The original source of these data is the SSPO website https://www.scottishsalmon.co.uk/ however their archives do not include the full time series of data available.

Sea lice count data - average ovigerous female lice per fish

three different reporting systems cover the data:

December 2010-2012 data was reported averaged over 6 large regions and over variable intervals (specified in data).
Area level reporting 2010 – 2012 and 2013 – 2017
Averages represent mean lice per fish for all farms within a defined area for each month. The number of farms in each region that have reported within the month is also provided.  
From the code of good practice:
ii) Where there are more than five pens per site, five fish should be sampled from each of five pens to give a total of 25 fish.
iv) Where a site contains less than five pens all pens should be sampled to give a total of 25 fish. A similar number of fish should be selected from each pen

Raw data to obtain these averages were obtained by the SSPO from all salmon farms in Scotland
Counts are obtained as specified in the Code of Good Practice for Finfish Aquaculture
Counting of sea lice is audited by the Fish Health Inspectors of Marine Scotland Science
Making and recording regular counts are legal obligations on all salmon producers
Raw data is not regularly obtained or recorded by SSPO
Data on numbers of farms in an area was published by SSPO on a quarterly basis, hence is only identical through the quarter.