Effects of rearing density on rainbow trout welfare, determined by plasmatic and tissue parameters

The work aimed to determine the welfare status of rainbow trout (50±15 g), intensively reared to commercial size in two concrete raceways (V1, V2) at different stocking densities, to reach final load of 40 kg/m3 and 20 kg/m3, respectively. Throughout the out-phase, plasma parameters and hepatic glycogen content were determined every three months and compared with those of rainbow trout of the same age and mean weight, reared extensively (VE). At the end of the trial significant differences of the monitored parameters were found between rainbow trout reared in raceways (V1, V2) and those held in the reservoir (VE). Plasma triglycerids, total cholesterol and alkaline phosphatase in fish reared intensively were significantly different from fish reared in extensive conditions. The highest levels of these parameters in V1 and V2 could be justified mostly by the administration of balanced feeding and consequent modifications of energetic metabolism and in small part to the high density of the intensive rearing conditions. Transaminases (AST, ALT) increased in V1 and V2 only at the first sampling. Afterwards, the activity of these two enzymes returned to the normal range at the subsequent assays, suggesting a capacity of the rainbow trout to adapt to the severe conditions of confinement, typical of out-basins. No significant differences among groups were observed for total protein, glucose, CK and LDH. A similar situation was also shown for liver glycogen content.