Function and trigger of thyroxine surge in migrating chum salmon Oncorhynchus keta fry

Chum salmon, Oncorhynchus keta, fry were reared in the Otsuchi Hatchery, fed for 5 weeks after yolk absorption, and then released at 09:30 to a hatchery pond. Diel rhythm of plasma thyroxine (T-4) was not observed in fresh water or in seawater. Plasma T-4 levels were less than 3 ng/ml in fish reared in well water in the hatchery raceway, whereas the level was in a range of 2 - 6 ng/ml and showed a T-4 surge of 14.3 +/- 1.4 ng/ml when they were reared in stream water. Fry released from the raceway began to form schools, and then actively swam downstream headfirst. The T-4 levels of early migrants increased from 2-7 ng/ml to 20-35 ng/ml during downstream Migration in the afternoon in three study years. In 2001, we sampled two groups of fry that spent 2 h for school formation in the hatchery pond, and a T-4 surge was observed. In both cases, the T-4 levels decreased in the late afternoon during downstream migration to the river mouth. Plasma T-4 increased again in the Fisherman Port before departure to the Otsuchi Bay. Another T-4 increase was detected in the bay before offshore emigration. Stimuli such as rainfall, turbid water and cold water also triggered T-4 surges. The duration of the T-4 surge was short, usually less than two It. These results suggest that T-4 surges occur when fry schools change their habitats. (C) 2003 Elsevier Science B.V. All rights reserved.