Enriched rearing has been demonstrated to shape the phenotype of hatchery-reared salmonids and improve their post-release survival in the wild, thus having an important applied value in conservation. However, it is unclear if rearing conditions or survival selection during the early life stages induce long-term fitness effects on adult phenotypes. Using a paired full-sib set-up, we investigated the influence of the environmental enrichment at the egg and fry stages on the milt quality and skin colouration of the adult brown trout (Salmo trutta L.). Overall, males had a higher number of skin spots than females. Notably, the total numbers of spots and black spots were significantly lower in fish raised in an enriched environment than in their full siblings reared in a conventional hatchery environment. However, neither sperm motility nor sperm swimming behaviour differed between fullsib males reared in different environments. Our results suggest that rearing method may shape the colouration of brown trout, either by ecological carry-over effects or by selective survival during the rearing process. This, in turn, indicates that ecological conditions at early life can have long-prevailing phenotypically plastic or microevolutionary effects on the adult traits of fish. These effects should be taken into consideration to better understand the ecological role of rearing methodology in salmonid conservation.
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