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Japanese dace (Pseudaspius hakonensis) foraging and predator avoidance behaviors were studied in dace schools with and without vicinity predators in Lake Nojiri, Nagano Japan. Dace and invasive smallmouth bass (Micropterus dolomieu), which were the only predator observed, were found interacting in shallow habitat along the shoreline. Dace exhibited remarkably different total foraging, foraging modes, and predator avoidance behavior when smallmouth bass were in vicinity. Total foraging was significantly reduced, foraging mode shifted from predominantly benthic foraging to water column foraging, and dace schooled tighter indicated by a significant decrease in nearest neighbor distance and increase in neighbor alignment. These findings show that in Lake Nojiri, dace recognize smallmouth bass as a predator and utilize predator avoidance behaviors. However, the severely decreased total dace foraging when smallmouth bass are in vicinity indicates that even when direct predation is avoided, the indirect effects of reduced foraging and heightened vigilance likely negatively impact dace growth and survival.

In Lake Nojiri, the Japanese smelt, Hypomesus nipponensis McAllister, 1963, population has been thought to be maintained by artificial spawning, and stocking efforts as natural spawning in in-flowing streams is unlikely due to the lack of inflowing stream habitat. In this study, novel resident Japanese smelt spawning was observed along the lakeshore in shallow areas with clean gravel and flow. Spawning occurred at night during early March to mid-April and eggs, confirmed on gravel substrate, progressed to the eyed state in 5–6 weeks. Although natural spawning was observed, the ratio of eggs that progressed to the eyed stage was minimal, possibly due to thick algae cover in areas with low flow velocity. Therefore, resident Japanese smelt spawning along the lake shoreline is physically possible and potentially contributes to the overall population in Lake Nojiri, but in-lake spawning alone presumably is not a biologically viable method for population sustainability due to low survival rates and egg density at present. Environmental improvements such as construction of fish ladders to suitable in-flowing spawning habitat, introduction of clean gravel to shorelines, and water level management adjusted to the smelt spawning run would contribute to higher recruitment by natural smelt reproduction, and consequently enhance the smelt production in Lake Nojiri.

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.

Spawning season and habitat of invasive smallmouth bass were studied for two seasons in the Chikuma River, Nagano, Japan. Smallmouth bass spawning started in early May, when required water temperatures were met, and continued as long as spawning was not disrupted by high water levels. Spawning sites were found in patches throughout the study area that had significantly different physical characteristics than typical stream habitat features. Utilized spawning sites had low current velocity, were relatively shallow, and had pebble-sized substrate. Individual nest variables showed correlation with nesting male size as larger males constructed larger and deeper nests in faster flowing and deeper areas of the river. Nest guarding rates were low while egg presence rates were high compared to the native range, which may indicate that factors such as favorable water levels and water temperature and prevalence of spawning habitat facilitate spawning and the spread of smallmouth bass in the Chikuma River.

Foraging behavior and microhabitat use of native Japanese dace, pale chub, and invasive smallmouth bass were studied in the Nogu River, Nagano, Japan. Chub, dace and smallmouth bass all used drift feeding as their dominant foraging behavior. Chub drift fed most frequently, followed by dace, and smallmouth bass drift fed significantly less frequently than both native species. Chub and dace utilized microhabitat that had significantly higher flow velocity than smallmouth bass. However, smaller chub and dace were found in areas with lower flow velocity compared to larger individuals. Prey resource competition between smallmouth bass and native cyprinids, especially smaller individuals, is suspected due to similar feeding methods and microhabitat use.