Abstract The freshwater fish fauna on the southern islands of Japan is largely composed of peripheral and diadromous fishes, with few genuinely freshwater species. Although paddy water systems are known to provide important habitats for the latter in Japan, the utilization patterns by such on the southern islands are poorly understood. A paddy water system on Iriomote Island, where traditional style rice farming is maintained, was investigated so as to evaluate its function as a habitat for insular freshwater fishes. A total of 24 species were found, including 6 brackish, 3 peripheral, 12 amphidromous, 2 catadromous species, plus
Gambusia ajfinis, the only genuine, and freshwater fish, which had been introduced. Of the 12 amphidromous species, five belonged to Eleotridae, including the endangered species
Hypseleotris cyprinoids, the remainder being gobies. Based on binary data (presence/absence of dominant fishes), logistic regression analyses were conducted to construct predictable models. Dominant fish occurrence was explained by sig-nificant environmental conditions, that were expressed as principal components. Openness, species' richness, impoundment, stony substrate, deep water column and brackish water were all factors primarily preferred by particular fish groups, the paddy water system on Iriomote Island supplying several types of habitat avail-able for insular diadromous fishes. In particular, the lack of hazards along the mi-gratory course was a significant habitat factor for fishes with less upstream-migra-tory ability, such as eleotrids.
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