C.M.19921N:17-Ref.B

MARINE MAMMALS COMMITIEEREFERENCE FISH CAPTURE COMMITIEE

This is areport of work in progress and, as the information given below may be revised, itis recommended that the authors are consulted before any citation.

PROGRESS IN THE DEVELOPMENT OF EFFICIENT WARNING DEVICES TO PREVENT

THE ENTRAPMENT OF CETACEANS (DOLPHINS, PORPOISES AND WHALES) IN

FISHING NETS

Margaret KlinowskaResearch Group in Mammalian Ecology and Reproduction, Physiological Laboratory, University of Cambridge,

Downing Strcet, Cambridge CB2 3EG, UK.

• David GoodsonSonar and Signal Processing Research Group, Electronic and Electrical Engineering Department, Loughborough

University of Technology,Loughborough LEII 3TU, UK.

PeterB100mThe Dolphinarium, Flamingo Land, Kirby Misperton, Nr Malton,

North Yorkshire YüI7 DUX, UK.

Abstract

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Every year many thousands of cetaceans die worldwide through accidental entanglementin passive fishing gear. Since it is not feasible to prohibit all such fishing, practical ways areneeded to reduce this morta1ity. Such techniques must also not interfere with theoperating efficiency of the fishery, compromise safety or entail excessive capitalinvestment, if they are to be widely accepted and implemented. Our approach addressesthe problem through detailed practical and theoretical consideration of the animals'behaviour and sensory abilities. Particular progress has been made in designing and testingpassive acoustic reflectors, to enhance the detectability of nets by the dolphins' own sonaremissions. Such enhanced sonar detectability, if it proves practical and successful, wouldalso assist in the retrieval of lost nets using ship's sonar, and thus help to prevent anothersource of danger to marine life as weIl as mitigating financial loss to the fishing industry.Two recent field tests are described. The first demonstrated that, over aperiod of severaldays, no wild dolphins attempted to penetrate a barrier of prototype acoustic reflectors.This technique, which presented no risk to the animals because no net webbing waspresent, genera ted a great deal of unique high-quality detailed information about animalmovements (through theodolite tracking) and sound emission (through underwateracoustic monitoring), which could not be obtained from conventional fishery monitoring.The second trial was successful in providing sidescan sonar images of normal andmodified net panels deployed at sea from a commercial fishing boat. The experimentalmethod for attaching the reflectors proved unsatisfactory in several respects in a workingenvironment, and is being re-designed. Further techniques invoking other cetaceansenses, such as chemical or visual warnings, and modifications of gear deployment toallow animals to pass nets ITlore safely, are also being investigated.

2Introduction

Large numbers of cetaceans (whales, dolphins and porpoises) die every year during fishingoperations world-wide. Accidental entrapment (as opposed to mortalities caused bydeliberate setting of nets on schools, for example in the Eastern Tropical Pacific tuna purse­seine fishery) also causes expensive damage to fishing gear and loss of valuable fishingtime. Any kind of gear (drift, towed or fixed) presents some hazard to these animals, and toother non-target species. While accidental cetacean entanglements have occurredthroughout fishing history, the strength of modern materials and the vast quantity of geardeployed today, as weIl as increasing knowledge and public concern about cetaceans, haveled to pressure for entanglements to be reduced or eliminated.

Our knowledge of how and why accidental entanglements occur is limited (IWC,1990). Some cetaceans seem to be trapped through unsuccessful attempts to "poach" caughtfish, or by using nets as "walls" to prevent the escape of their prey (which is not necessarilythe target species of the fishery). However, the root of the problem may be that as moderncetacean species evolved over millions of years in an environment which did not containunexpected temporary obstacles, they may simply not be well-equipped to deal with suchsituations. Banning all fishing throughout the world would certainly prevententanglements, but is obviously not feasible, since very many people (especially ,developing countries and in remote areas) depend on fishing to provide basic foorequirements (and/or employment) for which there is no real substitute. Practicaltechniques are therefore needed which reduce cetacean mortality. Such techniques mustalso demonstrably not interfere with operating efficiency, compromise safety, entailexcessive capital investment, or be difficult to monitor, if they are to be widely accepted andimplemented.

