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Do all sea trout undergo smolting?

No, it seems not. The process of smolting occurs in spring once a threshold size is reached and transforms relatively sedentary and highly-coloured parr which live close to hiding cover in the stream bed into free-swimming, streamlined, silvery fish, better-adapted to feeding at sea.  [Smolting is not obligatory for the transition to saline water. Non-smolting brown trout from inland sources survived and grew well when released in Norwegian fjords, indeed some brown trout strains gave better returns than most of the sea trout strains that were tested.]  Under natural conditions, most young sea trout become smolts and migrate to sea after one or more years (commonly two or three years in the UK) of growth in fresh water. However, some move down in a less advanced silvery condition, or as parr, into brackish estuaries, or sea lochs and develop into ‘ordinary looking’, or semi-silvered brown trout, the so-called slob trout, others eventually becoming fully silvered after a further period of growth.  Lastly, some smolts may not manage to migrate downriver, perhaps due to inability to get past barriers at the appropriate time for their physiological state to be maintained. These fish may revert to the appearance of ‘ordinary’ brown trout, remaining in fresh water to maturity, or they may become smolts again in the following year.