Lice levels high on sea trout sampled in the 'Two Brooms' area

Posted: Monday 5 August, 2013 @ 16:20:37

Results from sea trout monitoring have been mixed so far in 2013. At each of our RAFTS sweep netting sites at Kanaird, Boor Bay [Loch Ewe] and Flowerdale [Gairloch]), the required samples of 30 post-smolt sized sea trout (of less than 260mm) were caught during the month of June.

 

Sweep netting sessions were carried out at the Kanaird (Loch Broom) on 11th and 27th June and a total of 52 trout were caught, including many fish larger than post-smolts. These fish had generally between 20 and 50 lice; some had over 100 lice or damage from sea lice. Condition factors were mostly below 1.0, so the fish were thin. At Boor Bay (Loch Ewe), successful sweep net sessions were carried out on evenings of 10th, 21st and 25th June and a total of 31 sea trout were caught, mostly small post-smolts of less than 200mm in length. Few fish carried any lice; and nearly all of these fish were thin. Many of these fish may have been in the sea for only a few days prior to sampling.   At Flowerdale (Loch Gairloch) 41 sea trout were caught over two sessions (7th Jun and 8th July). Nearly all the fish were small post-smolts of less than 20cm, and as at Boor Bay, very few carried sea lice, and the fish were generally thin.

 

Sea trout were also sampled from the mouth of the Dundonnell River using a fyke net by Dundonnell Estate supported by WRFT. Between the 6th June and 20th July, over 70 sea trout were taken. The average number of lice on sea trout of less than 260mm in length was over 150 (the highest average lice counts at Dundonnell since the year 2000). Some fish had over 200 lice and a few had over 500 lice. This sample included some of the most heavily louse infected sea trout seen during the past 10 years.  

 

No sea trout were caught on a sweep netting exploratory visit to the mouth of the River Broom on 26th July; one finnock was taken using rod and line which carried 56 lice. Spotting suggested higher numbers of lice on this fish earlier in the summer.

 

Rod and line samples of sea trout were taken from the River Ewe on 8th July and 23rd July. 14 fish were taken, mostly finnock of between 200 and 300mm in length, larger than those that had been sampled at Boor Bay nearby. They were also fatter (higher condition factor) and carried between 3 and 135 sea lice. 

 

Reports from Loch Long (near Loch Alsh) and Loch Carron indicate that lice numbers on rod caught sea trout taken in these areas were relatively low during the period May – July.

 

The Scottish Salmon Producers Organisation’s sea lice reports are published on-line at:

http://www.scottishsalmon.co.uk/science/sea_lice/regional_reports%281%29.aspx

 

Note that during the period January to March 2013, the average numbers of adult female lice on farmed salmon between Kennart and Gruinard, the area which includes the SSF Summer Isles farms and Wester Ross Fisheries farms, was over 4 lice per fish, i.e. over 8 times the CoGP lice treatment threshold level. This infers a very high on-farm sea louse population in the area, and correlates with the high numbers of generally early stage lice subsequently recorded on sea trout sampled at the mouth of the River Kanaird and on fish taken in the fyke net at the mouth of the Dundonnell River in June and July.

 

In contrast, the Loch Ewe area, which contains only the Isle Ewe salmon farm, reported very low numbers of lice, well below the CoGP treatment threshold levels.

 

For help with sweep netting so far this year, thank you to Arran Matheson, Ian McFadyen, Marcus Munro, Ailsa McLellan, Angus Davidson, Gunnar Scholtz, Hugh Richards, Colin Milne, Jim Buchanan, Andrew Ramsay, Katarzyna Maykowska, Johnny Whitteridge, Alasdair MacDonald, Alan McGillivray, Farquhar and Jamie Renwick and friends, Louis Neate and Scott; Steve Kett, Andy Vicks, Hugh Whittle and family and friends, Ray Dingwall, Mark Williams, Pete Bridgman, David Mullaney, Garry Bulmer, Terry Doe, Mark Everard, Carl Forbes, David Barclay and Bill Anderson. Sweep netting is supported by the Scottish Government via RAFTS.

 

The sea trout monitoring report for 2012 can be found on the WRFT website via links at: http://www.wrft.org.uk/news/newsitem.cfm?id=162 ; further comments are welcome.

 

Further efforts to obtain a samples of sea trout may take place later in the summer: please contact the WRFT biologist at info@wrft.org.uk if you would like to help.