Our approach attempts to address the problem through detailed consideration of theanimals' behaviour and sensory abilities. While it is clear that no single modification ofgear or deployment is likely to provide complete protection, a combination of techniques,possibly specific to the fishery and area in question, could be effective. A major problem intesting new techniques is that cetacean entanglements are relatively rare (for example inlarge drift net fisheries of the order of one per 20 - 30 km of net set), and it has thereforebeen necessary to monitor many sets to genera te statistically significant results (IWC, 1990).

Non-Acoustic Approaches

A review of the non-acoustic abilities of cetaceans indicates two general approach.(Klinowska, 1990; 1992a and b; Klinowska and Goodson, 1990). The first involvesmodification of gear deployment, and would apply to animals using environmentalinformation such as the geomagnetic field, currents, water temperature or salinitygradients as a travel cue. It simpl y involves orienting the gear parallel to theenvironmental cue providing the cetacean travel path rather than across it. It should notbe difficult, expensive or disruptive to collect the base information required to test theseideas during the routine fishery monitoring which is already taking place in many parts ofthe world. Obviously, if the fishery target species happens to be using the same travel cuesas the cetaceans, reorientation of gear will not be practical. Nor would it be practical forgear which must remain in a particular place, such as anti-shark nets for bathing beachprotection. Nevertheless, this approach deserves serious consideration, because it is easy totest and, if effective, could be suitable for many modern fisheries to implement.

The second approach involves gear modification. Initial work in Australia (Hembreeand Harwood, 1987), more extensive investigations in Japanese albacore tuna (Tasman Sea)and flying squid (North Pacific) drift gillnet fisheries (Hayase, Watanabe and Hatanaka,1990), and some recent observations in thE', North Atlantic (Collet, Antoine and Danei,1992), indicates that lowering the head rope of the net below the surface greatly reduces theby-catch of sea birds, and may mitigate cetacean entanglements, without serious effects on

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3catch-per-tiriit-effort for the target species (although insufficient sets have so far beeilmonitored to provide statistically signifiCant results). The rationale for this approachappears to be that, as cetaceans must visit the surface from time to. time to breathe, thelower head rope provides dear space for these excursions. ,There is also space fOT cetaceanstravellirig near the surface to pass, and for birds to fish. Perhaps even more significantly,thci suh-surface head rope provides a very strong sonar target, giving clear advance warningof the presence of an obstruction. Increasing the visibility of underwater gear (in wayswhich are relevant to cetacean perception, but not to the fishcry target species) might beuseful in some restricted. circumstances, and it might even be worth corisidering exploringwhether improving visibility above waterwould be helpful (becatise of speculation in theliterature to the effect that these animals may sometimes use orientation informationvisible above the water). ,

Althotigh little is known of the role chemoreception (taste or smell) phiys in cetaceanfood finding and social behaviour, it is asense which can be invoked from a distance inlvater. Natural fibre nets, traditionally treated with a variety of oils, tar5, ete. \vould leave adist!nctive "trail" in the water, as would treatments applied to modern nets. The eonteritsof any nets would be likely to provide a chemical trail of excreta and othersubstances. Wepropose to investigate the role such eues may have in attraeting or alerting eetaeeans tonets, because such broadcast chemical signals eould weIl negate, any other efforts to prevententariglements. There are some anecdotes in the literature indicatirig that startled animalsmay produce a chemical signal \vhich alerts other animals to danger and induces flight. Ifthis is indeed the case, it might be possible to treat :flets \vith a similar substance, or to use itin conjunetion with other methods, to provide a dear general warning. .

Acoustic Approaches

Genciral Corisidcrations . .P~evious attempts tomitigate the problem using active and passive acoustie devic~s

attaehed to nets have generally had disappointing results (Dawson, 1991), which appear tobe related to three factors: (1) inadequate ':lnderstariding of the way in which dolphin activesonar functions, (2) insufficient attention to the engineering and operatiorial requircments,and (3) incomplete consideration of the behaviour of the animals.. Ftirthermore, theproblem is now seen to be one of target classification and not simply a problem of deteetionin noise (Au and Jories, 1991). In other words,it rriay be difficult for dolphins to in~erpret

weak difftised echoes from nets as a life-threatening hazard, when experience has taughtthat quite similar volume scattered echoes, returned by algae or by entrained air bubbles,are penetrable zones to be ignored, especially when a discrete fish target ean be deteeted onthe far side. .

