Underwater Noise

Underwater noise, primarily associated with increases in commercial shipping, has been doubling in intensity each decade in the open waters of the eastern North Pacific since the 1950s – a three-decibel(dB) increase every 10 years. Research suggests that this increase in noise is not all due to commercial shipping in deep water, but that additional contributions come from nearshore vessel traffic and other anthropogenic activities such as marine construction and small vessel traffic. Overall, human activities in and near the water are a growing concern as these activities appear to change the soundscape everywhere in the marine environment and these changes are affecting marine animals negatively.

What is sound and what is noise? The answer depends on who is listening. Essentially, a sound is an acoustic signal when it is important to the listener, but becomes noise when it interferes with the reception or transmission of a sound. For example, to an acoustician, someone who studies the properties of sound, listening for the underwater sounds of earthquakes, whales make noise; but to a whale listening for the call of another whale, ships and earthquakes make noise. The use of sound is the main form of transmitting and receiving information for most marine animals, but unfortunately, human-caused underwater noise has been increasing at an astonishing rate, interfering with the ability of marine animals to listen for predators, successfully find prey, communicate with each other, maintain contact, and navigate.

When we describe sounds, we consider the frequency, loudness, and the time period over which they occur. The following diagrams, called spectrograms, illustrate underwater sounds by depicting the frequency on the vertical axis, the duration on the horizontal axis and the loudness through colouration (blue to green to yellow represent increasing loudness). The spectrograms shown include the continuous sound of a nearby ship (Figure 1A), the repetitive and impulsive noise of pile driving (Figure 1B), the narrowband sound of a killer whale whistle (Figure 1C), and the echolocation signals of a resident killer whale (Figure 1D).

Figure 1. Spectrogram of underwater sounds. A: Continuous (always on) sound from a ship at a distance of 850 meters. Noise from vessels actually extends up to over 100 kHz depending on the vessel type. (Click here for a sound clip of freighter.) B: Pile driving, which is an impulsive (on/off) sound of high-energy. This illustration shows 14 pulses (representing the underwater sound produced when the hammer strikes the pile to drive it into the ground) over 21 seconds. (Click here for a sound clip of pile driving.) C: A killer whale whistle, which is a narrowband sound, extending over a very limited range of frequencies. This depicted sound centers around nine kHz most of the time but ranges from six to 10 kHz along the entire signal. (Click here for a sound clip of killer whale whistle.) D: Killer whale echolocation, which can range from very low frequencies (less than one kHz) to over 100 kHz. The display shows the recorded frequencies of the sounds between 0 and 11 kHz (Y-axis). The time (X-axis) shows three seconds of vertical lines, which represent echolocation signals. (Click here for a sound clip of echolocation.)

Low frequency sounds (under 100 Hz) associated with commercial shipping or seismic explorations (air guns) can travel hundreds if not thousands of kilometres underwater. Due to the large number of ships traversing all ocean basins at any given time, the noise pollution created by ships travels far and wide with little loss of energy, particularly in deep ocean channels (Figure 2).

Figure 2. Low frequency sound can travel for hundreds or even thousands of kilometres, especially when travelling in deep ocean channels. These channels occur as a result of changes in water temperature (surface water changes with outside temperature while deeper water maintains more constant temperatures), differences in salinity, and increased pressure with depth, which compresses water molecules. The red arrows show the transmission distance of a tone that was broadcast at less than 100 Hz.

Underwater noise has emerged as a conservation concern globally, as it is a threat to a number of declining marine mammal populations, and also has impacts on fish and invertebrates (e.g., shellfish). Even zooplankton are affected by activities such as seismic surveying, during which air-gun blasts can kill these small and often microscopic animals at distances of up to 1.2 kilometres away from the source.

Data gathered before and after 9/11 illustrated the unseen costs of underwater noise on an endangered population of North Atlantic right whales. Researchers had been collecting fecal samples from this population well in advance of 9/11 for stress hormone analysis. For three days following the events of 9/11 airspaces were silenced and shipping traffic was significantly reduced (resulting in a reduction of underwater noise by six dB) in the Bay of Fundy. Stress hormone levels in North Atlantic right whales were measurably and significantly lower during this time. Once air and ship traffic resumed to normal, stress hormone concentrations returned to pre 9/11 levels.

We also know that increased underwater noise interferes with the echolocation and communication of whales, a phenomenon known as acoustic masking. Figure 3 illustrates how the distance at which killer whales can detect their prey is reduced by underwater noise: R represents the distance which killer whales under quiet conditions can detect prey through echolocation, r1 is the distance under current ambient conditions which killer whales can detect prey, and r2 is the detection distance that is predicted to occur if noise levels increase in future as a result of increased commercial shipping.

Figure 3. The hypothetical reduction in detection distance of prey for killer whales using echolocation under increasing underwater noise levels. R represents the detection distance under quiet conditions, r1 represents the detection distance under current background noise levels, and r2 represents the detection distance that is predicted to occur if noise levels increase in future as a result of increased commercial shipping.

As killer whales dive to chase their prey into deeper and darker water, their ability to use vision to follow prey declines, which is why they rely on acoustic cues – echolocation signals – to find and capture their prey. Acoustic masking is one of the most obvious impacts of underwater noise on marine mammals, and Figure 4 shows how this overlaps with the echolocation and communication signals of killer whales.

