Natural Barriers and Internal Sources for the Reproductive Isolation in Sympatric Salmonids from the Lake–River System

Within the sympatric evolution framework a range of ecological variables are considered as potential initiators and controllers of the diversification process. Here, we identify the proximate factors providing the reproductive isolation among seven sympatric ecomorphs of the genus Salvelinus charr dwelling in the Lake Kronotskoe basin (North-East Asia). We demonstrate that the slope profile of the lake tributaries determining the water flow velocity and position of the groundwater discharges serves as a barrier between the reproductive sites of the ecomorphs and provides the basis for selective pressure affecting the Lake Kronotskoe fish during spawning. The main characteristic under selection is a migratory ability, which is determined by the swimming performance and the amount of energy reserved in the body and depends on fish morphology and physiology. The flex points indicating abrupt slope changes along the spawning watercourse restrict the upstream migration of the groups having a comparatively low swimming performance and energy reserve. A thorough analysis of the ecomorphs’ migratory and spawning activity indicated two energy expenditure strategies: the fish could invest most of the energy either on migration or spawning. Our findings supported with data on fish ecology, morphology, and thyroid hormone status allow us to put forward a following hypothesis. We suggest that the interplay of spatially heterogeneous environmental variables affecting life history decisions via ecomorphological and physiological traits could serve as a trigger for the reproductive isolation among the ecomorphs in a single ecosystem.