Active acoustie devices appear to have IÜtle potential at present. Cetaeeans do notseem to have any "dariger - keep a\vay" calls which could be imitated~ killer whales OrcinusDrell (one of their few natural enemies) only eall when hunting fish and are silent whenhunting mai-ine mammals, it is diffieult to see how sufficieilt numbers of eetaceans eouldbe tatight, that artificial noises mean "danger - keep away", and gerieration oE sounds oEhigh intensity to provide a painful aversive experieriee fOf the ariimals throughout netdeployment may be undesirable' for ethieal reasons as \vell as teehnically diffieult,expensive rind almost impossible to police.

, aur initial idea \vas to irivestigate the enhancement of the aeoustie eharacteristics ofmodern, nets by the additiori of passive sonar reflectors (analogous to "eat's eyes" in iheroad reflectirig the car heridlights back to tIle driver). Some limitations of dolphin sonar inthe context of net-like targets and parameters to be eonsideI-ed \vheri allernpting to ripplyacoustie engirieering techniques to the design of effident passive aeoustic refleetors havebeen discussed (Goodson, 1990; Goodson, Klinowska and Bloom, 1990; Goodson and Datla,1991; 1992a and b; Mayo and Goodson, 1992).

4The sonar behaviour of a solitary wild bottlenose dolphin (Tursiops truncatus)

observed while resident in the sea elose to Amble, (Northumberland, UK) providedrepeatable and interpretable patterns while the animal searched for and caught fish. Thepulse repetition frequency (PRF) during foraging ineludes identifiable rate modulationcharacteristics, which can be used to elassify the emissions into Foraging Search (no targetdetected), Initial Target Detection (Iocking-on) and subsequent Interception (range-Iocked).A further mode appears to occur at short ranges, where the high PRF may be used tosustain the swim bladder of the target fish in aresonant condition (Goodson et al., 1990;Goodson and Datta, 1991).

It should be emphasised that, although most work so far has concentrated on thebottlenose dolphin, this is only for convenience during the development stages, becausebesides the extensive literature on this species, access to both wild and captive specimens isrelatively easy. The final aim is to make the techniques applicable to as wide a range ofspecies as possible, particularly the lesser-known species such as the harbour porpoise(Phocoena phocoena) which are subject to high rates of entanglement in many parts oftheir range (IWC, 1990).

Echo Perception oE Small TargetsSound propagating underwater obeys physical laws, and the mechanisms of reflection a.weIl established in the literature. For a surface to reflect an echo, the material must offer adiscontinuity to the propagation of sound, i.e. the product of density and sound velocity(pe) of the material must differ appreciably from that of seawater. Water / air interfacesreflect weIl, as do most water /metal transitions, whereas polymer materials,e.g. nylon/water, do not. In this latter case, a significant proportion of the incident energyis simply transmitted through the interface.

For a target to return geometrical specular reflections or "glints" with directionalproperties, the dimensions of the reflecting surface need to be large with respect to thewavelength of the incident radiation. If the reflector dimensions are too small (inwavelength terms), the intercepted energy is simply scattered, and the proportion ofreflected energy returned towards the source falls very rapidly with decreasing size(Rayleigh scattering).

For a dolphin fis hing at night and/or in turbid waters, the use of eyesight as a sense toassist the detection and capture of prey can be assumed to be ineffective, and sonar isprobably its primary sense. The dolphin transmits abrief, intense broad-band soundimpulse or dick, and detects the echoes returning from objects ensonified by this clirAduring the interval between the transmissions. The dolphin's melon, functioning as ?'beam former of very limited acoustic aperture, is unable to project this wide band signalwithout severe frequency dispersion or "colouration". The higher frequency spectralcomponents in the dick are therefore seen to be concentrated into an intense, tight (10°),forward-looking beam, with lower frequency components being spread over progressivelywider angles. When sampled on-axis, the dick energy of the bottlenose dolphin isnormaIly found to have a strong peak near 120 kHz (Au, 1980), although the spectral peakmay appear to shift if the animal transmits at reduced source levels. The bottlenosedolphin is known to perceive sound frequencies up to 140 kHz. The published audiogramsfor this species (Johnson, 1966) show that the animal has the sensitivity to detect highfrequency echoes at 120 kHz fficiently. The exploitation of such high frequencycomponents is essential to resohre the presence of small fish-like targets.

