From a scientific perspective, there is little difference between the terms sounds and noise – essentially being pressure waves created by a vibrating object. In common parlance, however, noise translates into something that is not pleasing to the ear and is hence subjective. The frequency (periodic motion of the vibrating object) and intensity (energy carried by the wave per unit area) of sound is measured in terms of Hertz and Decibels respectively. Whether or not a living being can hear a particular sound depends on the frequency range that the being has the ability to perceive.
While the verbal exchange between humans has evolved over time, animals have always used sounds to communicate with each other. Sound is of particular importance to aquatic life – for instance, blue whales are believed to be able to sense sounds from about a thousand miles away and the auditory cortex and cerebellum (the parts of the brain that process sound) in a dolphin brain are rather larger relative to the human average. Marine animals depend significantly on echolocation i.e., they use sound to navigate, communicate, escape predators, find mates and even get food. It is therefore evident that marine animals are impacted by sounds underwater; whether natural or otherwise.
Let’s try to dive deeper into sounds in seas and oceans to understand this better!
The levels of anthropogenic (human-generated) noise have doubled every decade for the past sixty years in some regions. This picture represents the comparative scale of some known noises underwater:
Some of the natural forces causing noise beneath the water’s surface are ice cracking, rains and storms, earthquakes and the like. Anthropogenic underwater noise is usually due to the following:
- ship traffic
- seismic surveys using airguns
- military SONAR exercises
- explosives and
- construction work
While there are multiple consequences of underwater noise when it comes to the impact on the auditory faculties of marine animals, it is manifested in two forms:
- temporary or permanent loss of hearing and
- masking of biologically meaningful sounds, such as the predator or mating calls
These are of particular harm to animals such as dolphins and whales that use echolocation for survival. Echolocation involves making a sound and determining what objects are nearby based on echoes – thereby lowering dependence on sight.
Hearing loss in mammals depends on multiple factors, including the hearing sensitivity of the animal in comparison to the intensity, frequency and duration of exposure to the sound.
According to Discovery of Sounds in the Sea, a website curated to synthesise research and other efforts in underwater acoustics, the softest sound that an animal can hear at a specific frequency is called it’s hearing threshold at that frequency. Sounds below this threshold cannot be heard while those above can be, up to a particular combination of intensity and duration beyond which the threshold of hearing may be temporarily or permanently damaged. When this happens, sounds must be louder in order to be detected. If the threshold returns to near normal levels after some a while, this condition is called a Temporary Threshold Shift (TTS) and if it does not, it results in a Permanent Threshold Shift (PTS).
A recent study undertaken by a collaborative research team including scientists from Woods Hole Oceanographic Institution conducted on turtles demonstrates that not only mammals but also reptiles suffer from such hearing loss.
Masking occurs when any kind of noise comes in the way of an animal’s ability to perceive a sound and (similar to hearing loss) is influenced by the intensity, frequency and duration of the noise in comparison with the sound of interest. This phenomenon affects animals most significantly when the noise is at frequencies similar to those of biologically important signals, such as mating calls.
Animals respond to masking noise in one of the following three ways:
- stopping vocalisations
- increasing the intensity of their vocalisations (Lombard Effect)
- changing the frequency of their vocalisations
A good example of these responses would be the study of the effects of noise on the vocal behaviour of beluga whales inhabiting the St. Lawrence River Estuary in Canada.
This region is a significant route for commercial shipping as well as a popular spot for whale watching. The belugas were exposed to noise from a small motorboat and a ferry and were noted to have reduced their calling rate as the boats neared and increased the repetition of certain calls when boats came within a distance of 1 kilometre.
Other consequences of underwater noises include:
- stranding – a phenomenon where whales, dolphins, and porpoises (cetaceans) are found dead, either on the beach or floating in the water, or alive on the beach and unable to return to the water and
- avoidance behaviour, which can lead to the abandonment of habitat or migratory pathways and disruption of mating, feeding, or nursing
All of the above are likely to alter relationships between species by changing who can effectively catch food, find a mate or hide from predators which is a cause for concern. While mankind has come too far ahead to be able to do without some of the activities that cause noise underwater, we can certainly take steps to regulate the noise generated. To encourage the reduction of the negative effects of underwater radiated noise on marine species, in 2014, the International Maritime Organization (IMO) published Guidelines for the Reduction of Underwater Noise from Commercial Shipping to Address Adverse Impacts on Marine Life. The IMO is the United Nations specialised agency responsible for the safety and security of shipping and the prevention of marine and atmospheric pollution by ships.
Hopefully, over the next sixty years, human-generated noises do not double every decade!