Enhancement of spermiation in European sea bass (Dicentrarchus labrax) at the end of the reproductive season using GnRHa implants

Spermiating European sea bass (Dicentrarchus labrax) were treated at the end of the reproductive period (22 February 2002) with implants of gonadotropin-releasing hormone agonist (GnRHa), in order to examine their potential in enhancing expressible milt volume, sperm quality and in vitro fertilization success. The GnRHa implants at all doses (20, 40 and 80 mug GnRHa kg(-1) body weight [bw]) stimulated a significantly higher production of milt compared to controls over a 27-day period, increasing total volume from 0.5 to 2.2 ml kg(-1) sample time(-1). No differences in milt production were observed,among the three GnRHa doses. Histological evaluation of the testes indicated that, by day 21, control fish had entered into gonadal regression, containing almost exclusively spermatogonia. At the same time, testes from GnRHa-implanted fish still contained large numbers of spermatozoa. in the testes, indicating that spermiation was still underway. Sperm density varied between 7.4 and 14.4 x 10(10) spermatozoa ml(-1), but remained unchanged in response to GnRHa implantation. Sperm motility was high at the beginning of the experiment (92-100%) and decreased significantly by day 27 (32-55%), whereas motility duration was unchanged (0.9-1.2 min). Neither sperm motility nor motility duration was affected by the GnRHa treatment. Finally, fertilization success was high (40-80%) and not significantly different in response to GnRHa implantation. These results demonstrate that GnRHa implants are effective at increasing milt production at the end of the spermiation period, maintaining high sperm density and not affecting, either positively or negatively, sperm quality and fertilization capacity. (C) 2003 Elsevier Science B.V. All rights reserved.