Re-introduction of the Itasenpara bitterling to the Yodo River in Osaka Prefecture, Japan

Summary

The Itasenpara bitterling (Acheilognathus longipinnis), a Vulnerable cyprinid that spawns in freshwater mussel gill chambers and had gone undetected in Japan's heavily engineered Yodo River since 2006, was targeted for an experimental reintroduction trial by Osaka Prefecture's Aquatic Life Conservation Research Center and the Yodogawa River Office after river-restoration work reconstructed several riparian embayments that had since been naturally recolonized by mussels and other bitterling species. Drawing on a captive population maintained since 1972 from original Yodo River specimens, which retained high genetic diversity, the Yodo River Itasenpara Study Committee released 500 mature adults (1:1 sex ratio) into restored embayments in September and October 2009, following removal of predatory alien fish and in line with national fish reintroduction guidelines. A large typhoon-driven flood struck shortly after the first release, potentially washing bitterlings and mussels out of the embayments. In May 2010, 133 wild-born juveniles were observed, confirming the released adults had successfully bred and meeting one project goal, but numbers were low and no juveniles at all were found in spring 2011, indicating none of the 2010 cohort survived, likely due to continued heavy flood disturbance degrading mussel and bitterling habitat. Major difficulties included balancing the flood disturbance the species needs against disturbance strong enough to wash it out entirely, mussel population declines, invasive species increases during low-flood years, and illegal poaching pressure from aquarists. The project rates itself a Failure, while noting the trial generated valuable ecological and hydrologic data and that its still-healthy captive population keeps future reintroduction attempts viable.