30 September – 2 October 2015
MASTS: Annual Science Meeting
Implications on farm siting in a dynamically wide sea loch 1
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Berit Rabe , Jenny Hindson , Nabeil Salama 1 2
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Marine Laboratory, Marine Scotland Science, Aberdeen –
[email protected] Marine Laboratory, Marine Scotland Science, Aberdeen
Area being submitted to (delete as appropriate): 1) General science session Preferred presentation medium (delete as appropriate): (ii) e-poster format Are you a student? (Delete as appropriate): No
The Scottish west coast is characterized by a number of sea lochs that host economically valuable aquaculture farms. A multi-disciplinary study was conducted in Loch Linnhe, one of the largest systems, to investigate the dynamics of the system influencing sea lice dispersal. One outcome of the study links the oceanographic conditions – with fresh outflow along the NW coast – to particle-tracking outcomes, and highlights possible implications on farm siting in the system. The locations of fish farms in relation to the loch geometry and dynamics and other farms is therefore crucial. Two dimensionless numbers, the Ekman (Ek) and the Kelvin (Ke) number, were investigated for the Loch Linnhe system. The Ek number determines if the Coriolis force or the viscose force are more important in the system. The Ke number compares the channel width with the internal Rossby radius of deformation, which takes into account the density and depths of different layers and the Coriolis force. In the study area the Rossby radius ranges from about 1800 to 8150 m depending on location, season, conditions, etc. For the range of Ek numbers for Loch Linnhe we find that the exchange flow in the loch is preferentially vertically sheared but it can also exhibit lateral variations. If the Ke number is larger than one, as occurs in Loch Linnhe, the Earth’s rotational effects become important. In some parts of the loch the Ke number is larger than two and the system is considered wide with outflow and inflow separated laterally, which can be seen in observations. We will also compare our results to model results from Valle-Levinson (2010) in Ek versus Ke space. In Loch Linnhe exchange flow is horizontally sheared due to weak friction and large width. The dense inflow into the loch occurs in the lower layers along the SE coast and the lighter outflow is located
along the NW coast (the right hand side looking down the loch) in the surface layer. For Loch Linnhe we find that sea lice distributions are spatially and temporally heterogeneous. The predominant wind direction follows the orientation of the loch but is not reflective of the predominant dispersal structures within the loch. Considerable variability in particle density exists between model runs but there is a general tendency for particles being transported out of the loch along the right hand side when looking downstream, leading to relatively higher particle densities along this coast. This is a direct result of the internal Rossby radius of deformation distance. Sites located on the right-hand side of the outer Loch may experience substantially more lice exposure, compared to the left-hand side of the outer loch. This is a different result than in previous studies in Loch Torridon (Amundrud & Murray 2009) which showed that Torridon was predominantly wind-influenced. Loch Linnhe in comparison is much wider and has a much larger freshwater inflow influencing the dynamics in the system. To conclude Loch Linnhe is a dynamically wide system with outflow of fresh water and particles along the right-hand side and this could therefore inform the optimal siting of farms in the future. References T.L. Amundrud and A.G. Murray (2009) Modelling sea lice dispersion under varying environmental forcing in a Scottish sea loch. Journal of Fish Diseases. 32. 27-44. A. Valle-Levinson (2010) Contemporary Issues in Estuarine Physics. Cambridge University Press.
