Anastácio, state of Mato Grosso do Sul, Brazil.

The animals were acclimatized for 30 days in net cages prior to the experiment, during which they were manually fed twice a day (at 08:00 and 16:00), until apparent satiety, with pellets (5 – 8 mm in diameter) of commercial feed containing 280 g kg^-1 of crude protein, 50 g kg^-1 of ether extract, 100 g kg^-1 mineral matter and 3.5 g kg^-1 of crude fiber, as described by the manufacturer. The commercial feed used during acclimatization was the same as that used during the two experimental phases. The amount of feed supplied in phase I was calculated as 5% of live weight while in phase II it was 3%. Throughout the entire experimental period (both phases), the fish were fed at the same times as for the acclimatization period.

Water quality parameters were measured daily in the morning and were as follows: temperature of 26.01 ± 4.50 °C and dissolved oxygen of 6.50 ± 0.49 mg L^-1 (measured with an AT 160 Alfakit oximeter); pH of 6.70 ± 0.59 (measured with a portable pH meter Quimis); and toxic ammonia of 0.005 ± 0.003 mg L^-1 (determined by Alfakit Labcon colorimetric test).

Phase I – Effect of feed restriction

In phase I used 960 juveniles of P. mesopotamicus (368.44 ± 155.05 g and 26.52 ± 5.01 cm) and lasted 30 days. The animals were distributed in six net cages, at a stocking density of 40 fish per m³, in two treatments with three replications each, namely: FR – fish subjected to 15 days of feed restriction followed by 15 days of refeeding (n = 480 fish per treatment) and CF (control) – fish fed continuously (n = 480 fish per treatment). Biometry was performed on days 0 and 30 of the experimental period and survival was assessed on the last day of biometry.

Phase II – Effect of feed restriction

In phase II used 600 specimens of P. mesopotamicus (780.40 ± 96.07 g and 34.25 ± 1.58 cm) and lasted 60 days. The animals were distributed in six net cages, at a stocking density of 25 fish per m³, in two treatments with three replications each, namely: FR – fish subjected to 15 days of feed restriction followed by 15 days of refeeding (n = 300 animals per treatment) and CF (control) – fish fed continuously (n = 300 animals per treatment). Biometry was performed on days 0 and 60 of the experimental period and survival was assessed as described for phase I.

2.2 Biometric index

All animals were weighed and measured during each biometry and the data were used to calculate biometric indices. Total length was measured using an ichthyometer (²⁰) and individual weight using a pendulum digital scale, which allowed feed quantities to be readjusted according to the biomass calculated for the net cages. Fish were fasted for 24 hours prior to each biometry so that the gastrointestinal tract could be emptied. The following variables were determined: mean final length (FL, cm); mean final weight (FW, g); length gain (LG, cm) = mean final length (cm) – mean initial length (cm); weight gain (WG, g) = mean final weight (g) – mean initial weight (g); biomass gain (BG, kg) = final biomass (kg) – initial biomass (kg); survival rate (SR, %) = (final number of fish / initial number of fish) × 100; feed intake (FI, kg) = total feed provided (kg) / duration of the experimental period (days); hepatosomatic index (HSI) = 100 x (liver weight (g) / body weight (g)) and condition factor (K) = 100 × (W / L³), where W = weight (g) and L = length (cm).

2.3 Blood analyses

Prior to the end of each experimental phase, all animals remained fasting for 24 h. Blood was then collected from ten fish from each replicate (n = 30 animals per treatment), which were carefully captured randomly and subjected to anesthesia using eugenol (50 mg L^-1) (²¹). After deep anesthesia, blood (1.5 mL) was removed by puncture of the caudal vessel, using syringes bathed in anticoagulant (EDTA, 3%, ethylenediaminetetraacetic acid).

Aliquots of blood were used for hemoglobin quantification, using the cyanomethemoglobin method, as described by Collier (²²). Meanwhile, hematocrit values were determined using the microhematocrit method (²³). Total erythrocyte count was performed in a Neubauer chamber using a ProWay® light microscope (XSZ-PW206BT). Erythrocyte indices, including mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC), were calculated based on hemoglobin, hematocrit, and erythrocyte values. Another portion of the blood was centrifuged (Bioplus S200) at 9,000 RPM for five minutes at 4 °C, to obtain supernatant plasma. The aliquots of plasma were then used to quantify total glucose, triglycerides, cholesterol and total plasma protein, using colorimetric kits (Gold Analisa®).

