By Carole Baber
Sound travels 4.8 times faster underwater than in air and in the sea frequencies below 1000 Hertz can transmit for thousands of kilometers. Sound is a form of energy that is manifest as a wave of changes in pressure. It can change in frequency, wavelength, and intensity. The nature of the medium through which sound moves (e.g., air or water), its temperature, and the pressure affects the speed and path of the wave. Generally, lower frequencies travel further than higher frequencies. For reference, the range of audible frequencies for human beings is generally said to be from 20-20,000 Hz.
Cetacean stranding (2023, April 28) Wikipedia
Sound in the environment comes from a range of phenomena. That generated by natural sources is known as ambient noise, that generated by human activity is referred to as Anthropogenic noise. Since the Industrial Revolution, anthropogenic noise has increased with increased industrial activity, and we know that anthropogenic noise can cause a variety of problems in humans; serious damage can range from physical, physiological, neurological and endocrinological to increased risk of coronary disease, cognitive impairment, and sleep disruption. So, there is legislation to monitor and manage noise exposure for humans, in everyday life.
Noise in the ocean also comes from a variety of sources. Natural sources include waves, rain, wind, seismic activity (tectonic plate movements, volcanic and earthquake incidents), ice movements, and marine organisms (ranging from invertebrates to fish and mammals). Anthropogenic sources include harbour developments, offshore windfarm construction, tidal and wave energy generation, cable and pipe laying, shipping, aircraft, seabed drilling and mining, dredging, air guns used for seismic exploration, navigation, and communications such as sonar and imaging echosounders.
Many marine animals have evolved to create and receive sound for various functions. They have also developed, over evolutionary timescales, to accommodate natural sound sources in their soundscape. Sounds used for communication within a species can be for social bonding and information exchange, for selection of and competition for mates, and for warnings of danger. Other sounds are used to avoid predators, detect prey or for habitat selection. Sound is also a tool for navigation through the environment, often by echolocation.
The types of sound and the mechanisms used to make them in the sea are varied and fascinating. The snapping or pistol shrimps (Alpheidae) are major contributors to ambient ocean noise. They use sound to stun or kill the fish they eat. Because of the special structure of their claws, they can snap them so fast and hard that bubbles are propelled at high speed and the pressure stuns or kills its fish prey. Amazingly, this action makes one of the loudest ocean noises. Invertebrate hearing is not well understood, but many species have statocysts, structures that are thought to be involved with hearing and there is evidence that some invertebrates, including the pistol shrimps, can detect sound as well as make it, and use sounds for communication.
Jesse De Meulenaere, Unsplash
Bony and cartilaginous fish have inner ears similar to those of land vertebrates and it is thought that virtually all fish are capable of hearing. Some fish species can even use their swim bladders to interpret noise and to generate noise for communication. Marine mammals have internal ears that are structurally like those of land mammals, although they have developed a broader detection range, and some cetaceans (whale, dolphin, and porpoise) also use specialized fat deposits as auditory tissues. The use of bio-sonar by cetacean species to navigate, communicate and hunt is well documented, although not necessarily completely understood. So, it is clear that use of, and detection of noise in the marine environment is essential to fitness and survival of individuals, whole species, food chains, and therefore the marine ecosystem.
Marine mammals collectively have hearing ranges from 10-200Hz, fish as a group detect sound from well below 50Hz up to 500-1000Hz. Anthropogenic frequencies overlap with these relatively low frequency ranges. Therefore, it can be deduced that noise generated by human activity will ‘bump into’ and therefore interfere with the generation and reception of sound by marine organisms. It must be a bit like being in a noisy pub, or a disco when having a conversation with a friend, or worse, like working with equipment where the noise causes physical damage to hearing structures. Direct effects of this noise include masking, when the polluting noise is so loud that biologically important sound is drowned out or distorted, resulting in no or reduced communication, or transmission of misleading information, as happens in ‘Chinese whispers’. If the noise intensity is high enough, hearing organs can even be damaged, either temporarily or permanently. It is the same for us if our ears are regularly exposed to over 85 decibels. Not surprisingly, the interference and damage are not restricted to hearing organs alone. Human divers have suffered resonance of body cavities, dizziness, nausea, and sight disruption after experiencing low frequency sound at high intensity, such as that from some sonar systems. Swim bladders of cod can be ruptured when too close to pile driving activity. In one study, at 100m from a pile driving site 90% of swim bladders were ruptured and 92% of fish had internal bleeding. The extent of damage did decrease with distance, but 20% rupture was still seen at nearly 1.5km away, and up to a half of the fish had internal bleeding. Damage to sensory tissue linings of fish has also been observed, following exposure to air-gun use, and repair did not occur for up to 2 months. Cephalopods (octopus, squid, cuttlefish and nautilus) have structures called statocysts for orientation, they have a similar role to the mammalian inner ear. They are damaged by low frequency sounds of 50-400 Hz at 157 decibels, consistent with noise produced by operations such as shipping and fisheries. Furthermore, physiological, and developmental interference was demonstrated in wild scallop larvae exposed to seismic survey pulses; significant levels of body abnormalities and delays in development were found, compared to the controls.
