With animal vocal communication, underwater landslides, volcanic eruptions, and more, the oceans have never been a silent place. Humans have been adding their own noise to the soundscape for some time, but over the last century that noise has increased substantially. Shipping routes now cross the globe - including the Arctic which was, up until recently, largely inaccessible due ice. Military exercises as well as scientific and extractive resource exploration makes use of sonar and seismic airguns. Construction of in-water infrastructure as well as maintenance is also a noisy affair. Sound, especially low frequency sounds (1000 hertz and under), travel well in the ocean. Noise can be heard for hundred, even thousands of kilometres away from the source.
One of the earliest scientific assertions that anthropogenic (man-made) noise may impact marine species came in 1971 from Roger Payne (then at Rockefeller University) and Doug Webb (then at Woods Hole Oceanographic Institute). They noted that baleen whale vocalisation occurs in the same frequency band as vessel propeller noise, and that this may have detrimental impacts on the whales.
Determining the impact of noise on a particular species is not straightforward. It is not just the source of sound that needs to be considered but its characteristics, like the duration, intensity, and frequency, as well as environmental conditions, such as water depth and water temperature that can alter sound propagation through the water. One must also consider the physiology of species that may be impacted by sound. A frequency may be inaudible to one species whilst appearing very loud to another. Nevertheless improvements in technology that allow us to monitor the oceans as well as increases in our understanding of animal behaviour is providing an increasing body of evidence that ocean noise may have on the health and long-term persistence of a variety of different marine species.
A significant proportion of research on noise pollution in the ocean centres on marine mammals, particularly cetaceans who use sound for communication with their group (including mother–calf pairs), mating, hunting, and predator avoidance. A number of in-situ studies have shown how cetacean species, including North Atlantic right whales, orca, and beluga whales effectively ‘shout’ over boat traffic noise operating at the same frequency as the cetacean’s communication, to be heard. Some researchers postulate that if anthropogenic noise becomes too great, then the cetaceans may stop communicating all together. As anyone who has shouted for a long time knows, shouting involves much more energy expenditure than communicating at normal volume.
In early 2015, NOAA researchers trained captive Atlantic bottlenose dolphins to whistle at different sound levels under a hood that measures oxygen consumption. The dolphins consumed approximately 80% more oxygen when whistling at the highest vocal energy levels than they did at rest, demonstrating the increased metabolic cost of high-volume communication. Whilst occasional shouting may have few long-term impacts on the dolphins, the continuous noise that is now found in the ocean could, NWFSC biologist Dawn Noren reasons, have detrimental impacts; “If they are going to have to compensate for long periods, day after day, then that cumulative impact could be a concern, how much more fish will they need to eat to compensate for that”.
Not all anthropogenic noises are chronic. The use of sonar, whether for gas, oil or scientific exploration, and especially for military exercises has been correlated with numerous cetacean strandings around the globe. The UK’s largest mass stranding of short-beaked dolphins occurred whilst naval exercises using mid-frequency sonar were underway in the area. In 2008 100 melon-headed whales stranded in Madagascar’s Loza Lagoons system, an event later linked by the International Whaling Commission with a high-power 12 kHz multi-beam echosounder system, operated on behalf of ExxonMobil Exploration and Production (Northern Madagascar) Limited.
Construction work – and indeed maintenance of structures - is also a source of noise in the oceans. Focusing on harbour seals in the vicinity of an offshore wind turbine installation in England, researchers from University St Andrews determined that pile driving – driving foundation posts into the seabed – produced short pulsed sounds were exposed to noise levels that may induce hearing damage. Whether such damage could be temporary or permanent remains unclear, but hearing damage could impact reproductive success, predator avoidance, and finding prey. The team are continuing their study, using 'seal headphones' to take hearing measurements as well as monitor their at-sea movements and long-term data on their growth, reproduction and survival.
Understanding how anthropogenic noise may impact marine invertebrates is even more challenging than for marine mammals. Julia Samson (Woods Hole Oceanographic Institution) and colleagues set about answering a crucial question in determining if common cuttlefish may be impacted by noise pollution – how to measure their hearing. By watching for known behavioural cues, such as inking (which occurs when the animal perceives a threat) or twitching (a very mild response to stimulus) in captive cuttlefish exposed to different frequencies and levels of sounds, the researchers discovered that the cuttlefish hearing range is approximately the same as fish – and in the same range as the bulk of anthropogenic noise.
Exactly what this means in terms of anthropogenic noise impacts on cuttlefish is yet to be resolved, but the impacts to some species of fish is a little more clear. One study published last year by Stephen Simpson from University of Exeter and colleagues from University of Bristol found that the critically endangered European eel is 50% less likely to respond to an ambush from a predator when exposed to simulated shipping nose. In those that did respond, their reaction times were 25% slower, and were caught twice as quickly. In 2013 Sophie Holles, a PhD researcher at University of Bristol and colleagues published research detailing how tank-based experiments on longspine cardinalfish larvae (a coral reef species) indicated that boat noise can prevent the larvae hearing ‘reef noise’ -a vital cue for directing the juveniles to suitable habitat. Potentially this could reduce the number of larvae settling (and thus surviving) on reefs. In earlier work again by Stephen Simpson, it became apparent that the larval damselfish can be drawn to shipping noise, taking them away from their habitat with similar repercussions for population survival.
With growth in marine-based industries, anthropogenic noise in the ocean is not likely to halt anytime soon. The varying sources and types of noise, as well as the differing biology of marine species and variations in sound perception due to environmental factors makes finding workable solutions all the more difficult. Nevertheless there are adaptations that can be made to help keep noise pollution as low as possible. In 2014 the International Maritime Organization (IMO) adopted voluntary guidelines for reducing underwater noise from commercial shipping, primarily focusing on designing quieter ships and offering advice on operational and maintenance activities that can decrease noise. As an additional benefit, some of the measures suggested to reduce noise could also increase fuel efficiency – a win-win for humans and marine life. Measures to reduce Pile-driving noise include the use of bubble curtains, isolation cases, and cofferdams such as those designed to meet Germany’s more stringent low noise exposure criteria. Other solutions look towards different technologies and methods to drive in the piles as well as working on alternative platform types that do not require pile-driving in the first place.
Where noise occurs in sensitive areas, activities could be moved spatially and/or temporally to minimise the risk to marine life. One may, for example, move a shipping lane during whale migrations, or have a motor vessel free marine protected area. Arguably, the best way to combat noise pollution is for government to force ocean users to comply with strict, scientifically-based noise limits, whether the noise occurs during use, construction, decommissioning, or as a result of maintenance. Such measures will inevitably drive management actions to reduce noise pollution, but may encourage the development of technological solutions as well.
This story appeared in The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST).