In order to examine whether or not a germ cell derived from a donor fish, which has been transplanted into a recipient fish of a different species by a surrogate fish technique, grows or matures in the gonad of the recipient fish, it is necessary to use, as an indicator, a trait that is specifically expressed in the germ cell and can be used to distinguish the recipient fish from the donor fish. Vasa gene, which is a germ cell-specific gene, is specific to a primordial germ cell and a spermatogonium/an oogonium, and it is not expressed in a somatic cell. In the present invention, the Vasa gene sequences of a tuna, a chub mackerel, a spotted mackerel, an eastern little tuna, and a drumfish are determined, and the expression of such gene is used as a marker for a germ cell. In addition, according to the present invention, it is possible to specifically detect only a tuna Vasa gene in Vasa gene sequences that are highly conserved in fishes, without sequencing. Thus, a tuna-derived germ cell can be reliably and simply identified in the gonad of the recipient fish. As a result, the growth or breeding of tuna can be carried out with good efficiency. Moreover, utilizing the aforementioned findings, even in a case in which not only a tuna but also another Perciformes fish is used as a donor, a germ cell derived from the donor fish can be efficiently detected from the gonad of a recipient fish of a different species.