Smallmouth bass (Micropterus dolomieu) are a freshwater fish native to North America, which have been introduced globally for angling opportunities, and are known to cause native fish decline primarily due to predation. In Japan, starting in the 1990s, smallmouth bass populations have spread from a few mountain lakes into many neighboring river systems. Currently there is considerable concern as smallmouth bass continue to spread and establish in major river systems and pose threats to economically important freshwater fisheries. This presentation will focus on recent research at Lake Nojiri (Nagano Prefecture), one of the lakes where smallmouth bass were originally introduced, and in which removal is currently not a viable management option due to the economically important recreational bass fishery. Specifically examining the role of a recent resurgence of aquatic vegetation in possibly providing refuge for prey fish. The aquatic vegetation of Lake Nojiri had been severely reduced since the 1970s after grass carp (Ctenopharyngodon idella) introduction, but since 2017 a remarkable recovery is underway. Currently over 10 native vegetation species have reestablished and have been shown to provide valuable cover habitat for prey fish, especially young-of-year. In systems such as Lake Nojiri, where invasive predator removal is difficult, this type of natural management to enhance native species survival is promising.

In addition, a unique smallmouth bass population in Lake Katsura (Toyama Prefecture), where prey fish are extremely limited, will also be introduced. This population preys primarily on small freshwater shrimp and terrestrial insects, and prey-fish predation was not observed (excluding a few conspecific smallmouth bass (4 out of 446 stomachs)). This unique non-piscivorous diet likely leads to the observed reduced growth rate but may not be severe enough to induce high rates of cannibalism. This apparently diet plasticity of smallmouth bass may facilitate their establishment and pose complications for native species recovery. In conclusion, the current status of smallmouth bass spread throughout Japan will also be discussed, especially in light of recent reports in major river systems such as the Sho-river in Toyama. Best management strategies will be considered, while referencing current research in other non-native areas such as the Western United States.