Abstract
Hatchery‐reared honmasu salmon Oncorhynchus rhodurus × masou parr released into the Yanagisawa River, Tochigi Prefecture on 24 June 1996 increased significantly in length and weight by November, while siblings retained in hatchery tanks did not. Plasma growth hormone (GH) levels of the released parr increased from 1 day after the release until 23 August, while the GH levels of the wild parr in the river did not increase. The released parr started to consume various natural prey organisms within a day after release. However, stomach fullness of the released parr was considerably lower than that of the wild parr during the first week following release. Trypsin activity in the released parr increased after 23 August, 2 months post‐release. The results demonstrate that the released parr adapt to the natural river environment within weeks. Endocrine control by GH and exocrine pancreas activity appear to play important roles in the post‐release adaptation processes.