EFFECTS OF UNILATERAL OVARIECTOMY ON RECRUITMENT AND GROWTH OF FOLLICLES IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Virgin female rainbow trout, Oncorhynchus mykiss, were unilaterally ov ariectomised at various stages of ovarian development to investigate t he effect of the removal of one ovary on subsequent recruitment and gr owth of follicles in the remaining ovary. The right ovary was removed from groups of 12-15 fish, 12, 7 and 4 months before they were due to ovulate, and the gonadosomatic index and follicle number and size dete rmined just prior to ovulation. There were no differences in fecundity or follicle size in fish unilaterally ovariectomised at 12 and 7 mont hs prior to ovulation compared to the controls. However, in the female s unilaterally ovariectomised 4 months prior to ovulation, the remaini ng ovary either had the normal number of follicles for a single ovary but of a significantly larger size than follicles in the controls, or alternatively had almost 70% more than the normal number of vitellogen ic follicles but comprising two distinctly different size populations. Differences in plasma oestradiol-17 beta concentrations at the final sample were seen only in females unilaterally ovariectomised 4 months prior to ovulation, where the levels were significantly lower than bot h the sham operated and control fish (p < 0.05). These data show that in the rainbow trout, complete compensatory ovarian hypertrophy follow ing unilateral ovariectomy can occur throughout a major part of ovaria n development, but that follicle recruitment is limited to stages up t o (and therefore fecundity is determined by) mid-vitellogenesis.