Once the fish have developed markings on their sides, they’re known as ‘parr’. The parr will live in the river for two to three years depending on water temperature and food availability. On reaching about 12cm in length, the parr undergo a physiological transformation that lets them survive at sea.
The young fish, now called ‘smolts’, continue to change in appearance, becoming silver. These ‘post-smolts’ begin to leave rivers for the sea in late spring, with most fish gone by June.