Gonadotropin-releasing hormone analogue (GnRH-A) induces multiple ovulations of high-quality eggs in a cold-water, batch-spawning teleost, the yellowtail flounder (Pleuronectes ferrugineus)

Abstract

Sustained delivery systems for a gonadotropin-releasing hormone analogue (GnRH-A) were used for the first time in a cold-ocean teleost for inducing multiple ovulations. Intramuscular implants of 75-224 µg/kg GnRH-A ((D-Ala 6 ,Pro 9 -NHEt)LHRH) were administered to female yellowtail flounder (Pleuronectes ferrugineus) either as a cholesterol pellet or as biodegradable microspheres. Females implanted with GnRH-A ovulated 3-4 weeks earlier than controls, producing multiple batches of high-quality eggs (average rates of fertilization 66-68% and hatching 55-60%); control fish eggs had significantly lower fertilization (39%) and hatching rates (25%). GnRH-A treatment doubled the egg production, synchronized the females, and shortened the interovulatory periods. The proportion of round, clear, and floating eggs lacking a perivitelline space before fertilization was an indicator of the fertilization (r 2 = 0.60) and hatching (r 2 = 0.56) success. Moreover, egg collections of less than 10 mL varied greatly in quality, whereas larger batches, generally promoted by GnRH-A, consisted of relatively high-quality eggs. GnRH-A treated fish had increased plasma levels of estradiol-17 beta 4 days after implantation. A model for prediction of the time between hormone treatment and first spawning (T; days) was developed using the initial follicle diameter (D; µm) as the only predictor (T = 365(D - 380) -0.82 ; r 2 = 0.91).