Intensive aquaculture production challenges fish health and welfare, affecting both economic and ethical sustainability. Increasing environmental complexity (environmental enrichment, EE) has been shown to improve fish physiology and welfare and is assumed to promote natural, adaptive behaviours. This study investigated how early exposure to environmental complexity affects growth and welfare-related behavioural responses in juvenile rainbow trout. After 90 days of rearing in EE or barren environments (BE), fish were transferred to individual isolation for two weeks to assess stress coping ability. No differences in feed intake was observed between the treatments, but mass gain was more than twice as high in fish transferred from enriched compared to barren rearing. EE fish also showed significantly lower frequencies of stereotypic vertical swimming after transfer to the novel environment, suggesting greater behavioural flexibility and resilience. The increased occurrence of stereotypic behavior in BE fish, emerging only after exposure to the novel environment, supports the conclusion that growth differences are influenced by behavioural correlates of stress coping ability in EE- and BE-reared fish. While behavioural and growth responses differed markedly between rearing environments, physiological measures of stress remained similar, supporting the idea that behavioural expression may serve as a more sensitive welfare indicator. These results point to the value of broadening behavioural indicators used in aquaculture, as behavioural outcomes of welfare translate, to some degree, directly to growth performance following environmental change. This observation should serve as a strong motivation to maintain a clear focus on using behavior to assess welfare status in aquaculture production.