ICES AGISC 2005 ICES Advisory Committee on Ecosystems ICES CM 2005/ACE:06 Report of the Ad-hoc Group on the Impacts of Sonar on Cetaceans and Fish (AGISC) (2nd edition) By correspondence International Council for the Exploration of the Sea Conseil International pour l’Exploration de la Mer H.C. Andersens Boulevard 44-46 DK-1553 Copenhagen V Denmark Telephone (+45) 33 38 67 00 Telefax (+45) 33 93 42 15 www.ices.dk [email protected] Recommended format for purposes of citation: ICES. 2005. Report of the Ad-hoc Group on Impacts of Sonar on Cetaceans and Fish (AGISC) CM 2006/ACE:06 25pp. For permission to reproduce material from this publication, please apply to the General Secretary. The document is a report of an Expert Group under the auspices of the International Council for the Exploration of the Sea and does not necessarily represent the views of the Council. © 2005 International Council for the Exploration of the Sea ICES AGISC report 2005, 2nd edition | i Contents 1 Introduction...................................................................................................................................1 1.1 Participation...........................................................................................................................1 1.2 Terms of Reference...............................................................................................................1 1.3 Justification of Terms of Reference.......................................................................................1 1.4 Framework for response........................................................................................................2 1.5 Overview by the chair............................................................................................................2 1.6 Acknowledgements...............................................................................................................3 2 Physical background.....................................................................................................................4 2.1 The nature of sound...............................................................................................................4 2.2 Units for measuring sound.....................................................................................................4 2.2.1 Use of the decibel scale in water...............................................................................4 2.3 Parameters for estimating noise.............................................................................................4 2.3.1 Source level..............................................................................................................5 2.3.2 Impulsive sound........................................................................................................5 2.3.3 Sound propagation and transmission loss.................................................................6 2.4 Ambient noise........................................................................................................................7 2.5 Sonar in general.....................................................................................................................8 2.5.1 Low-frequency sonar................................................................................................8 2.5.2 Mid frequency sonar.................................................................................................9 2.5.3 High frequency sonar................................................................................................9 3 Biological Background - Cetaceans............................................................................................10 3.1 Hearing in cetaceans............................................................................................................10 3.1.1 Anatomy and physiology........................................................................................10 3.1.2 Hearing in smaller odontocetes...............................................................................11 3.1.3 Hearing in mysticetes.............................................................................................12 3.2 Potential effects of sound on cetaceans...............................................................................12 3.2.1 Direct damage to hearing........................................................................................12 3.2.2 Non-auditory tissue damage...................................................................................13 3.2.3 Masking and changes in vocal behaviour...............................................................15 3.2.4 Behavioural reactions.............................................................................................16 4 Cetaceans and sonar....................................................................................................................17 4.1 Marine mammals.................................................................................................................17 4.2 Beaked whales.....................................................................................................................17 4.2.1 Review of literature on effects of sonar on beaked whales.....................................20 4.2.2 Case study: Greece.................................................................................................22 4.2.3 Case study: Bahamas..............................................................................................25 4.2.4 Case study: Canary Islands.....................................................................................27 4.3 Other cetaceans and sonar...................................................................................................28 4.3.1 Research on LFA and cetaceans.............................................................................28 5 Mitigation measures for cetaceans.............................................................................................29 5.1 