Variation in pre‐treatment count lead time and its effect on baseline estimates of cage‐level sea lice abundance

Treatment efficacy studies typically use pre‐treatment sea lice abundance as the baseline. However, the pre‐treatment counting window often varies from the day of treatment to several days before treatment. We assessed the effect of lead time on baseline estimates, using historical data (2010–14) from a sea lice data management programme (Fish‐iTrends). Data were aggregated at the cage level for three life stages: (i) chalimus, (ii) pre‐adult and adult male and (iii) adult female. Sea lice counts were log‐transformed, and mean counts by lead time relative to treatment day were computed and compared separately for each life stage, using linear mixed models. There were 1,658 observations (treatment events) from 56 sites in 5 Bay Management Areas. Our study showed that lead time had a significant effect on the estimated sea lice abundance, which was moderated by season. During the late summer and autumn periods, counting on the day of treatment gave significantly higher values than other days and would be a more appropriate baseline estimate, while during spring and early summer abundance estimates were comparable among counts within 5 days of treatment. A season‐based lead time window may be most appropriate when estimating baseline sea lice levels.