Since spring 1999, lice levels have been measured in the intertidal areas at the head of Loch Shieldaig1. Measurements are undertaken by trawling a plankton net along a fixed transect. The longer of the three long-term data sets involves towing the net by boat along a 360m transect taken across the river mouth at the head of the loch. This survey has been undertaken since 1999, although initially only during the spring period. The second long-term data set involves a 50m transect by foot tow taken near the mouth of the river Shieldaig which has been undertaken since 2001. The third involves a 100m foot tow near the mouth of the River Shieldaig and has been undertaken since 2010. After each tow has been completed, the plankton net is washed down with sea water, the filtrate is stored and the larval lice present are counted. During 1999-October 2009, a subsample of the filtrate was examined. After November 2009 the whole of the sample was examined for the presence of lice. This change to the process does not introduce a bias in the measurements but does make them more precise. The tow data in relation to the production cycle on the local farms is presented below. It has previously been shown that numbers of lice in the environment tend to be higher during second years of production1,2 and this pattern is also evident in the recent tow data.
Density of sea lice (square root transformed) measuring by plankton tows in Loch Shieldaig 1999-2017. Green points are those in the first year of production, blue are in the second year.
1McKibben, M.. & Hay, D. W. (2004). Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, Western Scotland in relation to salmon farm production cycles. Aquaculture Research, 35, 742-750.
2Penston, M. J. & Davies, I. M. (2009). An assessment of salmon farms and wild salmonids as sources of Lepeophtheirus salmonis (Kroyer) copepodids in the water column in Loch Torridon, Scotland. Journal of Fish Diseases, 32, 75-88.