Sea Trout in West Sutherland
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Here are two interesting snippets on sea trout by the West Sutherland Fisheries Trust, extracted from their Annual Report 2015, courtesy of Shona Marshall, the Trust’s Biologist.
A project started in 1977 to give an indication of the migration and growth of sea trout smolts and adults within the area. Estuary sweep netting is used to catch trout samples and collect data; some fish are tagged before return. In addition to these data, the number of sea lice on the fish is also assessed.
In 2015, three estuaries, LAxford Bay, Kyle of Durness and Polla estuary, were samples monthly where possible from April to September. A total of 407 fish were individually measured and scale samples were takes, of which 185 were tagged using a visual implant tag, inserted in clear skin tissue behind the eye. The fish were also examined for the presence of sea lice, recording numbers and stage in the life cycle.
Post-smolts were caught at all sites throughout the year, indication extensive usage of estuaries by this group, presumably for feeding and shelter. That the sea trout populations are relatively static can be inferred from information on recaptures, where all bar one of the tagged fish recaptured during 2015 were taken in the same location as originally tagged. One exceptional fish was originally tagged in the Laxford and recaptured in the Polla.
in 2015, estuary netting recaptured 55 trout: I originally tagged in 2010, 3 in 2013, 21 in 2014 and the rest earlier in 2015, a pattern common to the sampling programme over the past 18 years, suggesting that the majority of sea trout do not stray fat from their home rivers in this area.
Average growth rates within the Laxford were 5.81mm, and 12.38g per month and 10.40mm and 39.07g for the Polla. Both growth rates are lower than seen in 2014, but still good within the Laxford and within the range seen in the Polla. The is encouraging and was apparent in the appearance of the fish caught in the netting: plump and well-conditioned.
It is now very widely accepted (except apparently by some state governmental departments), based on scientific evidence, that salmon farms pose a risk to wild salmon and sea trout postulation, for example, by acting as a ‘reservoir’ for parasitic sea lice that affect young, wild fish. Some recent published work in Norway has demonstrated a method to increase mortality risk to salmonids using the number of lice present per gram of fish, based on physiological effects determined from laboratory experiments and the use of sentinel cafes in fjords. Individual parasite loads are then extrapolated to a possible population-level effect. The graph below (bottom right) gives the results by year for each studied estuary in West Sutherland, with the banding indicating whether the risk to sea trout from sea lice is low (<10%), moderate (10-30%) or high (>30%). Within the low zone, there is probably a minimal risk to the population from sea lice infestation, while the other zones show potentially population-altering impacts.
From this, it appears that the potential risk within the Polla Estuary are low throughout the study period, with the exception of 2006, when increased lice levels were observed. This is a positive reflection on the situation within the estuary, not perhaps seen in previous analyses, bases solely on lice abundance.
In contrast, the Laxford analysis would indicate the sea lice populations are creating on a regular basis a potential sea trout population-changing effect. while there is an apparent biennial effect, primarily giving a moderate effect, in 2 years, 2011 and 2013, this was identified as high.