Species‐specific foraging behavior and diets of stream salmonids: An implication for negative impacts on native charr by nonnative trout in Japanese mountain streams

Salmonids have been introduced globally as a food source and recreational fishing target. In Japan, brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) were introduced in the 19th century and have since spread. In many headwater streams, native white‐spotted charr (Salvelinus leucomaenis) are thought to be experiencing negative impacts from these species. The current study examined foraging behavior, microhabitat use, and diet overlap of these three species in Kamikochi, Nagano Prefecture: one of Japan's premier mountain areas. In Kamikochi, many spring‐fed headwater streams are currently dominated by these invasive salmonids and white‐spotted charr have declined drastically over the last half century. Underwater video analysis revealed that while total foraging rates and foraging modes were similar between the three species, brook trout and white‐spotted charr foraged benthically more frequently than brown trout. Microhabitat water depth and flow velocity were similar between species, and fish size had a positive effect on water depth and flow velocity in all three species. Diet analysis indicated that brook trout and white‐spotted charr diets were nearly identical, comprised primary of aquatic invertebrates, while brown trout preyed on a mix of terrestrial and aquatic invertebrates, as well as amphibians and fish. These results indicate that in Kamikochi, the decline of white‐spotted charr is likely most influenced by direct competition with brook trout for prey resources. However, brown trout likely also predate on juvenile white‐spotted charr, while also possibly causing a foraging niche shift of white‐spotted charr, and have ecosystem‐level impacts due to predation on terrestrial prey. This study examines habitat, foraging, and diet niche overlaps of invasive salmonids (brown trout and brook trout) and native white‐spotted charr in headwater streams. All three species are found in similar habitat, while brook trout and white‐spotted charr have nearly identical foraging and diet niches. The combined effects of brown trout and brook trout have likely led to the decline of white‐spotted charr over the last century.