In a middle reach of the Shigenobu River (Ehime Prefecture, southwestern Japan), a floodplain marsh has been created as a part of a habitat rehabilitation project. On 1 July 2023, an extreme high-flow event occurred in this river during our fyke-net survey on fishes immigrating to the marsh from the river main channel. In this report, we described a mass immigration of fishes caused by this high flow event, by comparing with monitoring data on fish immigration to the marsh from the river during 2015-2016 (152 days through 18 months). During the high-flow event, the total number of immigrating fishes caught by the fyke net was 503 individuals (per two days), which was 10-100 times higher than the usual values recorded by the 18-month monitoring. The dominant immigrating fishes were fat minnow (Rhynchocypris oxycephala: 383 individuals), crucian carp (Carassius spp.: 44 indiv), and two gudgeons (Gnathopogon elongatus: 49 indiv; Pseudorasbora parva: 12 indiv), the latter three of which were uncommon species in the river main channel adjacent to the marsh. In particular, the two gudgeons are species that had never been caught by the 18-month monitoring during 2015-2016, and had rarely been found both in the marsh and adjacent river reach. Therefore, the two gudgeons and crucian carp caught by the fyke net during the high-flow event are likely to have immigrated not from but through the river main channel from other lateral aquatic habitats. This mass immigration shows that an extreme high-flow event can facilitate colonization of a distant habitat by freshwater fishes.

We investigated the impact of 16 wind power generators on the Mountain Hawk-Eagle Nisaetus nipalensis during the pre-construction, post-construction, and pre-replacement stages of new wind power generators, 18 years after the construction of the existing generators. Two pairs were observed during the pre-construction stage, three pairs were observed during the post-construction stage, and four pairs were observed during the pre-replacement stage. During the pre-replacement stage, two of the four pairs （pair A and B） were succeeded in breeding and other two （pair C and D） were not observed to be successful, even though many breeding behaviors, such as courtship displays, mating behaviors, and delivering prey were observed. Of the pairs confirmed the pre-replacement, pair A included an existing wind power generator located 553 meters from the nest site, and pair C included an existing wind power generator located 925 meters. Pair A was also succeeded in breeding. Additionally, within the breeding territory, pair A included 2 existing wind power generators, and pair C included 5. This indicates that all pairs continue to occupy their breeding sites in areas where existing wind power generators are in operation, it is thought that Mountain Hawk-Eagles can occupy their breeding sites near the wind power generators in this area.

The Kanogawa River runs from Amagi Mountains in the middle of Izu Peninsula toward to the foot of Mt. Fuji and discharges into Suruga Bay. The length of the river is 46 km, and the annual mean rainfall of the catchment scale amounts to 2,830 mm at the Yugashima observation site. The quality of river water was examined using public data sources spanning the nearly 30 years from 1990 to 2019. A decrease in organic compound pollution was shown by an apparent change in BOD concentration for all the examined observation points from upstream to downstream. The concentrations of COD and SS also supported this tendency. Other examined parameters such as pH and DO did not support this decrease and they likely came from biogeochemical activities in the river water. The reason for the improvement of water quality can be ascribable to the development of sewage treatment systems in the mid- and downstream catchment throughout the examined 30 years.

Smallmouth bass (Micropterus dolomieu) is an invasive fish native to North America, and in recent year its distribution in Japan has been expanding rapidly, causing concern about its negative impact on the ecosystems of rivers where it has introduced its species. Therefore, management of the invasive species has been carried out in several rivers but there have been no successful eradication attempts. Since smallmouth bass tends to inhabit large rivers, it is necessary to determine the extent of their distribution and the reaches where population densities are high to promote more efficient management. In this study, environmental DNA (eDNA) was used to determine the distribution of smallmouth bass in the Kushida River, Mie Prefecture and the condition of their invasion into the tributaries. The eDNA surveys were conducted a total of four times in the summer (August 30-31, 2021 and September 12-13, 2022) and winter (January 27-28, 2021 and February 8-9, 2022). The eDNA of smallmouth bass was detected in the Kushida River mainstream, approximately 1 - 40 km from the mouth and eDNA detection site at the most upstream was consistent across all surveys. Spatial distribution seems to be limited by artificial barriers located upstream of the uppermost detection sites. The eDNA was detected in a relatively large tributary river, suggesting a potential risk of colonization in tributaries. Even in the small tributaries, the eDNA was detected in summer, indicating that they were invading, albeit temporarily. These findings suggested that when smallmouth bass invade a river, they disperse and colonize a broad area, including tributaries. Widespread management is costly and difficult to eradicate. Therefore, it is most important in the management of invasive alien species in rivers to not introduce in the water system and to exterminate them when their populations are low.