Urbanisation has caused significant changes in material cycles, alterations in river flow rates and depths and modifications to habitats for aquatic organisms. These changes have led to disruptions in the movement and dispersal of aquatic species, as well as shifts in biodiversity. The loss of habitats due to urbanisation has resulted in a decline in species diversity across a variety of taxonomic groups. Aquatic organisms, in particular, are adversely affected by habitat loss caused by river channel modifications and the invasion of non-native species. It is generally reported that, in regions where invasive species have established and native species have declined, fish fauna tend to homogenise, leading to a substantial reduction in diversity. However, the impact of urbanisation on fish fauna varies depending on local ecosystems, the degree of urbanisation and the time elapsed since urban development. Therefore, it is crucial to gather region-specific information on how fish fauna have been altered by urbanisation. The study area in this research, the Kitakyushu metropolitan area, is one of Japan's oldest heavy industrial zones and many of its rivers have been affected by channel straightening and concrete lining.

A fish survey was conducted at 100 locations within the Kitakyushu metropolitan area during the summer period (from 16 July to 4 October 2024), focusing on river sections modified by channel straightening and concrete lining due to urbanisation. The survey resulted in collecting 30 genera and 32 species from nine orders and 19 families, for a total of 9,253 individuals. Amongst the species collected, rare species, such as the Bitterling, Liobagrus reini and Misgurnus anguillicaudatus, were also found (The Japanese Red List). Although the number of individuals was limited, these native rare species were found in inhabited rivers significantly altered by human activity. Additionally, invasive species including Lepomis macrochirus, Micropterus salmoides (Lacepède, 1802) and Channa argus (Cantor, 1842) were recorded. Despite accounting for only 0.5% of the total fish population, the highly predatory nature of these invasive species warrants proactive management, including the enhancement of impounded water environments through structures such as weirs, to mitigate their potential ecological impact.