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Article

Effects of Stocking Densities on the Growth Performance and Physiology of Juvenile Piaractus brachypomus in Recirculating Aquaculture System

by
Imaculada de Morais Carvalho Ananias
,
Sidney dos Santos Silva
,
André de Sena Souza
,
Fábio Aremil Costa dos Santos
,
Thamara Bentivole Magalhães
,
Wagner Junio de Freitas Martins
,
Gisele Cristina Favero
and
Ronald Kennedy Luz
*
Laboratório de Aquacultura, Departamento de Zootecnia, Universidade Federal de Minas Gerais, Avenida Antônio Carlos, n° 6627, Belo Horizonte CEP 30161-970, Brazil
*
Author to whom correspondence should be addressed.
Fishes 2025, 10(4), 171; https://doi.org/10.3390/fishes10040171
Submission received: 10 March 2025 / Revised: 4 April 2025 / Accepted: 10 April 2025 / Published: 11 April 2025
(This article belongs to the Section Fishery Economics, Policy, and Management)
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Abstract

The effects of different stocking densities on the growth and physiology of juvenile Piaractus brachypomus were evaluated in two experiments. Experiment I used juveniles weighing 1.5 ± 0.4 g at the following densities for 20 days: D0.68—0.68 kg/m3; D1.45—1.45 kg/m3; D4.41—4.41 kg/m3; and D7.17—7.17 kg/m3. Experiment II used juveniles weighing 6.66 ± 1.3 g at the following densities for 20 days: D1.0—1.00 kg/m3; D1.95—1.95 kg/m3; D5.63—5.63 kg/m3, and D7.90—7.90 kg/m3. Both experiments showed a reduction in dissolved oxygen levels in the water, with Experiment II showing a plateau effect from 2.34 kg/m3 (p < 0.05). Final weight, final length, weight gain, daily weight gain, and specific growth rate were inversely proportional to density (p < 0.05), while final biomass, feed intake, and feed conversion were directly related to density in both experiments (p < 0.05). At the end of Experiment II, plasma triglycerides decreased as stocking density increased (p < 0.05), and hemoglobin and mean corpuscular volume were higher at the lowest density (D1.0) (p < 0.05). High stocking densities reduced dissolved oxygen, characterizing a hypoxic state in both experiments, affecting growth and some physiological parameters. Therefore, studies testing stocking densities for P. brachypomus in normoxic situations are still needed.
Keywords:
growth; hypoxia; intensive production; pirapitinga; water quality
Key Contribution: The present study is the first to address different stocking densities in the production of juvenile pirapitingas Piaractus brachypomus (initial average weight of 1.5 g), in RAS, and with excellent growth responses.

1. Introduction

Closed production systems allow for greater control over production since water quality parameters can be managed compared to traditional systems [1]. Recirculating aquaculture systems (RASs) have the advantage of reduced water use, as only a small amount is replaced with clean water [2], which limits the risk of environmental contamination [3]. In addition, the versatility of the RAS allows its implementation in different climates and environments [4], enabling production in locations closer to markets. However, the assembly and maintenance of these filters and blowers, along with the uninterrupted consumption of electricity, generate costs [4], and there is a need for specialized labor to monitor and maintain the equipment.
Therefore, to make a RAS rearing system viable, it is necessary to determine the fish stocking density that allows maximum animal production without compromising their performance. Defined as the biomass or quantity of animals produced per tank [5], stocking density is an aspect of management that is of the utmost importance to produce any species [6], whatever the production system used. In addition to the efficiency of the system and the bacterial community present in the RAS [7], stocking density can directly influence fish growth and health [8,9,10,11]. Animal growth is inversely linked to stocking density due to competition for food and space [12,13,14], with influence on fish stress levels [10,15].
The health and well-being of fish can be determined through physiological analyses performed by evaluating hematological and blood biochemical indices [16]. Poorly dimensioned stocking density is considered a chronic stressor capable of affecting the physiological parameters of fish [17,18]. The so-called physiological chain is the name given to a series of physiological mechanisms that seek the adaptation of the animal to stress conditions, classified into three levels of responses [19]. In stressful situations, studies carried out with Puntius sarana show the influence of stocking density on hematological variables, increasing hematocrit and hemoglobin values [20]. Biochemical values can also be altered, since secondary responses to stress are related to glycolytic pathways, through gluconeogenesis to obtain energy for escape and/or adaptation [21], as reported by the increase in glycemia and changes in cholesterol and triglyceride levels of Ictalurus punctatus [22] and increased cortisol, glucose, cholesterol, and triglycerides of Megalobrama amblycephala [23]. Linked to energy metabolism, thyroid hormones can also be influenced by stocking density [24], as seen in I. punctatus [22].
Pirapitinga, also known as cachama blanca or “pacu de barriga vermelha” (Piaractus brachypomus), is a native fish of the Amazon and Orinoco River basins, and is easily found in northern Brazil, Colombia, Peru, Venezuela, and Ecuador [25,26]. In addition to its countries of origin, the species has been of importance in Indian [27,28] and Thai [29,30] aquaculture. In Brazil, the species is used for both consumption and for producing the hybrid tambatinga (Colossoma macropomum x P. brachypomus). It is widely produced in excavated tanks [27], and results for P. brachypomus production in intensive systems such as the RAS remain limited [31,32,33,34]. Thus, there is a need to carry out further studies to demonstrate the efficiency of producing the species in the RAS, with the aim of establishing adequate stocking densities for the different cultivation phases.
The present study, therefore, aimed to evaluate the effects of different stocking densities on the growth performance and physiological parameters of juvenile P. brachypomus.