In seawater a frequency of 120 kHz corresponds to a wavelength (A) of 12.5 mm, andhence any echo-producing target needs to be assessed in terms of this dimension. Sub­wavelength dimensions do not produce specular reflections; they simply scatter theintercepted energy in all directions, and thus generate very weak echoes back towards theensonifying source

Typical gill-net mesh is made from very thin polymer filaments or twine, joined atintervals by knots to form square or diamond apertures, the size of which is chosen to trap

, 5the target species. The twine or monoEilament material is signiEicaritly smallef in diameterthan the 12.5 inm critical \vavelerigth, and as a result intercepts a minute proportion oE theincident acoustic energy. TI1e length oE filamerit or t\vine between knots provides the onlydimension to the structtire that exceeds A. For any echo to be detectable depends criticallyon the incident energy arrh'ing perpendicular to such components,. so that the. scatteredsound energy rettirried towards the source sums coherently from the length dimensiori.Unfortunately the knotted structurc arid the overall flexibility oE the net ensure that onlysinall zones oE mesh meet these criteria at any given instant. Gill-net echoes returnedtowards the dolphin comprise many very \veak glints which appear to come from ri zone(defined by the range rind beariHvidth oE the dolphin's transmission) wÜh diffused and 'variable position. In contrast to the characteristic discretc echo oE a fish, the di{{erence isdearly significant. Fish echoes (especially those Erom fish with a swim-bladder) appearvery strong and return from a specific position. Sequeritial echoes from a swimming fishwill be intensity-modulated cyclically, due to the tail beat action. The (non-sphefical) swimbladder is thtis presented at changirig angles to successive dolphiri. dicks. Suchcharacteristic changes in the fisIl target-strength are likely to assist tIle' dolphin in~lassifying the echoes as "alive arid moving" arid also provide dues to thephysical size oEthe target. The discrete fish echo is easily detectable at ranges weIl beyond those at whichthe fishing net can be percciyed, and the comparative ~coustie transparency oE the net isprobably a major cause oE the dolphin's perception problem.

Fciragirig BehaviourIn Eoraging mode, the (Amble) dolphin emits loud dicks (source levels oE the order oE 210 -217 dB re 1 JlPa have beeri measured), repeated at slow repetition rates, normally in the

range oE 8 to 20 Hz. Since the transmission oE such an intense sound must mask \veakechoes returning Erom very long range, and since the transmitted sound is attenuated bysquare law sjJreading and by absorption (as is the rehirning echo), the exploitable detectionrange must be limited to the period bet\veen transmitted impulses. In shallO\v ,vater,reverberations oE the preceding pulse mise the noise noor, which further restricts thedetection range.

, From repetition rates, the maximum search ranges observed in 2 to 5 m ,vater depthsOimited by reverberation noise) appear to be oE the order oE 80 or 90 in, with 70 m beingmost typical. Fish seen to be regularly caught and swallowedwhole are around 35 - 40 cmin length (salmonids) (ßloom, 1991; 1992; in press). Occasionally larger fish are taken (max.60 cm), but these all appear to require energetic slapping activity by the ,dolphin to breakthern down to swallowable size. (We noted that the fish head may be discarded,iri. suchcases.) The pattern which has emerged, suggests that Eish oE sizes much larger than 35 ­40 cm are opportunist, rather than primary targets. The acoustic target strengih oE

swimbladdered fish oE these dimensions is approximately oE the order oE -35 dB (re a 2mradius sphcre).

Nct Dctcctability "The acoustic target strength oE polymer gill-net inaterial is difficult to assess in simpleterms. Ho\vever, the most recent published figures (Au and Iones, 1991), rneasured at veryshort range, provide useful ri1aximum values applicable to angles oE approach in theazimuth plane only.Worst-case detection must also consider the combined cffect ofazirriuth and elevation approach angles that are not normal to the plane oE the net.However, it is very dear that the echoes retumed by gill-riettirig are extremely ,veak incomparison ,to those from fish. A simple numerical comparison of fish against net target­strength indicates that dolphins ought to be able to detect rriany types oE gm-net, fromranges perhaps as Ear away as 9 metres. If this theoretical detection distance is rcgarded asproviding an adequate stopping distance, then c1early other factors must be involved inentanglements. It should be noted that a floating headline on the sea surface does not addto the detectability of the structurc, as wave-trough masking effecls and strong thermal

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gradients near the surface hinder their ensonification. When deployed sub-surface, suchheadline components provide dramatically better sonar targets than the suspended netmesh.

