Why ecologically similar species can coexist has been a central theme in ecology. Congeneric competitive fishes with parapatric distributions along with water temperatures, southern Asian Dolly Varden (SADV, cold-water species) and white-spotted charr (WSC, warm-water species), are one of the most well-known models to explore coexistence mechanisms. However, some past experiments have shown that WSC always dominates SADV regardless of temperature, which could not explain why SADV is dominant in cold water in the wild. To tackle this, we conducted a field survey at 35 stream habitat mosaics. Interestingly, we found a new pattern: the dominant WSC was abundant even in cold habitats if the subordinate SADV was absent. Therefore, we proposed a new coexistence mechanism determined only by the dominant species' cost-benefit balance regardless of the subordinate species. To test this hypothesis, we installed enclosures in four cold streams with a BACI design. We excluded SADV and then released 50 WSC in two “impact” streams and 50 SADV in two “control”. After a month, WSC remained in the cold streams at a similar rate (26–46%) as the SADV. This supports our hypothesis that some costs from the SADV, not water temperatures, can reduce the WSC’s cold-water preference and decide their upper limitation.