Female northern whitefish were treated with salmon [D-Arg6Pro9Net]-sGnRHa emulsified in Freund′s incomplete adjuvant as a sustained release treatment (sGnRHa-FIA) or with [D-Arg6Pro9Net]-sGnRHa dissolved in physiological saline as acute release treatment (sGnRHa-PS). Females in four experimental groups (A, B, C, D) and one control group (E) were injected intraperitoneally as follows: A and B – sGnRHa-FIA at doses of 50 and 25 μg kg−1, respectively; C – double injection (DI) of sGnRHa-PS at 25 μg kg−1 administered 3 days apart; D – single injection (SI) of sGnRHa-PS at 25 μg kg−1; and E – control group receiving a 1:1 mixture of FIA and physiological saline. All treatments induced and synchronized ovulation. Mean time to ovulation was significantly reduced (P < 0.05) in GnRHa-treated groups compared with control. Hormone treatments did not affect the relative fecundity. Slight differences were found in ovarian fluid pH between group A and D (P < 0.05). Except for group C, egg size was significantly reduced (P < 0.01) in hormonally synchronized groups compared with the control. Survival to the eyed stage of eggs from females in groups A, B and C was significantly lower (P < 0.01) than in groups D and E. Per cent hatched alevins was lower (P < 0.01) in groups A, B and C than in groups D and E. We conclude that synchronization of ovulation in northern whitefish can be induced by a single acute injection of [D-Arg6Pro9NEt]-sGnRHa at 25 μg kg−1 BW if given near the natural spawning time determined by water temperature below 2°C.