The effects of partial and full feed restriction on the plasma levels of thyroxine (T 4 ), triiodothyronine (T 3 ) and testicular activity were studied in the male air-breathing catfish, Clarias gariepinus. Partial and full feed restriction for 3 weeks significantly decreased the plasma T 4 levels during the progressive and breeding phases, and decreased the plasma T 3 levels only during the breeding phase. However, full feed restriction for 3 weeks increased plasma T 3 levels and T 3 /T 4 ratio during the quiescent, progressive and regressive phases. Histologically, 3 weeks of partial and full feed restriction markedly reduced the testicular activity during the progressive and breeding phases. These results indicate that partial and full feed restriction adversely affects the thyroid hormonogenesis and testicular activity in a phase-dependent manner in C. gariepinus.

The purpose of this study is to evaluate seasonal variations in thyroxine (T 4 ) and triiodothyronine (T 3 ) levels and testicular activity in the male air-breathing catfish Clarias gariepinus. Plasma T 4 levels increased from March to a peak in July and then decreased throughout autumn, declining to their lowest level in January. Plasma T 3 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.

This review article focuses on the effect of pesticides on reproductive functions, haematological and biochemical changes in catfishes. Pesticides are chemical substances that are released into the environment to control the populations of harmful pests. Pesticide exposure leads to toxicity in aquatic organisms, including fishes which are particularly sensitive to pesticides. The acute and sublethal concentrations of pesticides in the aquatic environment result in different lethal alterations, including changes in reproductive functions, histology, haematology, proteins, glucose, lipids, enzymes, etc. Pesticides act as endocrine disruptor compounds and have the potential to impair reproductive function in catfishes. Alterations in haematological and biochemical parameters are used as efficient biomarkers in assessing the toxicity of pesticides in fishes.

Male air-breathing catfish, Clarias gariepinus were exposed to hyperosmolar mannitol solution (250 mM) to determine its effect on the plasma levels of thyroxine (T 4 ) and triiodothyronine (T 3 ) during the winter, spring, summer and autumn seasons. During the winter season, hyperosmolar mannitol solution significantly increased the plasma levels of T 4 throughout the duration of exposure, whereas the plasma T 3 levels significantly increased only at 12-48 h. During the spring season, significant decreases in the plasma levels of T 4 were observed only at 12 and 48 h, and significant increases in the plasma levels of T 3 were observed only at 6-24 h. During the summer season, the plasma levels of T 4 significantly increased at 6 h and decreased at 24-48 h, while the plasma levels of T 3 significantly increased only at 6 and 24 h. During the autumn season, the plasma levels of T 4 significantly increased at 6, 12 and 48 h and the plasma levels of T 3 significantly decreased at 6, 24 and 48 h. These results indicate the season-dependent and differential effects of the hyperosmolar mannitol solution on the plasma levels of thyroid hormones in Clarias gariepinus.