2.4 Hepatosomatic index and hepatocyte histomorphometry

After blood collection, the fish were euthanized with an overdose of clove oil (450 mg L^-1) (²¹). The liver of each animal was collected to determine the HSI. A portion of the liver was then removed and fixed in 10 % buffered formalin for 24 h and subsequently stored in 70 % alcohol until histological processing.

Histomorphometry of hepatocytes involved collecting portions of liver, which were embedded in paraffin, sectioned at a thickness of 4 μm, and stained with eosin and hematoxylin. The histological sections were analyzed under a microscope (Opticam 500R) in 1000x magnification. Ten photomicrographs of each slide were taken using a Opticam (LOPTC 14003) camera. Sixty hepatocytes were randomly sampled to measure the area (μm²) and perimeter (μm) of the cytoplasm, as well as the area (μm²), perimeter (μm), and diameter (μm) of the nucleus. The measurements were taken using Motic 2.0 software (MoticAsia, Hong Kong), according Rodrigues et al. (¹⁴). These values were used to calculate the following parameters: nucleus area / cytoplasm area ratio (Racn) = (nuclear area / cytoplasm area) x 100; nucleus perimeter / cytoplasm perimeter ratio (Rpcn) = (nuclear perimeter / cytoplasm perimeter) x 100; hepatocyte nuclear volume (NV, μm³) = 4/3.π.r³, where r³ = diameter/2); and hepatocyte nucleus circularity (NC) = p²/4.π.a, where p² = nuclear perimeter and a = nuclear area.

2.5 Statistical analysis

All data were tested for normality using the Shapiro-Wilk test and for homogeneity of variances using Levene’s test. Since the data met the assumptions of normality and homoscedasticity, productive performance, blood parameters, and hepatocyte histomorphometry data were analyzed by analysis of variance (ANOVA), followed by the F-test at a 5% significance level. Statistical analyses were performed using Infostat software.

3. Results

The P. mesopotamicus submitted to treatments with (FR) and without (CF) feed restriction did not differ significantly (p > 0.05) for any of the productive performance parameters (FL, FW, LG, WG, BG, HSI, K, S) (Table 1). However, feed consumption was significantly lower (p < 0.05) for FR fish compared to CF fish during both experimental phases.

Table 1.
Productive performance parameters (mean ± standard deviation) of Piaractus mesopotamicus raised in net cages, subjected to biweekly cycles of feed restriction and refeeding (FR) or continuous feeding (CF), during two experimental phases (30 and 60 days).

Variables	Phase I (30 days)
	Treatments
	FR	CF	p-value	CV (%)
FL (cm)	28.40 ± 3.19	28.78 ± 2.05	0.8695	9.38
FW (g)	480.71 ± 151.13	521.49 ± 118.18	0.7314	27.07
LG (cm)	2.56 ± 2.10	2.77 ± 1.71	0.9013	71.84
WG (g)	133.27 ± 116.14	153.59 ± 118.26	0.8422	81.71
BG (g)	13.50 ± 11.15	18.82 ± 15.56	0.6559	83.79
FI (kg)	24.15 ± 2.83 b	49.63 ± 1.49 a	0.0002	6.14
HSI (%)	1.14 ± 0.17	1.20 ± 0.34	0.8714	14.03
K	2.15 ± 0.24	2.16 ± 0.02	0.9115	11.79
S (%)	95.04 ± 3.83	93.54 ± 2.82	0.614	3.57

	Phase II (60 days)
	FR	CF	p-value	CV (%)
FL (cm)	34.56 ± 2.76	34.99 ± 1.51	0.8362	6.76
FW (g)	796.23 ± 144.14	818.40 ± 76.89	0.8257	14.31
LG (cm)	0.85 ± 0.52	1.15 ± 0.58	0.6104	65.09
WG (g)	30.52 ± 28.58	45.92 ± 28.14	0.5425	74.2
BG (g)	6.48 ± 5.71	6.02 ± 1.35	0.8988	66.43
FI (kg)	49.24 ± 7.48 b	101.78 ± 28.25 a	0.0357	27.36
HSI (%)	1.17 ± 0.16	1.25 ± 0.10	0.8978	12.76
K	1.83 ± 0.19	1.79 ± 0.26	0.7898	10.58
S (%)	94.12 ± 0.58	94.34 ± 2.08	0.9902	1.62

The FR fish had significantly higher (p < 0.05) hemoglobin and erythrocyte values compared to CF fish during Phase I (Table 2). Additionally, erythrocyte indices were significantly affected in this phase, with FR fish showing higher mean corpuscular volume (MCV), while CF fish presented higher mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) (p < 0.05). In Phase I, glucose, triglycerides, cholesterol, total proteins and hematocrit did not differ significantly (p > 0.05) between treatments. Similarly, in Phase II, no significant differences (p > 0.05) were observed between treatments for biochemical parameters, hemoglobin, erythrocytes, or erythrocyte indices.

