Porpoises are entirely dependent on their hearing for survival. They navigate, hunt, and communicate by emitting rapid click sounds and listening to the returning echoes. However, with increasingly noisy oceans, it is getting harder for porpoises to "hear their way." Noise from shipping is a particu lar problem. While ship engines primarily emit low-frequency noise, they also produce high-frequency sounds that can drown out the porpoises' own clicks. These clicks are sharp, brief, and only travel limited distances, making them highly vulnerable to noise sources in their immediate vicinity. ...read more read less