Effect of timing of count events on estimates of sea lice abundance and interpretation of effectiveness following bath treatments

Gautam, R and Vanderstichel, R and Boerlage, A S and Revie, C W and Hammell, K L (2017) Effect of timing of count events on estimates of sea lice abundance and interpretation of effectiveness following bath treatments. Journal of Fish Diseases, 40 (3). pp. 367-375. ISSN 0140-7775 (https://doi.org/10.1111/jfd.12519)

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Abstract

Effectiveness of sea lice bath treatment is often assessed by comparing pre- and post-treatment counts. However, in practice, the post-treatment counting window varies from the day of treatment to several days after treatment. In this study, we assess the effect of post-treatment lag time on sea lice abundance estimates after chemical bath treatment using data from the sea lice data management program (Fish-iTrends) between 2010 and 2014. Data on two life stages, (i) adult female (AF) and (ii) pre-adult and adult male (PAAM), were aggregated at the cage level and log-transformed. Average sea lice counts by post-treatment lag time were computed for AF and PAAM and compared relative to treatment day, using linear mixed models. There were 720 observations (treatment events) that uniquely matched pre- and post-treatment counts from 53 farms. Lag time had a significant effect on the estimated sea lice abundance, which was influenced by season and pre-treatment sea lice levels. During summer, sea lice were at a minimum when counted 1 day post-treatment irrespective of pre-treatment sea lice levels, whereas in the spring and autumn, low levels were observed for PAAM over a longer interval of time, provided the pre-treatment sea lice levels were >5-10.

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