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

Abstract

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 (T4) was not observed in fresh water or in seawater. Plasma T4 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 T4 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 T4 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 T4 surge was observed. In both cases, the T4 levels decreased in the late afternoon during downstream migration to the river mouth. Plasma T4 increased again in the Fisherman Port before departure to the Otsuchi Bay. Another T4 increase was detected in the bay before offshore emigration. Stimuli such as rainfall, turbid water and cold water also triggered T4 surges. The duration of the T4 surge was short, usually less than two h. These results suggest that T4 surges occur when fry schools change their habitats.