Smolt sampling on the Bladnoch

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25 Apr 19
by Kacie Jess

GFT are presently sampling smolts caught in a fyke net at the inflow to Torhouse Fish Farm on the lower River Bladnoch. The fyke net also helps provide data on various other fish species within the Bladnoch catchment.

Since early April, once a week Kacie carefully measures and take a small scale sample from any salmon smolts which are caught in the fyke net during the previous 24 hours.  For the other six days of the week any fish caught are counted and released below the fish farm by their staff.  While permanent screening stops any smolts entering the fish farm, the net ensures smolts do not become trapped in the lade.  From the 2019 sampling work to date, it has found that the smolt run started on the 10th April and dropped off with the warmer water temperatures and low flows experienced in mid April.  Smolt numbers have leapt up since 21st April and the main run has now started.  The fish counts for each day are compiled into a data set, with one day a week being dedicated to sampling smolts to provide representative data covering the whole smolt run.  Each salmon smolts’ length is measured, checked for signs of predator damage and a scale sample is taken from every 3rd individual to ensure a variety of sizes can be aged.  These scale samples will be read by GFT staff once the project has ended.  Once all fish have been sampled they are released carefully back into the main river.

Atlantic Salmon begin smolting when they are 1-3 years old and undergo physiological changes to prepare for the transition between fresh and salt water.  They spend time in estuaries where they can gradually adapt to salt water.  They also lose their parr marks and develop a silver colour which improves their camouflage in the lighter and more open sea.  Most smolts should have transferred to the sea by June where they will live typically for 1-3 years before returning to freshwater for spawning.

This project will help determine the ages and health of smolts leaving the Bladnoch catchment.  In the past around 70 – 80% of the smolts will be aged 2+ years old but things may be changing with climate change.  Smolts that are only just over a year old are typically small and particularly vulnerable to predation.  Any apparent reductions in survival of fry to parr in electrofishing surveys could be a result of more juvenile salmon simply smolting after their first year so it is important to understand the age structure of the smolt run. This work will continue to build on data from previous years, helping to understand and thus protect future smolt runs.