Bottlenose dolphins are widely distributed in the North Atlantic, West African, Mediterranean and UK coastal waters. They occur predominantly in two areas in UK inshore waters – Cardigan Bay and the Moray Firth. The population in the inshore waters of the UK is probably between 300 and 500 individuals.

A fully-grown bottlenose dolphin is a large and formidable adversary, its only predators being killer whales and sharks which tend to take only the small or sick among the group. Studies have shown that most coastal bottlenose dolphins live in small groups of about 10 individuals, and are restricted to a 'home range'. The size of this 'range' varies, however, from population to population in different areas of the world.

When the 'home range' is in areas of human habitation, solitary dolphins frequently encounter divers, sailors and fishermen. After a while, they accustom themselves to human presence, and become very friendly and approachable. Tour operators tend to make the most of these encounters. Paying visitors may be inclined or encouraged to touch and feed the dolphins and this behaviour, if not monitored, may have long term effects on the dolphins’ natural instincts to return to the group.

The sensitivity of the bottlenose dolphin depends largely on the dolphins’ choice of home range, the activities which take place in the area, and the water quality. The species will tend to inhabit areas where there is adequate food and clean, hazard free waters in which to swim. Heightened sensitivity to recreation may result from a decline in water quality caused, for example, by increased sewage discharge or levels of marine litter. This could have a direct influence on the lower trophic species within the dolphins’ food chain, with the knock on effect of driving the dolphins’ favoured higher trophic species out of the vicinity, subsequently followed by the dolphins themselves. Discarded nets and fishing debris are also prime examples of potential hazards to dolphins.

The potential for ‘noisy’ activities to disturb marine mammals such as bottlenose dolphins has not been sufficiently quantified to reach a generalised conclusion. However, animals which stay in an area despite disturbance, possibly because of a lack of an alternative site, may suffer from elevated levels of stress. While able to ‘cope’ with such stress in the short term, repeated exposure could have long term impacts on the animal.

A useful conceptual framework for the management of noise and disturbance is a ‘zone of influence’ model. This specifies the various ranges at which a particular sound source might be expected to illicit a certain level of response. The sound thresholds at which these zones are set depend on the frequency components of the sound and the frequency sensitivity of the animals concerned. In addition, the actual ranges at which these threshold sound levels will be reached, depend on the intensity and frequency components of the sound source and the propagation conditions in the area.

For bottlenose dolphins both the horizontal range from the source of the noise and the depth of the receiver (underwater range) should be taken into account. However, fundamental data on the effects of recreational noise disturbance to marine mammals is sparse, and the understanding of the long term effects of noise-making activities is far from comprehensive.

The tables below summarise the potential threats to bottlenose dolphins from land and waterborne processes.

Water based processes

Land based processes

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