Harbour noise and marine life

Underwater-radiated noise has received an increasing amount of attention because of the growing evidence of its potential threats to marine life, especially if exposed in a cumulative fashion (i.e., from various sources). Many marine animals depend on sound as their primary means of communicating. Vision is usually less important when navigating, looking for prey or communicating. Sound travels long distances underwater, which implies that impact on marine life is not limited to the direct surrounding of the source of noise.

Underwater noise in harbour areas can have various sources, i.e. shipping, seismic survey (air guns), dredging and pile driving. Pile driving and seismic survey are perhaps the most intrusive sound sources giving off high intensity, low frequency noise. Shipping noise is not the most intrusive sound source in port areas, but scientists express concern about its vast expansion in its ability to mask biological sounds marine animals need for their biological functions. Shipping has increased by 50 percent over the last 15 years and this number is expected to double in the next 20 to 30 years.

There are various ways in which ships can produce noise. Aside from engine noises there’s also the process known as cavitation, around the propellers and which is the most important underwater noise produced by shipping. The intensity of the noise produced depends on the propeller, engine, size and shape of the ship, cargo load, speed and mode of operation. Considering the impacts of shipping noise on the marine environment and the prospected further increase in the number of vessels, it appears imperative that regulations must be put forth to either minimise the amount of noise released (by engineering quieter engines, strict maintenance rules for propellers, etc), or redesigning shipping lanes in such a way that sensitive and critical biological habitats be avoided. (Jasny 2005). To initiate a regulatory process, in 2008 the International Maritime Organisation included underwater noise as a new item on the agenda of its Marine Environment Protection Committee (MEPC): Agenda Item 19 - “Noise from commercial shipping and its adverse impact on marine life” Pile driving is the act of hammering steel monopiles that support wind turbines into the seafloor. These impulsive hammerings generate a considerable amount of underwater-radiated noise, which raise concerns for its impacts on marine life. (de Jong&Ainsley 2008)

Underwater noise has most impact when it concerns intrusive, high intensity, low frequency sound sources, like pile driving. Shipping noise however, is of concern because it is continuous and covers a broader region. In the case of shipping, it is harder for fish and other animals to escape background noise. Most of the energy is concentrated in the low frequencies. (Jasny 2005)

Harbour noise sources and impact on marine life

Shipping

Most information available dealing with underwater noise document effects on whales for the most part. Some animals were observed to avoid ships, while others seem attracted to them. Some studies have shown certain whale species (like the humpback or grey whale) to displace from their natural habitat, where shipping had increased substantially.

The greatest concerns lie in shipping noise’s potential for masking. (Jasny 2005) Only single studies have reported the effects of shipping noise on fish, and the results are summarised below.

One study by Scholik and Yan investigated the effects of boat engine noise on the auditory sensitivity of the fathead minnow. The fathead minnow is a hearing specialist with a wide frequency range and low hearing threshold. Small boats have been reported to produce sound pressure levels exceeding 175 dB re 1 microPa, and are thus capable of elevating auditory thresholds in fish. Vessels and large boats are of concern because of their vast distribution around the globe and their increasing number. Very little research has been done on sounds resulting from increasing shipping activity (propeller, engine and cavitation noise). Scholik and Yan found that a boats’ noise spectrum ranged from 0.3 to 6.0 kHz (this falls within the fish’s hearing range). Auditory thresholds of the fish were found to be significantly elevated. The authors also found a significant loss of hearing ability of the fish after exposure to the boat noise.

Previous studies also looked at how loud sounds must be to induce startle responses. One study looked at the startle responses of the Pacific herring to sudden changes in vessel speeds. The authors concluded that the abrupt temporal changes of sound characteristics when a boat suddenly changes speed, elicit most startle responses. Other higher powered vessels were found to elicit fleeing response in several cyprinid fish species, with noise levels of 125 dB re 1 microPa.

These results raise the concern of not only the effects on the fathead minnow or the other species used in previous studies, but also what this means for potential damage to other fish species with similar hearing ranges Since the fathead minnow has similar auditory thresholds as the goldfish and many other cyprinid species it seems very likely that boat engine noises have similar effects to these species. (Scholik and Yan 2002) Some studies were also conducted with research vessels and their effects on fish avoidance behaviour and fish abundance estimation. Results were that low frequency noise causes fish to avoid the vessel. (Mitson and Knudsen 2003).

