Effect of GnRHa injection on milt volume in recently stripped rainbow trout Oncorhynchus mykiss

In this study, the effect of gonadotropin-releasing hormone agonist (GnRHa) injection on milt production in spent rainbow trout was investigated. On day 0, 25 newly matured male rainbow trout (Oncorhynchus mykiss) were stripped manually, and sperm quantity (vol: mL fish⁻¹) and quality, spermatocrit (%), sperm count (cell mL⁻¹), motile sperm percentage and motility duration (s) were evaluated. After stripping, fish were randomly divided into five groups: intact; sham (injected with propylene glycol as a hormone vehicle); and groups receiving 4, 8 or 16 μg kg⁻¹ BW of [ d-Ala⁶ Des-Gly¹⁰] mGnRHa. On day 7, the fish were stripped again and the same sperm characteristics as on day 0 were measured. At the beginning of the experiment, there were no significant differences in any of the sperm quantity characteristics between groups. On day 7, expressible milt volume was significantly reduced compared with day 0 (P0.05, t-test). The present study shows that GnRHa injection with a concentration as low as 4 μg kg⁻¹ BW after first stripping could prevent a significant reduction in milt quantity collected 7 days later without any adverse effects on sperm quality.