Table 2.
Biochemical, hematological and erythrocyte indices (mean ± standard deviation) of Piaractus mesopotamicus reared in net cages and subjected to biweekly cycles of feed restriction followed by refeeding (FR) or continuous feeding (CF), during two experimental phases (30 and 60 days).

Phase I (30 days)
Treatments	Biochemical parameters
	Glucose (mg dL-1)	Triglycerides (mg dL-1)	Cholesterol (mg dL-1)	Total proteins (mg dL-1)
FR	111.29 ± 18.06	179.21 ± 38.86	299.21 ± 38.86	3.06 ± 0.90
CF	119.81 ± 18.79	171.50 ± 30.65	291.50 ± 30.65	3.49 ± 0.94
P-value	0.2172	0.5485	0.5485	0.2172
CV (%)	15.93	19.82	11.76	28.03

	Hematological parameters
	Hemoglobin (g dL-1)	Hematrocrit (%)	Erythrocytes (106 mL-1)
FR	14.30 ± 3.44 a	42.36 ± 2.31	1.42 ± 0.56 a
CF	12.06 ± 2.74 b	41.25 ± 5.78	0.89 ± 0.36 b
P-value	0.0008	0.5081	<0.0001
CV (%)	23.59	10.8	41.33
	MCV (fl)	MCH (pg)	MCHC (g dL-1)
FR	400.43 ± 135.59 a	1.09 ± 0.50 b	26.59 ± 6.11b
CF	263.30±74.65 b	2.08 ± 1.04 a	37.04 ± 4.88a
P-value	0.0029	<0.001	<0.001
CV (%)	34.25	52.10	18.32

	Phase II (60 days)
	Biochemical parameters
	Glucose (mg dL-1)	Triglycerides (mg dL-1)	Cholesterol (mg dL-1)	Total proteins (mg dL-1)
FR	91.33 ± 27.03	187.63 ± 29.86	347.05 ± 93.15	5.80 ± 2.01
CF	83.67 ± 26.35	192.31 ± 24.32	356.72 ± 64.85	6.86 ± 2.97
P -value	0.2705	0.5130	0.7115	0.1108
CV (%)	30.5	14.36	23.25	40.03
	Hematological parameters
	Hemoglobin (g dL-1)	Hematrocrit (%)	Erythrocytes (106 mL-1)
FR	9.13 ± 2.46	64.77 ± 10.76	4.57 ± 1.32
CF	9.72 ± 2.37	68.73 ± 11.87	4.90 ± 1.33
P-value	0.3482	0.2535	0.3438
CV (%)	25.61	16.97	28.10
	MCV (fl)	MCH (pg)	MCHC (g dL-1)
FR	140.26 ± 58.55 a	4.65 ± 1.57 a	14.59 ± 3.97 a
CF	153.24 ± 58.94a	4.18 ± 1.69 a	13.88 ± 5.35 a
P-value	0.4536	0.2793	0.6279
CV (%)	37.42	37.63	33.85

All evaluated hepatocyte histomorphometric parameters (NA, CA, Racn, NP, CP, Rpcn, NV, and NC; Table 3] did not differ significantly between treatments during both experimental phases (p > 0.05).

Table 3.
Hepatocyte histomorphometric variables (mean ± standard deviation) of Piaractus mesopotamicus raised in net cages, subjected to biweekly cycles of feed restriction and refeeding (FR) or continuous feeding (CF), during two experimental phases (30 and 60 days).

