Increasing attention is being paid to the ecological consequences of underwater noise generated by human activities such as shipping and maritime industries including, but not limited to, oil and gas exploration and extraction, sonar systems, dredging and the construction of offshore renewable energy devices. There is particular concern over the extension of these activities into previously undeveloped areas of the oceans, including Polar Regions and areas of coral reef habitat. Most of the concern by regulators and others has focussed upon effects upon marine mammals and other protected species. However, examining the impacts upon the overall ecology of affected habitats is also important as it may be dominated by effects upon the far larger biomasses of fishes and invertebrates, which do not have the same degree of legal protection. Many of these assessments of the impact of noise on fishes and invertebrates have overlooked important issues, including the sensitivity of a substantial proportion of these species to particle motion rather than sound pressure. Attempts have been made to establish sound exposure criteria setting regulatory limits to the levels of noise in terms of effects upon mortality levels, injury to tissues, hearing abilities, behaviour, and physiology. However, such criteria have almost exclusively been developed for marine mammals. Criteria for fishes and invertebrates have often had to be assumed, or they have been derived from poorly designed and controlled studies. Moreover, the metrics employed to describe sounds from different sources have often been inappropriate, especially for fishes, and invertebrates, as they have been based on sound pressure rather than particle motion. In addition, the sound propagation models employed to assess the distances over which effects might occur have seldom been validated by actual measurements and are especially poor at dealing with transmission under shallow water conditions, close to or within the seabed, or at the surface. Finally, impacts on fish and invertebrate populations are often unknown and remain unassessed. This paper considers the problems of assessing the impact of noise upon fishes and invertebrates and the assessment procedures that need to be implemented to protect these animals and the marine ecosystems of which they form an integral part. The paper also suggests directions for future research and planning that, if implemented, will provide for a far better scientific and regulatory basis for dealing with effects of noise on aquatic life.

Directional hearing may enable fishes to seek out prey, avoid predators, find mates, and detect important spatial cues. Early sound localization experiments gave negative results, and it was thought unlikely that fishes utilized the same direction-finding mechanisms as terrestrial vertebrates. However, fishes swim towards underwater sound sources, and some can discriminate between sounds from different directions and distances. The otolith organs of the inner ear detect the particle motion components of sound, acting as vector detectors through the presence of sensory hair cells with differing orientation. However, many questions remain on inner ear functioning. There are problems in understanding the actual mechanisms involved in determining sound direction and distance. Moreover, very little is still known about the ability of fishes to locate sound sources in three-dimensional space. Do fishes swim directly towards a source, or instead “sample” sound levels while moving towards the source? To what extent do fishes utilize other senses and especially vision in locating the source? Further behavioral studies of free-swimming fishes are required to provide better understanding of how fishes might actually locate sound sources. In addition, more experiments are required on the auditory mechanism that fishes may utilize.

Hearing, as defined by numerous investigators of vertebrate hearing, involves not only detection of a sound, but the ability to respond to the sound behaviourally. Thus, in order to understand what a fish (or any animal) hears, it is important to use behavioural methods that allow an animal to “tell the investigator” which sounds are detected and which are not. However, since the 1980’s, most studies that suggest they are measuring fish hearing have used electrophysiological approaches to measure the response of the ear or lower levels of the central auditory system to sounds. In this paper, we argue that these electrophysiological measures do not represent hearing since they do not indicate what sounds a fish can potentially respond to behaviourally. Thus, while the electrophysiological measures provide some useful and interesting insights into fish sound detection mechanisms, the most valid measures of hearing sensitivity, and understanding which sounds (anthropogenic or otherwise) that fish are able to respond to, are obtained using behavioural studies. We also argue that these studies must be done in carefully designed acoustic setups so that the nature of the stimulus to which the fish responds is appropriate and well understood.

Airguns used for offshore seismic exploration by the oil and gas industry contribute to globally increasing anthropogenic noise levels in the marine environment. There is concern that the omnidirectional, high intensity sound pulses created by airguns may alter fish physiology and behaviour. A controlled short-term field experiment was performed to investigate the effects of sound exposure from a seismic airgun on the physiology and behaviour of two socioeconomically and ecologically important marine fishes: the Atlantic cod (Gadus morhua) and saithe (Pollachius virens). Biologgers recording heart rate and body temperature and acoustic transmitters recording locomotory activity (i.e. acceleration) and depth were used to monitor free-swimming individuals during experimental sound exposures (18–60 dB above ambient). Fish were held in a large sea cage (50 m diameter; 25 m depth) and exposed to sound exposure trials over a 3-day period. Concurrently, the behaviour of untagged cod and saithe was monitored using video recording. The cod exhibited reduced heart rate (bradycardia) in response to the particle motion component of the sound from the airgun, indicative of an initial flight response. No behavioural startle response to the airgun was observed; both cod and saithe changed both swimming depth and horizontal position more frequently during sound production. The saithe became more dispersed in response to the elevated sound levels. The fish seemed to habituate both physiologically and behaviourally with repeated exposure. In conclusion, the sound exposures induced over the time frames used in this study appear unlikely to be associated with long-term alterations in physiology or behaviour. However, additional research is needed to fully understand the ecological consequences of airgun use in marine ecosystems.