2. Materials and Methods

The experiment was carried out at the Laboratório de Aquacultura (LAQUA) of the Universidade Federal de Minas Gerais (UFMG, Brazil), and was divided into two experiments with all procedures previously approved by the Comissão de Ética no Uso de Animais (CEUA/UFMG—n° 17/2023), in 05/08/2023.

2.1. Experiment I

Experiment I used 798 juvenile P. brachypomus, with an initial weight (IW) of 1.5 ± 0.4 g (media ± standard deviation) and measuring 4.5 ± 0.5 cm (media ± standard deviation) in total length. The animals were distributed in 16 tanks with 28 L of useful volume, maintained in a RAS with homemade mechanical filters, made of acrylic blankets changed weekly, and homemade biological filters that we use as a biofilter. The crushed stone is stored in submerged boxes that allow water to contact the surface of the stones. We used a heating system (thermostat with heater, Warner, 200 w, Ocean Tech Professional Aquarium®, Porto Alegre, Brazil) and water pumping systems (submersible pump, SB2000, 1950 L/h, 30 w, Salobetter®, São Caetano do Sul, Brazil), and supplementary aeration from a compressor (2 HP radial compressor, CR-5, Ibram Ltd.a, São Paulo, Brazil) and porous air stones connected by 4 mm silicone hoses as diffusers. The water flow was kept at 0.59 ± 0.98 L/min (media ± standard deviation) for all tanks, resulting in a total of 30.24 renewals of tank volume per day. The following four stocking densities were tested, with four replicates each: D0.68—0.68 kg/m3; D1.45—1.45 kg/m3; D4.41—4.41 kg/m3; D7.17—7.17 kg/m3.
Experiment I lasted 20 days due to rapid growth and limited space in the 28 L tank. The photoperiod was 12 h of light (Key West DNI group, digital timer).
The animals were fed a commercial extruded diet (1.3–1.5 mm in diameter) from the Wean Prime line (Total Rações®, Três Corações, Brazil) with 45% crude protein, 5% ether extract, 15% mineral matter (max.), 40% crude fiber (max.), 0.2% calcium (min.), 0.3% calcium (max.), 0.04% vitamin E (min.), and 0.1% vitamin C (min.) (manufacturer’s data). Feeding was carried out three times a day (8:00, 12:00, and 16:00), with a feeding rate of 10% of the daily biomass [35]. The amount was readjusted after the 10th day of production through partial biometrics performed to obtain the average biomass of each tank, ultimately calculating the adjustment. The remaining feed was collected, dried in a forced circulation oven at 105 °C (Nova Etica/Ethink®), and subsequently weighed to calculate consumption and apparent feed conversion.

