Wild trout responses to a stress experience following confinement conditions during the spawning season

Salmo (trutta) marmoratus is an endemic specie in the North of Italy, subjected to hybridization with domesticated strains of trout. Native populations are managed by supportive release in the rivers. Wild breeders are captured, confined in facility for short periods and then released in the river after artificial fertilization. Premature mortality during confinement and post release mortality in river have been observed in breeders supporting the view that confinement stress could be the cause. Twenty-six adult trout were captured from a river by electrofishing and stocked in two tanks. The first one (RF) was provided with artificial refuges to simulate the natural environment and covered by dark panels. The second tank (TR) was only partially covered by dark panels and without artificial refuges. All the other conditions were identical and the animals were fed ad libitum with natural food collected in the same river. After 50 days, from a third group of 8 trout (WD) captured in the same river by a 5-min electrofishing session, blood samples were sequentially collected for the assessment of serum cortisol response to serial repeated handlings. With the same sequential method, individuals of the RF and TR experimental groups were sampled. Cortisol levels were compared between groups by ANOVA. Biomass densities decreased during the experiment due to premature mortality of the largest individuals in both the RF (7.69%) and TR (30.77%) groups. At the end of the experiment, data clearly demonstrated that after a stressing confinement, the TR group showed a reduced post-stress response to the successive serial handlings. Group RF, which experienced a more careful confinement, responded to the second serial acute stressing manipulation in conformity as the group WD that was not confined. Cortisol data support the hypothesis of impaired cortisol response as a consequence of oversecretion due to uneasiness during the short-term confinement in artificial environment without refuges or due to "chronic" stress. Furthermore, our data demonstrated that cortisolaemia is not adequate as a marker for chronic stress but cortisol response to serial acute stress, in relation to time, can be used for the monitoring of stress level and welfare in trout breeders during confinement. The use of refuges and shadow in the artificial environment provided a relevant role in the survival of wild breeders of trout during confinement in the spawning season.