Physiological effects of stocking density on the fish Piaractus mesopotamicus fed with red seaweed (Pyropia columbina) and β-carotene-supplemented diets

This study aimed at investigating the physiological responses of Piaractus mesopotamicus exposed to high stocking density and the potential protective role of supplemented diets. Fish were fed with basal, red seaweed (Pyropia columbina) or beta-carotene-supplemented diets for 90 days. Then, fish were distributed at low (1.5 g/L) and high (22 g/L) stocking densities for 15 days. Fish exposed to the high density showed increased hepatosomatic index, haemoglobin content and mean corpuscular haemoglobin concentration (all diets); decreased haematocrit, mean corpuscular volume (basal and seaweed) and white blood cells count (all diets) were observed. High density-exposed fish showed decreased plasmatic metabolites as well as the hepatic lipids content in basal and seaweed diets. Regarding oxidative stress, increased activity of glutathione S-transferase in high density-exposed fish muscle (all diets), and lower lipid peroxidation in liver (basal and beta-carotene) and intestine (basal and seaweed) were evidenced. Interactions between diet and stocking density were recorded regarding the triglycerides (decrease in fish exposed to high density fed with basal and seaweed) and hepatic lipids (decrease in fish exposed to high density fed with basal). The major changes occurred in haematologic and metabolic parameters as strategies to cope with overcrowding stress. Fish response to stocking density was not affected by diets.