2.2. Experiment II

Experiment II used 2850 juvenile P. brachypomus with an initial weight (IW) of 6.6 ± 1.3 g (media ± standard deviation) and measuring 8.0 ± 0.5 cm (media ± standard deviation) in total length. The animals were distributed in 12 tanks, each with a useful volume of 100 L, maintained in a RAS with homemade mechanical filters made of acrylic blanket, changed weekly, and homemade biological filters that we used as a biofilter. The crushed stone was stored in submerged boxes that allowed contact between the water and the surface of the stones. We used a heating system (thermostat with heater, Warner, 200 w, Ocean Tech Professional Aquarium®, Porto Alegre, Brazil) and water pumping systems (submersible pump, SB2000, 1950 L/h, 30 w, Salobetter®, São Caetano do Sul, Brazil), along with supplementary aeration with compressor (2 HP radial compressor, CR-5, Ibram Ltd.a, SP, Brazil) and porous air stones connected by 4 mm silicone hoses as diffusers. The water flow was kept at 1.12 ± 0.12 L/min (media ± standard deviation) for all tanks, totaling 16.12 renewals of tank volume per day. The following four different stocking densities were tested, with three replicates each: D1.0—1.00 kg/m3; D1.95—1.95 kg/m3; D5.63—5.63 kg/m3, and D7.90—7.90 kg/m3. Experiment II lasted for another 20 days due to rapid growth and limited space in the 100 L tank.
Due to the larger size of the animals, they were fed a commercial extruded diet of 2–3 mm in diameter from the Aquos Alevinos 45 line (Total Rações®, Três Corações, Brazil), containing 45% crude protein, 12% moisture, 8% ether extract, 15% mineral matter, 4% crude fiber, 0.002% calcium (min.), 0.003% calcium (max.), 0.8% phosphorus, and 0.06% vitamin C (manufacturer’s data). Feed was offered three times a day (8:00, 12:00, and 16:00), with a feeding rate of 10% of daily biomass [35]. The amount was readjusted after the 10th day of production through partial biometrics performed to obtain the average biomass of each tank and finally calculate the adjustment. The remaining feed was collected, dried in a forced circulation oven at 105 °C (Nova Etica/Ethink®), and subsequently weighed to calculate consumption and apparent feed conversion.

2.3. Water Quality Analysis

In both experiments, temperature was measured three times a week using a multiparametric probe (Hanna Instruments HI98130, Hanna®, Barueri, Brazil) together with total ammonia (LabconTest Alcon® colorimetric kit, Camburiú, Brazil). The toxic ammonia (NH3) was calculated according to the table found in the kit. Dissolved oxygen (DO) (YSI multiparametric probe, EcoSense® DO200A, Yellow Sprongs Instruments Co., Inc., Yellow Sprongs, OH, USA) and pH were measured once a week.

2.4. Growth and Survival

Biometrics were performed after 10 and 20 days in both experiments. Weight was measured using a Marte 5000 digital analytical scale (accuracy of 0.001 g), with juveniles previously anesthetized with 20 mg/L of eugenol [36], while total length was measured using an ichthyometer (accuracy of 0.1 cm). The following data were obtained:
-
Initial weight (g) (IW);
-
Initial total length (cm) (IL);
-
Final weight (g) (FW);
-
Final total length (cm) (FL);
-
Weight gain (g) (WG) = FW − IW;
-
Daily weight gain (g) (DWG) = (FW − IW)/ΔT, where ΔT is the duration of the experiment in days;
-
Specific growth rate (%/day) (SGR) = 100 (ln FW − ln IW)/ΔT, where ΔT is the duration of the experiment in days;
-
Final biomass (FB) (kg/m3), where all the weights of all the fish were added together and the value found was converted to kg/m3;
-
Feed consumption (FC) (kg) = (feed offered (g) − feed leftover (g))/100
-
Feed conversion rate (FCR) = consumption/(FB − IB), where IB is the initial biomass.
Survival was also determined after 20 days in both experiments and was calculated considering the following formula: survival (%) = (number of live juveniles × 100)/(total number of juveniles per tank).

2.5. Hematological and Biochemical Analysis

At the end of Experiment II (after 20 days of rearing), the animals were fasted for 24 h. Four fish from each tank (n = 12 per treatment) were anesthetized with eugenol (50 mg/L) [33], restrained with a damp cloth, and subjected to blood collection by caudal venipuncture with previously heparinized 1 mL syringes, collecting a total of 300 µL of whole blood per fish.
Blood samples (50 µL) were dispensed into microtubes containing sodium heparin anticoagulant (10%) for determination of hemoglobin (tHb) using a commercial colorimetric kit (Quibasa-Bioclin, Belo Horizonte, MG, Brazil) and hematocrit (Ht) by the microhematocrit method [37] using capillary tubes.
Total plasma protein (TPP) was determined with an analog refractometer (0 to 90% Brix)—RHB0-90 after rupture of the microhematocrit tube. The number of erythrocytes (RBCs) was determined by diluting 10 µL of whole blood in 2 mL of citrate formalin and then counting them in a Neubauer chamber.
The remaining whole blood (250 µL) was centrifuged at 4000 RPM for 10 min (microcentrifuge SL-5AM SPINLAB CO®, Shenzhen, China) to separate the plasma (125 µL) and determine the following biochemical parameters: glucose (GLU), triglycerides (TG), cholesterol (TC), alanine aminotransferase (ALT), and aspartate aminotransferase (AST). All analyses were performed by colorimetric method using commercial kits (Quibasa-Bioclin, Belo Horizonte, MG, Brazil), with readings on a Biochrom Libra S22 UV-VIS spectrophotometer (Biochrom Instruments, Cambridge, UK).
The hematimetric indices (mean corpuscular volume—MCV, mean corpuscular hemoglobin—MCH, and mean corpuscular hemoglobin concentration—MCHC) were calculated according to the formulations established by [38]. These analyses were not performed at the end of Experiment I due to the small size of the juveniles for blood collection.

