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There are natural sources of noise in the ocean in the form of wind, ice, waves and earthquakes. However over the last century there has been a significant increase in the level of noise in the ocean. The locations of this noise also tend to be along well travelled routes in the ocean and along critical coastal and continental shelf habitat (Weilgart, 2007). There is increasing concern that this degraded acoustic environment is having a negative effect on marine mammals that inhabit these areas. These potential effects have only recently become a research priority (approximately in the past 20 years) and hence there are still significant gaps in knowledge (Gill et al, 2010). However the evidence that ocean anthropogenic noise is having a detrimental effect on whales in mounting.
Anthropogenic noise can come in the form of shipping traffic, research activities, military sonar, machinery and propeller noise, acoustic deterrent devices (ADD), oil and gas exploration and production, marine wind farms and dredging (Sultana and Zhi, 2007). Sound travels very effectively underwater, so the potential area impacted can be thousands of square kilometres or more (Weilgart, 2007). Noise pollution tends to be both high intensity and acute or lower level and chronic. In some areas it is likely that both these noise events may be happening simultaneously. Traffic noise tends to be chronic low frequency noise. This has resulted in an overall increase in background noise by 10-20Db (Cummings and Brandon, 2004). Air guns used in seismic surveying produce noise pulses with very high peak levels (Salter and Wallace, 2003). Underwater nuclear tests and ship-shock trials although conducted rarely, produce the highest overall sound pressure levels, sufficient to cause physical damage (Weilgart, 2007).
Effects of Noise
Whales communicate with each other via sound. It is their principle sense (Weilgart, 2007). Toothed whales use echolocation to visualise their landscape and monitor prey. They also engage in complex vocalizations exchanges. Baleen whales use long range acoustic communication systems to initiate mating and social interaction. There are a range of effects of increased ocean noise on whales and depend on the type of noise and the species. Whales may encounter feeding and breeding problems, abandonment of desired habitat, stress, damage of tissue and organs, hearing loss, behavioural changes, masking and communication problems and even stranding (Sultana and Zhi, 2007).
One of the major issues with increasing noise is the potential masking of important natural cues (Richardson and Würsig, 1997). Whales used sound from natural sources to detect prey, navigate and avoid predation. When background noise increases, it may decrease a whale's ability to pick up these natural cues.
Most of the evidence for hearing loss has come from beached populations of whales. 'Nearly 50% of beached whales show evidence of some form of auditory compromise or pathology that correlates with low to profound hearing loss' (Cummings and Brandon, 2004). Mann et al (2010) examined a stranded short-finned pilot whale and found profound hearing loss. High-intensity, impulsive blasts can damage cetacean ears (Ketten et al. 1993). Cetaceans exposed to high level mid frequency sonar have been found with haemorrhaging in the ears, lesions caused by bubble formation and/or bubble expansion in other tissue (Cummings and Brandon, 2004). However it is possible that other factors over the mammals' life may have caused or contributed to this, and there are limitations in what can be concluded from post mortem analysis.
Research has been conducted to try to estimate the hearing capacity of whales. In a study by Parks et al. (2007) morphometric analyses of 18 inner ears from 13 stranded North Atlantic right whales (Eubalaena glacialis) were used to development a preliminary model of the frequency range of hearing. Their estimated hearing range was 10 Hz-22 kHz (Parks et al., 2007). Previous research on the hearing of marine mammals has shown that these functional models are reliable estimators of hearing sensitivity of whales (Parks et al., 2007). However there can be high intraspecific differences so measurements from more specimens will be required to develop a more accurate model of the right whale hearing range. Hearing losses are classified as either temporary threshold shifts (TTS) or permanent threshold shifts (PTS). Since whales are extremely dependant on their acoustic sense, both TTS and PTS are considered serious (Weilgart, 2007). These hearing losses may have population consequences, through reduced echolocation and communication ability (Weilgart, 2007). Functional models allow you to predict specific response thresholds (TTS and PTS) for different noise types. However this has only been achieved for a limited number of species (Richardson and Würsig, 1997).
Behavioural responses of whales to noise are complex and poorly understood (Richardson et al. 1995) partly due to the difficulty in observing these animals effectively in the wild. Responses seem highly variable within species and between species. Gill and Tsoflias (2010) found no obvious responses by whales to a loud, anthropogenic, low frequency sound. Some cetaceans seem highly tolerant to noise and remain in areas despite extensive human activity and noise (e.g. oil and gas production areas) (Richardson and Wursig, 1997, Madsen et al., 2002). However their continued presence doesn't necessarily confirm no effect. There may be sublethal effects that are impacting the animal in more subtle ways over long time scales. Other whales show greater sensitivity. Bowhead whales showed changes in surfacing and blowing rates up to 54-73km from an active airgun array (125 Db).
