Growth, biochemical and physiological responses of Salminus brasiliensis with different stocking densities and handling

The dourado Salminus brasiliensis is a freshwater species found in major South American rivers and has recently been cultivated in cage systems. As stocking density and handling are known stress factors in cage systems, we studied the combined effect of stocking density (of 30, 150 and 300 fish m−3) and fish handling (consisting of the pursuit, capture and exposure of the fish to air for 30 s) on growth as well as biochemical and physiological responses of S. brasiliensis fingerlings. The endpoints measured were as follows: standard length and weight, blood hematocrit (Ht) and hemoglobin (Hb), plasma glucose and lactate, hepatic activities of the antioxidant enzymes glutathione peroxidase (GPx), glutathione reductase (GR), glutathione-S-transferase (GST) and catalase (CAT), and the concentrations of Na+, K+, Cl− and Ca2+ in the gills and plasma of S. brasiliensis fingerlings. Samples were taken every 20 days until the end of the experiment, on the 80th day. S. brasiliensis fingerlings that were not handled exhibited reduced growth at increased densities. In contrast, handled fish exhibited slower growth at all densities, suggesting that the effect of handling was greater than the effect of density in these fish. Enzymes related to oxidative stress were influenced by handling, indicating that stress is an important modulator of the antioxidant response. Antioxidant defenses were negatively influenced by handling, but only slightly influenced by density. On the 80th day, the Hb concentration, enzymatic activity of GR, GST, GPx and CAT in the liver, and concentrations of Na+ and Cl− in the gills were higher in fish that had not been handled, in contrast to the glucose concentration, which was higher in the handled fish. However, the activity of GPx was influenced by stocking density in handled fish. The Ht and the concentrations of lactate, Na+, K+, Cl− and Ca2+ were not influenced by the stress factors of stocking density or handling. The growth of S. brasiliensis was negatively affected by frequent handling stress, whereas stocking density only affected growth when fish were not handled, thus overcoming the effect caused by high densities.