Phase I (30 days)
Variablesns	Treatments
	FR	CF
Na (μm2)	106.00 ± 0.01	105.00 ± 0.01
CA (μm2)	8.92 ± 0.04	8.94 ± 0.05
Racn	12.38 ± 0.17	12.31 ± 0.18
NP (μm)	3.74 ± 0.01	3.73 ± 0.01
CP (μm)	11.76 ± 0.06	11.79 ± 0.06
Rpcnns	32.19 ± 0.27	32.07 ± 0.27
NV (μm3)	0.63 ± 0.02	0.64 ± 0.01
NC (0-1)	0.94 ± 0.01	0.97 ± 0.03

Phase II (60 days)
	FR	CF
Na (μm2)	99.00 ± 0.43	93.00 ± 0.34
CA (μm2)	6.09 ± 2.42	7.60 ± 2.71
Racn	16.14 ± 5.30	13.30 ± 8.90
NP (μm)	3.33 ± 0.78	3.45 ± 1.25
CP (μm)	9.19 ± 4.90	9.93 ± 1.95
Rpcn	37.66 ± 6.75	35.52 ± 11.89
NV (μm3)	0.42 ± 0.32	0.55 ± 0.22
NC (0-1)	0.95 ± 0.42	0.94 ± 0.09

4. Discussion

This is the first study to evaluate feed restriction followed by refeeding with P. mesopotamicus raised in net cages under real production conditions. The responses of productive performance and physiological parameters of fish subjected to periods of feed restriction followed by refeeding can vary widely among species and due to physiological status and animal health (²⁴). In the present study, the P. mesopotamicus raised in net cages (average initial weight in phase I of 368 g and in Phase II of 780 g) and subjected to biweekly cycles of feed restriction followed by refeeding exhibited similar growth performance to that of control fish (continuously fed).

In the present study, fish of the feed restriction treatment experienced an average reduction of 48% in feed consumption compared to animals fed continuously, without significant changes to the other growth performance parameters evaluated, during both experimental phases (30 and 60 days). This result was similar to that observed for matrinxã, Brycon amazonicus, which experienced a 40% reduction when subjected to two restriction days followed by three refeeding days, when raised in a running and aerated water system (²⁵). On the other hand, other studies with the freshwater fish tambaqui, Colossoma macropomum (⁹), and pirapitinga, Piaractus brachypomus (²⁶), raised in a recirculating aquaculture system (RAS) subjected to cycles of feed restriction of one day followed by six days of refeeding, experienced reductions of 17% and 22%, respectively. These results endorse the importance of adopting food restriction strategies followed by refeeding in different fish production systems to reduce feed costs (²⁷; ²⁸) and environmental degradation (⁶).

Glucose, triglycerides and cholesterol concentrations were not influenced by biweekly periods of feed restriction followed by refeeding, during both experimental phases of the present study. Glucose values remained unchanged, probably due to glycogenolysis (breakdown of hepatic glycogen into glucose) as a way of meeting animal energy demand (²⁹). The feed restriction applied consisted solely of limiting the supplied ration, which may have contributed to maintaining metabolic stability during the experimental period. The feed restriction applied consisted solely of limiting the supplied ration. Triglycerides are an energy source found in the muscle and liver of teleost fish and are mobilized during excessive metabolic activity (³⁰). Cholesterol (structural lipid) can be synthesized and/or used in response to dietary restriction (³¹). However, the responses of cholesterol and triglyceride concentrations to food restriction in fish are conflicting, as they may vary according to species (³²), and feed restriction period (³³; ³⁴), decreasing, increasing (³⁵), or remaining unchanged (³⁶).

The lipostatic model proposed by Jobling and Johansen (²²) suggests that post-fasting hyperphagia and rapid lipid replenishment promote compensatory growth via IGF signaling, resulting in hyperanabolism (³⁷). This mechanism was evidenced in P. mesopotamicus raised in a recirculating aquaculture system (RAS), subjected to 30 days of fasting followed by 50 days of refeeding, with recovery of hepatic parameters and significant compensatory growth (³⁸). In the present study, despite the absence of intermediate physiological assessments, the biweekly protocol in net cages also maintained biochemical stability and growth performance comparable to the control group, except for hemoglobin and erythrocytes in phase I. These results reinforce the high physiological plasticity of the species, capable of recovering under different regimes and production systems, highlighting the potential of the feed restriction followed by refeeding strategy to optimize production performance and, furthermore, to be a viable strategy for reducing operational costs in aquaculture, such as feed and labor.