There are varying responses by whale species to vessel exposure. Migrating humpbacks demonstrate vessel avoidance at 4-5km (Frankel & Clark, 1998). While Minke whales in the Great Barrier Reef Area are attracted to vessels and maintain contact for prolonged periods (Mangott et al., 2011). Gordon et al. (1992) found that surface times of sperm whales when vessels were near tended to be reduced; with fewer blows per surfacing, reduced frequency of dives with raised flukes and shorter intervals between blows (Richardson and Würsig, 1997). Baleen whale responses to boats appear inconsistent (Richardson and Würsig, 1997). While Miksis-Olds et. al (2007) found that grass bed usage was negatively correlated with increased boat presence in the morning, possibly due to the associated noise.
Investigations in the stress responses to whales are limited. Blood samples from a white whale (Delphinapterus leucas) before and after varying sound exposure showed increased catecholamine levels with increasing sound levels and were significantly higher after high-level sound exposures, suggesting some potential stress. However Thomas et al. (1990) found no significant changes in catecholamine levels or behaviour after exposure to playbacks from an oil drilling platform (Romano et al., 2004). Captive beluga whales also showed no stress response to playbacks (oil rig sounds at 153 dB). However it is possible that wild belugas may respond differently. It is evident that extensively more research needs to be conducted in this area.
There is growing evidence that noise, particularly from airguns may be responsible for some mass stranding events (Simmonds et al., 2003). In the Sea of Cortex in 2002 strandings and deaths of whales coincided with seismic surveying in the area (Simmonds et al., 2003). This was also seen in the Bahamas in 2000, in Kryparissiakos Gulf in 1996, Canary Islands in 2002 and Madiera in 2000 (Salter and Wallace, 2003). In 2002, in Abrolhos Bank there was an unusual increase in the stranding rate of adult humpback whales. This coincided with the 3D seismic surveys conducted in the area (Engel et al., 2004). Seismic surveys use airguns and each pulse is highly intense and acute, however these surveys can take months to complete potentially making them more damaging. Beaked whales seem particularly sensitive to high-intensity sound exposure (e.g. seismic surveys). When exposed to such noise, they strand and die in mass (Weilgart, 2007). Beaked whales are toothed whales and hence rely on echolocation for orientation and feeding. Seismic activity can cause hearing defects and hence this could lead to their stranding (Mann et al., 2010). This suggests that there is varying sensitivity between species to these high-intensity noise events.
There has been a worldwide decline in tonal frequencies of blue whale songs (McDonald et al., 2009). One of the hypotheses for this trend is the increase in ocean noise however there is limited evidence at this stage to demonstrate this. Some studies have shown significant differences in whale vocal behaviour to sound exposure (Gill and Tsoflias, 2010). A significant number of sperm whales ceased calling in the Indian Ocean during seismic surveying (seismic pulses were 10-15dB above the background ambient noise and produced over 300km away). However other studies have reported no change in vocal patterns during seismic surveying (Gill and Tsoflias, 2010). Few studies have concentrated on this aspect of anthropogenic noise so there is a big knowledge gap in this area. These potential changes in vocal behaviour may lead to decreased foraging efficiency or mating opportunities and could have major population consequences (Weilgart, 2007).
It is evident that noise pollution is a threat to whales. However reactions vary between species, within species and depending on the type and level of noise. There is also variability in the results recorded (particularly behavioural studies). Although understanding on the effects of noise on whales has improved there are still significant knowledge gaps (Gill et al, 2010). Hearing ranges and thresholds are yet to be established for all whale species and effects of noise on each whale species still requires significantly more research and improved understanding. Short term studies (most of the literature) although important do not capture potential long term effects and population consequences. It is extremely important that we increase long term monitoring to understand whether any sublethal effects are impacting on forging ability, mating, predator avoidance or echolocation. Reduced hearing capacity, increased stress and/or masking could have lethal consequences over time (e.g. stranding of beaked whales).
Research also needs to focus on the synergistic effects of noise as many marine mammals are likely to be subjected to multiple stressors due to the large temporal and spatial scales that anthropogenic noise encompasses. The use of non-invasive monitoring methods need to be exercised where possible (e.g. aerial observation and systematic shore based tracking) when analysing behaviour so that results are not confounded (Wiirsig et al., 1991). Recent technological advancements in the form of acoustic tags are likely to make assessment and monitoring easier and some studies are already beginning to assess their usefulness in the field (Ward et al., 2011).
Ocean noise is only going to increase in the 21st century so it is important that we have a thorough understanding of how anthropogenic noise affects whales and other marine biota. We need to understand the scope of the effects both short term and long term on each species so that we can avoid unnecessary harm and implement the appropriate management strategies (e.g. safe exposure levels and ranges).