Effects of thermal regime on ovarian maturation and plasma sex steroids infarmed white sturgeon, Acipenser transmontanus

Recently, commercial aquaculture farms in Northern California have exposed gravid, cultured white sturgeon females to cold water (12 +/-1 degreesC) th roughout the late phase of vitellogenesis and ovarian follicle maturation r esulting in improved ovulation rates and egg quality. However, the optimum timing for transfer of broodfish to the cold water and the capacity of tran sferred broodfish to maintain reproductive competence over an extended time in cold water had not been evaluated. Gravid white sturgeon females that h ave been raised at water temperatures of 16-20 degreesC were transported to either cold water (12 +/-1 degreesC; Group 1) in November 1997 or maintain ed in ambient water temperatures (10-19 degreesC; Group 2) until early spri ng. In March 1998, half of the fish in Group 2 had regressed ovaries, but t he remaining females had intact ovarian follicles and were transported to t he cold water. Ovarian follicles and blood were collected from females unti l they reached the stage of spawning readiness (determined by germinal vesi cle position and an oocyte maturation assay) or underwent ovarian regressio n. Exposure of gravid sturgeon females to ambient water temperatures (14.5 +/- 2.3 degreesC, mean +/- S.D.) from October to March led to a decrease in pla sma sex steroids and a high incidence of ovarian regression in fish with a more advanced stage of oocyte development. Transfer of females with intact ovarian follicles to cold water (12 +/-1 degreesC) in the fall or early spr ing resulted in normal ovarian development in the majority of females. Hold ing females in cold water does not seem to override their endogenous reprod uctive rhythms but extends their capacity to maintain oocyte maturational c ompetence over a longer period of time. A temperature-sensitive phase in ov arian development may occur during the transition from vitellogenic growth to oocyte maturation, and the degree and timing of sensitivity to environme ntal temperature are dependent on the female's endogenous reproductive rhyt hm. (C) 2001 Elsevier Science B.V. All rights reserved.