Habitat preference by the endangered bitterling fish Rhodeus atremius suigensis in an agricultural channel in southern Okayama, western Japan

Rhodeus atremius suigensis is an endangered bitterling species designated as a Nationally Endangered Species of Wild Fauna and Flora by the Ministry of Environment of Japan. In order to create preferable microhabitats for the threatened bitterling in agricultural channels, environmental factors affecting its occurrence need to be identified. The present study investigated environmental preference of the fish inhabiting an agricultural channel in southern Okayama Prefecture, western Japan, through monthly surveys of fishes and physical environments between June 2014 and May 2015. The effects of physical environmental factors on the occurrence of the bitterling were analyzed using generalized linear models （GLM） with binomial probability distribution or generalized estimating equations （GEE）, seasons separately. The best models yielded by model selection based on AIC or QICu revealed seasonal changes in the key environmental factors for the occurrence of the bitterling. The variables with positive effects on the bitterling occurrence included submerged macrophyte coverage during the spring and the summer to autumn period and presence of submerged structures and above-water cover during the winter. Water velocity had a negative effect on the occurrence during the spring and the summer to autumn periods. At the same time, mean hydraulic environment at the locations with the bitterling occurrence was consistent throughout the study period, with water velocity less than 11.5 cm/s （3.4 cm/s on average） and water depth ranging 20-50 cm （37.7 cm on average）. The findings insist on the importance of the above mentioned lentic environment, in addition to covering materials represented by submerged structures and above-water cover, when creating preferable microhabitats for the bitterling in agricultural channels.