Acceleration of gonadal maturation in anadromous maturing sockeye salmon by gonadotropin-releasing hormone analog implantation

The effects of a three week intramuscular implantation of gonadotropin-rele asing hormone analog (GnRHa) on gonadal maturation and changes in serum gon adal steroid hormone levels were examined in anadromous, maturing sockeye s almon Oncorhynchus nerka during the pre-spawning season. Spermiation occurr ed in both GnRHa-implanted and control males, but serum 17 alpha,20 beta-di hydroxy-4-pregnen-3-one (DHP) levels in the implanted males were higher tha n in controls. No differences were found in serum testosterone (T) levels b etween GnRHa-implanted and control males. Serum 11-ketotestosterone (11KT) levels in control males increased for three weeks, but those in GnRHa-impla nted males did not change. All GnRHa-implanted females ovulated completely for three weeks, while 25% of control females ovulated partially and the re st of females ovulated 17-24 days after sampling. The number of ovulated eg gs of GnRHa-implanted females was greater than that of control females, and percentages of eyed eggs were not different between GnRHa-implanted and co ntrol females. Serum levels of DHP increased dramatically in GnRHa-implante d females, but those of T and estradiol-17 beta (E-2) was similar to contro l females. These results confirm that GnRHa accelerates gonadal maturation of both sexes through elevation of serum DHP levels in maturing sockeye sal mon.