2.6. Viscerosomatic and Hepatosomatic Indexes

After blood collection, the same four animals from each tank (n = 12 per treatment) were euthanized with 285 mg/L eugenol solution [39], and their viscera (stomach, intestine, pyloric cecum, liver, gallbladder, and perivisceral fat) and liver were collected to determine the following indices:
-
Viscerosomatic Index (VSI) (%) = 100 × ((weight of total viscera (g)/body weight (g)).
-
Hepatosomatic Index (HSI) (%) = 100 × (weight of liver/body weight).

2.7. Statistical Analysis

The data from both experiments were tested for homoscedasticity and normality using Shapiro–Wilk and Levene’s tests, respectively. The data were then subjected to ANOVA followed by regression tests. Variables that were not significant in regression tests were subjected to mean comparison using Tukey’s test at 5% probability.

3. Results

During Experiment I, dissolved oxygen levels decreased linearly with increasing stocking density (p < 0.05) (Figure 1A). Dissolved oxygen levels also decreased with increasing density during Experiment II, but only up to D1.95, with constant values from 2.34 kg/m3 onwards, reflecting a plateau effect (p < 0.05) (Figure 1B). The other water quality parameters were not influenced by stocking density (p > 0.05) (Table 1).
Survival was not affected in either experiment, being 94.05 ± 12.13% for Experiment I and 99.28 ± 1.19% for Experiment II.
Final weight (Figure 2A), final length (Figure 2B), weight gain (Figure 2C), daily weight gain (Figure 2D), and SGR (Figure 2F) were inversely related to increasing stocking density (p < 0.05) at the end of Experiment I. Final biomass (Figure 2E), feed consumption (Figure 2G), and feed conversion (Figure 2H) were directly related to increasing stocking density (p < 0.05).
Final weight (Figure 3A), final length (Figure 3B), weight gain (Figure 3C), daily weight gain (Figure 3D), and SGR (Figure 3F) were inversely related to increasing stocking density (p < 0.05) at the end of Experiment II. Final biomass (Figure 3E), feed consumption (Figure 3G), and feed conversion (Figure 3H) were directly related to increasing stocking density (p < 0.05).
At the end of Experiment II, plasma triglycerides were inversely related to increasing density (Figure 4 and Table 2) (p < 0.05). There were no differences in the regression analyses for tHb and MCV; however, higher values were observed for the lowest density (D1.0) (p < 0.05) (Table 1). The other variables analyzed, as well as VSI and HSI, did not differ significantly among treatments (Table 2).

