Acoustic pollution

In addition to all the other influences, noise can also be a source of pollution for the sea life. In the world under water, sound plays a different, and sometimes more important role below the sea surface than above. From recent studies, it has been shown that noise from ship motors and shock waves produced by geological research sometimes influence sea life more than man has realized up till now.

Under normal circumstances, it is very quiet under the water surface. Many marine animals such as crustaceans, fish and marine mammals produce all kinds of sounds which add up to a noise level of around 40 decibels.

Sound behaves differently in water than in the air. It travels 5 times more quickly and carries much further due to the lack of obstacles. Many marine animals make grateful use of this. They use sound (and especially echoes) for tracking prey, recognizing obstacles in the dark and for their mutual contact. The tooth whales (dolphins, porpoises and sperm whales) are grand masters in this: they live in a world of sound, just like man lives in a visual world, and dogs mostly in a world of odours. In addition, baleen whales also communicate with each other using sound signals over enormous distances.

Acoustic mist

The tranquillity found under water is sometimes disrupted by natural situations. Especially in coastal areas, a considerable storm or a heavy rainfall can lead to an increased noise level up to 75 decibels. It has been shown that dolphins migrate to deeper waters under these conditions. This seems logical since they are not able to orientate as well with all the intensified racket, nor can they probably communicate as well. This underwater noise is referred to as 'acoustic mist'.

Human sources can also cause acoustic mist. Oceanographer Morris performed sound measurements in Cardigan Bay (in the Irish Sea). Under normal circumstances, Cardigan Bay is a quiet area (up to 50 decibels), however during the weekends and vacations between May and September, the local aquatic sports (in particular, the motor boats) increase the background level up to 80-90 decibels. In these periods the dolphins, which are normally observed close by the Cardigen Bay coast during the day, only come to this area during the night to hunt in the fish-rich coastal waters.

porpoise: victim of acoustic mist?

It is known that humpback whales are affected by noise from sonar, which is used by submarines for navigation. During mating season, the males 'sing' ballads in order to attract the females. When scientists play a tape with sonar sounds during the singing of the whales, then it would take 30% longer to sing the song. The scientists think that this could disturb the mating ritual.

Low frequency sonar

Nowadays, submarines are practically untraceable with conventional sonar instruments. However, a special low-frequency system (LFA SURTASS) can trace submarines by sending out exceptional penetrating signals. A region in the sea in the vicinity of the Canary Islands is used by navy ships from the NATO to practice regularly with these powerful sonar signals. During these practices, it is a matter of course that beaked whales wash ashore on the beaches of the islands. Their ears have been damaged and they have deadly air bubbles in their blood veins (Caisson disease). Scientists discovered in 2004 that beach whales often show signs of Caisson disease. It seems as if whales are frightened by disturbances and then rise too rapidly, get the Caisson disease and end up stranding.

Similar practices are occurring in America. The American environmental organization National Resources Defence Council had indicated that an unnatural number of beachings were occurring when the navy tested the LFA sonar system. Therefore, a judge declared that the American navy must avoid dolphins and whales.

Sound explosions

Up till 1965, one used explosives during (seismic) studies of the constitution of the sea bottom when searching for oil and gas. By analysing the behaviour of the shock waves in the bottom, one could draw conclusions concerning the various layers in the earth. However, the shock waves did not just penetrate the bottom. They led to such massive numbers of death among the fish in the water column that other alternative methods had to be found.

The most often used method as of 2000 works with air guns. These produce a series of powerful yet low frequency sound waves, which are easy to direct according to the oil companies. They claim that in the direct vicinity of the research, seismic sound waves are hardly perceptible nor are the frequencies used (less than 1 kHz) disturbing for the marine mammals. Critical oceanographers such as Morris believe otherwise. It is known that the bottlenose dolphin hears noises as low as 75 Hz (0.075 kHz). Porpoises also make low frequency noises: their 'grunt' is the reason the Germans call them "Schweinswal" ('pig whale'). Studies made at the Dutch Dolfinarium in Hardewijk (reported in the book by van Nachtigall et al. in 1995) showed that the low frequency sound sources exercise a special attractive force on porpoises: they show tendencies to move towards the source.

Seismic research also causes some acoustic effects in the wide surroundings of the research vessels. In 1992, Morris measured an increase in the noise level from 40 to 76 decibels at a distance of 5 sea miles from a seismic research vessel. During another study in 1994, the noise from the air guns was measurable up to 80 sea miles from the ship. During the last study, very few marine animals and seabirds were observed in the vicinity, when there are normally high numbers seen. Morris mentions in his article published in the North Sea Monitor of September 1995 that there are plans for seismic research with even more powerful sound waves. He suggests that further studies of the effects from this research should be made.

The strength of the sound at the source during normal seismic research lies between 230 and 250 decibels and the shock waves have a force of 0.5 and 12 million Watt. This can increase to 100 million Watts in the future. In comparison: the sound waves produced by a minke whale (the sea animal which produces the most noise), used for communicating throughout the world seas, are around 30 Watts strong with a strength of 150 dcibels.

Racket in the North Sea?

The environmental quality in the North Sea is intensively kept track of with the help of measurement networks. But these only measure the concentrations of materials. Sound measurements are not systematically measured. But noise, particularly in the southern North Sea, could play an important determining factor in the underwater environment. It is one of the busiest navigational regions in the world. The area is rich in ground minerals which are regularly being searched for with seismic instruments. Recent studies of the distribution of cetaceans in the North Sea have shown that porpoises and white-nosed dolphins are relatively common in the North Sea, but avoid the southern trough. In earlier years, porpoises and dolphins (bottlenose dolphins) were a normal sight along the Dutch coast, particularly when the herring and anchovies swam past. The herring still swim along the coast but the dolphins and porpoises are rarely sighted. A study of sound levels in the southern North Sea would give more insight as to why these animals stay away.

Weblink

More information about low-frequency sonar:
http://www.surtass-lfa-eis.com/

Chemical and acoustic pollution of the North Sea:
http://www.ngo.grida.no/wwfneap/Projects/Reports/PDF_Maps/North_Sea_offshore_poll.pdf

Source: de Vleet, Ecomare