Effects of Vessel Noise on Walleye Pollock (Theragra chalcogramma) Behavior

The results of experiments on assessing the reaction of walleye pollock (Theragra chalcogramma) to the noise of passing research vessel are presented. A free-drifting buoy with built-in scientific echo sounder (38 kHz) was used. Pollock reactions were accompanied by horizontal and vertical movements from the noise source. The response distances to vessel noise ranged from 140 to 180 m, which is consistent with the model estimates. The total acoustic density curve has a tendency of a small initial rise and then a steady decrease with further approach of the vessel. A stronger decrease in the total density of fish in the upper horizons due to lateral avoidance is typical. The diving of fish when the vessel approaches causes a consistent increase in their density in a deeper layer. In the depth range of 0–100 m, the ratio of pollock density to the background at the closest point of approach was 0.56. At depths of 100–200 m, due to the deepening of fish, the decrease in total density was less significant (Kavoid = 0.86). The diving speed of pollock in the vessel’s noise field is an order of magnitude higher than during daily vertical migrations. Such avoidance behavior can be a source of error in echo integration and trawl fish stock estimations.