4. Discussion

Dissolved oxygen was reduced at the higher stocking densities in both experiments, which may have affected the growth of juveniles kept at D4.41 and D7.17 during Experiment I, and D5.63 and D7.9 during Experiment II. The high survival and physiological changes observed in tHb, MCV, and TG highlight the oxiconforming characteristics of P. brachypomus. However, the adaptability of the species and the possibility of its production in the RAS are reinforced by survival rates exceeding 90% after the two experiments, along with unchanged somatic indices, as observed in previous studies [31,32,33,34].
Among the physico–chemical parameters of the water, only dissolved oxygen was influenced by stocking density in both experiments, which influenced the growth of juveniles. The ideal concentration of dissolved oxygen for rearing P. brachypomus has not yet been determined. However, rivers in the Amazon region have a large amount of organic matter and high water temperatures, variables that result in low levels of available dissolved oxygen [40,41,42]. These conditions have led Amazonian species to develop morphological and physiological adaptations to resist low oxygen concentrations [43,44]. The reduction in oxygen with increasing density in both phases of the experiment was a result like that found in studies with tambaqui (C. macropomum) in the RAS, also an Amazonian species [12,14]. For example, C. macropomum can withstand hypoxic conditions [45,46], including excellent recovery after periods of exposure to air [47]. The pacu (Piaractus mesopotamicus), although endemic to other river basins, belongs to the same genus as P. brachypomus and can also be reared in lower concentrations of dissolved oxygen, around 3 mg/L [48]. The high survival and growth of the juveniles after the experimental period of this study confirms the adaptation and production capacity of P. brachypomus in conditions of low oxygen availability, demonstrating its oxiconforming capacity even though its growth is reduced. However, there is still a need for more studies examining the tolerance of P. brachypomus in hypoxic situations, how their adaptation mechanisms are activated, as well as research on stocking density in normoxic conditions.
However, the maintenance of the other parameters means that the RAS used was efficient in maintaining water quality. As all the tanks were managed in the same way, future studies should aim to improve the aeration system of the RAS at high densities so that dissolved oxygen levels remain higher than those recorded here. Little is known about NH3 tolerance levels for P. brachypomus culture; however, P. mesopotamicus, used as a study object for NH3 toxicity, could withstand concentrations of up to 1.49 mg/L of total ammonia without symptoms of discomfort [49]. Similar studies were carried out with the tambacu hybrid (C. macropomum x P. mesopotamicus), which indicated an LC of 1.63 mg/L of NH3 [50]. Even with the low temperature for Amazonian fish in Experiment II, the daily weight gain, although affected by oxygen concentrations and stocking densities, together with the high survival rate, demonstrates the possibility of rearing P. brachypomus in regions with mild temperatures. Results like production in C. macropomum have been observed, allowing Amazonian species to be produced in colder regions [51,52].
The two experiments showed similar patterns in relation to growth parameters, except for biomass, feed consumption, and feed conversion ratio, all of which were inversely proportional to the increase in stocking density. These results suggest that space and low dissolved oxygen concentrations are limiting factors for the growth of P. brachypomus. Factors such as the species’ biology, age, growth stage, and the type of system used determine the stocking density variable [53]. In addition to space limitations, there is an increase in energy demand due to competition for food at high stocking densities [13,54]. The low oxygen concentrations at the highest densities tested in the two experiments should also be considered when reducing the growth of juveniles. In situations of chronic stress, such as hypoxia resulting from the experiments and stocking densities, growth is reduced due to energy reorganization for homeostasis [55]. A study carried out with C. macropomum (0.54 g) at different densities (0.3, 0.6, and 0.9 kg/m3) also showed similar results, with a reduction in dissolved oxygen levels at higher densities and consequently lower growth at the same densities [14]. However, even under normoxic conditions, juvenile C. macropomum with different size classifications suffered the influence of the stocking densities tested (34.88 g—0.5; 1.0; 1.6 kg/m3); (150.61 g—1.5; 3.0; 4.5 kg/m3); (300–400; 400–500 g—3.9 kg/m3) on their growth [13]. High stocking densities also reduced the development of individuals of Prochilodus cearaensis (0.09; 0.15; 0.3 g/L) [56], Micropterus salmoides (50; 75; 100 peixes/m3) [9], and Clarias gariepinus (100; 200; 400 kg/m3) [10], which is a common response for different species.
One of the indices used to assess the productivity of the system is the final biomass produced [57]. The final biomass, feed consumption, and feed conversion ratio increased with the increase in stocking density in both experiments. The increase in biomass is explained by the greater number of animals kept at the higher densities. However, the feed conversion ratio values remained between the averages of 0.86 and 1.13, results that have already been observed for the species [32,58]. Stocking density influences feeding competition behavior, oxygen consumption, and the response to stress in fish, consequently affecting the feeding rate [59], an influence that varies according to the species [60]. Feed consumption is linked to nutrient retention in fish; when it increases, retention is reduced, interfering with growth [60] and increasing feed conversion. However, in situations of hypoxia, feed consumption and feed conversion ratio are worsened, as reported for C. macropomum [14,61], channel catfish hybrids (Ictalurus punctatus x Ictalurus furcatus) [62], and Atlantic salmon (Salmo salar) [63].
Hematological and biochemical parameters and somatic indices can be used to assess an animal’s stress condition. Somatic indices are linked to an animal’s nutritional status [64], while biochemical and hematological parameters show the influence of stress factors on its health and metabolism [65]. The limited influence of stocking density in Experiment II of the study on unchanged biochemical and hematological parameters, combined with survival and growth performance data, shows the species’ adaptation to the RAS. Changes in triglyceride levels are related to energy metabolism linked to glycolytic pathways [66]. Decreases in value may be linked to energy consumption as a way of adapting to stressful situations. Triglycerides are considered important indicators in conditions of high stocking density [67]. The reduction in triglyceride levels observed with the increase in stocking density and the hypoxic conditions resulting from overcrowding may indicate a form of energy consumption by the organism to adapt to its environment. The reduction in the lipedogram of fish under stressful conditions is related to glycolytic pathways, mainly gluconeogenesis, to produce glucose [21]. Nutritional studies carried out on juvenile P. brachypomus, testing cycles of food restriction [32] and low water temperature and feeding time [34] as stressors for the species, resulted in a decrease in plasma triglyceride levels. Different supplementations at two different stocking densities for P. mesopotamicus under normoxic conditions for the species promoted a reduction in triglyceride levels at the high density tested, regardless of the type of supplementation used [68]. However, studies with different densities of juvenile C. macropomum have also reported variations in lipid concentrations, although with an increase in triglyceride levels as stocking density increased [13,14]. The results suggest species-specific responses to this production management.
Hematimetric indices and variables such as tHb, Ht, and RBC are related to oxygen transport capacity in stressful situations [65]. The low MCV and tHb values observed at the highest densities (D1.95, D5.63, and D7.90) may be related not only to the increase in stocking density but also to the low oxygen concentrations available at the respective densities tested. In stressful situations, an increase in the indices related to the fish’s blood count is to be expected, given the increased demand for oxygen in order to achieve homeostasis [65]. However, the results observed can be explained by the reduction in oxygen pressure in the water, which facilitates its uptake by increasing the affinity of hemoglobin for oxygen [69]. The results observed in this study may indicate an oxyconforming mechanism of the species. However, more studies are needed to prove this hypothesis and to report on the low oxygen conformation characteristics of P. brachypomus. Even with the change in hemoglobin and MCV values, they are still within the limits observed in other studies with the species [32,33,34], indicating that even with the influence of low oxygen and stocking densities, the well-being of P. brachypomus has not suffered major damage. Juveniles of C. macropomum kept at densities of 4 and 6 kg/m3 in a normoxic situation also showed lower hemoglobin levels than those kept at a lower density (2 kg/m3) [14]. Specimens of Paralichthys olivaceus kept at five different stocking densities and two dissolved oxygen concentrations showed changes in hemoglobin levels, but their quantity increased with the increase in available oxygen [70].
The other hematological and plasma biochemical indices were not influenced by stocking densities and hypoxia resulting from overcrowding. The responses to stress depend not only on the species and age of the animals, but also on the intensity of the stressor [65], so the conditions of this study did not cause serious problems for the physiology of P. brachypomus. C. macropomum juveniles kept at three stocking densities also showed no changes in TC, ALS, and AST levels [14].

