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 degrees C for up to 3 months prior to the natural time of o vulation, in experiments conducted over three different spawning seaso ns, The majority of fish held at 9 and 12 degrees C ovulated, variable numbers ovulated at 15 and 18 degrees C and only one fish ovulated at 21 degrees C. Egg production was similar at 9, 12 and 15 degrees C, s ignificantly lower at 18 degrees C and near zero at 21 degrees C. Egg survival to the eyed stage after incubation at 11 degrees C was simila r at 9, 12 and 15 degrees C, and nil at 18 and 21 degrees C. Histologi cal examination of oocytes from fish held at 12, 15 or 18 degrees C fo r 1 or 2 months showed no evidence of gonadal atresia. Plasma levels o f gonadotropin (GtH) were measured in samples taken 1, 2 and 3 months after introduction to temperatures of 9, 12, 15, 18 or 21 degrees C, a nd showed no differences between temperatures at any time. Plasma leve ls of testosterone (T) and 17 beta-oestradiol (E(2)) were similarly un affected. Repeat measurement of T and E(2) in another spawning season also showed that holding temperature had no effect on plasma steroid l evels. In contrast, in vitro basal steroidogenesis by isolated ovarian follicles was generally lower at 18 degrees C than at 12 or 15 degree s C. Follicles from fish held at 18 degrees C for 2 months did not ret ain responsiveness to stimulation with steroid precursors or GtH, wher eas those from fish held at 12 and 15 degrees C did. The results indic ate that elevated autumn holding temperatures have a deleterious effec t on ovulation, egg production and fertility but have equivocal effect s on endocrine parameters associated with vitellogenesis. This suggest s that the effects are exercised on processes associated with final ma turation and ovulation rather than vitellogenesis.