Anthropogenic marine noise has long been recognized as a major threat to many marine species. It is caused by military and civil sonar, shipping and vessel traffic, geophysical surveys, explosive detonations, marine construction work (e.g., pile driving, drilling, welding, and dredges), acoustic deterrent devices (e.g., “pingers”), acoustic data transmission, wind turbines, and much more [1-2].

Marine wildlife relies on sound for vital functions, including communication, prey and predator detection, orientation, and for sensing their surroundings. Therefore, animals exposed to elevated or prolonged anthropogenic marine noise can suffer direct injury and temporary or permanent hearing loss (threshold shifts), inhibiting their communication, ability to detect threats and find food [3-6]. Exposure to very high sound pressure levels or to the shock wave of an explosive can also be directly lethal [5,7-8]. Elevated sound levels that are not high enough to cause injury can still cause behavioural disruptions that may mask communications or cause animals to leave or avoid important foraging or resting habitats [9-11].

These impacts are experienced by a wide range of species. For some, such as cetaceans (whales, dolphins, and porpoises), these impacts have been widely studied [12-13]. However, other species which are also likely to be affected, including polar bears [14], sirenians (the dugong and manatees), pinnipeds (seals, sea lions and walrus) [15], marine turtles [6], otters, and even fish [16] crustaceans and cephalopods, have received less attention with respect to underwater noise [17].

 

 Hearing Ranges and Vocal Communication

Marine turtles are able to hear both in the air and underwater and are most sensitive to low-frequency sounds below 1000 Hz. While there are some differences in species- and age-specific hearing, research has shown that sea turtles can detect sounds between 50 and1,600 Hz (Figure 1) [18-23] .

Figure 1: Anthropogenic noise [1-2, 9] that overlaps with hearing ranges of common marine turtle species [18-21, 23-24]. Darker colours indicate the ranges where most sound energy concentrates and where marine turtles hearing is most sensitive.

 

Additionally, despite the common misconception that they do not produce sounds, vocalizations in marine turtles do exist. Studies of Green [25], Kemp’s Ridley [26], and Leatherback Turtles [27] have shown that they emit sounds during nesting and in the waters in front of nesting beaches. It is possible that all marine turtle species communicate acoustically underwater.

Anthropogenic noise that overlaps with the hearing range of sea turtles includes noise produced by military sonar (100 – 500 Hz low-frequency [2,9], 1 – 10 kHz mid-frequency), vessel noise which has most energy between 10 Hz and 1000 Hz [1-2, 9], airgun seismic surveys with most energy between 5 and 300 Hz [1], and pile driving, involved in constructions, for example, of wind turbines, with most energy between 100 and 1000 Hz [1]. Many of these activities occur in coastal waters, which turtles use for foraging, breeding, nesting, and as habitat for the juveniles [25, 28-30]. Turtles may use acoustic cues to select high-quality habitats with abundant food [30]. If the received sound levels from anthropogenic sources are above background levels, they can mask important environmental acoustics cues meaning that turtles will be hindered in utilising otherwise suitable habitats in noisy areas [25]. Further, masking may obscure propeller sounds affecting sea turtles’ ability to avoid collisions [6], cause changes in their behaviour [6,31], and impair their ability to navigate oceans, locate prey, avoid predators, and lower their general environmental awareness [6,24]. In turn, “such alterations can lead to a reduction in individual fitness […], potentially causing detrimental effects at a population level” [6].

 

 

 Effects of noise on turtles

Photo: Alessandro Cere

Photo: Alessandro Cere

There is clear evidence of changes in behaviour of marine turtles in response to underwater noise. The behavioural changes have the potential to result in impacts on their energy budgets, reproductive success, and safety. Loggerhead turtles surfaced and showed avoidance behaviours in response to low-frequency sounds, airguns, and acoustic deterrent devices [31]. Green and hawksbill turtles appear to avoid areas after exposure to seismic sources [32]. Faster swim speeds, avoidance, changes in swimming direction, alarm responses, and erratic behaviour have also been reported in response to small and large airguns [33] and seismic vessels [34].

Furthermore, there have been repeated cases of dead or injured turtles found after explosive removal of offshore structures [35-37]. Noise could also cause physiological damage to their ears, resulting in either temporary or even permanent hearing threshold shifts as has been seen for other marine animals [3-4, 38-40]. Stronger shock waves likely result in injury to other internal organs as well [35]. Small explosions could directly lead to injury of the auditory system or lungs of turtles in the region [41-42]. Marine turtles might recover from some injuries, such as a lung haemorrhage and gastrointestinal tract contusion [41-42]. However, massive or combined trauma to internal organs, such as extensive lung haemorrhage, concussive brain damage, cranial, skeletal, or shell fractures, or massive inner ear trauma, are injuries that sea turtles would not survive, even if not killed instantly [35, 41-42].

To understand impacts of underwater noise on marine turtles better, it is important that comprehensive post-mortem examinations are undertaken, assessing their health, including the hearing system. This can help evaluate the effects of noise exposure on hearing, but also monitor the efficiency of mitigation measures that are being implemented.

 

 

 Implications for Policy

The existing literature shows that anthropogenic noise pollution in our oceans may seriously affect turtles in similar ways as it does other species. Noise can cause changes in behaviour, or mask important environmental acoustic cues. These responses to noise may affect their survival rates and individual fitness. Thus, policy needs to take this threat into account for effective conservation, developing mitigation strategies and requiring mitigation measures to be applied for marine noise-generating activities to reduce negative noise-related effects on marine turtles.

Mitigation strategies based on sightings and acoustic detections, which are frequently used for cetaceans, might be less effective for marine turtles given their cryptic nature [33]. These strategies could include delaying the start of activities or halting the noise-generating activities when turtles are sighted. The effectiveness of these strategies for reducing injury risks in marine turtles should be evaluated [43]. Nevertheless, marine turtles need specific consideration and measures should be enforced to ensure effective conservation.

Parties to the Convention on the Conservation of Migratory Species of Wild Animals (CMS) in Resolution 12.14 Adverse Impacts of Anthropogenic Noise on Cetaceans and Other Migratory Species commit themselves to endeavour to control the impact of anthropogenic marine noise pollution in habitats of vulnerable species. This should be done by restricting the emission of underwater noise; and where noise cannot be avoided, by developing an appropriate regulatory framework or implementing relevant measures to ensure a reduction or mitigation of anthropogenic marine noise. One such measure is to undertake Environmental Impact Assessments (EIAs) that take full account of the effects of activities on CMS-listed marine species, their prey and their habitats to identify best management practices and mitigation measures to support continued recovery efforts.

To assist with this, CMS Parties endorsed the CMS Family Guidelines on Environmental Impact Assessment for Marine Noise-Generating Activities (Annex 1 to Resolution 12.14), which outline the information required to properly assess effects on all species of interest, including marine turtles. Further, Technical Support Information is available, which includes species-specific expert advice on marine turtles.

For more information on the work of CMS on marine noise, please visit this website.

 

 

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