Hot spots in a cold river synchronize temporarily separated salmon runs in their offsprings development
The identification of thermal heterogeneity in the environment and its inclusion in adaptive strategies are key to habitat management of cold-water fish, including salmonid species. This study tested the hypothesis that upwelling of groundwater (GW) from a tributary catchment through a relatively deep aquifer (tributary GW) affects salmon redds selection in an urbanized gravel-bed river, and that the spawning preference of such areas depends on the seasonal context. The field study was conducted between 2001 and 2015 in an approximately 6-km long segment of the Toyohira River, Northern Japan. Chum salmon redds distribution data over 15 years (2001-2015) were combined with spatial distribution data of hyporheic water affected by tributary GW in the riverbed to examine seasonally variable redds site selection in relation to the presence of unique GW. Furthermore, models that predicted the hyporheic water thermal regime were coupled with redds count data to estimate the approximate timing of fry emergence from the riverbed. The redds site selection was seasonally variable, with a higher dependence on tributary-GW-affected areas with a decrease in water temperature. The time until fry emergence from spawning was shortened when the tributary-GW area was chosen during the cold winter. Overall, the present study identified hotspots for salmon spawning redds in winter with a disproportionately high level of site selection because of their warmer temperature compared to surface river water in winter. Thermally diverse spawning habitats allow the diversification of spawner strains in synchronized descents to the sea. Signs of tributary-GW pollution was suggested, and thus the conservation of the groundwater pathway and its sources, followed by improvements in quality, can be beneficial to the Chum salmon populations in the Toyohira River.