Unveiling the effects of interspecific competition: ecological consequences of competitive release after damming on Salvelinus curilus populations in a three-salmonid species coexistence system

Abiotic factors and biotic interactions, such as interspecific competition, are fundamental determinants of animal growth and survival. However, comprehensively and quantitatively assessing the impacts of interspecific competition while accounting for confounding abiotic factors remains a challenge in ecological research. We investigated changes in the ecological and morphological characteristics of southern Asian Dolly Varden (Salvelinus curilus, SADV) with competitive release in river systems fragmented by small dams. In sympatric reaches downstream from dams, where SADV coexisted with masu salmon (Oncorhynchus masou) and white-spotted charr (Salvelinus leucomaenis), these populations exhibited reduced growth, apparent survival, and body condition. The sympatric SADV developed subterminal jaws and smaller eyes, adaptations that allowed them to shift to benthic prey. Conversely, SADV in allopatric reaches upstream of dams exhibited increased population densities, enhanced growth rates, and generally improved body condition. Morphological traits, such as terminal jaws and enlarged eyes, reflected adaptations for enhanced foraging efficiency on terrestrial prey. These findings highlight the demographic, behavioral, and developmental effects of interspecific competition and competitive release on the morphology and ecology of these charr, providing valuable insights into how habitat fragmentation influences population dynamics and adaptive traits.