Impacts of geophysical seismic surveying on fishing success

Transcription

1 Reviews in Fish Biology and Fisheries 10: , Kluwer Academic Publishers. Printed in the Netherlands. 113 Points of view Impacts of geophysical seismic surveying on fishing success Andrew G. Hirst & Paul G. Rodhouse British Antarctic Survey, Natural Environment Research Council, Madingley Road, High Cross, Cambridge, CB3 OET, UK; Present address: Department of Biological Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK (Phone: +44(0) ; Fax: +44(0) ; Author for correspondence Received 3 October 1998; accepted 16 February 2000 Introduction Over many years acoustic sources of various types have been used in the search for oil and gas in the marine environment. These sources have included sub-marine explosions and airgun blasts, which in turn have been shown in the laboratory, in large scale enclosures, and in situ, to have lethal and sub-lethal effects upon marine mammals, birds and fishes (Richmond and Jones, 1973; Yelverton et al., 1973; Sakaguchi et al., 1976; Wright, 1982; Linton et al., 1985; Sverdrup et al., 1994; Goold, 1996). Although explosive charges were commonly used until the 1960s, by 1985, 97% of seismic surveys used airgun devices (Holliday et al., 1987), and we therefore concentrate on the latter in this review. Organisms may not only be immediately killed on exposure to airgun detonations (Turnpenny and Nedwell, 1994; McCauley, 1994), but their mortality may also be delayed as a result of direct physiological damage, or indirectly from increased predation. The effects of close range airgun discharge on short-term (i.e. minutes to days) mortality of eggs, juvenile and adult fish have been examined in some detail and reviewed in Turnpenny and Nedwell (1994), and impacts of airguns at close range are briefly considered herein for completeness. Our objective is to review effects on a larger spatial and temporal scale than is typical for close range studies, specifically for the first time bring together published information on fish catch success. Airgun sources and surveys Typical seismic airgun surveys involve towing 24 to 32 airguns at 2 4 vessel lengths behind the surveying vessel. Their depth is controlled so that they lie between 5 to 10 metres below the water surface. Airguns usually have pneumatic chamber volumes of around 0.16 to 32.8 litres, and arrays have a combined gun volume of between 9.1 and 130 litres, producing sound levels of around 241 to 265dB re 1µPa-m (Richardson et al., 1996). Signals from airguns are emitted as short, sharp pulses, with most energy at Hz, although significant contributions may extend up to 500Hz, and frequencies as high as 1,000Hz may be produced (Malme et al., 1986). The guns within an array are discharged simultaneously at a rate of one firing every 6 to 20 seconds (McCauley, 1994), and shooting may proceed for 24 hours a day. Over a 24 hour period a single surveying vessel may cover a track of 96 nautical miles (178 kilometres) when travelling at a typical speed of 4 knots, and discharge over 14,000 shots. Seismic surveys have conventionally used a single array source and a single hydrophone streamer, with tracklines several kilometres apart and vessels only being within a localised area for brief periods, this technique is termed a 2D survey. More recently 3D surveying has increasingly been used for localised surveys (McCauley, 1994). With this technique, vessels deploy multiple hydrophone streamers and airgun arrays. Firing of arrays may be staggered to reduce interference, with an increase in the length of the duty cycle. These surveys examine localised areas for longer periods, with individual tracklines separated by as little as metres. As sound travels from a source, its level generally decreases. The major component of such transmission loss is the spreading loss, while absorption loss may be minor in comparison. The zone over which a sound has a given sound level may vary in radius 10-fold

2 114 or more depending upon the site and depth of operation. Changes in the water temperature, salinity and stratification are also of importance (Richardson et al., 1996). In areas where water depths are greater than 50 metres, sound typically spreads spherically, that is at a rate of 20log 10 R (where R is the ratio of the distance to which the sound levels are to be predicted [in metres] to the distance from source for which source levels are given [as a standard this is typically 1 metre]). Simple spherical spreading rules may only be applied with caution to shallow waters <10 50 metres. Sound propagating in lesser depths is much more affected by factors such as surface reflections, bottom composition