5. Conclusions

Dissolved oxygen levels were reduced at the higher stocking densities, resulting in a hypoxic situation, directly influencing the growth of P. brachypomus juveniles in both experiments and the levels of triglycerides, hemoglobin rate, and MCV in Experiment II. The increase in feed consumption and feed conversion ratio made it impossible to produce P. brachypomus juveniles weighing 1.5 g at stocking densities above 4.41 kg/m3 and juveniles weighing 6.6 g at stocking densities above 5.63 kg/m3. More studies are still needed on the production of P. brachypomus juveniles at high stocking densities under normoxic conditions.

Author Contributions

Conceptualization, methodology, validation, formal analysis, resources, investigation, writing—original draft, writing—reviewing and editing, visualization: I.d.M.C.A.; conceptualization, methodology, validation, formal analysis, investigation: S.d.S.S., A.d.S.S., F.A.C.d.S., T.B.M., and W.J.d.F.M.; conceptualization, methodology, validation, formal analysis, investigation, writing—reviewing and editing, visualization, supervision, project administration, funding acquisition: G.C.F. and R.K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-Brasil—402952/2021-9, 310170/2023-0, 316901/2021-0 and 402840/2023-2); Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG-Brasil); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brasil—finance code 001).

Institutional Review Board Statement

The animal study protocol was approved by the Ethics Committee on Animal Use of Universidade Federal de Minas Gerais. (Protocol number: n° 17/2023; Approval date: 5 August 2023).

