| Although many fish passages have been constructed to facilitate fish migration through weirs, it is difficult to determine their effectiveness. Many weirs in Japan <0.5-m tall have never been considered as candidates for fish passages, and we have not been able to determine whether they present barriers to any fish species. In this study, molecular methods were used to evaluate fish migration in the Ota River in Hiroshima, Japan. Study sites included a 55-km reach with 9 weirs and a 10-km reach of a tributary that had 3 dams. Eight of 9 weirs allowed fish passage, and the remaining weir had no opening. One of the weirs was situated downstream from its study site, and the other sites were located upstream. Dams on the branch of the Ota River were built in 1959, 1961, and 2001, and none allowed fish passage. Fin samples from two fish species, Nipponocypris temminckii and Rhinogobius flumineus, were collected over a 2-year period for DNA extraction and analysis of amplified fragment length polymorphisms (AFLPs). To estimate genetic differentiation between populations in each region and between generations, genetic diversity and population structure were estimated using AFLP-SURV and STRUCTURE programs. Local populations on the upstream side of dams were genetically distinct from other populations. Few differences were found throughout the Ota River in both upstream and downstream regions. Although small differences were detected between populations upstream and downstream of several weirs, these patterns were not consistent between years. Genetic relationships between populations also revealed different patterns for each species. Based on these results, we concluded that both fish species migrate throughout the Ota River with no significant changes in population genetics. We also demonstrated that molecular techniques could be used to evaluate fish migration, particularly in rivers where fish passages have been constructed. |
