Acceleration of Gonadal Maturation in Anadromous Maturing Sockeye Salmon by Gonadotropin-Releasing Hormone Analog Implantation

Abstract

The effects of a three week intramuscular implantation of gonadotropin-releasing hormone analog (GnRHa) on gonadal maturation and changes in serum gonadal steroid hormone levels were examined in anadromous, maturing sockeye salmon Oncorhynchus nerka during the pre-spawning season. Spermiation occurred in both GnRHa-implanted and control males, but serum 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) levels in the implanted males were higher than in controls. No differences were found in serum testosterone (T) levels between GnRHa-implanted and control males. Serum 11-ketotestosterone (11KT) levels in control males increased for three weeks, but those in GnRHa-implanted males did not change. All GnRHa-implanted females ovulated completely for three weeks, while 25% of control females ovulated partially and the rest of females ovulated 17-24 days after sampling. The number of ovulated eggs of GnRHa-implanted females was greater than that of control females, and percentages of eyed eggs were not different between GnRHa-implanted and control females. Serum levels of DHP increased dramatically in GnRHa-implanted females, but those of T and estradiol-17β (E₂) was similar to control females. These results confirm that GnRHa accelerates gonadal maturation of both sexes through elevation of serum DHP levels in maturing sockeye salmon.