Informed Consent Statement

Not applicable.

Data Availability Statement

All data needed to evaluate the conclusions in the paper are present in the main text. Detailed numerical data will be made available to individuals upon request.

Acknowledgments

The authors thank Conselho Nacional de Desenvolvimento Científico e Tecnológico, Fundação de Amparo à Pesquisa do Estado de Minas Gerais and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior for financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fishes 10 00171 g001
Figure 1. Dissolved oxygen level (means and standard error) in culture tanks during the production of pirapitinga (Piaractus brachypomus) at different stocking densities. (A) Phase 1. (B) Phase 2.
Figure 1. Dissolved oxygen level (means and standard error) in culture tanks during the production of pirapitinga (Piaractus brachypomus) at different stocking densities. (A) Phase 1. (B) Phase 2.
Fishes 10 00171 g001
Fishes 10 00171 g002
Figure 2. Growth performance (means and standard error) of juvenile pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 1 at different stocking densities in RAS. (A) Final weight, (B) final length, (C) weight gain, (D) daily weight gain, (E) final biomass, (F) specific growth rate, (G) feed consumption, and (H) feed conversion.
Figure 2. Growth performance (means and standard error) of juvenile pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 1 at different stocking densities in RAS. (A) Final weight, (B) final length, (C) weight gain, (D) daily weight gain, (E) final biomass, (F) specific growth rate, (G) feed consumption, and (H) feed conversion.
Fishes 10 00171 g002
Fishes 10 00171 g003
Figure 3. Growth performance (means and standard error) of pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 2 at different stocking densities in RAS. (A) Final weight, (B) final length, (C) weight gain, (D) daily weight gain, (E) final biomass, (F) specific growth rate, (G) feed consumption, and (H) feed conversion.
Figure 3. Growth performance (means and standard error) of pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 2 at different stocking densities in RAS. (A) Final weight, (B) final length, (C) weight gain, (D) daily weight gain, (E) final biomass, (F) specific growth rate, (G) feed consumption, and (H) feed conversion.
Fishes 10 00171 g003
Fishes 10 00171 g004
Figure 4. Plasma triglycerides (means and standard error) of pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 2 at different stocking densities in RAS.
Figure 4. Plasma triglycerides (means and standard error) of pirapitinga (Piaractus brachypomus) after 20 days of culture in Phase 2 at different stocking densities in RAS.
Fishes 10 00171 g004
Table 1. Water quality parameters (means ± standard deviations) in culture tanks during the production of pirapitinga (Piaractus brachypomus) after 20 days of production in Experiment I and Experiment II at different stocking densities in RAS.
Table 1. Water quality parameters (means ± standard deviations) in culture tanks during the production of pirapitinga (Piaractus brachypomus) after 20 days of production in Experiment I and Experiment II at different stocking densities in RAS.
Stocking DensitiesNH3
(mg/L)		Temperature (°C)pH
Experiment I
D0.680.0153 ± 0.0128.44 ± 0.456.53 ± 0.05
D1.450.0153 ± 0.0128.34 ± 0.386.50 ± 0.08
D4.410.0153 ± 0.0128.29 ± 0.276.48 ± 0.10
D7.170.0153 ± 0.0028.19 ± 0.576.53 ± 0.10
p-value0.79350.99280.8015
Experiment II
D10.0256 ± 0.0026.60 ± 2.637.07 ± 0.05
D1.950.0270 ± 0.0025.75 ± 0.027.03 ± 0.05
D5.630.0265 ± 0.0025.80 ± 0.097.03 ± 0.05
D7.90.0261 ± 0.0025.77 ± 0.10;7.03 ± 0.05
p-value0.0770 ± 0.000.25270.8592
ANOVA and Tukey’s test.
Table 2. Hematological and biochemical parameters and viscerosomatic and hepatosomatic indices (means ± standard error) for juvenile pirapitinga (Piaractus brachypomus) after 20 days of production in Experiment II at different stocking densities in RAS.
Table 2. Hematological and biochemical parameters and viscerosomatic and hepatosomatic indices (means ± standard error) for juvenile pirapitinga (Piaractus brachypomus) after 20 days of production in Experiment II at different stocking densities in RAS.
ParameterStocking DensityOverall p-ValueLinear p-ValueQuadratic p-Value
D1D1.95D5.63D7.90
GLU (mg/dL)95.07 ± 1.2181.55 ± 8.4981.35 ± 6.2693.75 ± 3.170.15050.71750.0520
TC (mg/dL)100.29 ± 3.62114.42 ± 9.29109.87 ± 9.7484.45 ± 7.790.11210.08910.0858
TG (mg/dL)207.21 ± 26.46 ab246.78 ± 53.22 a175.54 ± 19.73 ab142.88 ± 18.13 b0.02330.0067 *0.7374
tHb (g/dL)8.20 ± 0.48 a6.45 ± 0.47 b7.11 ± 0.22 ab6.59 ± 0.32 b0.04720.10680.5191
Ht (%)33.50 ± 1.2323.83 ± 2.4726.78 ± 2.6928.92 ± 2.400.08290.72550.1201
TPP (g/dL)4.88 ± 0.094.98 ± 0.165.07 ± 0.074.72 ± 0.150.29050.33330.1028
RBC (×106/μL)1.12 ± 0.181.38 ± 0.101.24 ± 0.071.28 ± 0.120.53760.78200.7500
AST (UI/L)39.42 ± 7.2728.91 ± 2.9131.01 ± 2.7033.09 ± 4.280.46090.66700.3561
ALT (UI/L)9.29 ± 0.167.23 ± 0.368.48 ± 0.839.43 ± 0.830.11990.25890.2082
MCV (ftl)317.56 ± 44.49 a178.57 ± 5.41 b232.13 ± 32.25 ab198.76 ± 16.27 b0.03870.13000.4233
MCH (pg)74.42 ± 11.7048.92 ± 3.5459.29 ± 2.5755.44 ± 4.600.12100.33710.4499
MCHC (g/dL)24.77 ± 1.3227.56 ± 2.6227.85 ± 3.3321.06 ± 1.840.23920.21030.1124
VSI (%)7.96 ± 0.467.28 ± 0.207.01 ± 0.317.42 ± 0.310.30290.39000.1329
HSI (%)2.00 ± 0.071.85 ± 0.162.00 ± 0.141.90 ± 0.260.89520.94700.8391
ANOVA, Tukey’s test, and regression tests. Means in the same row followed by different letters differ significantly by Tukey’s test (p < 0.05). GLU—glucose; TC—total cholesterol; tHb—hemoglobin; TTP—total plasma protein; RBC—erythrocytes; AST—aspartate aminotransferase; ALT—alanine aminotransferase; MCV—mean corpuscular volume; MCH—mean corpuscular hemoglobin; MCHC—mean corpuscular hemoglobin concentration; VSI—Viscerosomatic Index; HSI—Hepatosomatic Index. * Equation: Y = −26.424x + 259.16.
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de Morais Carvalho Ananias, I.; dos Santos Silva, S.; de Sena Souza, A.; dos Santos, F.A.C.; Magalhães, T.B.; de Freitas Martins, W.J.; Favero, G.C.; Luz, R.K. Effects of Stocking Densities on the Growth Performance and Physiology of Juvenile Piaractus brachypomus in Recirculating Aquaculture System. Fishes 2025, 10, 171. https://doi.org/10.3390/fishes10040171