Introduction.........................................................................................................................29 5.2 Control at source..................................................................................................................30 5.3 Mitigation of death and injury caused by the direct effects of sound..................................30 5.3.1 Marine Mammal Observers (MMOs).....................................................................31 ii | 5.3.2 Passive Acoustic Monitoring (PAM) or Active Acoustic Monitoring (AAM)....................................................................................................................31 5.4 Other control methods.........................................................................................................31 5.4.1 Scheduling..............................................................................................................31 5.4.2 Warning signals......................................................................................................32 5.5 Monitoring...........................................................................................................................32 5.5.1 Noise monitoring....................................................................................................32 5.5.2 Marine mammal observation..................................................................................32 5.6 Mitigation measures in use for military sonars with regard to marine mammals................33 5.6.1 Guidelines for sonar research testing by NATO and marine mammal risk mitigation research at the NATO Undersea Research Centre..........................33 5.6.2 Mitigation on UK naval vessels or in UK sonar tests.............................................33 5.6.3 Mitigation on Australian naval vessels...................................................................35 5.6.4 Mitigation on Italian naval vessels.........................................................................35 5.6.5 Mitigation on US naval vessels during training and during exercises....................36 5.6.6 Future mitigation development...............................................................................36 6 Summary of gaps in understanding for marine mammals.......................................................36 7 Other relevant items....................................................................................................................37 7.1 Noise pollution as a more serious problem?........................................................................37 8 General conclusion for marine mammals..................................................................................38 9 Fish 40 9.1 Biological background.........................................................................................................40 9.1.1 Hearing in fish........................................................................................................40 9.1.2 Impact of sonar on fish...........................................................................................45 9.1.3 Mitigation measures for fish...................................................................................47 10 Recommendations........................................................................................................................47 10.1 Future investigations and research for marine mammals.....................................................47 10.2 Future research on fish and sonar........................................................................................48 11 References....................................................................................................................................48 ICES AGISC report 2005, 2nd edition | 1 1 Introduction This is the second edition of our report. A first edition, covering effects on physical background, sonar usage and effects and mitigation in relation to marine mammals was produced early in 2005. This second edition contains improved sections on physical background and mitigation for marine mammals, and an entirely new section on fish. 1.1 Participation The following members of the Ad hoc Group on the Impact of Sonar on Cetaceans and Fish (AGISC) participated in producing this report (see Annex 1 for addresses). Tony Hawkins UK Finn Larsen Denmark Mark Tasker (chair) UK Chris Clark USA Antonio Fernández Spain Alexandros Frantzis Greece Roger Gentry USA Jonathan Gordon UK Tony Hawkins UK Paul Jepson UK Finn Larsen Denmark Jeremy Nedwell UK Jacob Tougaard Denmark Peter Tyack USA Tana Worcester Canada 1.2 Terms of Reference At the MCAP meeting January 2004, an Ad hoc Group on the Impact of Sonar on Cetaceans and Fish (AGISC) was established and was given the following terms of reference: i. Review and evaluate all relevant information concerning the impact of sonar on cetaceans and fish; ii. Identify the gaps in our current understanding; iii. Prepare recommendations for future investigations and research; iv. Prepare draft advice on possible mitigation measures to reduce or minimize the impact of sonar on cetaceans and fish. 1.3 Justification of Terms of Reference The terms of reference derive from a letter from Catherine Day (Director General of EC DG Environment) to David Griffith (General Secretary, ICES), dated 25 September 2003. In this letter, the European Commission indicated that it had for some time received complaints about the impact of sonar on marine mammals. These complaints claimed that the emission of intense, low and medium frequency tone bursts has a disturbing effect on cetaceans. Information had also been forwarded indicating that these sonars might have an impact on fish and fish behaviour. European legislation (mainly the Habitats Directive (92/43/EC)) requires Member States of the European Union to take measures to establish a system of strict protection for all cetaceans in European waters. The European Commission does not have a comprehensive and 2 | ICES AGISC report 2005, 2nd edition authoritative review of information concerning the impact of sonar, and thus finds it difficult to develop a clear position on the issue. The Commission therefore asked ICES to undertake a scientific review and evaluation of relevant information concerning the impact of sonar on cetaceans and fish, to identify the gaps in current understanding and to make recommendations for future investigations or research. The Commission is also interested in advice on possible mitigation measures to reduce or minimise the impact of sonar on cetaceans and fish. 1.4 Framework for response The Group’s response to these terms of reference has been compiled by correspondence. Sections were initially drafted by Group Members and then agreed by circulation to all members. Much of the report is based on existing review literature (not all relating to sonar directly), updated and amended as appropriate. 