Salmon lice loads on Atlantic salmon smolts associated with reduced welfare and increased population mortalities

Modelling potential impacts of salmon lice Lepeophtheirus salmonis on Atlantic salmon Salmo salar requires estimation of the levels that cause serious sub-lethal physiological impacts and direct mortality. Here we analysed results from existing laboratory experiments to identify 2 thresholds; the lower threshold (T1) estimates the level at which lice loads cause systemic sub-lethal effects on smolts likely to impact performance, and the upper threshold (T2) identifies the lice load causing direct mortality. T1 is an empirical value based on a catalogue of lice impacts according to impact type (superficial or systemic) and concentration, converted from a range of experiments. The onset of these indicators of welfare and behavioural impacts indicates T1 ≈ 0.08 lice g−1 of host. T2 represents 50% probability of onset of mortality under laboratory conditions quantified using a dose-response curve based on data collated from studies where mortality has occurred or not at specified lice concentrations. T2 ≈ 0.24 lice g⁻¹ with a bootstrapped 95% confidence interval between 0.1 and 0.67 lice g⁻¹.