Achieve QUieter Oceans by shipping noise footprint reduction (Aquo) (KB-24-002-002)

This study has been realized in the scope of AQUO, a collaborative research project supported by the 7th Framework Program through Grand Agreement N°314227, whose final goal is to provide to policy makers practical guidelines to mitigate underwater noise footprint due to shipping, in order to prevent adverse consequences to marine life. The present document is the deliverable D4.3 “Masking effects of shipping noise on harbour porpoises”.

Sound is a primary means for harbour porpoises (Phocoena phocoena) to sense their aquatic environment. Like other toothed whales, they use echolocation to probe their marine environment and rely on the detection of echoes reflected from objects in their surroundings for orientation, navigation, foraging and predator avoidance. Anthropogenic sound could reduce a harbour porpoise’s ability to detect echoes. Broadband sound is generated as a by-product by the machinery of ships or emitted due to propeller cavitation into the marine environment.

 

The results show that ship noise has the potential to mask the auditory perception of harbour porpoises over the entire frequency range tested in this study. Modelling a ship’s masking range is a complex task as it depends on the sound source, the sound propagation, ambient noise level and the receiver. Determining the exact amount of masking is equally difficult, as numerous (mainly physiological) factors contributing to masking exist as well as masking release mechanisms and strategies. Nevertheless, the detection ratios (Signal-to-Masker Ratio, SMR) determined in this study prove that masking through ship noise occurs if an animal is close enough to a ship. Furthermore, they allow assessment of the scale of the masking. The masking effect of ship noise on the hearing in harbour porpoises has been proven to be frequency dependent.

 

Masking through ship noise changes the entire auditory scene for harbour porpoises and the range over which they can operate acoustically. This, in turn, can negatively affect the animals’ energy budget and ultimately their fitness and population dynamics. Ideally, masking should be considered as a significant conservation issue when assessing existing or new anthropogenic sound sources/activities at sea. Contrary to the common understanding that only the low frequency part of ship noise has to be considered with regards to marine fauna, this study provides further evidence that, despite the decrease in acoustic energy contained in the ship signature at the higher frequencies, the actual masking effect increases. This needs to be taken into account in assessing the potential impact zones and durations, but also in defining the noise monitoring parameters as in regulatory frameworks.