An extremely noisy ocean
Sound can travel for thousands of miles in the oceans, whether that be from natural sources of sound such as communication in animals or noises produced by anthropogenic activities such as shipping, seismic activity or renewable energy resources. Sound travels 4.5 times faster in water (1500 m/s) than in air and low frequency sounds, such as those produced by anthropogenic activities, are the fastest moving. The increase in the chaotic chorus of anthropogenic noise in the oceans is proving to be detrimental to the lives of marine mammals, such as Cetaceans. These mammals use vocalisations and hearing as their primary sense and rely on it for; hunting, navigation, group recognition, social behaviour, coordination and finding mates.
The Odontoceti (the toothed whales) are the most advanced when it comes to acoustics, they can produce echolocation clicks, which are highly specialised to detect objects and prey. They also produce pulse and tonal sounds which are used for communication. The detection of sound is thought to be through the fats and inner jaw of cetaceans rather than directly through the ear canal. The effects of anthropogenic noise on these animals can cause permanent ear damage, displacements and stranding’s.
Noise is in the ocean is a huge conservation problem as sounds produced by anthropogenic activity can have global affects. Shipping is one of the biggest noise sources and other contributing factors include seismic activity and in the installation of offshore wind farms.
The behaviour of the North Atlantic right whale (Eubalaena glacialis), a critically endangered species, has been influenced by shipping activity. Within their lifetime, they have had to shift their communication frequency up to vocalise over shipping noise, as seen in figure 1. It has also been found that they are vocalising less when there in an increase in shipping activity. This reduced calling behaviour is presumed to be due to a realisation of the other communicators inability to hear over the noise, however another possibility is that it is a stress induced response.
Seismic activity includes the use of explosions in order to obtain oil and gas for human usage. The frequency bands from air guns and explosions have been found to be the same as blue whale calls. Therefore seismic activity has influenced and disturb the behaviour of the Blue whale. When seismic activity has coincided with important time periods such as feeding and social encounters, the Blue whale has had to compensate to the elevated noise by increasing calling (Figure 2). A visualisation of this for humans is the behaviour of shouting in a club over the loud music.
Strandings occur due to noise which is often associated with naval activity. Proving that these stranding’s are due to naval activity can be difficult. However thought post-mortem there are obvious signs of trauma such as bubble lesions which are thought to be due to reverberation of air filled lungs i.e. too much reverberation bursting blood vessels. Further physical damage can include decompression sickness in deep divers as they become stressed and so rise to the surface too quickly. Other animals are found stranded with no indication of sickness around areas of naval activity. As an example, during a seven minute use of sonar on a US navy vessel, 34 short finned pilot whales, one minke whale and two pigmy sperm whales were stranded in North Carolina in 2005.
Renewable energy- offshore wind farms
Increasing demand for offshore wind renewable energy has risen in popularity over the past thirty years. Offshore wind farms are known to cause disturbance to marine mammals and the most disturbance occurs during the construction phase as opposed to the operational phase. The construction phase involves the use of pile- driving which can have sever implications to marine mammals. Injury induced by the noise can cause temporary and permanent threshold shift and stress (neurological disturbance). The acoustic disturbances during the construction phase can also cause the marine mammals to show avoidance behaviour especially during activities such as drilling and dredging. Further, operation noises can also mask the hearing frequencies of marine mammals which can reduce their ability to detect vital signals such as communication in the North Atlantic right whale. Displacement due to sound can also detrimentally affect mating opportunities of marine mammals and calf survival.
Although it seems doom and gloom that marine mammals are so heavily affected by the noises produced by man in the ocean, there are mitigation methods in order to protect these creatures. Some of these methods include, reduced seismic activity in the presence of marine mammals and the use of bubble curtains to reduce the sounds produced by the construction of offshore wind farms.