and topography. Consequently, rates of loss may vary widely in shallower waters. Refraction of sound in shallow water can bring about both reduced or enhanced sound transmission, and between areas differences may be dramatic. If the water depth is less than a quarter the wavelength of the sound, then very large losses in sound can occur very rapidly with distance. Close range effects There are various sources of information about the close range, short time scale effects of airguns and sub-sea explosives. Adults, juveniles and eggs of fish appear to suffer immediate mortality only if within very close range of an airgun detonation, typically a few tens of metres or less. Physiological damage at 180dB re 1µPa and above includes: inner ear damage, haemorrhaging, eye damage, blindness, swimbladder rupture and even death (Turnpenny and Nedwell, 1994). Fish with swim bladders appear more prone to physiological damage and mortality from explosions than fish lacking swim bladders. Air-filled structures may become over extended and rupture when decompressed as a result of shock waves, hence the greater vulnerability of this group (Christian, 1973). The swim bladder may be one of the major sites for damage (Wright, 1982). Christian (1973) concluded that all biota will be killed at very short ranges from an underwater explosion, but fish that have gas-filled swimbladders are sometimes killed at much longer ranges than other organisms such as crabs, lobsters, oysters, and even other types of fish that do not have a swimbladder. Crustaceans and benthic molluscs may be more tolerant of airgun exposure, and may be apparently unharmed at distances of less than 2 metres from an airgun discharge (Matishov, 1992). In addition to immediate mortality, factors such as physiological damage and behavioural change, leading to delayed mortality may also be caused by airgun sound. Matishov (1992) found that exposure of cod (Gadus morhua) at a distance of 2 and 4 metres from an airgun led to eye damage, transient stunning, and as a result of internal injuries, death within 48 hours. Unfortunately, no exposure levels were determined in their study, although these may be estimated at 214 to 220dB re 1µPa (Turnpenny and Nedwell, 1994). Sound levels at which physiological damage and the mortality of adult, juvenile and larval fish have been found are all at, or in excess of, 180dB re 1µPa (see Turnpenny and Nedwell, 1994). Most studies of close range impacts of airguns have measured impacts under ex-situ, laboratory controlled conditions, and over short periods, where natural predators are excluded, and natural mortality rates as a result of increased predation and disease susceptibility through behavioural change and physiological damage may go underestimated. Physiological damage and behavioural changes may be missed in such studies because of the simple methods and short periods over which almost all studies have been conducted. Severe damage including hearing loss, blindness and internal injuries may lead to the subsequent death of individuals, even though under artificial laboratory conditions mortality may not have been observed. Much experimental work has also been completed using single airguns and single shots, and yet in the field airgun arrays comprise many guns, and are shot repeatedly. As such, close range exposure in the field may be more damaging than the typical experimental procedures suggest. Commonly, lethal effects have only been observed at close range, i.e. have only been found within a few metres to tens of metres of airgun discharges. However, given the amount of ground covered in a survey, the water depth, and the distance between tracklines, direct immediate mortality may be expected to be relatively small. What may be of greater significance to fisheries however, are the more diffuse behavioural impacts which occur at much greater distances. Impact on catch success We have introduced observed close range impacts on fished species, we now turn our attention to impacts measured on fishing catch. Changes in fishing success resulting from seismic operations have been

