Atlantic salmon Salmo salar were infected with sea lice Lepeophtheirus salmonis(0·08 ± 0·007 sea lice g−1) over a period of 4 h. Both infected and non‐infected fish were swim tested in sea water (SW) and
fresh water (FW). The ventral aorta of each fish was fitted with a Doppler cuff in order to measure cardiac output, stroke volume and heart rate during swim testing. Blood samples were taken at rest and after exercise. Critical swimming speed of infected fish in SW (2·14 ± 0·08
body lengths, bl s−1) was significantly lower (P < 0·05) than infected fish switched to FW (2·81 ± 0·08 bl s−1) and non‐infected fish in SW (2·42 ± 0·04 bl s−1)
and FW (2·61 ± 0·08 bl s−1). Cardiac and blood results indicated infected fish exposed to FW did experience stress, but relief from osmotic and ionic distress probably reduces energy expenditure, allowing the increase in performance.
As the performance of sea lice‐infected fish improved upon transfer to FW, it is likely that heavily infected salmonids do return to FW to restore compromised osmotic and ionic balance, and remove sea lice in the process.