AMA Style

de Morais Carvalho Ananias I, dos Santos Silva S, de Sena Souza A, dos Santos FAC, Magalhães TB, de Freitas Martins WJ, Favero GC, Luz RK. Effects of Stocking Densities on the Growth Performance and Physiology of Juvenile Piaractus brachypomus in Recirculating Aquaculture System. Fishes. 2025; 10(4):171. https://doi.org/10.3390/fishes10040171

Chicago/Turabian Style

de Morais Carvalho Ananias, Imaculada, Sidney dos Santos Silva, André de Sena Souza, Fábio Aremil Costa dos Santos, Thamara Bentivole Magalhães, Wagner Junio de Freitas Martins, Gisele Cristina Favero, and Ronald Kennedy Luz. 2025. "Effects of Stocking Densities on the Growth Performance and Physiology of Juvenile Piaractus brachypomus in Recirculating Aquaculture System" Fishes 10, no. 4: 171. https://doi.org/10.3390/fishes10040171

APA Style

de Morais Carvalho Ananias, I., dos Santos Silva, S., de Sena Souza, A., dos Santos, F. A. C., Magalhães, T. B., de Freitas Martins, W. J., Favero, G. C., & Luz, R. K. (2025). Effects of Stocking Densities on the Growth Performance and Physiology of Juvenile Piaractus brachypomus in Recirculating Aquaculture System. Fishes, 10(4), 171. https://doi.org/10.3390/fishes10040171

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