3 115 examined in teleost fish, crustaceans, and molluscs (bivalves, gastropods and cephalopods). Table 1 gives a compilation of the studies of fish catch success to seismic gun exploratory work, with details of the estimated temporal and spatial limits of disturbance. The degree of disturbance, measured as reduction in catchper-unit-effort is also included. All of these studies examined teleost fish, except La Bella et al. (1996) who examined crustaceans and molluscs. Information from the appropriate investigations were extracted including, the distance to which any impact or lack of impact was detected and the time this impact was studied or the period for return to pre-catch levels detected. The method of fishing and the species being caught is also included, and an estimate of the level of sound at the limit of the effect (or at the location where no effect detectable) was derived assuming that the sound spread spherically from the source, as discussed earlier (see Table 1 and Discussion). Several important short-term effects, which may result in reduced catchability of teleost species, have been observed. Reactions of fish to airgun operations have been demonstrated to range from brief interruptions of normal activities e.g. feeding, mating, startle response (Sverdrup et al., 1994), to short periods of horizontal or vertical migration away from the source (Chapman and Hawkins, 1969; Dalen and Knutsen, 1987; Engås et al., 1993; Løkkeborg and Soldal, 1993). It is possible that longer term displacements of fish populations may occur, although this has apparently almost gone unstudied (e.g. see Engås et al., 1993). Observed behavioural effects on exposure to sound in Gadidae (the cod family), Clupeidae (the herring family) and other species (including bass, eel, Atlantic salmon and goldfish), have shown that sound levels eliciting behavioural responses are between 102 and 205dB re 1µPa (Turnpenny and Nedwell, 1994). The responses including startle and alarm, avoidance, migration, fatigue, loss of equilibrium, with consequences including reduction of catch rates. Airgun detonations have been reported to appear alarming to cod and haddock at distances of nautical miles ( km) from ships performing seismic investigations (Engås et al., 1993). Our review suggests that the lowest levels of airgun sound in the open sea shown to elicit a behavioural response which results in altered fish catch rates is estimated at less than 160dB re 1µPa (see Table 1). Declining fishing harvests have been reported in areas where gun detonations have recently been carried out (Skalski et al., 1992; Engås et al., 1993; Løkkeborg and Soldal, 1993), in some cases at distances in excess of 33 kilometres (Engås et al., 1993). Løkkeborg and Soldal (1993) showed that fishing catch rates could be significantly reduced during airgun discharges, with these effects lasting for 24 hours and at distance of at least 9 kilometres. As behavioural change by an individual will almost certainly rely upon that individual being able to hear the seismic source, then the auditory ability of species needs to be addressed. The zone of audibility is the area in which a specified organism can differentiate the sound of interest from background noise levels. Identifying this zone has obvious importance in estimating areas of disturbance, as behavioural response will probably occur closer to a source than that to which a sound is audible. The extreme limit of the zone of audibility is influenced by the character of the sound signal, ambient noise levels, local propagation conditions (which may change on a daily and seasonal basis), and the hearing sensitivity and threshold of the species (McCauley, 1994). Hearing capabilities also change with life-cycle stage and to a certain extent between individuals of the same age group within a species. Hearing sensitivity in animals may vary with season, locality, duration of shooting, whether they are migrating, and availability of food. No effect on short-term catch success in areas localised to the seismic operations was observed by La Bella et al. (1996) on the trawl catch success of cephalopods or Norway lobster (Nephrops norvegicus), or the gill netting success of mantis shrimp (Squilla mantis). Most invertebrates are likely to only be able to hear seismic survey sounds at very close range (perhaps less than 20 m), via their mechanoreceptors; cephalopods are an exception to this, and are known to possess far-field sensitivity to certain types of sound (McCauley, 1994). Far-field is the distance from a source where sound pressure and particle velocity are related to each other by the plane wave equations, and the source appears to have point source properties. For a point source the boundary of the near-field is approximately one wavelength from the source. In regions of poor propagation, seismic shots may only be above ambient levels for many vertebrate species at km, whereas in regions of good propagation they may be above ambient for well over 100km (McCauley, 1994). Catches of both cod and haddock (Melanogrammus aeglefinus) have been reduced at distances as great as 33km from an airgun

