<p>The efficacy of stock enhancement for conservation has long been limited by low survival rates of hatchery-reared fish after release. While environmental enrichment is a promising strategy to improve their ecological adaptability, the optimal practices for specific species remain poorly understood. This study investigated the effects of different levels of environmental complexity on the physiological and behavioral traits of juvenile rock bream (<i>Oplegnathus fasciatus</i>). Fish were reared in tanks with no structure, low-level structures, or high-level structures for four weeks, after which the growth performance, basal cortisol levels and exploratory behaviors were examined. Fish reared with low- and high-level structures had longer latencies to leave a shelter, and locomotor activity tended to decrease with increasing environmental complexity, suggesting enhanced caution and antipredator abilities in the fish. Contrary to expectations, the presence of the low- or high-level structures did not improve growth performance or reduce basal cortisol levels. Overall, these results indicate that increased environmental complexity potentially benefits antipredator behaviors in juvenile rock bream. However, further research is needed to refine this strategy and promote more ecologically adaptive phenotypes, thereby contributing to stocking success of this species.</p>