High-frequency vessel noise can mask porpoise echolocation

Ultrasonic cavitation noise from fast vessels overlaps spectrally with echolocation clicks of toothed whales and therefore has the potential to degrade echolocation performance through auditory masking of returning echoes. Here, we tested that hypothesis by exposing two trained echolocating porpoises carrying DTAGs to two different levels of decidecade noise centered on 2 kHz (non-masking) and 125 kHz (masking) during an active target discrimination task. We found no click level adjustments or effects on discrimination performance in trials with non-masking noise or low-level masking noise. However, when exposed to high-level masking noise of 113±3 dB re. 1 µPa root mean square (RMS), the porpoises increased their mean click source levels by 7-17 dB. Despite this Lombard response of 0.2-0.5 dB_signal/dB_noise, and longer time and more clicks used by the porpoises to perform the task in noise, both animals were still significantly poorer at discriminating the targets (64-85% success rate) than in the other treatments (94-100%), thus demonstrating adverse masking effects. When the porpoises were offered spatial release from masking by relocating the noise source off-axis relative to the animal-to-target axis, echolocation performance was regained. We conclude that moderate levels of high-frequency noise, such as from cavitating vessel propellers several hundred meters from a vessel, can mask porpoise echolocation in a way that cannot be fully compensated for. As biosonar is vital for foraging and navigation around hazards such as gillnets for porpoises and other toothed whales, this study highlights that masking effects should be considered in impact assessments of cavitating vessels around echolocating toothed whales.