INFLUENCE OF DIETARY VITAMINS E AND C AND HUFA ON RAINBOW TROUT (ONCORHYNCHUS MYKISS) PERFORMANCE UNDER CROWDING CONDITIONS.

The present study was conducted to evaluate the effects of vitamins E and C and HUFA on the stress response to crowding of rainbow trout and whether the relative levels of these nutrients could influence fish welfare. Two levels of vitamin E (25.6 and 275.6 mg/kg diet), vitamin C (0 and 1000 mg/kg diet), and HUFA (12.5 and 30.5 g/kg diet) were used to formulate five practical diets (−E−HUFA, −E+HUFA, +E−HUFA, +E+HUFA, −C+E+HUFA). Each diet was randomly assigned to triplicate groups of rainbow trout (53.1±5.2 g initial mean weight) reared at low (20 kg/m3) and high (100 kg/m3) stocking densities for 42 days. Plasma cortisol, growth performance, food intake, nutritive indices, mortality, and weight distribution were evaluated. Plasma cortisol levels increased only in +E+HUFA and −C+E+HUFA groups as a consequence of chronic crowding. High stocking density reduced growth and food intake and induced the establishment of hierarchies, although severity of these negative effects was not the same for all groups. The synergistic effect of diet and stocking density was strongly evident in crowded −E+HUFA group which underwent marked mortality. Under crowding conditions, diet +E−HUFA promoted the best performance of the fish. Our results appear to confirm that plasma cortisol would not be a suitable indicator of chronic stress in fish, since other parameters evaluated in the present study clearly showed that trout were chronically stressed. Since rainbow trout is able to synthesise HUFA, decreased dietary levels of such fatty acids would enhance stress resistance and welfare of this species under intensive culture.