1.5 Overview by the chair The effects of human inputs to natural systems have been a topic of interest and study for many years, however much the greatest amount of work has been carried out on chemical inputs, both in the form of contaminants and nutrients. The subject of energy input has historically received much less attention. The anthropogenic input of sound to the marine environment started with the coming of mechanically propelled ships, but until the advent of sonar, nearly all sound input was a by-product of another activity as opposed to deliberate. Both forms of input though carry the risk of affecting other marine life. Evidence has been available for some time that anthropogenic noise has the capacity to disturb those forms of marine life dependent on sound for communication and sensing in the seas. Much less evidence has been available on damage, injury or lethal consequences at the individual level, and none at the population level. A series of incidents in recent years when certain deep- diving whale species stranded or died co-incident with the use of high-powered sonar alerted many more to the risks posed by sound. Research elsewhere indicated that other forms of loud sound might affect fish. The behaviour of sound in the marine environment is complex and is equally complex to describe. We have attempted to describe the physical background briefly in the first main part of this report, but are aware that this may be too brief and simplistic for some. We refer those readers to standard texts for further information. This section includes a brief description of the types of sonar in use today. It proved difficult to find information on the characteristics of many forms of sonar. The next section of the report deals with the mechanisms for hearing in cetaceans and describes the potential effects of sound on these mechanisms and the behaviour of these animals. Until recently, most concern has focussed on the effects on hearing and communication systems of cetaceans but recent evidence is indicating that damage may also be caused through other mechanisms, and perhaps indirectly through dangerous alterations in behaviour. There is however little experimental evidence currently in the public domain of the effects of sonar on the acoustic systems, physiology or behaviour of cetaceans. Logistically, any such experiments are easiest to conduct in the laboratory on individual animals, but extrapolating the few available items of information to wild populations is at present very difficult and uncertain. Section 4 reviews observations and deductions from cases where whales have stranded or been found dead in association with the nearby use of military sonar. As with many observational cases, obtaining the best and most pertinent evidence proved difficult both from the corpses and in other cases of strandings, from the military authorities. In order to improve ICES AGISC report 2005, 2nd edition | 3 deductions, we ideally need to complete three of four cells in a 2 x 2 matrix - naval operations occurred or not occurred versus marine mammal strandings occurred or not occurred. We have knowledge of some stranding events associated with naval operations, and possibly some information on the number of stranding events without naval sonar being present, but we do not know how many naval sonar operations occurred (in suitable beaked whale habitat) without any observed marine mammal strandings. It is though agreed that high-powered, mid- frequency sonar can affect beaked whales in particular. These effects can lead to death, either at sea or as a consequence of stranding ashore. These effects may be caused by a lethal behavioural change leading to physiological damage, or possibly by direct physiological damage. Hypotheses exist to explain these effects. It seems likely that these effects also occur at lower received sound levels than previously thought likely to cause damage and as a consequence the sphere of effect of these sonars is not known. Coupled with the lack of knowledge of the population size or distribution of beaked whales, we cannot be certain of whether population level effects might occur. However, at present it appears that these military sonars are not used widely. This could change in the future if these sonars were more widely deployed on ships or were used in non-exercise situations. Effects would be most severe in areas important for beaked whales. Section 5 provides an overview of possible measures to mitigate the effects of sonar on marine mammals as well a brief description of the measures being undertaken by some the Navies of some nations in relation to military sonar. Section 6 outlines some of the gaps in understanding around this issue and makes some suggestions as to how they might be addressed. Section 7 notes that other facets of the issue of noise in the ocean could have potentially more significant effects than direct lethal effects on individuals. In particular, the apparently increasing levels of anthropogenic low-frequency noise (mostly from shipping) may have consequences for the large baleen whales that use these frequencies for communication. General conclusions on marine mammals are summarised in Section 8 Section 9 describes relevant aspects of fish biology and the potential