Target Detection BehaviourWhen the (Amble) dolphin detects a fish target its transmission behaviour changes. Whilesearching, in the absence of targets, the slow PRF is characterised by some inter-dick timeirregularity (a dearly visible effect when viewed on an oscilloscope display). The onset oftarget detection normally involves a sudden increase in the PRF, which may be preceded bya very brief cessation of transmissions. The PRF typically takes several pulse/echo periodsbefore settling to a steadily increasing rate with precise inter-dick timed intervals (range­locked interception behaviour) (Goodson, Datta and Van Hove, 1991).

Target InterceptionAlthough referred to as range-locked behaviour, and in the examples examined the pulserate during interception appears dosely correlated to the target's range, it is morereasonable to interpret this behaviour as an attempt by the animal to extract the maximumpossible number of echoes from the target during interception. Transmitting a PRF abovethat determined by the target range must result in masking the arrival of the wanted echAby the succeeding transmission. The tight PRF lock on the target is normally maintained(in the absence of a successful evasion manoeuvre by the target) from the initial detectionrange down to a relatively short range. However, the terminal PRF observed duringseveral, known-to-be-successful, fish takes did not appear to exhibit a consistent pattern.Frequently, the steadily increasing PRF appeared to stabilise before termination, settlingbriefly at a particular frequency. Given that a physiological limit to the maximum PRFmust exist, it seems that attempting to maintain the range-locked data rate at very shortranges « 4m) is unprofitable. However during interception of the larger swimbladderedfish, the PRF may match and stimulate the swim bladder bubble into sustained resonance.Spectrograms of some fish echoes (detected as they passed very dose to a hydrophone)appear to demonstrate the onset of a narrow-band low-frequency tonal component whichwould support this hypothesis. (Such sustained resonance behaviour by a wild bottlenosedolphin examining other types of target has been observed, and experimentallydemonstrated to be feasible - Goodson, Klinowska and Morris, 1988.) Although the hearingof the bottlenose dolphin is relatively poor at such low frequencies, and the animal isunlikely to need further data to define the position of the prey, it would still seem that tIadolphin can benefit from sustaining this swim bladder stimulus behaviour as the fish maylose its ability to exploit the Mouthner escape reflex under these conditions (Canfield andEaton, 1990).

ImplicationsThe sonar guided target intercept behaviour of the bottlenose dolphin may be seen toexdude detection of secondary targets once the PRF locks to the fish. That dolphins areobserved to forage successfully and to catch fish in an obstade filled environment (i.e. doseto rocks, cliffs, harbour walls, etc.) would appear to support an argument that muchenvironmental data is retained hom prior exploration oE the habitat and the animal maynavigate within a memory mapped environment during target interception. Thishypothesis requires that unfamiliar or temporary (net-type) obstructions be madeacoustically detectable at the dolphin's maximum sonar search range. Based on the Ambledolphin's observed behaviour, the net target strength must therefore significantly exceed-35 dB (at 120 kHz) when observed within the animal's high frequency (l00) beamwidth.Several prototype designs for small efficient acoustic reflectors which return this echostrength, regardless of the direction of the incident sound, have been cornpleted. Thesedevices are now at the early stages of testing using both wild and (naive) captive dolphins.The prelirninary results frorn these tests appear encouraging, as they clearly indicate long

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range detection and seem to sÜmuhite positive avoidance behaviour (Goodson, Kliriowska,Bloom, Datta, Mayo, Wilson, Prickett and Sturtivant, 1992; Mayo and Goodson,1992; Mayo,Goodson, Sturtivant and Hayers,1992). However, the minimum distribution pattern'necessary to deter penetration of the net stnicture at close range has still to be optimisedand. the effects in a commercial fishery have nöt yet been evaluated, although. someprelirriinary feasibility tests iri a working fishing environment have taken plaee (Swarbrickarid Goodson, iri preparation).

hlitiai Rcsults froin the 1991 Moral' Firth Tririi

The site cllosen for the first fie~d test was the Moral' Firth, NE Scotland. A current photo­ideritity catalogue of approxirriately 150 individuals for the local bottleriose dolphiripopulation has been compiled (Wilsciri, Thompson and Hainmond, 1992).. Animals are,regularly sightcd within 200-600m of shore near the entrance to the Crorriai-ty Firth, \vherethere is good visibility from adjacent 50 in cliffs and ,the seabed in the zone of iriterest is flat

