Effect of photoperiod on the incorporation of 3H-thymidine into the gonads of Carassius auratus

Gldfish, Carassius auratus of varying sizes were conditioned in continuous light or darkness f or 10, 20 and 30 days and the incorporation of ³H-thymidine into the gonads was investigated to attempt to develop a bioassay for fish gonadotropins.
The 24-hour gonadal ³H-thymidine of 10-day conditioned fish was significantly less at the dose level of 0.5 μCi ³H-thymidine/fish compared to 1.0, 2.0 and 3.0 μCi/fish which gave gonadal activities not significantly different from each other. Thus, for all subsequent work the dose of 1.0 μCi/fish was used.
Photoperiod of continuous light or darkness had little effect on fish weighing less than 11 g but in fish 11-15 g conditioned for 20 days in darkness and fish greater than 16 g conditioned for 10 days in darkness, depression in gonadal ³H-thymidine incorporation occurred. In fish 11-15 g, prolonged conditioning for 30 days in darkness induced more gonadal activity than was observed at 20 days.
The effect of injection of Channa striatus pituitary extract at doses of 1 mg/10 g body weight and 5 mg/10 g body weight induced a significant increase in ³H-thymidine incorporation in the gonads over saline injected controls. The results suggest the potential of using photoperiod as a means of inducing regression of gonads of suitable fish in a bioassay of gonadotropins.