The overall objective of the study was to investigate changes in quantitative parameters of goldfish (Carassius auratus) semen, testosterone (T), and gonadosomatic index (GSI) during the four seasons of the year (spring, summer, autumn, and winter). Simple environmental and hormonal treatments were used to induce out-of-season spawning in goldfish. The semen was taken from goldfish in different periods during the four seasons, and the characteristics of sperm and pH were analyzed. Plasma levels of T, GSI, and histological studies of the testes, as well as a range of indices of ovarian development, were measured. No significant differences were observed between volumes of semen which can be extracted per fish, in the four seasons (P>0.05). Significant differences were found between sperm motility at different seasons (P<0.05), as the maximum total duration of motility was observed in autumn (109.25±14.00 s). Sperm density showed a higher value during summer (57.30±10.41spermatozoa (spz)ml−1) and winter (65.09±80.40 spz ml−1) than values that were obtained from spring (48.00 ± 7.08 spz ml−1) and autumn (40.42±16.54×109 spz ml−1) (P<0.05). However, spermatocrit (in percent) was higher inwinter (39.90±4.74) compared with other seasons (P<0.05). Values of pH were higher in autumn (7.87±0.05) and in winter (7.83±0.03) than values that were obtained from other seasons. The peaks of T and GSI during spermiation in spring (T, 21.08 ng/ml, and GSI05.21 %) and in summer (T, 23.32 ng/ml, and GSI06.10 %), when most gonadal development took place, were statistically significantly higher than the levels observed during autumn (T, 15.08 ng/ml, and GSI0 3.21 %) and winter (T, 22.18 ng/ml, and GSI02.78 %) (P< 0.05). Our results provided the statistically significant evidence of seasonal variation in sperm characteristics, T and GSI, for goldfish. These findings may be used to: (1) optimize semen collection for hatchery production and (2) characterize the potential impact of seasons o
