The NDSFB team completed the last ‘smolt’ pass clearance of the season at Dochfour Lock on the Caledonia Canal this morning.
Between March and June each year the largest juvenile salmon, known as parr, begin to drift downstream in shoals towards the sea. During this journey their bodies become elongated, their fins darken and they become more silvery in colour. They then become known as salmon ‘smolts’.
A typical example of an early Ness system salmon ‘smolt’ which average approximately 120mm in length (Photograph by Chris Conroy)
Smolts have a very limited period of physiological readiness in which they must enter the sea. In some cases it is believed that this ‘smolt window’ may be as short as a week. This means that any human activity that delays or obstructs the migration of salmon smolts can lead to increased mortality.
The Caledonian Canal and the rivers of the Ness system are interlinked in a number of places. This creates potential ‘bottle necks’ where smolts may drift into the mouth of the canal rather than continuing down the river. A series of ‘smolt passes’ were therefore installed in these locations to provide a route by which any smolts trapped in the canal can escape back into the river and continue their journey to the sea without delay.
The entrance to the smolt pass upstream of the lock gates in the Caledonian canal at Dochfour (Photograph by Chris Conroy)
Given the nature of these smolt passes, they are prone to blockage by branches and other debris. NDSFB and Scottish Canals staff therefore regularly inspect them during the smolt run and clear any blockages to allow free passage of the fish.
Billie Orrock from the NDSFB clearing the smolt pass at Dochfour (Photographs by Chris Conroy)
Water following freely through the pass from the canal back into the River Ness below Dochfour Weir (Photograph by Chris Conroy)
With the survival of smolts at sea believed to be at its lowest for many years, it is vitally important to ensure maximum natural smolt escapement from the riverine environment. This simple, routine fisheries management activity is just one of a number of actions being undertaken by the NDSFB and its partners in an attempt to achieve this.