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Distribution of Larval Sea Lice in a Scottish Sea Loch

Loch SheildaigWork to Date

This research was undertaken to increase our understanding about distribution of planktonic (free-swimming) larval lice within a sea loch.

Observations provide temporal and spatial information about planktonic louse distribution, while environmental datasets are used to provide background for the development of predictive transport models within Loch Torridon. It involves scientists from several programmes in Marine Scotland Science (MSS); Freshwater Fisheries, Aquatic Environment (AE) and Aquaculture and Aquatic Animal Health (AAAH), and builds on the work of the Freshwater programme which has monitored the sea trout population in the River Shieldaig for several years.

Drogue movementThis body of work began in 1999 and will continue up until 2011 at least. Since 1999, scientists from the Freshwater programme have sampled in inter-tidal areas close to river mouths in Loch Shieldaig and Upper Loch Torridon in order to estimate the abundance of planktonic sea lice. Sampling is done fortnightly throughout the year. With this sampling regime, it is hoped to quantify the infective pressures on sea trout post-smolts migrating from the rivers into the marine environment. MSS Freshwater scientists have also monitored lice levels on post-smolts returning early to the river Shieldaig.

Open-water Plankton Sampling in Loch Torridon

Complementing this work, scientists from the AE programme have been sampling for the planktonic sea louse nauplii and infective stages in the open waters of Loch Torridon.

Sea LiceThese larvae have a limited swimming ability which enables them to remain near the sea surface and are transported by currents. Weekly samples were collected at a number of key locations within the loch until 2006; ongoing sampling is on a fortnightly basis. The aim of this work is to establish the abundance of lice in the wider marine environment, including sites near fish farms and inshore areas. The ultimate aim of the project is to try and identify cost-effective management strategies to reduce the infection risk to sea trout without unacceptable impacts on the environment.

Observations of Environmental Conditions and Models of Larval Transport and Dispersion

Further, observations of local winds and extensive measurements of currents using both moored instruments and GPS-equipped drifters have been collected to help predict the distribution patterns of sea lice.

These data have been used to support the development of 3-dimensional numerical circulation models within the loch. Comparing observed wind driven transport with model predictions has revealed the importance of wind to the transport of larvae in the surface layer, with observed currents showing a stronger response to wind forcing than predicted.

The results of the circulation model provide forcing input to particle tracking models that incorporate information on lice maturation and mortality under various environmental conditions. Modelled 'lice' are released at a location and their dispersion tracked over two weeks, producing a map of regions of infectivity based on the assumed points of origin. The coupled bio-physical model of lice distribution involves scientists from both the AE and AAAH programmes and is based on the results of previous and ongoing research.

Results

A large and unique data set on planktonic louse distribution has been gathered and combined with preliminary model results indicates several important points:

  • the main species of larval louse recovered was Lepeophtheirus salmonis (3, 4);
  • larvae were more abundant in the water-column when local salmon farms were in the second year of the two year production cycle (4);
  • local Atlantic salmon farms were a likely source of L. salmonis larvae at times in the farming production cycle (4, 5);
  • infective stages (copepodids) were more widespread than the younger (nauplii) stages (5);
  • the importance of wind as a factor affecting the distribution of the larvae (1, 2, 4, 6);
  • conventional modelling of surface circulation may underestimate the wind driven circulation. (6, 7).

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(1) Penston, M.J., McKibben, M.A., Hay, D.W. & Gillibrand, P.A. (2004) Observations on open-water densities of sea lice larvae in Loch Shieldaig, Western Scotland. Aquaculture Research 35 (8), 793-805.

(2) Penston, M.J., McKibben, M.A., Hay, D.W. & Gillibrand, P.A. Sea Lice Larvae Distributions in Loch Shieldaig, Western Scotland. Poster

(3) Penston, M.J., Davies, I.M. & Zuur A. A Two-Year Plankton Survey of Sea Lice Larvae in Loch Torridon. Poster

(4) Amundrud, T.L., M. J. Penston, and A.G. Murray (2006) A summary of the effects of environmental factors on the simulated dispersal of sea lice larvae and the findings of the 04/05 plankton survey in the Loch Torridon System, Western Scotland. Fisheries Research Services Internal Report, No 15/06.

(5) Penston, M.J., Millar, C.P., Zuur, A. and Davies, I.M. (2007) Temporal and spatial distribution of Lepeophtheirus salmonis (Krøyer) larvae in a sea loch containing Atlantic salmon, Salmo salar (L.), farms on the north-west coast of Scotland. Poster

(6) Amundrud, T.L., and A.G. Murray (2007) Validating particle tracking models of sea lice dispersion in Scottish sea lochs. ICES CM 2007/B:05. 12 pages.

(7) Gillibrand P.A., and T.L. Amundrud (2007) A numerical study of the tidal circulation and buoyancy effects in a Scottish fjord: Loch Torridon. Journal of Geophysical Research (Oceans) 112(C05030), doi:10.1029/2006JC003806.


Results to date have been presented at the ICES Annual Science Conference (Copenhagen 2002, Helsinki 2007), the Challenger Society UK Marine Science Conference (Plymouth, 2002), the 6th International Sealice Conference (Canada 2003), ICES/NASCO Interactions between aquaculture and wild stocks of Atlantic salmon (Norway 2005), World Aquaculture Society (Texas 2007), CompareLice meetings (Scotland, Canada and Norway 2007).