Estimation of the frequency of maturity and sexual differences in the maturation period of the roughskin sculpin Trachidermus fasciatus

In Japan, the catadromous roughskin sculpin Trachidermus fasciatus, belonging to the family Cottidae, is found in Ariake and Isahaya Bays, and in the rivers flowing into these water bodies. Several studies have been published on the reproductive ecology of this fish. However, estimates on the maturation of this species are not available. In this present study, the authors estimated the frequency of maturity stages and sexual differences in the maturation periods of T. fasciatus collected from the Kashima River and Ariake Bay. Specimens were collected 3 km upstream from the mouth of the Kashima River in Kyushu Island, Japan, between November 1994 and January 1995. Until 1995, no specimens have been collected from Ariake Bay because the spawning grounds of this species were unknown. However, in a previous study conducted in 1999, the spawning grounds of this species were found for the first time to be 1.1– 2.3 km away from the mouth of this river. In addition, the individuals caught in May 1997, at approximately 1.5 km upstream the mouth of the Naka River (the tributary of the Kashima River), were used as the specimens in postspawning season. The males and females that were collected in this study were also used as specimens in the present study. In total, 69 (85.3–161.3 mm body length [BL]) female and 60 (82.4–146.7 mm BL) male specimens were collected. These specimens werefixed in10%buffered formalin, and thegonads were removed. The gonad weight (WG; mg) and body weight (WB; mg) were measured, and the GSI (I) was calculated using the following equation:

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