Interspecific variation in diel‐scale temporal niches is common in natural communities. Such variation changes population dynamics via effects on the growth and reproduction of individuals. Also at the community level, theory predicts that animals can reduce competition for shared resources by changing diel activity in certain situations. However, the role of diel activity at the community‐level has not been examined sufficiently. In this study, to examine whether the diel‐scale temporal niche act as a competition‐mitigating mechanism for stream fishes at the community level, we surveyed diel changes in microhabitat use and foraging, and the pattern of interspecific diet overlap in the middle reaches of a temperate stream where various fish species that seemed to be either nocturnal or diurnal coexisted. Our results suggest that the fishes forage during both daytime and night, but change their foraging mode at different times of the day, so that the foraging habits of these fish species cannot be divided simply into nocturnal and diurnal. Furthermore, fishes appeared to aggregate in the vicinity of common food resources during time zones with high availability of the resources, and therefore, inter‐guild diet overlap was high during certain time zones. On the other hand, when inter‐guild diet overlap was low, each fish species used foods or microhabitats that did not any have the potential to be used by species of another guild. Therefore, we conclude that variation in diel niche use is influenced by variation in the fundamental niche and food supply or availability rather than by competitive interaction between fishes in the stream fish community.

The cephalic lateral line systems of seven Far Eastern Phoxinus species (P. phoxinus, P. kumgangensis, P. semotilus, P. lagowskii, P. oxycephalus, P. perenurus, and P. czekanowskii) were investigated. In this genus, the infraorbital canal is not connected with either the supraorbital canal or the preoperculomandibular canal. Phoxinus phoxinus is unique for having an underdeveloped condition, such as canal formation or remaining as uncovered. A unified supraorbital canal was observed in all species, but the infraorbital canal of both P. perenurus and P. czekanowskii was not unified into a single canal throughout their development. Unification between both sides of the supratemporal canal occurred in larger individuals of P. lagowskii, P. oxycephalus, and P. czekanowskii. The preoperculomandibular canal of P. kumgangensis, large P. lagowskii, and large P. oxycephalus was unified. The pore number of each part of the canal system also varied depending on the species. Intraspecific variations were observed between Korean and Japanese specimens of P. lagowskii in the unification of the supratemporal canal and the preoperculomandibular canal, and the number of pores of the supratemporal canal. It was inferred that the specific characteristic patterns of their cephalic lateral line systems reflected the following two factors: different environmental requirement for their microhabitat and different maximum body size.

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