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River estuaries are influenced by terrestrial and marine areas, and have a unique environment that is constantly fluctuating. They are also important habitats for biodiversity conservation. Tanegashima Island is significantly influenced by the Kuroshio Current. Although nearby Yakushima Island has been recognized as a World Natural Heritage site and information on many species has been collected, there is little information on species inhabiting the river estuaries of Tanegashima Island. In this study, the river estuarine ichthyofauna of 26 small and medium‐sized rivers on Tanegashima Island was surveyed and a total of 2758 individuals of 29 species and one genus belonging to 15 families were collected. The fish fauna of the river estuaries of Tanegashima Island were classified into three groups, namely the river estuaries where coastal terraces are underdeveloped and drowned valleys are formed, the river group where the riverbed gradient at the mouth is high, and other river groups. Environmental factors selected as being important for ordination of fish fauna were the altitude of the headstreams at the watershed scale, the gradient of the river estuarine area, and the presence or absence of rapids in the estuary. The importance of the geohistorical factor of drowned valley formation, in addition to habitat and watershed scale environmental factors, as a factor on river estuarine fish fauna is an important finding for future conservation of local biota diversity. [ABSTRACT FROM AUTHOR]

Highlights • The Japanese archipelago was classified into 15 groups using fish fauna classification. • Classification results were closely related to the process of formation of the Japanese archipelago, the ocean current in Japanese coastal waters, and the connection of the water system in the glacial age. • The classifications system used in this study enable to compare the biological integrity of fish fauna among watersheds for managing river environment. Abstract An evaluation of ecological integrity is required for ecosystem conservation and restoration. The ecological region, or "ecoregion", has been adopted as a unit of geological area to enable a comparison of the ecological integrity of different regions. The delineation of an ecological region is difficult in countries in East Asia, including Japan because of complex topographies (i.e., several peninsulas and islands) and fauna that are very finely delineated based on climate or geology. Therefore, it is important to appropriately determine the ecoregions when determining their biological integrity and comparing it among that of other ecoregions. I attempted to delineate an ecological region of the Japanese archipelago based on the similarities among fish fauna by integrating the information on fish fauna that was collected by the researchers and the national government and local governments. In addition, quantitative analyses to investigate the relationship between fish fauna classification and meteorological and geographical factors were conducted to discuss the factors that influence fish fauna classifications. The archipelago was classified into 15 fish fauna groups, and the results of these grouped classifications were closely related to the process by which the Japanese archipelago was formed, the ocean current in its coastal waters, and the connection of the water system to the glacial age. Our findings suggest that rivers within geographical areas that are different from those within the Japanese archipelago might have different fish fauna classifications based on our results and potential fish fauna depending on the characteristics of the watershed, such as the scale of the floodplain, river conflicts, or river formation process. By applying the results of our fish fauna classification, we are able to make a comparison of the biological integrity of fish fauna among different watersheds for managing the river environments or establishing conservation policies. [ABSTRACT FROM AUTHOR]

Selecting sites for river restoration projects or performing ex post evaluation must be done in ecoregions with comparable biological integrity. Moreover, to evaluate the integrity of a river ecosystem that varies from the headwater to downstream regions, adjustments of the ecoregion according to the longitudinal change in biota must be made. We defined regions where biota are identical according to the direction of change along the stream path as “segment-based ecoregions.” We delineated segment-based ecoregions using fish fauna data for two ecoregions in Japan: the northeast Kyushu, and south Kyushu. The northeast Kyushu ecoregion was divided into three segment-based ecoregions, and south Kyushu ecoregion into five segment-based ecoregions. Classifying fish fauna within the same ecoregion enables classification by differences in the longitudinal change in the stream path rather than by differences in watersheds. Specific topographic regions such as the amphitheater areas of the south Kyushu ecoregion was classified as independent segment-based ecoregions. We further defined indicator fish species for each segment-based ecoregion. Indicator fish species reflected the fish fauna of each ecoregion in the midstream area, such as migratory Gobiidae of the south Kyushu ecoregion. Applying the segment-based ecoregion concept helps compare the biological integrity of fish fauna depending on a common environmental type between watersheds. [ABSTRACT FROM AUTHOR]