The piscivorous chub (Opsariichthys uncirostris) is a Japanese predatory cyprinid with a native distribution restricted to a few large lakes. It has now established in a large region of Japan following accidental translocation. Although large water bodies with abundant food resources were long considered essential for establishing this species, the chub has settled in small irrigation ditches in Kyushu. In this study, we explored the phenotypic responses of the chub in these small water bodies by comparing life history traits and morphology with those of chub inhabiting a native lake. Growth rate, fecundity-related traits, and trunk length shifted markedly following translocation to the new habitat. These phenotypic shifts were typical reactions to characteristic conditions of irrigation ditches, such as habitat instability, lotic conditions, and limited food. Adaptability via a rapid phenotypic shift by the chub may have facilitated establishment of populations in small irrigation ditches.
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