Figure 4. Spectrogram illustrating an example of acoustic masking, recorded by Orcalab on Hanson Island in Johnstone Strait. During the
first nine seconds, the calls and echolocation signals of a fish-eating northern resident killer whale pod are clearly visible. Approximately
nine seconds into the recording a large ship comes out from behind an island, masking the sounds of the whales. Whale calls can no longer be distinguished (heard or seen in the spectrogram), which means it is likely that the whales can no longer hear calls from each other as well, especially at greater distances. At 19 seconds into the recording there is a very loud killer whale vocalization, which can be heard above the ship noise. (Click here for a sound clip of acoustic masking).

Research suggests that killer whales increase the amplitude of their calls in noisy environments but this comes with an increase in energetic cost of sound production and may increase their stress levels, and/or affect their activity budgets. If the noise levels are high enough and sustained over long periods, the ability of the whales to communicate could be completely impaired.

Underwater noise and its potential impacts on marine life vary among vessel types because they emit very different sound pressure levels (which are called sound source levels, or sound amplitudes, or simply, loudness) at different frequencies (Figure 5). Generally, tankers and especially container ships, produce much more noise in lower frequencies (less than 250 Hz) than most other vessels (Figure 5). Container ships also have much higher sound levels across all frequencies, but container ships also travel much faster than most other commercial vessels. Sound source levels of small pleasure craft also vary greatly with speed and some can emit much higher noise levels in higher frequencies than shown in Figure 5 when they travel quickly. Low frequency noise, however, travels further underwater than higher frequency noise. This means that large commercial vessels drown out the noise of smaller vessels in lower frequencies when travelling in the same area. At higher frequencies (especially above 10,000 Hz) noise from smaller vessels can be louder than that from large ships depending on how fast the small vessel is travelling. This is important for smaller whales and dolphins that usually hear those higher frequencies better than large whales. Because all vessels do still have high sound levels at frequencies above 10,000 to 20,000 Hz – which is in the range where killer whales’ hearing starts to become most sensitive – vessel noise is loud enough to mask not only large whale signals but also signals of smaller whales such as killer whale communication and echolocation.

Figure 5. Median source sound levels of ships for a range of vessel classes (see legend). Each vessel generates noise over a range of frequencies (x-axis) and the sound pressure (loudness) (y-axis) of the noise is greater at lower frequencies. (The frequency range audible to humans is roughly 20 to 20,000 Hz for children, while most adults do not hear well above 15,000 Hz. The red vertical line indicates 20,000 Hz.) Source: Veirs et al. 2016.

Humpback whales and large vessels often share the same marine space. (Photo: Karina Dracott)

From 1972 to 1999, the number of ships increased globally from 57,000 to 87,000 and their total cargo carrying capacity (gross tonnage) has increased from 268 million tons to 543 million tons. Increases in carrying capacity and newer faster ships could explain the observed increase in underwater noise, as bigger ships are generally noisier and faster ships are generally noisier. However, it is not that simple because on average newer vessels are quieter than older ones due to improvements made to ship design to reduce fuel consumption. Removing the loudest, and often oldest, vessels from the fleet could reverse or slow the trend in increasing underwater noise.

The recession in 2008 and 2009 took its toll, but at the beginning of 2014, the world’s commercial fleet consisted of 89,464 vessels, with a total tonnage of 1.75 billion deadweight tons (dwt). From 2010 to 2030, the total tonnage and number of vessels is projected to increase for all major ship types between 1.8 and three times (this includes bulk carriers, containerships and liquefied natural gas vessels). The increases in tanker capacities are expected to be less than other ships – 1.7 to 1.8 times greater.

Underwater noise levels are being monitored at a number of locations on the B.C. coast (Table 1), as well as in the nearby waters in northern Washington State, part of the home range of the endangered southern resident killer whale (SRKW). The Canadian Department of Fisheries and Oceans (DFO) also has a number of hydrophones deployed that focus on detecting fin, humpback and killer whales. In the future, these may be useful for monitoring underwater noise.

Individual and Organization Actions:

As an individual boater or a member of the shipping industry, you can:

• Slow down to reduce noise.
• Clean your hull and maintain your propeller.
• Insulate your engine and use resilient mountings for onboard machinery.
• Incorporate vessel quieting considerations during re-fits (e.g., modify your propeller to minimize cavitation).
• Modify your route to avoid whales.
• Shut off your sounders, especially when in the vicinity of whales and only when it is safe to do so, as this will quieten noise levels at higher frequencies.
• As a member of the shipping industry, you should be familiar with and follow the 2014 International Maritime Organization guidelines on underwater noise.

Government Actions and Policy:

Both Transport Canada and DFO are moving towards adopting policies aimed at regulating underwater noise generated from shipping, recognizing that increased noise levels impact marine life. Future recommendations to increase the foraging and communication efficiency of southern resident killer whales (SRKW), as well as other marine life include:

• Develop marine environmental water quality objectives based on all sources of underwater noise.
• Incorporate the cumulative impacts of multiple projects on the underwater soundscape in environmental assessment processes.
• Expand the critical habitat for SRKW to include waters off the southwest area of Vancouver Island and Swiftsure Bank.
• Harmonize whale-watching regulations that are consistent with those in U.S. waters, and maintain and improve the capacity for enforcement of these regulations.
• Establish trans-boundary collaborations that focus on management actions to quantify and reduce underwater noise associated with vessel traffic, particularly in SRKW critical habitat.
• Implement temporal and spatial vessel regulations and/or guidelines to quiet the acoustic habitat of SRKW. These may include vessel speed and/or routing restrictions, no-go periods for large ships (e.g., midnight to 4 a.m.) and the use of convoys.
• Support incentive programs and regulations that reduce the acoustic footprints of vessels that regularly travel in the critical habitat of species at risk.
• Identify and create acoustic refuge areas for SRKW.
• Apply D-tags on northern resident killer whales to better understand killer whale behavior, especially foraging at night.