Post-release adaptation processes of hatchery-reared honmasu salmon parr

Hatchery-reared honmasu salmon Oncorhynchus rhodurus x masou parr released into the Yanagisawa River, Tochigi Prefecture on 24 June 1996 increased sig nificantly in length and weight by November, while siblings retained in hat chery 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 level s 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. Howe ver, stomach fullness of the released parr was considerably lower than tl;a t of. the wild parr during the first week following release. Trypsin activi ty in the released parr increased after 23 August, 2 months post-release. T he results demonstrate that the released parr adapt to the natural river en vironment within weeks. Endocrine control by GH and exocrine pancreas activ ity appear to play important roles in the post-release adaptation processes . (C) 2000 The Fisheries Society of the British Isles.