Abstract
Sea lice can flourish when salmon are farmed in open sea‐cages, necessitating treatments to control outbreaks and reduce larval export. However, mobile ectoparasitic stages can be dislodged during crowding or other procedures, and potentially reinfest farmed or wild fish. We studied vertical movements and host‐finding behaviours of mobile salmon lice ( Lepeophtheirus salmonis ) released into the water column, and used those data to parameterise a biophysical dispersal model. Over 0–0.4 m in tanks, larger stages sank more quickly than smaller stages (1.5 cf. 0.6 cm s −1 ), with sinking speeds of adult lice validated in deeper tanks (0–3 m) and a fjord (~15–24 m). Adult males had the greatest behavioural component, sinking more slowly with host cues present and faster when dead. Detached lice were able to intercept new hosts in a tank (23% within 5 min). We also investigated Caligus elongatus , but found their strong swimming not amenable to study. A hydrodynamic dispersal model indicated that detached lice can reach neighbouring cages but rarely neighbouring farms before sinking below cage depth. Simulations comparing farm sites highlighted the influence of site‐specific current conditions on dispersal kernels, and indicated that crowding/handling fish during favourable tides can reduce downstream risk.