Impacts of aquaculture fish cages and their biomass on the local water current field is investigated with extensive, full scale field measurements. This is important for the fish welfare as well as for waste transport from open fish cages. The relationships between the current flow rate at various locations close to fish cages are examined at an aquaculture marine site close to Frøya, Norway. This is an area with a complex current pattern due to the bathymetry, rocks and islands, coast current and strong wind conditions. An array of ADCPs (Acoustic Doppler Current Profilers) measured currents. Moreover, CTD and separate temperature profiler instruments were made in order to analyse flow stratification. Two different measurement periods are analysed with 1) small fish size (0,2–0,4 kg) and 2) large fish size (1 kg–5 kg). The field measurements indicate some impact from the biomass on the measured current flow at distances from 90 to 320 m around it. The results are supported by results from the numerical ocean model SINMOD (SINtef MODel).