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Sea trout monitoring around Wester Ross in 2011: initial findings

Posted: Wednesday 16 November, 2011 @ 13:27:02

WRFT Biologist and volunteers after a successful sweep in Loch Gairloch; a recaptured sea trout

As part of the ‘RAFTS Aquaculture Project’ funded by the Scottish Government, WRFT sweep netting teams carried out a programme of sampling that included excursions to the River Kanaird estuary, Gruinard Bay, Loch Ewe, Loch Gairloch and the River Carron estuary. Data are initially being analysed by RAFTS Aquaculture Project officers along with data from other parts of Scotland, and will subsequently be reported in full.


Outwith the RAFTS project, further sweeps were carried out by WRFT.  On the 4th August, 6 sea trout were caught in Loch Gairloch. Lice levels were mixed. Two of these fish were over 450mm & 1kg in weight. One of these fish carried over 200 pre-adult and adult lice and had a raw eroded dorsal fin; the other had only 12 lice and was in much better condition (see photo). This fish was recognised as one caught earlier in the year in February at the mouth of the River Kerry (click here or visit the Trout Scale Catalogue on the ‘downloads’ page for further details). On 27th September, 28 sea trout were caught of which 13 were over 500g in weight; the two largest were 1900g and 1617g respectively. Three of these fish were identified from photographs as recaptures based on scale patterns, including both of the fish caught on 4th August described above. The fish that had over 200 lice on 04/08, had only 80 lice on 27/ 09, and was by far the lousiest fish in the 27/09 sample. Many fish had eroded, but generally healing, dorsal fins indicative of earlier lice damage.


Around Loch Ewe, on 12th July, a sea trout of 345mm with 90 lice was caught off spit at Inverasdale with sweep net and a further 10 post-smolts/finnock were caught using rod and line in the River Ewe with between 5 and 67 lice per fish (filmed by BBC). At Boor Bay nearby, 4 sea trout were caught in the sweep net on 31st September, ranging in size from 181mm – 272mm, with low sea lice levels (2 to 11 lice per fish). In addition to WRFT sampling, Dundonnell Estate with support from WRFT operated a fyke net at the mouth of the Dundonnell River, Little Loch Broom; and data has been collected by the Marine Scotland Science Shieldaig project team in Loch Torridon. At Dundonnell several fish carried well over 100 lice.


Lice levels may have initially been exacerbated by warm sunny conditions in April 2011, then subsequently kept partly in check by high river discharge of freshwater into coastal areas in May. The relationship between sea lice levels on wild fish and the salmon farming industry continues to be the subject of investigation by Marine Scotland Science in collaboration with RAFTS, as recently reported by BBC Scotland (follow link to interview with Government Scientist, John Armstrong).  


Scales from some of the post-smolts and finnock caught around Wester Ross show reasonable growth in 2011, suggesting adequate food in nearby waters. Sea trout of over 1kg were recorded in Gruinard Bay, Loch Ewe, and Loch Gairloch. Terns, which also feed on sandeels and ‘whitebait’ (sprats and herring fry), bred successfully in Loch Ewe.


A more detailed report for 2011, including all results from the WRFT sampling area will follow.


For help with sweep netting, thank you to keepers Stuart Alison, Marcus Munro, Brian Fraser, Alasdair Macdonald, and Ray Dingwall; Colin Milne & Hugh Richards (Wester Ross Fisheries), Gunnar Scholtz, and volunteers Dr John Ogle, Jane Murphy and friends and familes, Dr Steve Kett; Alan, Greg and Frank Choonara; David, Dougie and Flora Foreman, Drew Davies, and Bill Anderson.