MECHANISMS CONTROLLING EGG SIZE AND NUMBER IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Female rainbow trout (Oncorhynchus mykiss) produce a single batch of e ggs each year; synchronous growth of oocytes, all of which are ovulate d at the same time, occurs in the two ovaries. To examine the regulato ry mechanisms controlling egg size and number, virgin female rainbow t rout were subjected to unilateral ovariectomy (ULO) during early vitel logenesis, and oocyte recruitment and growth in the remaining ovary we re monitored. The study also set out to determine whether the presence of a second population of smaller oocytes in the maturing pool (induc ed by ULO) affected the timing of ovulation and/or the size of the egg s ovulated. Two months after ULO, there was no difference in the gonad osomatic index between ULO fish and controls. Compensatory ovarian hyp ertrophy resulted from the recruitment of a second population of prima ry oocytes into the vitellogenic pool. This population of smaller matu ring oocytes in the ULO fish displayed growth rates up to twice those of the population of larger oocytes in the same ovary and of oocytes i n controls. The growth rate of the population of larger oocytes in the ULO fish was not altered by the recruitment of a second maturing popu lation. One month after ULO, fish had a lower concentration of plasma estradiol-17 beta than did controls; subsequently the concentrations o f plasma estradiol-17 beta in the ULO and control groups were similar. After ULO, plasma levels of vitellogenin in the ULO fish did not diff er from those in the control group throughout the study. At or close t o ovulation, the fecundity of ULO fish was 75-80% that of controls. In the control group, oocytes appeared to reach a certain critical size before they were ovulated, and fish with higher fecundity ovulated lat er than their less fecund counterparts. ULO did not affect the timing of ovulation, and ULO fish ovulated eggs with a considerably greater s ize-range than did controls.