Metabolic rates, swimming capabilities, thermal niche and stress response of the lumpfish, .

The lumpfish () is a semi-pelagic globiform teleost native to the North Atlantic with a ventral suction disc that allows for attachment onto surfaces. Some local populations are in decline and the species has recently become important in salmonid sea cages as cleaner fish. Little is known about the basal physiology of the lumpfish, and a characterization of thermal performance, aerobic capacity, swimming behaviour and stress response is therefore warranted. In the present study, swim tunnel respirometry was performed on lumpfish acclimated to 3, 9 or 15°C. Higher temperatures were also attempted, but at 18°C their behaviour became erratic and 15% of the fish died over 3 weeks of acclimation. Water current tolerance was assessed in two size classes (∼75 g and ∼300 g) both with and without the ability to voluntarily use the ventral suction disc. Lastly, blood samples were taken from resting, exhausted and recovered fish to assess haematological effects of exercise stress. Lumpfish had relatively low aerobic scopes that increased slightly with temperature. Critical swimming speed was poor, increasing within the tested temperatures from 1.3 to 1.7 body lengths s in 300 g fish. They struggled to remain sucked onto surfaces at currents above 70-110 cm s, depending on size. Acute stress effects were modest or non-existent in terms of changes in cortisol, lactate, glucose, erythrocytes and ion balance. These results describe a typical sluggish and benthic species, which is contradictory to the pelagic nature of lumpfish in large parts of its lifecycle.