The relationship between water quality parameters and response to density stress in Pacu fish (Piaractus brachypomus)

ABSTRACT:Backgrounds: Pacu, Piaractus brachypomus is an ornamental imported fish that is reared as an edible fish in some countries. There have been very few studies as food fish in Iran. Objectives: This study aimed to evaluate the relationships between water quality parameters and stress response indices in different densities in a relatively recirculation system. Methods: An experiment carried out in 3 recirculation treatments with 3 replications in 250-liter tanks with a filled volume of 200 litres water. An extra 100-liter tank prepared as biological filter. Tanks were stocked in 3 densities 50, 75 and 100 fingerling.m-3 with an average weight as 3.52±0.11 for duration of 3 months. Water replacement was 10% per day. Water quality parameters of total ammonia, nitrite and nitrate determined every 15 days and dissolved oxygen, alkalinity and electrical conductivity every day. Blood collected to determine plasma cortisol and glucose to evaluate the effect of water quality on stress response in different densities. Results: The results showed that in densities 50 and 75 fish/m3, there was no significant effect on stress response indices of cortisol and glucose (P > 0.05). But, alkalinity decreased nitrite and nitrate parameters increased in 100 fish/m3 (P < 0.05). Likewise, in this density stress response indices of cortisol and glucose significantly increased in third month of rearing. Conclusions: It suggests that density of 100 fish/ m3 can be a chronic stressor for fish which raise cortisol response.