Towed sentinel cages (TSCs) were deployed from Reinforced Inflatable Boats (RIBs) in Upper Loch Linnhe (ULL) and the Lynn of Lorne (LoL) in Lower Loch Linnhe during these trials. Transects were carried out starting next to a fixed sentinel cage (FSC) deployed for the same duration and were undertaken close to shore although in water depths of at least 5 metres to ensure vessel and staff safety. In ULL transects were carried out between the FSCs while in LoL this was not possible as the likely “finish location” fell within a busy ferry route. The following day transects were carried out in reverse to ensure variability due to tidal conditions was addressed where possible.
The sentinel cages were stocked with 30 Atlantic salmon smolts (mean length 225 mm (SE ± 0.86) and weight 123.81 g (SE ± 1.148)) supplied by Mowi Scotland from freshwater sites within the Loch Linnhe management area. At the end of each tow, the smolts were transferred into bins and euthanised using ethyl 3-aminobenzoate methanesulfonate salt (MS222; Sigma-Aldrich, Germany) at a dose of 1 g L-1. Sampling was undertaken in the wet laboratory aboard the Scottish Environment Protection Agency (SEPA) research vessel “Sir John Murray”. Each smolt was measured, weighed, and a visual examination of the external surface was undertaken and lice counts made. For any observed lice, the life stage and position of these was recorded for each individual smolt. Initial species identification of any lice found was as described by Johnson and Albright (1991) with specific stage identification using Hamre et al. (2013). Additionally, to increase the probability of finding dislodged sea lice, the anaesthetic water from each cage of smolts was filtered. As each cage was treated in a single container, detached lice could not be assigned to a specific host but could be assigned to the overall total for the tow.
During deployments one scientific team undertook the deployment of the TSCs while a second team simultaneously deployed FSCs anchored on the towed transect to allow comparison of the different methodologies. These tows were carried out on 10 occasions in ULL and 6 days in LoL. Tows in ULL were conducted on the east shore (4 dates) or west shore (6 dates) of Loch Linnhe. Tows were conducted from north to south or south to north on alternate days, where possible, to allow for different current directions due to tidal flow.

To allow data layout to be more understandable the information has been split into 2 files.

The ‘Towed_Sentinel_Cage_Trials_Data.csv’ file contains the following columns:
Date – The date the cage was deployed.
Year – The year the cage was deployed.
Duration – The duration, in hours, the cage was deployed for.
Fish_number – Identifier for individual fish within the specific cage.
Cage_type – Identifier for the type of cage deployed. Either ‘Fixed’ or ‘Towed’.
Tow_number – Identifier for the specific FSC or TSC deployed.
Site – Identifier for the site the cage was deployed at. For towed cages, deployment began or ended at specified site to account for differences in tides that can occur.
Fish_length – Length of the individual fish in mm.
Fish_weight – Weight of the individual fish in g.
Species – Identifier for the species of fish.
HEAD+OPER, MOUTH/GILLS, ANT.DORS, POST.DORS, ANT.VENT, POST.VENT, DORSAL, PECTORAL, PELVIC, ANAL, ADIPOSE and CAUDAL- these all refer to the locations on the individual fish where lice may occur, number of lice found in each location are recorded.

BAG/BIN – Refers to if lice were found in the filtered water (ie detached from the fish prior to the counting process).

TOTAL – Total number of lice recorded on the individual fish.

The ‘Towed_Sentinel_Cage_Location_lifecycle_info.csv’ file contains the following columns:
Date – The date the cage was deployed
Cage_type – Identifier for the type of cage deployed. Either ‘Fixed’ or ‘Towed’.
Tow_number – Identifier for the specific FSC or TSC deployed.
Start_location – Start of deployment location for specific cage in Degrees decimal minutes
End_location – End of deployment location for specific cage in Degrees decimal minutes
Life_stage – Life cycle of stage of any lice found on sentinel cage salmon. Either ‘Copepodid’, ‘Chalimus I’, ‘Chalimus II’, ‘Pre-Adult I’, ‘Pre-Adult II’ or ‘Adult’.
Lice_number – Number of lice found of the specific life cycle stage.
Notes – Any notes on tow/lice or fish.