Gonadotropin releasing hormone-analogue treatment increases sperm motility, seminal plasma pH and sperm production in yellowtail flounder Pleuronectes ferrugineus

Compared to control fish, gonadotropin releasing hormone-analogue (GnRHa) t reatment delivered either by mi crospheres or cholesterol pellets successfu lly increased sperm production (cells kg(-1)) and milt volume (ml kg(-1)) i n mature yellowtail flounder Pleuronectes ferrugineus during the spawning s eason. Spermatocrit decreased in both treated and control groups between 12 and 29 days post-implantation, indicating a seasonal decrease in sperm con centration, rather than an effect of the GnRHa treatments. Plasma levels of testosterone, 11-ketotestoterone and 17 alpha,20 beta dihy droxy-4-pregnen-3-one (17 alpha 20 beta P) showed no clear pattern either a cross treatments or over days, however this does not exclude the possibilit y that GnRHa had its effect on milt volumes via the stimulation of steroid production since the sampling protocol did not allow for the rapid clearanc e of steroids from the plasma. GnRHa treatment did not have a negative effe ct on sperm fertilizing ability, percentage hatch or larval morphology. Spe rm motility and seminal plasma pH were increased by GnRHa treatment.