Quentin Jean-Baptiste Horta-Lacueva

The ecology of salmonids is tightly linked to their spawning habitats, but the link between spawning site selection and phenology is poorly understood. To address this, we studied the Arctic charr ( Salvelinus alpinus ) from the postglacial lake Thingvallavatn (Iceland) through a multidisciplinary approach involving aerial surveys, behavioural observations, temperature monitoring and embryo rearing experiments. Aerial footage revealed that most nests (i.e. redds) were established in shallow parts of the spawning area, and we reported through direct observations trends for stronger male–male competition and for more frequent courtship behaviours in shallow than in deep redds. While water depth did not correlate with temperature at the time of spawning, the temperatures recorded at the shallow redds were consistently lower in the two months following the video recordings, likely because of the proximity of glacial outlets. Laboratory experiments demonstrated that the temperature regimes in shallow waters can delay hatching with about a month, likely impacting the phenology of the offspring. The viability of the Arctic charr in Thingvallavatn may thus depend on physical features like groundwater springs and upwelling water flows acting as “temperature shelters”.