Hatchery-reared fish often show different behavioral traits from their wild counterparts, possibly due to the lack of environmental stimuli. Here, we aimed to reveal the stage-specific effect of environmental stimuli on the development of learning capability in striped knifejaw Oplegnathus fasciatus. The fish were raised for 15 days (50-65 days post-hatch) or 30 days (50-80 or 90-120 days post-hatch) in either conventional rearing tanks (control) or in a structurally enriched tank containing bricks, artificial sea grass, and plastic pipes (enriched environment), and were examined for learning capability using Y-maze reward conditioning. The learning capability of wild juveniles was also examined, and their scores were compared with those of hatchery-reared fish (which we previously reported). Only fish in the 50-80 days post-hatch enriched-rearing group showed significantly better scores than those in the control group, and wild fish performed better than hatchery-reared ones. The present results indicate that, although the learning capability of hatchery-reared fish is inferior to that of wild fish, exposure to a highly structured environment at an appropriate stage promotes the development of learning capability. Such environmental enrichment can potentially improve the viability of hatchery-reared fish when they are released into the wild.