Sea lice counts on wild sea trout dataset https://portal.medin.org.uk/portal/start.php?tpc=012_Marine_Scotland_FishDAC_12394 Data comprise sampling for wild sea trout from named sampling locations. The number of fish, individual fish fork length and the number and life cycle stage of any sea lice found are all recorded. Notes on temporal and spatial coverage Data are in a single sheet however there are minor differences in spatial coverage, data covering 1997 – 2009 and 2011 – 2024 include available data covering all mainland west coast and western isles fisheries trusts regions. Data from 2010 features only data from mainland west coast fisheries trust regions (which includes some inner islands). There was no data collection in 2020. Column headings “site” contains the sampling site name “region” contains information on the general region of Scotland within which the sample site is located. “date” contains the date that sampling point was undertaken “fish_id” contains a number to identify individual fish “length” contains the fork length in mm of the individual sea trout “copepodid_chalimus” contains the number of this life cycle stage of lice found on the individual fish “preadult_adult” contains the number of this life cycle stage of lice found on the individual fish “ovigerous_females” contains the number of this life cycle stage of lice found on the individual fish Acknowledgements This work was funded by Scottish government Marine Directorate, and managed by Marine Directorate and Fisheries Management Scotland. Sampling was carried out by employees of the district fisheries trusts with the help of volunteers as required. 20220204