Physiological impact of sea lice on swimming performance of Atlantic salmon

Atlantic salmon Salmo salar were infected with two levels of sea lice Lepeophtheirus salmonis (0.13 +/- 0.02 and 0.02 +/- 0.00 sea lice g(-1)). Once sea lice became adults, the ventral aorta of each fish was flitted with a Doppler cuff to measure cardiac output ((Q) over dot), heart rate (f(H)) and stroke volume (V-S) during swimming. Critical swimming speeds (U-crit) of fish with higher sea lice numbers [2.1 +/- 0.1 BL (body lengths) s(-1)] were significantly lower (P < 0.05) than fish with lower numbers (2.4 +/- 0.1 BL s(-1)) and controls (sham infected, 2.6 +/- 0.1 BL s(-1)). After swimming, chloride levels in fish with higher sea lice numbers (184.4 +/- 11.3 mmol l(-1)) increased significantly (54%) from levels at rest and were significantly higher than fish with fewer lice (142.0 +/- 3.7 mmol l(-1)) or control fish (159.5 +/- 3.5 mmol l(-1)). The f(H) of fish with more lice was 9% slower than the other two groups at U-crit. This decrease resulted in (Q) over dot not increasing from resting levels. Sublethal infection by sea lice compromised the overall fitness of Atlantic salmon. The level of sea lice infection used in the present study was lower than has previously been reported to be detrimental to wild Atlantic salmon. (C) 2003 The Fisheries Society of the British Isles.