4 116 Table 1. Compiled airgun long-range experimental observations and effects upon catch rates of fish, molluscs including squid, and crustacea Species Survey Source level (db re Distance from CPUE reduction as % Fishing type Source description and (db re 1µPa)@limit of effect source to of pre-shoot catch water depth which effect period reduction occurred (km) lasted (post-shooting) Gadus morhua (Atlantic Cod) Continuous array survey 250dB re 1µPa@1m >33 3 Catch reduction 46 69% 2 Trawl Engås et al., 1993 over 5 days, <160dB re 1µPa@fish location 1 Lasting at least 5 days 3 10 nm area covered 250 db re 1µPa@1m Between 17 and 33 Catch reduction 17 45% 3 Long-lining Water depth = m dB re 1µPa@fish location 1 Lasting at least 5 days Intermittent sleeve gun array? >15 4 Catch reduction 55 79% Long-lining Løkkeborg and Soldal, 1993 survey 40 hrs over 10 days,? Lasting at least 24 hours nm area covered Water depth not given Continous sleeve gun array? >9 Catch reduction 79% By-catch in shrimp Løkkeborg and Soldal, 1993 survey, survey length not given? Period of effect not determined Trawl Water depth = m Continous sleeve gun array 254dB re 1µPa@1m 6 >9 Catch reduced by 83% By-catch in shrimp Løkkeborg and Soldal, 1993 survey, survey length not given <175dB re 1µPa@fish location 1 Lasting 24 hours Trawl Water depth = m Continous sleeve gun array 258dB re 1µPa@1m 6 Within surveyed area Catch increased by 525% By-catch in saithe Løkkeborg and Soldal, 1993 survey over 9 hrs on 2 days,? Lasting 12 hours Trawl 98 km covered Water depth = m Melanogrammus aeglefinus (Haddock) Continous array survey 250dB re 1µPa@1m >33 3 Catch reduction 70 72% 2 Trawl Engås et al., 1993 over 5 days; <160dB re 1µPa@fish location 1 Lasting at least 5 days 3 10 nm area covered 250 db re 1µPa@1m >33 3 Catch reduction 49 73% 3 Long-lining Water depth = m <160dB re 1µPa@fish location 1 Lasting at least 5 days Sebastes spp. (Rockfish) Survey around rock pinnacles 223 db re 1µPa@1m Not determined Catch reduction 52% Long-lining Skalski et al., 1992 (maximally 165 metres away) >186 db re 1µPa@fish location Effect period not determined using a single airgun during 3 20 minute long-line soak periods Water depth = m Merluccius merluccius [large individuals >21 cm] (Hake) 6 profiles, km fired 210dB re 1µPa@1m 7 Given width of study site No apparent catch reduction Trawl La Bella et al., 1996 Merluccius merluccius [small individuals <21 cm] (Hake) overall hours of firing 149dB re 1µPa@fish location 1 No effect at > day after prospecting Illex coindetti (short-finned squid) using airgun array Nephrops norvegicus (Norway lobster) Water depth = m Squilla mantis (Mantis shrimp) 6 profiles, 42.82km fired 210dB 210dB re 1µPa@1m 7 Given width of study site No apparent catch reduction Gill nets La Bella et al., 1996 re 1µPa@1m 7 overall dB re 1µPa@fish location 1 No effect at > day after prospecting hours of firing using airgun array Water depth = 15 m Paphia aurea (Golden carpet shell) 6 profiles, km fired 210dB re 1µPa@1m 7 Given width of study site No apparent catch reduction Hydraulic clam La Bella et al., 1996 Anadara inaequivalvis (Inaequivalvis ark shell) overall hours of firing 147dB re 1µPa@fish location 1 No effect at > days after prospecting Dredge Bolinus brandaris (Purple die murex) using airgun array Water depth = 15 m 1 Sound levels at which response elicited estimated by assuming spherical spreading (20 log10 R) from source. 2 Estimated from Tables 1, 2, 5 and 6 in Appendix E in original study. 3 Effects measured to limit of measuring activity i.e. 33 km, effect probably extends beyond these limits therefore. 4 Distance to which effect measured being defined as the distance to which the catch rate/fleet (kg/fleet) would appear to be less than that before the airgun survey began (i.e. an average of 2,500 kg/fleet). All data from the Frøyanes fleet as Frøde fleet did not fish prior to shooting. 5 Period of effect defined as period taken for catch rates to return to pre-shoot levels (i.e. 2,500 kg/fleet) for the Frøyanes fleet. 6 To allow estimation of the sound levels produced by airguns and airgun arrays, when these are not given, but volumes are, then an equation has been derived which allows prediction from total gun volume. Airgun volume and zero to peak (z p) sound levels in db re 1µPa-m have been taken directly from Richardson et al. s (1996) compilation. These levels have been converted to peak to peak (p p) values by adding 6dB. A linear regression of sound level (db re 1µPa-m) against log 10 Total gun volume (V in litres) giving the equation for conversion: db re 1µPa-m = (14.86 log 10 V) Intensity also quoted as 210 db re 1µPa-m/Hz in the abstract of the source, these units would appear not to be in the correct form however, and have therefore been taken as incorrect.

