Downstream migratory behavior and plasma thyroxine levels of biwa salmon Oncorhynchus rhodurus

Downstream migratory behavior, plasma T4 concentrations and histological changes in the thyroid gland of biwa salmon underyearlings were examined during late spring and early summer for three consecutive years. The downstream migration was observed specifically when the flux of the river increased following rainfall. A close correlation was seen between the number of migrants and the rate of flux increase of the river. Plasma T4 levels of the fry caught in the river were variable, and no correlation was seen between the plasma T4 levels in the fish remaining in the river and lunar phase, rainfall, water flux or water temperature. However, significantly higher T4 concentrations were observed in the fish during downstream migration when compared with those in the fish remaining in the river and postmigrants in the lake. The mean epithelial height of the thyroid folliciles was greatest in the underyearling in May, shortly before the onset of the migratory season. The thyroidal activity of the migrants in late May and June tended to decrease and postmigrants in the lake had inactive thyroid. These results suggest that the rainfall or the associated changes in turbidity and flux is one of the initiators of the migratory behavior in biwa salmon, and also that thyroid hormone is involved directly or indirectly in downstream movement.