Physiological performance of ballan wrasse (Labrus bergylta) at different temperatures and its implication for cleaner fish usage in salmon aquaculture

Ballan wrasse (Labrus bergylta) are used extensively as cleaner fish to control salmon lice (Lepeophtheirus salmonis) infestations in the Atlantic salmon (Salmon salar) aquaculture industry. Fish are either cultured or caught in the wild before being transferred to salmon sea cages. Ballan wrasse are a poorly studied species, and fundamental knowledge of physiological performances and environmental limits are therefore needed for better deployment strategies and to predict when animal welfare may be at risk. We acclimated ballan wrasse for a minimum of 2 weeks to 5, 10, 15, 20 and 25 degrees C, representing the full range of temperatures wrasse may experience in salmon sea cages. Swim tunnel respirometry was performed at each temperature to measure standard and maximum metabolic rates, aerobic scope, and critical swimming speed (U-crit). No mortalities occurred at any acclimation temperature. However, fish were generally inactive at lower temperatures, as evidenced by low metabolic rates. It was not possible to stimulate fish to swim continuously between 5 and 20 degrees C, and U-crit was only obtained at 25 degrees C as 27 cm s(-1) (1.1 body lengths s(-1)). The aerobic scope increased throughout the thermal interval tested from 129 +/- 7 mg O-2 kg(-1) h(-1) at 5 degrees C to 265 +/- 18 mg O-2 kg(-1) h(-1) at 25 degrees C. Owing to weak swimming capabilities, ballan wrasse deployment at locations with moderate to strong current speeds will likely result in poor welfare. Low metabolic rates and inactivity at 5-10 degrees C suggests that their efficiency as cleaner fish will be limited in winter and in higher latitude locations. Overall, ballan wrasse differs substantially from Atlantic salmon in physiology, behaviour and morphology, and may not thrive in some farm environments suitable for salmon.