Variations in the Plasma Levels of Thyroid Hormones and Testicular Activity in the Male Air-breathing Catfish (Clarias gariepinus) over the Annual Cycle

Abstract

The purpose of this study is to evaluate seasonal variations in thyroxine (T4) and triiodothyronine (T3) levels and testicular activity in the male air-breathing catfish Clarias gariepinus. Plasma T4 levels increased from March to a peak in July and then decreased throughout autumn, declining to their lowest level in January. Plasma T3 levels also varied seasonally, increasing from March to a peak in June. The increase in plasma levels of thyroid hormones coincided with an increase in the testicular activity of the fish. The gonadosomatic index (GSI) peaked in May and then declined slowly. In February and March, the testes were characterized by a new generation of spermatogenic cells. Mature spermatozoa appeared in April and became dominant in the testes during the summer months. Testicular activity decreased gradually with the gradual decrease in water temperature and day-length during the autumn season. Decreased testicular activity during the winter months was associated with low GSI values. The patterns of seasonal change in the levels of thyroid hormones as well as testicular activity were similar to those in water temperature and natural day-length.