. (hard sand) \vith a minimum ,vater depth of 7 m. A barrier, consisting of a buoyant head­rope from which thin rope tails were attached, was deploycd perpendicular to the shore,across the predicted path of the dolphins. One particular prototype reflectorwas attached at2 m intervals to the rope tails, which were spaced 2 m apart. The head-rope was 200 inlong, half used as control and half supporting ~ grid of reflectors, eomprising an obstrucÜon100 m x 7 m deep (Figures 1 and 2; Tables 1 and 2).

Equipmcnt ririd ProccdureA detailed list of equipment is given in Table 1. The experiment extended that described bySilber (1989), with the dolphins being tracked by their sui-facirig positions using anelectronic theodolite, and underwater acoustic activity moriitored. The theodoliteemployed was also capable of working as a distance measuring device, and in this mode theinstrument could be used to accurately measure its height above sea level. Subsequentmeasureinent of horizontal and vertical angles enabled the Northings an? Eastings of eachsurfacing position, and of the head rope barrier, to be calculated and plotted. The times ofthese readings were also recorded. As only one theodoliteof this type was available, it wasusually not possible to track more than the leading ariimal(s) from each passing rriaingroup, even when several distinet sub-groups \vere present. To back up the theodolitereadings two video cameras and voice-Ioggirig recorders were used. The tiriderwatersounds, received from the sonobuoy hydrophone by radio telemetry, ,vere recorded on afour-track instrumentation machine, together with timecode ,arid a voice log. A secondreceiver. simultaneously fed the telemetry to an R-DAT. digital recorder. In general,observations could be maintained only between dawn arid dusk, as the team "Jas too smallto provide fuH 24-hour cover.

RcsultsControl sightings and reeordings, made before the barder was d~ployed (e.g. Figure 3),confirmed that dolphins passing in small groups, and in loose associations of up to about30 animals, did swim parallel to the cliff, in both directions, at a predictable distanceoffshore. As the barder was first beirig deployed, a group of dolphins approachcd. Therewas considerable acoustic acti~..ity and all the rinimals diverted to avoid the barder, takingan inshore passage. Late the following ri10rnirig the irishore anchor of the barrier dragged,but for the first afternoon and most of the next morning animals \vere observed prissing, inboth direciions, between the end of the barrier and the shore in a narrow zone oE veryshallow water (e.g. Figure 4). After some difficulty in obtaining stable moorings closer toshore, the barrier was finrilly repositioned during the penultimate mornirig to obstiuet theinshore passage. ,

In the early afternoon oE the final working day" t\vo distinct main groups of about 30animals, each with several sub-groups, were observed sequentially prissing the outer end of

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the barrier over about an hour. The track of a pair of animals leading the first sub-group ofthe second main group is shown in Figure 5. After tracking this pair through the test zone,and while other sub-groups were still passing, a single animal was seen to surface some55 m from the centre of the barrier. The subsequent track of this animal is plotted inFigures 6, 7 and 8. The single animal retreated, at minimum speeds of about 3-4 m/sec, to adistance of 170 m. This apparent retreat from the barrier was interrupted several times,with the animal backtracking briefly while swimming at much slower speed. The animalfinally altered course to join the track of the rest of the group and, dosely following this,swam past the outer end of the barrier. Then, as had been observed for other passinggroups, the single animal appeared to investigate the back of the barrier (dosest point ofapproach 55 m) before leaving the area, continuing along the usual line of passage. Theremarkable similarity between these two main tracks can be seen by comparing Figures 5and 6. So far it has not been possible to establish whether the same or different animalspassed during the days of the experiment, although some dose-up video and photographstaken from tourist boats have still to be assessed. It is also possible that some individualidentification information may be obtained from analysis of "signature" whistles.However, from the experience of the photo-identification team (Wilson - personalcommunication), it seems likely that the groups were different.

Subsequent analysis of the recorded undervvater sounds demonstrated no obvioueecholocation activity which can be assigned to the approaching single dolphin until sevenseconds before the first surface plot made as it retreated. At that time a burst of dicks at arepetition rate indicative of target detection at 20 m range is apparent. Slow motion replayof the video record shows that at the first surfacing position the animal is swimmingrapidly away from the barrier. This is the single recorded close approach to the barriermade during the study period, although a large number of animals (SOor more duringdaylight hours) passed the site each day.