5 117 array producing source levels of 250dB re 1µPa-m, with catch rates being reduced for at least 5 days after the completion of shooting (see Table 1). Both these species are swimbladder fish, and as mentioned swimbladder fish may be more sensitive to sound from airguns in comparison to non-swim bladder species. The degree of linkage between the swimbladder and the ear is also of great importance because of the correlation with ability to detect sound (Hawkins, 1986). Many fish are believed to move diagonally downwards and away from airgun operations (e.g. Løkkeborg and Soldal, 1993; Engås et al., 1993). Fish catches may be reduced in airgun operation areas as a result of the decrease in the number of animals as these migrate away from the shooting area (Engås et al., 1993). Reduced catch rates may also result from fish changing their position in the water column, or changing their behaviour with regard to fishing gear. In either case, active or passive methods of fishing may have reduced catch rates in areas of airgun operation. The mobility of a species will also effect how catch rates may be influenced. Discussion The transmission of sound in water is variable and site-specific, and will influence the distance to which organisms are influenced. Most fishing catch effects are likely to be the result of changes in fish behaviour, manifested as horizontal and vertical dispersal, general activity levels and perhaps also their reaction to fishing gear. Fishing success has been shown to be reduced for at least 5 days after airgun shooting has finished, and to a distance of 33 km (see Table 1). Although these are the most extensive measurments to date, the studies in which they were recorded still did not determine the absolute outer limit of effect and the entire period over which the effects extended. More extensive impact distances and times may be demonstrated in future investigations. Although rates of sound loss may vary considerably, in none of the catch success impact studies have measurements of sound levels at distance from the seismic source been made. In this study (and many others) spherical spreading equations have been used to derive estimates. In some studies there have not even been measurements of the levels emitted by the source (e.g. Løkkeborg and Soldal, 1993; see Table 1). There seems little alternative at this point but to attempt to estimate these sound levels. Future investigations should consider such shortfalls. Any consideration of effect should also take scale into account. Conventional exploratory surveys (2D techniques) may cover large areas and are only briefly in a localized area. Such surveys are typical for speculative wide area coverage, with each trackline being several kilometres apart. However, 3D surveys methods have been developed in which several parallel hydrophone streamers and possibly airgun arrays are deployed from a single vessel. Tracklines for 3D site evaluation may be only metres apart. This type of survey may focus work on small spatial scale, and proceed for weeks at a time (McCauley, 1994). Such surveys may have more pronounced effects on organisms in surrounding areas, and also will have greater impacts on fishing success. No investigations appear to have been undertaken on long-term effects of seismic surveying on fishing success, although such impact can not be discounted. Disruption of behaviour during critical periods such as mating, spawning and migration could be particularly important. The success of a species is also dependent upon both its prey and predators. If these are affected, for example driven away by a seismic survey, then there may be consequences for commercially fished species which rely upon them as prey. One problem of interpretation of the sensitivity to sound is signal duration. Mammals have hearing systems which act so that longer signals can sound louder than shorter signals, even though both may have the same pressure. If this is true for other marine species then the time duration of seismic signal may be as important as the peak level in influencing behaviour (Malme et al., 1986). In teleost fish the shorter the duration of a sound signal, the louder the signal must be for the fish to be able to detect it (Hawkins, 1986). Engås et al. (1993) have however, expressed doubt that the pulse duration of an airgun signal at ms is short enough to influence the detection threshold. If a commercially exploited species is to detect a sound signal, the strength and frequency of the signal are of primary importance, although the natural background noise will also be important. This study highlights some seismic surveying effects, and some of the factors which will cause these to vary. It would appear that teleost fish and their associated fisheries are more sensitive to disturbance than crustacea and molluscs and their fisheries. Cephalopods have only been examined in a single study, in which no impact was detected, however, given their

