Tolerance, growth, ionic and osmotic response and gill enzymatic activity adjustments (Na + K + ATPase, COX, CS, PK and LDH) of Atlantic wolffish (Anarhichas lupus) exposed to different salinities (0%%, 7%%, 14%%, 21%%, 28%% and 35%%) were examined. Direct transfer from seawater (SW) to freshwater (FW) of 0 + juveniles situated the L 5 0 for salinity tolerance between 5%% and 6%%. The data from prolonged exposure to various salinities indicate efficient homeostatic control in Atlantic wolffish A. lupus and improved growth performance at 14%% compared to 28%% after 70 days (calculated growth trajectories in length and in weight). The latter is linked to indications of reduced metabolic costs of ion regulation, i.e. significantly reduced Na + K + ATPase and LDH activity at 14%% compared to 28%% (Na + K + ATPase: 5.67+/-1.41 compared to 7.27+/-1.23 and 5.44+/-0.86 compared to 6.86+/-1.34 U/mg protein at weeks 14 and 19, respectively, p=0.05 and 0.026, respectively; LDH: 10.4+/-2.14 compared to 12.39+/-2.02 U/mg protein at week 14 and 12.46+/-1.34 compared to 17.17+/-2.67 U/mg protein for LDH, p=0.026 and 0.008, respectively). A strong positive relationship between Na + K + ATPase, LDH and PK activity was also observed [log y=1.107log LDH-0.914, p=0.0000, r 2 =0.46 (n=33) at weeks 14 and 19 and with PK (n=18) at week 19, log y=1.143 log PK-0.595, p=0.001, r 2 =0.497]. This indicates that from week 14 onward, the activity of branchial Na + K + ATPase is coupled to adjustments in the level of activity of the glycolytic enzymes. Our results indicate that Atlantic wolffish is a strong osmoregulator that demonstrates haloplasticity which enables cultivation of this species at reduced salinities with expected benefits in productivity.