impact of sound, particularly sonar, on fish. There are very few studies of this and it should be noted that those studies cannot be regarded as being representative of the wide diversity of fish species that occur in the oceans. Effects of sonar have been noted at the individual level. Despite this, wide-ranging species of fish of commercial importance are unlikely to be affected at the population level with current rates of usage (and areas of usage) of military sonar. Other sonars (and noise sources) are more widespread. Research is suggested on the more subtle and on the cumulative effects of noise on fish. Section 10 provides the groups recommendations that may form the basis of advice from ICES. 1.6 Acknowledgements We thank Rene Swift and Jay Barlow for help in accessing some of the references used in this report. Gerald D'Spain, Jim Miller, and Dave Bradley provided comments on noise budgets. Bertel Møhl and Hans Lassen both provided helpful comments. Jake Rice read the whole report and provided many helpful comments. We thank John Polglaze, Mike Carron, Fernando Cerrutti, and Claire Burt for comments on the chapter on mitigation. Thanks to Arthur Popper for providing an update on his work on SURTASS LFA sonar and fishes. Jake Rice and Håkan Westerberg are thanked for more general comments. 4 | ICES AGISC report 2005, 2nd edition 2 Physical background 2.1 The nature of sound Sound consists of a symmetrical fluctuation in pressure around the hydrostatic pressure, accompanied by a back and forth motion of the component particles of the medium. For a plane wave travelling in open space without any interaction with objects or boundaries, the relationship between sound pressure (p) and particle velocity (v) is p = (ρc)v, where ρ (kg/m3) is the density of the medium and c (m/s) is the speed of sound in the medium. The acoustic energy flux or intensity (I) of a sound wave is the product of the pressure multiplied by the particle velocity, and has the units of Joule per square meter per second (J/m2-s) or watts per square meter (W/m2). For a plane wave the intensity (or energy flux) is given by I = p2/(ρc). It is equivalent to the amount of energy in Joules passing through a unit area per unit time as the sound wave travels unbounded in the medium. 2.2 Units for measuring sound Underwater sound is usually expressed using the logarithmic decibel (deciBel) scale. Underwater sound is conventionally presented in decibels referenced to 1 microPascal, i.e. as dB re 1 µPa, and this convention has been adhered to in this report. 2.2.1 Use of the decibel scale in water The fundamental unit of sound pressure is the Newton per square metre, or Pascal. However, when describing underwater acoustic phenomena it is normal to express the sound pressure through the use of a logarithmic scale termed the Sound Pressure Level. There are two reasons for this. First, there is a very wide range of sound pressures measured underwater, from around 0.0000001 Pa in quiet sea to around 10,000,000 Pa for an explosive blast. The use of a logarithmic scale compresses the range so that it can be easily described (in this example, from 0 dB to 260 dB re 1 µPa). Second, many of the mechanisms affecting sound underwater cause loss of sound at a constant rate when it is expressed on the dB scale. The Sound Pressure Level (SPL) is defined as: SPL = 20 log (P/P ) ref where P is the sound pressure to be expressed on the scale and P is the reference pressure, ref which for underwater applications is 1 µPa. For instance, a pressure of 1 Pa would be expressed as an SPL of 120 dB re 1 µPa. 2.3 Parameters for estimating noise In order to provide an objective and quantitative assessment of degree of any environmental effect it is necessary to estimate the sound level as a function of range. To estimate the sound level as a function of the distance from the source, and hence the range within which there may be an effect of the sound, it is necessary to know the level of sound generated by the source and the rate at which the sound decays as it propagates away from the source. These two parameters are: • the level of sound generated by the source or Source Level (SL) and • the rate at which sound from the source is attenuated as it propagates or Transmission Loss (TL) These two parameters allow the sound level at all points in the water to be specified, and in the current state of knowledge are generally best measured directly at sea, although acoustical models exist which may give reasonably reliable results for propagation from sonar systems in ICES AGISC report 2005, 2nd edition | 5 homogeneous deep water. However, these data have usually to be extrapolated to situations other than those in which the noise was measured; in these cases the commonest method of modelling the level is from the expression: Received Level (RL) = SL-TL Conventionally, the RL is calculated in dB re 1 μPa, but a similar expression may be used to estimate the received level of other measures of sound such as its impulse. Note that both RL and SL are absolute values, while TL is a rate. If the level of sound at which a given effect of the sound is known, an estimate may be made of the range within which there will be an effect. Sound can behave in very different ways in shallow water compared with deep water and is therefore much more difficult to model. 