Host-parasite interactions are moderated by the environmental conditions of the interaction medium (e.g. air or water). Encounter rate and the time available for a parasite to make physical contact with a host are both influenced by fluid dynamics, yet how they interact is poorly known. Here, we tested whether current velocities altered the initial attachment and post-settlement survival of an ecto-parasitic copepod (Lepeophtheirus salmonis) on Atlantic salmon. Current velocities strongly influenced attachment; infestation levels were 2.5 and 1.3 times higher in moderate than high and low velocity currents, respectively, while current velocities did not affect post-settlement survival. An interplay between a reduced host-parasite encounter rate in a low velocity current and reduced contact time in a high velocity current likely explains this result. Initial parasite attachment position was influenced by an interaction between current velocity and swimming behaviour, likely due to different fin positioning by fish in flows of different velocities. Our results imply that rapid swimming by salmon migrating out of coastal waters, usually described as adaptive against predation, could also be adaptive against parasitism. Infestation rates were also highest at the typical swimming speed of farmed salmon in coastal fish farms, which may be a hitherto unrecognised factor contributing to L. salmonis epidemics.