The reproductive biology and sex hormones of the rabbitfish (Siganus rivulatus) in the Suez Gulf

Samples of the rabbitfish ( Siganus rivulatus ) were collected monthly during January- December 2016 from the Suez Gulf. The maturity stages of the males and females were morphologically separated according to the changes in shape, size, and color of the gonads in different six successive stages. Stage I (Immature), stage II (Maturing 1), stage III (Maturing 2), stage IV (Mature), stage V (Ripe), and stage VI (Spent). Reproductive characteristics of Siganus rivulatus showed that the male reaches first sexual maturity at a smaller body length than the female (15.1 cm and 15.8 cm for male and female, respectively). The gonado-somatic index (GSI) of males was lower than that of females. Where, the maximum GSI values were recorded in July (7.9 % and 6.8 % for females and males, respectively). The absolute fecundity of the present species ranged from 147321 to 384086 eggs. The logarithmic relationship showed that fecundity increased with the increase of length, weight, and gonad weight. K values for combined sex began to increase from January to reach a maximum value (1.65) at April then decreased from May to August. The spawning season of Siganus rivulatus in the Suez Gulf may be extending from May to August for females and males. 17β - Estradiol level in females of S. rivulatus ranged from 0.076 ± 0.007 ng / ml (recorded in June 2016) to 0.003 ± 0.0005 ng / ml (recorded in December 2016). Estradiol level was low during February but afterward, it increased significantly in June 2016. Testosterone level in males of S. rivulatus recorded the highest value of 1.698 ± 0.022 ng/ml in July. Then, it continuously decreased, reaching the minimum value of 0.039 ± 0.007 ng/ml in December 2016 (17.4 ± 1.054).