CondusionsThe tracks reconstructed to date seem to indicate that the leading animals became aware ofthe barrier position at a maximum range of 150 to 170 m - a much greater range thanpredicted. However, two significant factors may help to explain this: 1) The dolphins wereapproaching in a direction normal to the plane of the barrier. At a range of 170m a 10·beamwidth will excite nearly simultaneous echoes from the reflectors spread alongapproximately 30m of the barrier, which effectively increases the target strength. Thiswould not be the case if the animals approached from a more oblique angle, as the multip*echoes then arrive sequentially. 2) The quiet sea (Sea State 2 or less) provided excellerPacoustic conditions, and a very flat sandy seabed contributed little confusing reverberation.

The single animal may have been travelling in a low-awareness or resting state (as hasbeen noted elsewhere for non-Ieading animals - Hatakeyama et al., 1990). Whether itsbehaviour was triggered by the activity of other animals beyond the barrier, or by one of therandom loud clicks that have been occasionally noted from other resting animals, has notyet been established. However, if areal gillnet had been in the position of the test barrier,this individual seems a likely candidate for entanglement.

Although the data obtained in this first field test are limited, and our detailed analysisis still incomplete, the test results appear promising and exceed our expectations. Theprotocol employed needs further refinement, but generates detailed interaction data at ratesfar faster than in conventional fishery monitoring, and without any risk to animals.Further work towards optimising various parameters, including the reflector design anddistribution spacings, will take place in a larger test planned for September 1992.

Initial Handling Trials at Sea

A short sea trial took place in June 1992, where both modified and unmodified panels ofgill-net were shot and hauled in order to evaluate handling problems. Additionally the

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, ' 9acotistic detectability of echo-enhanced net panels was compared with equivalent im­modified sections at different ranges and angles usirig a 100 kHz sidescan sonar.

The application of any such acouslic device to a coinmc~cial fishing net createspractical problems e.g. 1) There is a predictable iricrease in the voluine of the rriodified netwhich riuiy overEill a standard net storage bin. 2) The handling of the. riet duringdeployment, recovery and during transfer betwcen net pounds on board ship may beimpaired. 3) The change in buoyancy caused by the reflectors may affect the deployrrient ofthe net iri the \vater. ' , ,

Equipmerit and proccdure ,To investigate the significance of such cffects a short sea trial was arrangedon board a UKgill-riet fishing vessel, the 15.25 m (overall length) BRITTANIA V (FH 121). A test riet,based on a commerdal tuna net, was prepared (Table 2, Figurc 9). The reflectors had beenpreparcd, by ci commercial twine manufacturer, within a mixed fibre flat braid. Thistcchnique was chosen with a view to ease of handling and reducing the likelihood of"outioning", which would cause adjacent layersof netting to catch together. Braidirig alsoavoids the torque effects that occur in a conventional rape \vhen under tension.

Deployment Results "The fishing vessel deployed the fleet of four test nets in ri depth of 54 m abc)lit 6 kmoffshore in calm seas wHh rio ,vind. The fleet ,vas shot with no difficulty, the nets passingeleanly' from the net poimd and over the sterri raiL The unmodified riet parieIs deployedcorrectly and seUled into the water ,vHhout problems, out themodified panels appeared tobe prevented from unfurling immediately to fuH depth by the presence of the reflectorstrings. It seemed likely that once \vetted, the mixed-fibre tightly wovenbraid rernainedvery buoyant \vith trapped air, and the lead cored footrapc had insitfficient weight to dragthe rough-surfaced hanging lines off the hcadrope and mesh. After about 15 mins thiscondition did improve somewhat, although the problem remained through01,ii the90 mins total immersion time. Iri the opinion of the skipper, such a net would not becapable of fishing as it should. , ", , . ,

The nets were hauled. through a Tarbenson 750/500 drum-belt type hauler. Thehaulirig process was a smooth operation. Norie of the nets or reflectors were damaged, andall the reflectors remained in place ,within the braided tube. The hauler. pulls oothheadrope and footrope together, and the hanging lines of reflectors, \vhich \Vere notattached to the net webbing, tended to form into loose bights, presenting a potentialsnagging problem. ,

