As a result of the higher density of water compared to mixture of gases in the atmospheric, sound travels four times faster in the sea than in air. Marine mammals, including whales, dolphins and porpoises, are much more dependent on sound than land mammals due to the limited effectiveness of other senses in water. Some mammals like whales and dolphins highly developed their sense of hearing over millions of years to send and receive variety of complex sounds. They highly rely on sound to communicate with each other, navigate, find food, defend their territories and avoid the predators. Fishes also use sound for basic life functions.
People produce some sounds intentionally, such as military sonar and seismic tests for oil and gas exploration. Other sounds are an unintentional, such as shipping and underwater construction. Many human-produced sounds in the ocean are periodical, whereas shipping creates an almost constant rumble in the ocean. Even the motor of a fishing boat creates extra sound underwater.
All of these sounds add to overall ocean noise and contribute to the “soundscape,” which scientists define as the combined sounds made by humans, natural events, and marine animals. Because sound travels so well underwater, many of these sounds can be heard miles from their sources.
Noise pollution in the ocean can be split into 3 different categories:
– Environmental noise
– Noise of Ocean Life
– Anthropogenic noise
Environmental noise is caused by airbone noise which propagates into the sea, wind and waves, seismic noises (oils and gas). Many environmental events make non-biological sounds, such as low frequency rumbling of a thunderstorm the cracking of singing ice as ice sheets slide against each other and break apart. Vulcanic eruptions and earthquakes oscillate in a very low frequency band, sometimes only fractions of Hertz and the sound last longer compared to the most species living underwater. The environmental noises are responsible for noise floor, which its power is higher compared to the noise floor in the air.
Noise of ocean life produces a variety of different signals. Mammals are called ‘the birds of the sea’ and vocalise in a different frequency bands. The song of a blue whale consist mostly of low frequencies. Together with fin whale they produce sweeps at about 20-35 Hz, which results in fin whale sound masking content of the blue whale calls.
Anthropogenic noise Human activity in and around the ocean makes a variety of sounds, such as pinging from echosounders or low-frequency vessel noise from motorized boats and ships. Some anthropogenic sounds can be disruptive or harmful to marine life.
References:
1) https://www.fisheries.noaa.gov/national/science-data/sounds-ocean-environmental-and-anthropogenic
2) https://fast.wistia.net/embed/channel/tvcq9ovg1h?wchannelid=tvcq9ovg1h
3) Ocean Noise, from science to management – Journal of Marine Science and Engineering
4) https://dosits.org/animals/use-of-sound/how-do-marine-animals-use-sound/
