High stocking density produces crowding stress altering some physiological and biochemical parameters in gilthead seabream, Sparus aurata, juveniles

High stocking density has been shown to produce a wide variety of effects on cultured fish populations, such as alterations in behavior and poor feed utilization, resulting in mortality and poor growth. High stocking density has also been reported to produce chronic stress and mobilization of energy sources in fish. There are few studies focusing on the effect of high stocking density on one of the most important marine fish species for Mediterranean aquaculture, gilthead seabream (Sparus aurata). Consequently, this study investigates the effect of high stocking density on juveniles of this species, focusing on effects of stocking density on growth, biochemical composition, immune status and hematology.The results of this study showed that high stocking density produced a chronic stress situation. Plasma cortisol levels increased up to 16.25 ng ml–1 in fish held at high stocking; this value was 4 fold that of fish held at low stocking; density (3.91 ng ml–1 ). As a consequenceof this stressful situation, there was both haemoconcentration and a decrease in alternative complement pathway (ACP), an important component of the immune system of fish (from 167.23 U ml–1 down to 146.37 U ml–1). Haematocrit, haemoglobin concentration and red blood cell count were significantly higher in fish held at high stocking density (43.87%, 10.76 9dl–1 and 3.36 ×106 mm–3, respectively) compared with those fish held at low stocking density (37.21%, 9.32 g/dlg–1 and 2.82 ×106 mm–3, respectively).In addition, high stocking density produced a decrease in hepatosomatic index (from 2.26 down to 2.04) and altered liver fatty acid composition. Oleic acid (18: 1n-9) decreased in liver total lipids of fish held at high stocking density and arachidonic acid (20: 4-n6) and n-3 high unsaturated fatty acids (n-3 HUFA) were reduced in liver polar lipids of those fish. These alterations reflect the effect of stocking density on lipid metabolism to help meet the increased energy demand.