In light of our up and coming Dunbar Science Club workshop, ‘Communication in marine mammals’ (www.dunbarscienceclub.org.uk), it seems appropriate to address the topic of marine mammals and noise.
Marine mammals rely on sound to survive, using it to navigate, explore, hunt, communicate, locate lost conspecifics and find mates.
The ocean is a noisy environment, filled with both natural and manmade noise. Natural sources can be biological (e.g. marine mammals, snapping shrimp), or physical (e.g. precipitation, earthquakes, lightning strikes), whilst manmade sounds include unintentional noise, such as that produced by vessels, and intentional noise, for example from seismic surveys, or military SONAR.
Effects of manmade noise on marine mammals are not easy to determine, but depend upon a number of factors including characteristics of the sound source, background noise levels, sound propagation paths, water depth and hearing sensitivities of the individual receiving the signal. Substantial amounts of research have been carried out but results are mixed, and often conflicting. Reactions often go unobserved, and when they are, it is difficult to pinpoint a single source of disturbance, as many activities are likely occurring at the same time.
Potential impacts of high intensity sound include permanent or temporary damage to the auditory system, known as Permanent Threshold Shift (PTS) or Temporary Threshold Shift (TTS). Temporary Threshold Shift is a temporary increase in hearing threshold (i.e. the minimum intensity a sound can be before it is undetectable). Given that it is temporary, any effects should be short lived. Permanent Threshold Shift is a permanent increase in hearing threshold, and will impact upon an individual’s ability to detect sound. High intensity mid frequency naval SONAR has also been linked to mass strandings of deep diving beaked whales.
Lower intensity sounds are unlikely to damage hearing, but can affect behaviour. Observed reactions include altered dive/surfacing patterns, change in vocalisation characteristics and production rates, disturbance of feeding, and avoidance. Avoidance can be localised and short-lived, with animals returning to the area once the sound source has seized, or it can be longer term. Noise created over a prolonged period (e.g. construction noise) could cause habitat displacement, but cause and effect are difficult to determine. Stress is also a possibility when marine mammals are exposed to noise for prolonged periods. In addition, noise that overlaps with the frequency of marine mammal vocalisations can result in masking, where the reception of one sound is hindered by the presence of another. For example, low frequency vessel noise can reduce the area over which baleen whales can communicate with their low frequency calls.