6 118 sensitivity to sound they may yet prove to be adversely affected by seismic surveying in ways similar to fish. There is insufficient information to predict the longterm effects on fishery success in areas of geophysical surveying since studies have not made measurements over periods exceeding a few days. Much more work is needed before adequate predictions of impacts on fishing may be formulated. This review does, however, give a first indication of some of the potential impacts of seismic surveying on fisheries which may occur in the short term. Acknowledgements This work was supported by a grant from the Falkland Islands Government Fisheries Department. We thank Dr. Phil Smith for reading an early version of the manuscript. References Chapman, C.J. and Hawkins, A.D. (1969) The importance of sound in fish behaviour in relation to capture by trawls. FAO Fish. Rep. 62, Christian, E.A. (1973) Mechanisms of fish-kill by underwater explosives. In: Young, G.A. ed. Proceedings of the First Conference on the Environmental Effects of Explosives. Naval Ordnance Laboratory, White Oak, Maryland, USA. Dalen, J. and Knutsen, G.M. (1987) Scaring effects in fish and harmful effects on eggs, larvae and fry by offshore seismic explorations. In: Merklinger, H.M. ed. Progress in Underwater Acoustics. Plenum Press, NY, pp Engås, A., Løkkeborg, S., Ona, E. and Soldal, A.V. (1993) Effects of seismic shooting on catch and catch-availability of cod and haddock. Fisken og Havet, nr pp. Goold, J.C. (1996) Acoustic assessment of populations of common dolphin Delphinus delphis in conjunction with seismic surveying. Journal of the Marine Biological Association of the UK 76, Hawkins, A.D. (1986) Underwater sound and fish behaviour. In: Pitcher, T.J. ed. The Behaviour of Teleost Fishes. Holliday, D.V., Pieper, R.E., Clarke, M.E. and Greenlaw, C.F. (1987) The effects of airgun energy release on the eggs, larvae and adults of the Northern Anchovy (Engraulis mordax). American Petroleum Institute. Tractor Document No. T U. La Bella, G., Cannata, S., Froglia, C., Modica, A., Ratti, S. and Rivas, G. (1996) First Assessment of Effects of Air-Gun Seismic Shooting on Marine Resources in the Central Adriatic Sea. Society of Petroleum Engineers. International Conference on Health, Safety and Environment, New Orleans, Louisiana, 9 12 June, pp Linton, T.L., Landry, A.M. Jr., Buckner, J.E. Jr. and Berry, R.L. (1985) Effects upon selected marine organisms of explosives used for sound production in geophysical exploration. The Texas Journal of Science 37, Løkkeborg, S. and Soldal, A.V. (1993) The influence of seismic exploration with airgun on cod (Gadus morhua) behaviour and catch rates. ICES Mar. Sci. Symp. 196, McCauley, R.D. (1994) Environmental implications of offshore oil and gas developemnt in Australia-seismic surveys. In: Swan, J.M., Neff, J.M. and Young, P.C. eds. Environmental Implications of Offshore Oil and Gas Development in Australia. The Findings of an Independent Scientific Review, pp Malme, C.I., Smith, P.W. and Miles, P.R. (1986) Characterisation of Geophysocal Acoustic Survey Sounds. Minerals Management Service, Los Angeles, California. Report No. OCS/MMS86/ Matishov, G.G. (1992) The Reaction of Bottom-Fish Larvae to Airgun Pulses in the Context of the Vulnerable Barent Sea Ecosystem. Fisheries and Offshore Petroleum Exploitation 2nd International Conference, Bergen, Norway, 6 8 April. Richardson, W.J., Thomson, D.H., Green, C.R. Jr. and Malme, C.I. (1996) Marine Mammals and Noise. Academic Press, San Diego. 450 pp. Richmond, D.R. and Jones, R.K. (1973) Safe distances from underwater explosions for mammals and birds. In: Young, G.A. ed. Proceedings of the First Conference on the Environmental Effects of Explosives. Naval Ordnance Laboratory, White Oak, Maryland, USA. Sakaguchi, S., Fukuhara, O., Umezawa, S., Fuhiya, M. and Ogawa, T. (1976) The influence of underwater explosions on fishes. Bull. Nansei Reg. Fish. Res. Lab. 9, Skalski, J.R., Pearson, W.H. and Malme, C.I. (1992) Effects of sound from a geophysical survey device on catch-per-unit-effort in a hook-and-line fishery for rockfish (Sebastes spp.). Canadian Journal of Fisheries and Aquatic Sciences 49, Sverdrup, A., Kjellsby, E., Krüger, P.G., Fløysand, R., Knudsen, F.R., Enger, P.S., Serck-Hanssen, G. and Helle, K.B. (1994) Effects of experimental seismic shock on vasoactivity of arteries, integrity of the vascular endothelium and on primary stress hormones of the Atlantic salmon. Journal of Fish Biology 45, Turnpenny, A.W.H. and Nedwell, J.R. (1994) Consultancy Report. The Effects on Marine Fish, Diving Mammals and Birds of Underwater Sound Generated by Seismic Surveys. Fawley Aquatic Research Laboratories Ltd. 40 pp. Wright, D.G. (1982) A Discussion Paper on the Effects of Explosives on Fish and Marine Mammals in the Waters of the Northwest Territories. Canadian Technical Report of Fisheries and Aquatic Sciences pp. Yelverton, J.T., Richmond, D.R., Fletcher, E.R. and Jones, R.K. (1973) Safe Distances from Underwater Explosions for Mammals and Birds. Defense Nuclear Agency, Washington, D.C. Contract Nos. DADA 01-70C-0075 and DADA 01-71C