After a period of growing in the river, at some point the young salmon undergo a physiological transformation which takes place in the spring of the year. In April and May the fish develop of silvery coat and are then known as smolts. The age at which young salmon become smolts varies according to how fast they have grown. For example, in cold infertile highland tributaries it may take the fish three years to grow big enough but in highly fertile lowland streams such as the River Eden in Fife, they make actually become smolts after only one year. Most smolts in the Tay are two years old and they tend to measure from 12 to 15 cm long.
In the late spring the smolts leave their native tributaries
and start moving down the river. The migration seems
partly to be triggered by rising water temperatures,
in warm springs they will migrate earlier, but spates
also encourage them to move as well. Initially the smolts
only migrate on dark nights and hide up in cover during
the day out of the way of predators but as May progresses
they become less and less inhibited and eventually move
during the day too.
In addition to the main spring migration there is also a lot of evidence that many of the bigger parr do actually move out of the tributaries in the autumn and perhaps many fish actually accumulate in the main stem of the river over winter in readiness for smolting the following spring.
While they are moving downstream smolts tend to follow wherever the current takes them, and this does make them particularly vulnerable to obstacles and predators etc which may be in their path.
The purpose of the smolt migration of course is for the fish to reach the sea where they can then grow much more rapidly in the rich feeding grounds of the North Atlantic. The smolting process has adapted the fish for life in sea water. Once the sea is finally reached it appears that the smolts waste little time but move rapidly out into deeper water.