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2019 sweep netting season starts with sample of louse-infested sea trout in Loch Gairloch

Posted: Monday 22 April, 2019 @ 15:22:37

Sweep netting team, Flowerdale, 19th April 2019 and sea trout with (inset) lice-damaged dorsal fin

Thirty seven sea trout were caught in the first SWRFT sweep netting session of 2019 in Loch Gairloch. These fish were assumed to be a mix of overwintered finnock and older sea trout (rather than this year’s sea trout smolts) of lengths between 225mm and 450mm. Scale samples have been taken to confirm fish and sea age.

Sea trout were generally thin with an average condition factor (for the first 30 fish) of 0.88. The average estimated lice count on these fish was 107+ lice per fish (range 3 to 300+). Larger lice were Lepeophtheirus salmonis, smaller lice (too small to distinguish in the field) were also assumed to be L. salmonis. Most of the lice were very small copepodid or chalimus lice.

Details can be found at:

Most of these fish are assumed to have returned early to freshwater as a result of sea lice infestation (c. Taranger et al, 2014).

The sample is similar to the one taken in April 2017 at the same site in terms of fish size and lice infestation levels.

However, on 18th April 2018, the sample of 18 sea trout carried an average of just 4.17 lice per fish.

The nearest salmon farms to the Flowerdale, Loch Gairloch sweep netting site are in Loch Torridon and were in the second year of the two year production cycle. Associations between sea lice levels on farmed salmon in Loch Torridon and on wild fish in surrounding waters were investigated in 2015; please see:

Sweep netting has been supported by Wester Ross Area Salmon Fishery Board and The Scottish Government.


For help with sweep netting, thank you to Pat Brunton, Mike Longley, Jim Henderson, John Hedger, Anwen Page, Peter Davison, Janet Davison, Kate Jenner, Chris Beresford, Katherine Vine, Cameron and three SNH Beinn Eighe NNR volunteers, and Gary Bulmer.