Total plasma proteins are fundamental constituents of plasma, and most resting metabolism of fish occurs through protein catabolism; however, protein must be the last resource to be degraded by animals (³⁹). Total plasma protein did not differ between treatments in the present study, which may be related to the preservation of protein resources. Silva et al. (⁸) evaluated different feed restriction strategies for juvenile pacamã, Lophisolirus alexandri, and observed that protein values were higher after fasting, even for fish of the control treatment. These results demonstrate the ability of fishs to spare protein degradation during periods of food restriction. On the other hand, the levels of plasma proteins are regulated by the liver and are used to indicated the functioning of this organ in fish (⁴⁰).

The evaluation of hematological parameters are useful to verify the health status of fish in aquaculture (⁴¹), especially during periods of feed restriction. In Phase I of the present study, fish subjected to feed restriction followed by refeeding showed higher hemoglobin and erythrocyte values, which may be related to a compensatory response to optimize oxygen transport under stress conditions (⁴²), such as feed restriction. In addition, erythrocyte indices were also altered in this phase, with higher MCV observed in FR fish, while CF fish presented higher MCH and MCHC. These changes may reflect adjustments in erythrocyte morphology and hemoglobin content, indicating different physiological strategies to maintain oxygen-carrying capacity under distinct feeding regimes. Several studies have demonstrated that hematological changes, including hematocrit, hemoglobin, erythrocyte counts, and derived indices, can occur in fish subjected to feed restriction.

The structural organization of the liver is crucial for the growth performance and health of fish, as the liver plays a fundamental role in the synthesis, regulation, and degradation of substances involved in animal growth and immune response (⁴³; ⁴⁴). The histomorphometric variables of hepatocytes in P. mesopotamicus were similar during both experimental phases, indicating that, at least at the cellular level, there was no effect between restriction and refeeding treatments. Ostaszewska et al. (⁴⁵) observed changes in the size of hepatocytes and their nuclei in response to changes in food supply for P. mesopotamicus subjected to diets with different protein sources. Additionally, nucleolar characteristics, size, and number of diploid liver cells in fish can demonstrate the functional activity of the genomes in the nucleus and can be modulated by environmental factors, such as food availability, which can affect animal growth and condition. In general, hepatocytes with lipid deposition exhibit rounded vacuoles in the cytoplasm, and peripheral displacement of the nucleus may also be observed (⁴⁶; ⁴⁷). Therefore, it can be concluded that the fish in the present study that were subjected to biweekly cycles of food restriction followed by refeeding did not experience injurious conditions.

Finally, and reinforcing the positive effects of the feeding strategy evaluated in the present study with P. mesopotamicus, the final growth of the fish subjected to feed restriction followed by refeeding was not compromised, reaching the same weight and body size at the end of each experimental phase as the animals fed continuously. Thus, it can be inferred that P. mesopotamicus, when subjected to feed restriction protocols, has a remarkable ability to recover growth after refeeding, as also observed in other studies with the same species in different experimental conditions (⁵; ¹⁹).

5. Conclusion

The present study demonstrated that the use of 15 days of feed restriction followed by 15 days of feeding was satisfactory for growth performance of P. mesopotamicus and caused minimal changes in biochemical and hematological parameters. Furthermore, there were no changes in the hepatocyte histomorphometric variables in the two experimental phases. These results demonstrate the important applicability of feed restriction for P. mesopotamicus raised in net cages, as it can reduce production costs and improve water quality by reducing the excretion of nitrogenous compounds due to a 48% reduction in feed consumption. However, more studies are needed to evaluate the different development phases of P. mesopotamicus raised in net cages under real production conditions with different experimental protocols.

Data availability statement

The data and material that support the findings of this study are available from the corresponding author upon reasonable request.

• Generative AI use statement
  The authors did not use generative artificial intelligence tools or technologies in creating or editing any part of this manuscript.

Acknowledgments

The authors thank the Graduate Program in Animal Science at the State University of Mato Grosso do Sul (UEMS) and the Graduate Program in Animal Science at the Federal University of Mato Grosso do Sul (UFMS) for the academic and institutional support in carrying out this research. This study was partially funded by the Coordination for the Improvement of Higher Education Personnel – Brazil (CAPES) – Funding Code 001. The authors R. M. G. Acunha, F. C. Oliveira, and G. K. Vieira thank CAPES for providing scholarships. The authors also thank Fazenda São Judas Tadeu for making the experimental area used in this study available.

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Edited by

• Editor:
  Rondineli P. Barbero

Publication Dates

• Publication in this collection
  03 Aug 2026
• Date of issue
  2026

History

• Received
  22 Oct 2025
• Accepted
  11 May 2026
• Published
  16 June 2026