Increased background noise

The greatest threats posed by underwater noise impacts come from high intensity sound sources, like pile driving in wind farm construction. Lower intensity sound sources, like shipping and aquaculture facility pumps, are more continuous and cover a broader region. In this case, it is harder for a fish to avoid the background noise. Some studies have shown that long term exposure to background noise have more effects on hearing specialists than on hearing generalists. Hearing specialists have anatomical structures that enable them to better detect lower Sound Pressure Levels (SPLs), and are therefore more susceptible to temporary hearing loss than fish lacking these specialisations. These results cautiously suggest that the amount of hearing loss occurring in fish is related with the SPL and the fish’s hearing threshold. Hearing specialists therefore have a lower threshold and show more hearing loss than fish with higher thresholds, or generalists. (Popper & Hastings 2009)

Pile driving

A study in 2001 examined fish that died from underwater sound resulting from pile driving operations. Several fish species were held in cages at different distances from the source. According to the results, mortality was caused by pile driving exposure sound. Several species were found dead within 50m from the pile-driving source. Injuries found included damage to the swim bladder and bleeding. The extent of the damage was usually greater for the cages in closer proximity to the source, but the number of fish used was relatively low.

In 2004 a study monitored caged fish again during a seismic safety project, where fish were exposed to pile driving sounds at distances ranging from 23m to 314m from the source, and where duration to exposure varied between 1 min and 20 mins. Control fish were used placed in cages at the same locations as the test animals, only with absence of pile driving exposure sound. The authors reported more injury in the fish exposed to the pile driving operations and the same low level of damage in the controls. Lower levels of injury were observed to the fish exposed to the sound, but with presence of an air bubble curtain. This last statement was not statistically proven due to the small sample size used.

A similar study investigated the effects of pile driving on the salmon, O. kisutch. The fish were placed in cages and once again at different distances from the pile driving operations. Sound levels were measured, and the near cages had SPL that reached 208 dB re 1 microPa. Cumulative SEL reached 207dB re microPa during a 3-4hr period. Control cages were also used, that were kept far away from the pile driving region and where the sound levels did not exceed ambient levels.

The authors saw no mortality in either group, and no difference in external and internal anatomy was observed between the two groups. (revPopper&Hastings 2009)

The question that remains is whether each pile driving strike is a completely separate event in terms of damage to fish or if the problem lies in its cumulative damage resulting from multiple pile strikes. (popper & hastings 2009)

Dredging

Dredging is the act of picking up and transporting sediment with machinery on a vessel, and this machinery can produce noise underwater. Noise can be the source of either machinery noise, or noise of the scraping of the transported sediment.

Information on animal behavioural reactions to dredging is scarce, all is known is that the noise is audible to cetaceans and behavioural reactions should be expected at close range to the source. (Nedwel and Howell 2004)

Management recommendations

Several mitigation attempts have been made to reduce shipping noise. The design for quieter shipping technologies needs to be stressed. Techniques have been designed to mitigate propeller cavitation. The blades can be built in such a way that they are skewed and with special contour details, that will keep the blades from vibrating. Another possible design is to adjust the pitch of the blades for different loads, in that way requiring less power to operate. Maintenance is also important: scraping propeller blades, from, for example, barnacles can significantly decrease cavitation. Maintenance protocols could be a measure set to mitigate ocean noise from ships.

Engine noise can also be reduced. Most current diesel engines still radiate a large amount of acoustic energy. Using an electric generator would be a quieter option. On smaller ships, another feasible option would be to insulate the diesel engine with, for example, isolation mounts or damping tiles. The final strategy would be to redirect shipping traffic away from important biological habitats. A monitoring programme could be installed to evaluate which areas are of biological significance for fish and marine mammals and redirect shipping lanes away from these areas. (Jasny 2005)

As for pile driving and other sources, management options would be to use different alternatives to pile driving (ie gravity based structures) or use mitigating measures such as air bubble curtains.

References:

Jasny, M. (2005) Sounding the depths: The rising toll of Sonar, Shipping and Industrial ocean noise on marine life. Natural Resources Defence council. De Jong, CAF, Ainslie MA. (2008) Underwater radiated noise due to the piling for the Q7 Offshore wind park. Mitson, RB. Knudsen HP. (2003) Causes and effects of underwater noise on fish abundance estimation. Aquatic living resources, 16. 255-263. Nedwell, J., Howell, D. (2004) A review of offshore windfarm related underwater noise sources. Tech. Rep. 544R0308, Prep. by. Subacoustech Ltd., Hampshire, UK, for: COWRIE Popper, AN. Hastings, MC. (2009) The effects of human generated sound on fish. Integrative Zoology. 4: 43-52. Popper, AN. Hastings, MC. (2009) Review Paper: The effects of anthropogenic sources of sound on fishes. Journal of Fish Biology. 75: 455-489. Scholik, AR. Hong, YY.(2002) Effects of boat engine noise on the auditory sensitivity of the fathead minnow, Pimephales promelas. Environmental Biology of Fishes 63: 203-209.