Novel isolated cDNA encoding the precursor of the novel gonadotropin-releasing hormones in four species of the fish lamprey is disclosed. The use of such cDNA's in studying the phylogenetic relationship of various species of fish and vertebrates overall; and the use of such cDNA's for controlling the gonadal development and spawning of fish, including inducing and inhibiting maturation, spawning and reproduction, is also described. Methods by which the novel peptides may be administered to fish to control their reproduction are also described.