The real-time behaviour of herring schools exposed to a full-scale 3D seismic survey off Vesterålen, northern Norway, was observed using an omnidirectional fisheries sonar. Throughout the study period, the herring swam slowly against the predominant northeast current, with a net displacement along with the current. The mean swimming speed after subtracting the drift velocities was 0.35 m s−1, and the mean response speed in the direction away from the air gun array was 0.22 m s−1. No changes were observed in swimming speed, swimming direction, or school size that could be attributed to the transmitting seismic vessel as it approached from a distance of 27 to 2 km, over a 6 h period. The unexpected lack of a response to the seismic survey was interpreted as a combination of a strong motivation for feeding, a lack of suddenness of the air gun stimulus, and an increased level of tolerance to the seismic shooting.