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: D_0.68—0.68 kg/m³; D_1.45—1.45 kg/m³; D_4.41—4.41 kg/m³; and D_7.17—7.17 kg/m³. Experiment II used juveniles weighing 6.66 ± 1.3 g at the following densities for 20 days: D_1.0—1.00 kg/m³; D_1.95—1.95 kg/m³; D_5.63—5.63 kg/m³, and D_7.90—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 (D_1.0) (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.

Hematological and blood biochemical parameters are useful tools for monitoring fish physiological status; however, information on their variation between sexes during reproductive development remains limited for Colossoma macropomum. This study evaluated the hematological and blood biochemical profiles of male and female C. macropomum during growth and first sexual maturation under controlled conditions in a recirculating aquaculture system. Fish were sampled at seven ages from 590 to 1080 days after hatching (DAH). Hematocrit, total plasma protein, hemoglobin, leukocytes, erythrocytes, glucose, cholesterol, and triglycerides were evaluated, and reproductive development was assessed through macroscopic and histological examination of the gonads. Age significantly affected hematocrit, leukocytes, erythrocytes, glucose, triglycerides, cholesterol, and total plasma protein, whereas hemoglobin remained unchanged. Sex significantly affected only glucose, with females showing higher concentrations than males across ages. A significant sex × age interaction was detected only for total plasma protein, with higher concentrations in females than males at 1080 DAH. These findings indicate that developmental progression associated with age is a major source of variation in blood profiles, whereas sex-related effects are limited and parameter-specific. The results provide baseline physiological information for monitoring C. macropomum broodstock under controlled conditions.

This study evaluated the effects of water renewal rate and temperature on the growth performance and physiological responses of juvenile Piaractus brachypomus reared in a recirculating aquaculture system (RAS). A total of 336 fish (1.35 ± 0.24 g) were distributed in six RAS units under two water renewal rates (42 and 128 L h⁻¹) and three temperatures (26, 29, and 32 °C) for 45 days. Temperature was the main factor affecting growth, with higher final weight and total length at 29 and 32 °C throughout the experimental period. Water renewal rate significantly influenced feeding efficiency and energy allocation. Higher renewal (128 L h⁻¹) increased dissolved oxygen and daily feed intake and resulted in higher hemoglobin levels and hepatic lipid deposition, particularly at 32 °C, indicating greater metabolic activity. Conversely, the lower renewal rate (42 L h⁻¹) was associated with better feed conversion ratios at 29 °C and higher muscle lipid content at 26 °C, suggesting reduced energy expenditure. Hematocrit, total plasma protein, and cholesterol were primarily influenced by temperature, with higher values at 29 and 32 °C, while glucose, triglycerides, and liver enzymes were unaffected. Overall, temperatures of 29–32 °C optimized growth, while water renewal rate modulated feed utilization, physiological responses, and lipid deposition. These findings highlight the importance of jointly optimizing temperature and water renewal rate in RAS to enhance growth performance and metabolic balance in juvenile P. brachypomus.