Habitat dependent predation‐competition interaction shifts of invasive smallmouth bass (Micropterus dolomieu) and resident cyprinids in the Chikuma River, Nagano Japan

Abstract

Habitat use and diet composition of invasive smallmouth bass, Micropterus dolomieu, Japanese dace, Pseudaspius hakonensis, and pale chub, Opsariichthys platypus, were studied for two years in the Middle Chikuma River, Nagano, Japan. Habitat use of dace and chub overlapped both years but showed differences from smallmouth bass. Additionally, smallmouth bass habitat use differed greatly between years, as young smallmouth bass were found in uncharacteristically fast flowing habitat in 2018 and predominantly in more typical slow flowing habitat in 2019. This use of fast flowing habitat in 2018 appears to be influenced by the high overall fish density, especially for smallmouth bass. Diet composition, studied in 2018, overlapped significantly between the three species in riffle habitat and overlap became less significant in run and pool habitat where smallmouth bass fed increasingly on fish prey. We therefore suggest that the interactions between smallmouth bass and dace and chub shift from predominantly predation in slow flowing habitat to resource competition in faster flowing habitat.