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Sea lice problems for sea trout reviewed at Aultbea workshop

Posted: Tuesday 25 February, 2014 @ 14:50:54

The WRFT-AST sweep netting team (& a sea trout) at Flowerdale 19 Feb 2014 (photo by Ben Rushbrooke)

Summary of meeting

This meeting was arranged to provide those concerned about problems for wild and farmed salmonids caused by parasitic sea lice with an opportunity to meet Prof Mark Costello, during his visit to the Wester Ross area with Prof Ken Whelan, Tony Andrews and Ivor Llewelyn of the Atlantic Salmon Trust.

Prof Mark Costello, (from the Leigh Marine Laboratory, University of Auckland, New Zealand), summarised what is known about the biology and ecology of marine fish lice. Problems for wild and farmed salmonids caused by the salmon louse, Lepeophtheirus salmonis were reviewed and discussed.

Although the younger chalimus stage lice are usually the most numerous on fish, the adult lice cause the greatest damage to fish. To understand the threat to both farmed and wild fish, the total number of lice on farms in an area is more critical than the number of lice per individual salmon. Of the chemicals used to treat for lice on salmon farms, Emamectin benzoate (SLICE) had been the most successful to date. SLICE has a variable effect depending on when and how it is used; and was [typically] 80% effective (means 80% of sea lice are killed 20 % survive). As lice resistance to SLICE built up, the efficacy of SLICE as a means of controlling sea lice had reduced.

Several species of wrasse have been used to control sea lice on salmon farms.  Wrasse initially took longer to reduce sea lice populations than using bath treatment. However, [in trials?] wrasse were able to maintain lice at a lower level for longer compared with the bath treatment ‘Nuvan’ (where after a couple of weeks following bath treatment sea lice levels had risen again). The use of wrasse also produced faster growth rates in farmed salmon.

To minimise the impact of sea lice: farming areas should be (a) separated by 20km-30km to avoid drift of infective lice from one area to another; (b) have only one year class in each area; (c) entire bays should be fallowed synchronously; (d) sea lice treatment should be coordinated amongst neighbouring farms; (e) theraputants should be used; and (f) cleaner fish should be used.

Lice which emanate from salmon farms have been be identified from traces of veterinary medicines and in-feed pigments used in salmon production [ref?]. Strains of sea lice that are resistant to Emamectin can be concluded as coming from salmon farms.

Mark also described experiences with developing Marine Reserves [MRs] (no take zones) in New Zealand. These have been very popular with huge public support. Following designation there can be a rapid recovery in the size and number of marine fish and shellfish species. Currently there are 34 MRs around New Zealand, many of which are looked after by community groups including local schools. Commercial fishermen have caught more fish or shellfish around margins of MRs. In one example, the kelp forest was restored within 5 years of designation following the revival of the predator populations which preyed upon sea urchins.  Elsewhere, sometime a break in trophic cascades caused by the removal of a top predator had led to irreversible changes in the ecosystem (e.g. Newfoundland cod fishery).

Mark’s presentation [11MB] can be found on the downloads page of this website or by clicking here.

Keith Dunbar presented a summary of the 2013 fishing season for the Loch Oscaig system. Following a cold spring in 2013, the sea trout catch was one of the lowest on record; however the finnock catch was amongst the highest on record.  A 3 year programme of stocking sea trout eggs into the Oscaig system finished in 2012. To support production of juvenile trout, a spawning channel was created and gravel added: sea trout were seen finning over the gravel at spawning time in 2013.

Peter Cunningham focussed on the problem of being able to assess the impacts of sea lice epizootics on sea trout at population and fishery levels. Following many years of monitoring by west coast fisheries trusts, the Scottish Government now accepted that at locations in close proximity to a salmon farm in the second year of a two-year production cycle, elevated sea lice levels on wild sea trout could be expected. However, it was not possible to attribute elevated sea lice levels to changes in sea trout at the population level. The extent to which other factors including predation by seals and food availability contributed to changes in the performance of sea trout fisheries was not clear.

Peter proposed a way forward. Records for sea trout catches around Scotland showed much variation. In 2012, the NW of Scotland and the Moray Firth regions (away from salmon farming zone) had their lowest catch totals on the 61 year time series. In contrast, the North of Scotland region (Caithness and North Sutherland) experienced its highest rod catch in the record. In addition to variation in rod catch, the proportions of larger sea trout in recorded catches within Wester Ross and around Scotland also varied. Larger sea trout had been particularly scarce in some parts of Wester Ross and at other locations in the west of Scotland since the 1980s.

The presentation concluded with an outline for a proposed Scottish Sea Trout Project aimed at improving our understanding of how different factors affect sea trout in different parts of Scotland by analysing the distribution of larger sea trout in catches around Scotland. Peter also posed the question: is there a need to designate some special river systems in Scotland for the conservation of sea trout (e.g. Camasunarie & Coruisk on Skye)?

Peter’s presentation [7MB] can be found on the downloads page of this website or by clicking here.

Prof Ken Whelan, (Marine Institute, Burrishoole, Ireland; AST Research Director) raised a number of issues. Climate change / increasing temperature has a big effect on sea lice abundance. A climatic regime shift in 1988-89 resulted in very high lice levels with the biggest effect close to salmon farms. Effects had been observed on the east coast & Ireland as well as the west coast. At Burrishoole the adult sea trout population had fallen from 4,000 fish to just 30 adult sea trout. Juvenile salmon had displaced trout from some streams, hindering the recovery of sea trout.

The biggest problem to successful prosecution of salmon farms is the inability of the wild fish lobby to present accurate numbers data to substantiate claims that farms are impacting upon wild sea trout. Courts demand irrefutable evidence that lice from farm X have damaged sea trout from river Y and are able quantify the extent of the damage.

When asked, Ken suggested that a £5k budget for data collection could most usefully be expended on:  a) obtaining counts of  migrating smolts; b) count numbers for early returning post smolts; c)catch returns from skilled & trained anglers; d) trapping, counting & scale reading kelts to get a measure of larger sea trout present.

The importance of trapping kelts was emphasized. There are probably more large sea trout in river systems than anglers ever see or catch. It’s often best to trap outflows from 'side lochs' rather than main rivers. Kelts can give the best estimate of sea trout population composition and trends. Sea trout kelts recover very quickly after spawning, silver up and may commence feeding in fresh water before returning to sea where they are very susceptible to sea lice infestation. Copepodid stage sea lice cluster at the fresh /salt water interface to intercept migrating kelts and smolts.   

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Thank you to Tony Andrews and Ivor Llewelyn of the Atlantic Salmon Trust for organising the visit of Mark and Ken to Wester Ross and for helping at the workshop. Thank you Prof Dave Barclay for chairing the meeting, and to everyone else who attended and contributed to the discussion. If there were other points which should be included in the foregoing summary, please email them, or any other comments, to . Thanks to Keith Dunbar for taking notes at the meeting from which some of the foregoing is based.


Mark Costello, 2014 ‘The Problem with Sea Lice on Salmonids: wild and farmed’

Peter Cunningham, 2014 ‘Problems for Sea trout in Wester Ross’.