Effect of holding temperature on ovulation, egg fertility, plasma levels of reproductive hormones and in vitro ovarian steroidogenesis in the rainbow trout Oncorhynchus mykiss

Sexually mature female rainbow trout were held at temperatures ranging from 9–21 °C for up to 3 months prior to the natural time of ovulation, in experiments conducted over three different spawning seasons. The majority of fish held at 9 and 12 °C ovulated, variable numbers ovulated at 15 and 18 °C and only one fish ovulated at 21 °C. Egg production was similar at 9, 12 and 15 °C, significantly lower at 18 °C and near zero at 21 °C. Egg survival to the eyed stage after incubation at 11 °C was similar at 9, 12 and 15 °C, and nil at 18 and 21 °C. Histological examination of oocytes from fish held at 12, 15 or 18 °C for 1 or 2 months showed no evidence of gonadal atresia.Plasma levels of gonadotropin (GtH) were measured in samples taken 1, 2 and 3 months after introduction to temperatures of 9, 12, 15, 18 or 21 °C, and showed no differences between temperatures at any time. Plasma levels of testosterone (T) and 17β-oestradiol (E2) were similarly unaffected. Repeat measurement of T and E2 in another spawning season also showed that holding temperature had no effect on plasma steroid levels. In contrast, in vitro basal steroidogenesis by isolated ovarian follicles was generally lower at 18 °C than at 12 or 15 °C. Follicles from fish held at 18 °C for 2 months did not retain responsiveness to stimulation with steroid precursors or GtH, whereas those from fish held at 12 and 15 °C did. The results indicate that elevated autumn holding temperatures have a deleterious effect on ovulation, egg production and fertility but have equivocal effects on endocrine parameters associated with vitellogenesis. This suggests that the effects are exercised on processes associated with final maturation and ovulation rather than vitellogenesis.