Shiro Sagawa

We studied the habitat status of fishes in a Tanaka retention pond (TRP) , which was expected to become a foraging site for oriental storks (Ciconia boyciana) in Japan. By comparing the distribution of a fish community in a lateral drainage channel (LDC) of a paddy field before (December 2018) and after (November and December 2019) flooding by Typhoon No. 19 (Hagibis) , we examined the problems and future policy considerations for fish habitats in the TRP during periods of normal water level. Before flooding, the numbers of taxa and individuals in the LDC were small, and some study sites were no fish. In the LDC, most fish species that have been observed in the main drainage channel (MDC) during the same season were not captured. After flooding, the number of fish taxa and individuals increased significantly in the LDC compared to those before flooding, and some fish species such as the Gnathopogon elongatus elongatus and the Oryzias sp., which were not observed before Hagibis, were captured. In addition, oriental weather loaches (Misgurnus anguillicaudatus) were captured from LDCs that were completely dry before the flood. According to the paddy field survey in May 2019 (before flooding) , only Misgurnus dabryanus, the invasive loach species, were observed to breed in the study paddy fields during the cropping season. Breeding abundance may have been impacted by low continuity to the LDC during baseflow conditions, as many fish species could not migrate to paddy fields during the cropping season nor overwinter sufficiently in the LDC during the non-cropping season. For improving the function of the TRP as a breeding and wintering habitat for fish during baseflow conditions, gaps between the MDC and the LDC and between the LDC and paddy fields must be connected, and a wintering habitat for fish must be secured by actively preserving the channel sections with soft mud and connecting them with the inter-levee floodplain water bodies. However, since these restorations may expand the distribution range of invasive species, our goal should be to conserve the continuity of healthy water bodies.