Cetacean strandings in Greece, Madeira, the Canary Islands and the Bahamas have been attributed to use of military sonar. Autopsies of the individuals involved revealed hemorrhages in the fats that are used as acoustic organs and subarachnoid (space around the brain) bleeding. Also, gas bubbles and fat particles in the blood were indicative of changes to normal diving behaviour. Although there is a cause-and-effect relationship here, it is not clear whether the sound directly caused the damage, or indirectly by provoking a change in diving behaviour. Humpback whales off Newfoundland had damage consistent with blast injuries after blasting activity and, following underwater explosions around the McMurdo sound, 50% of a Weddell seal sample had tissue damage to their ears.
Studies on fish have shown a range of behavioural changes in response to anthropogenic noise. Reactions often differ by species and vary from startle responses such as collapsing to the bottom, becoming motionless, increasing swim speed, changing swim direction or changing depth in the water column. Changes in schooling behaviour differ too, from aggregating more closely to becoming less coordinated with consequent loss of school structure and even complete dispersal. These responses use up extra energy, threaten the cohesion of schools and potentially disorientate fish. The result is stress, increased vulnerability to predation, reduced foraging, reduced fertility and breeding success, loss of direction when migrating and general reduction in fitness to survive. Many fish migrate to spawn or find feeding grounds and it is thought that they use the soundscape as part of their navigation and habitat location system. It has been suggested that larval reef fish use characteristic sounds of their reefs to locate home. If high level noises cause disorientation, fish can end up losing their way, and habitats may become unrecognizable. There are also implications for ecosystem services here and anthropogenic noise has been reported to have caused a reduction in the catch rate of some commercial marine species.
Ray Harrington, Unsplash
Indirect effects of noise pollution can have wider implications than for the individuals and species affected by direct effects. For example, negative effects on prey and predator species affect the balance of food webs; the ramifications are potentially far reaching and complex. Species that perform ecosystem services such as water filtration and nutrient recycling will also have a fundamental impact on the health and wellbeing of marine communities. Although effects of anthropogenic sound on invertebrates have been less studied and are less understood, research has shown that some filter feeders involved with mixing surface sediments reduce their activities when exposed to anthropogenic noise. So, noise pollution not only poses a threat to individual marine organisms but also to the composition, and subsequently the health and service functions of the ecosystem.
Who is responsible for addressing these problems? Marine Protection Areas (MPAs) may have their own policies and strategies for noise control. But to be enforceable, national and government agencies need to empower MPAs with legislation and impose penalties for transgressions. To complicate matters, sound travels further and faster in water than in air and so boundaries cannot be controlled, and conversely, polluters may be remote from the location concerned.
NOAA’s Ocean Acoustics Program supports and conducts research. Ocean Noise Strategy to guide the agency towards more effective and comprehensive understanding and management of ocean noise impacts on marine life in the years 2016 to 2026.
So, what can be done? Monitoring species status and responses to anthropogenic noise in local areas is essential to inform management plans. Plans might be in relation to specific, vulnerable species or habitat, or to a particular type of local pollution. Responses could be restriction of noise to certain, less damaging times e.g., outside a breeding season, reduction of noise to a specified level and duration, mitigation by use of alternative technology, or, in extreme cases, a complete ban. Acoustic buffer zones around a sensitive area may help address the issue of boundaries. Every country that uses the sea should have noise reduction targets, in key habitats and for the open sea, and there is a need for noise limits everywhere.
And what about the high seas, where no one has jurisdiction? The new High Seas Treaty is something to watch. It requires environmental impact assessments before new exploitative activity can progress. But how will the impact of sound be assessed? It is a diffuse kind of pollution with often long-term effects that might be invisible in the short term of an assessment. And what about the noise already here? What about international shipping noise? Perhaps we need no-go MPAs in the high sea area? But who will monitor and react? These are difficult and frustrating issues, a truly challenging problem.
Blue growth and Blue Economy are new buzz words for the ‘Sustainable use of ocean resources for economic growth, improved livelihoods and jobs, while preserving the health of the ocean ecosystem’; the imperative is to ensure sustainability in the face of all possible stakeholders. There needs to be more international negotiation and cooperation to address the level of noise that exists today, control future noise, and to monitor and enforce compliance.