2.3.1 Source level The Source Level of a source is defined as the level of sound at a nominal distance of 1m, expressed in dB re 1 μPa. However, there are several assumptions implicit in this definition. Sound is composed not just of sound pressure but also a motion of the component particles of the medium (particle velocity). In the area very close to the sound source (the near field) there are very large particle motions for a given sound pressure and this has implications for organisms sensitive to particle motion – such as many fish, but less so for cetaceans. However, this area is very small (a few metres at the greatest) so population level effects are extremely unlikely and even effects on individuals would not be frequent. In addition, some sound sources, such as airgun arrays or steerable sonars, are composed of several sound sources that are operated simultaneously. When one is within several times the diameter of the source(s) the sound field is variable. It is therefore good practice to measure the sound pressure in the far field, at sufficient distance from the source that the effects of particle motion and complexities of multiple sound sources close together have reduced, and to use this pressure to estimate the apparent level at a nominal 1m from the source. However, this apparent level may not predict the actual level at ranges near an array of sources. An array of sources each of which operates at one particular source level may have a higher apparent source level far from the array. However as an animal approaches the array, it is unlikely that it would experience a sound level greater than one of the individual sources from which the array is made up. A ‘measurement’ of the apparent level can be made by assuming inverse dependence of pressure on the range, R, from the noise source, or by extrapolating the far field pressure. There is in general no reliable way of predicting the noise level from sources of man-made noise, and hence it is normal to measure the Source Level directly when a requirement exists to estimate far-field levels. 2.3.2 Impulsive sound Powerful impulsive sounds are generated by the use of explosives underwater, by the airgun arrays used in seismic surveying, and by some forms of construction activity such as underwater pile driving. These sources generate impulsive waves of short duration, high peak pressure, and a wide frequency bandwidth, and may consequently have an effect on marine organisms. Historically, two key parameters have been used to describe the severity of an impulsive source, the peak pressure and the impulse. The peak pressure of a blast wave P is the max maximum level of overpressure, that is, the pressure above the local ambient pressure caused by the sound. This is usually at the initial peak of the waveform and is easily read from a recording of the sound. The impulse I is defined as the integral of pressure over time and is given by 6 | ICES AGISC report 2005, 2nd edition ∞ I = ∫ P(t)δt 0 where I is the impulse in Pascal-seconds (Pa.s), P(t) is the acoustic pressure in Pa of the blast wave at time t and t is time. Impulse may be thought of as the average pressure of the wave multiplied by its duration. The importance of impulse is that in many cases a wave acting for a given time will have the same effect as one of say twice the pressure acting for half the time. The impulse of both these waves would be the same. Several workers (Johnstone, 1985; Ross et al., 1985; Larsen and Johnsen, 1992) have showed the impulse of the shock wave (Yelverton et al., 1975) to be the best predictor of damage to fish and other aquatic animals from explosives. However, this measure tends to give conservative estimates for shallow water (5-10m depth) and is not considered suitable in areas having hard, reflective beds or under ice (Engelhardt et al., 1985). The sound duration as well as the sound level is important in estimating the damage that may be caused by a sound. To compare sound events of varying duration and intensity the SEL or Sound Exposure Level (in dB) may be determined by converting the total noise energy measured during a noise event to an equivalent dB level for a single event that would be only one second in duration. The SEL accounts for both the magnitude and the duration of the noise event. The SEL may be calculated by summing the cumulative pressure squared (p2) over time and is expressed as dB re 1µPa2-s. Yet another method of quantifying the noise environment is to determine the value of a steady-state sound that has the same sound energy as that contained in the time-varying sound. This is the Equivalent Sound Level (Leq). It is important when measuring impulsive sounds to record examples of the waveforms, so that alternative metrics may later be applied. 2.3.3 Sound propagation and transmission loss The level of a sound diminishes as it spreads out from its source. A sound of 230 dB one metre from the source drops to 224 at 2m, 218 at 4 m, 212 at 8m out to 190 dB at 100m if the medium is homogeneous out to 100m. The reason for this extensive loss is that the energy emanating from the source expands in all directions, spread over a sphere of ever-increasing volume. This is called spherical spreading. Sound consists not only of a variation in pressure, but also of a back and forth motion of the medium. While sounds are normally monitored by measuring the sound pressure, many fish respond to the particle motion (measured as the particle velocity, particle displacement or particle acceleration). As we have seen in section 2.1, in a free sound field the particle velocity can readily be estimated from the sound pressure. However, close to a source, in the ‘near field’, much larger particle velocities are associated with a given sound pressure. Estimating the changes in particle motion with distance from a source depends on the nature of the source. Note that the ‘near field’ can be quite extensive (extending for several metres) for low-frequency sound. Losses far from the source are complex and depend on the depth of water, temperature, salinity and other factors. Not all frequencies propagate equally. High frequency sounds have a short wavelength and are absorbed by seawater, reflected by material in the water and converted to heat faster than low frequency sounds that have a long wavelength. For that reason, low frequency sounds propagate farther than a high frequency sound of the same source level. However, even though they propagate further, their levels continually decrease through spreading loss. These effects may be different in very shallow water, when low frequencies can attenuate faster than high frequencies. Sound rarely spreads out uniformly. The path that sound takes through water is affected by the presence of a reflecting bottom and surface, and by factors that change water density, mainly