Water carries sound efficiently and light poorly, so marine animals rely on hearing where terrestrial animals rely on sight. Added noise therefore operates on the sense these species depend on most.
Why sound dominates underwater
Sound travels roughly five times faster in seawater than in air and attenuates far more slowly, particularly at low frequencies which can propagate across ocean basins.
Light, by contrast, is absorbed within tens of metres, and chemical cues disperse slowly and unpredictably with currents.
Hearing is consequently the primary long-range sense for marine mammals, and many fish and invertebrates use sound for orientation, spawning and predator detection.
Where the noise comes from
Commercial shipping produces continuous low-frequency noise from propellers and machinery, and because vessel traffic is constant this forms a persistent background rather than an event.
Seismic surveys used in offshore exploration fire high-energy pulses repeatedly for weeks, and those pulses are detectable at very long distances from the source.
Pile driving during offshore construction, military sonar and small vessel traffic add further contributions, each with a different frequency range and duration.
Masking is the main effect
The most widespread consequence is not injury but masking, where background noise reduces the distance over which a signal can be heard.
A whale call that once carried across a wide area may now be audible only within a fraction of it, which matters for species that locate mates and coordinate feeding acoustically.
Baleen whales are particularly exposed because their calls occupy the same low frequencies that shipping produces, so the overlap is close to complete.
Behaviour changes before hearing damage
Animals respond to noise by moving away, diving differently, calling louder or falling silent, and these responses occur at levels far below those causing physical harm.
Interrupted feeding is the significant cost. An animal that abandons a foraging dive loses energy it needed, and repeated interruption accumulates over a season.
Strandings following intense sonar exposure have been documented in deep-diving beaked whales, associated with abnormal dive behaviour rather than direct acoustic injury.
Why noise is a tractable problem
Unlike most marine pressures, noise stops when the source stops. There is no residue, and the acoustic environment returns to its previous state immediately.
Quieting technology exists, since propeller design and machinery mounting affect radiated noise substantially, and the quietest vessels in a fleet are often also the most fuel-efficient.
Routing changes and speed limits in areas of known importance deliver measurable reductions without new equipment, which is why they have been adopted first.