Analysis of the gut contents of the internal exotic fish species opsariichthys uncirostris uncirostris in the futatsugawa river, Kyushu Island, Japan

Opsariichthys uncirostris uncirostris was introduced from Lake Biwa to several freshwater areas in Kyusyu Island, and the gut contents of this fish were observed in order to evaluate the effect of internal invasion on native aquatic organisms. The fish specimens were captured in the Futatsugawa river, northwestern Kyushu Island, Japan, between February 2007 and January 2008, and their gut contents were examined under a stereomicroscope. The gut contents of this exotic species comprised various prey animals, mainly fishes in most of the seasons. The empty gut ratio was approximately 80% in several seasons and 100% during February and March. This exotic species showed high selectivity for Zacco spp., Pseudorasbora parva, Tanakia spp., and Rhodeus spp., which had large populations at the study sites. In contrast, this exotic species showed no selectivity for Carassius spp. that also had a large population. Acheilognathus spp. that had a small population at this study site showed high selectivity as prey by this exotic species. A. tabira nakamurae, a native species, is listed as threatened in the Japanese Red Data Book. Native species with a small population and high prey selectivity were estimated to be the most negatively effected species by the internal invasion of the study site by this exotic species.