It was also noted that, when stowing the wct nets, the modified riets occupicd some30% more voluine than the unmodified riets, preseniing an unacceptable problem iri acommercial situation. Orice the nets were hauled, the process of transferring thern into rietbins (turrting the net over) ,vas more difficult \vith the rriodified panels, as there was somebuttoning effect of the reflectofs between layers of net. ,

Dolphiri and Sidescan Sonar Charaetcristics " ,The sidescari sonar equipment employed iri this test was seleded for its nomirially similarsignal charaderistics to those of thc bottleriose dolphin. The Waverley 3000 sonartransmits a Source Level (SL) of SOIl1e 227 dB re 1 J..lPa at 1m, whieh is the same SL as themaximumrecorded for a bottlenose dolphin. The sidescan 100 kHz operating frequency isalso elose to the high. frequency peak (l20 kI-Iz) riormally present in the dolphin's \videbariddick spectrum (Au, 1980). The sidescan' sonar transmils two fan-shaped, sonar beams, oneon each side of the "tow-fish" track, each with a horizontal beam\vidth of 1.5·. In thevertical plane, both beanl\vidths are 50·, although the transmission centre-line is depressedapproxirnately 10· below the horizontal in ,9rder to favour bottom search operations.(Note: some sidelobe sensitivity exists outside these angles for strang target echoes.) Thesidescan sonar images the seabed and midwater targets on each side of the tO\ving ship's

10track and progressively builds up a two dimensional picture on a thermal printer, due tothe vessel's forward movement. The tow-fish is fin-stabilised and should runhorizontally, without a significant roll angle, at a depth defined by the towing speed andlength of towing cable.

In contrast, the dolphin's biological sonar has evolved as a highly directional forwardlooking sonar. This system appears optimised to search for targets within a 10· conical"spotlight" beam projected ahead of the animal along its swimming axis. While travelling,small cetaceans are believed to stay relatively dose to the surface. However, when activelysearching for food, they may dive much deeper while employing slow sonar transmissionrepetition rates indicative of a long search range (of the order of 60 m - 90 m). Thebottlenose dolphin appears to adapt its search radius to the propagation conditions, and inshallow water its sonar appears to function as a reverberation limited system.

Interpreting the Acoustic Images of the NetAseries of 13 runs were made past the net at different distances and orientations. Thesescans were made by towing the sidescan tow-fish at a speed of about 4 knots with 50 m ofcable between it and the vessel. This combination of forward speed and short cable lengthresulted in the tow-fish running at about 15 m depth. For most of the runs, the tow-fishseems to have been running with a slight tilt (roll angle), so that one side favoured t~bottom and the other the surface. When the towing vessel turns between scanning run"the forward speed of the tow-fish falls, causing the device to run deeper. The short towcable, needed to avoid scanning too deeply, unfortunately results in more wake disturbanceappearing within the images. These echo "douds" are a result of propellor-inducedturbulence and entrained (very fine) air bubbles, which generate a significant volumescattering echo.

Since a drifting gillnet is expected to hang vertically from the surface, and the sidescansonar traces displaying slant ranges give little height information, it is apparent that a tow­fish view from the footrope depth should make this lead cored rope the first targetdetected, with the floating head rope echoes arriving a little later. Likewise, if the tow-fishis run at a depth equivalent to the middle of the net, then both floatline and leadlineechoes will arrive together. The problem is compounded in practice, as the wake of thetowing vessel applies a variable side thrust which tends to tilt the net curtain from thevertical. The very light breeze during the trial also tended to drift the head rope positionrelative to the bottom of the net. In any event, the 18 m deep net appeared compressed inthe images, with respect to its distance from the tow-fish, and some care was needed whe,identifying specific net components.

That said, the sidescan images of the experimental net are quite informative. In aninitial scan, the net position was dearly detectable at 120 m. At this distance however, thenet's component structure cannot be resolved. Closer scans made at 45 m distancedelineate both the headline and leadline but, even at 20 m, the dosest point of approach,did not yield any detection of the gillnet mesh. The acoustically modified panels showed asin-filled structures due to the reflectors. The gap between the modified panels was alsoc1early detectable in most scans and, on some runs even that between the unmodifiedpanels was just discernable.

DiscussionDetection of the drifting net by the sidescan sonar was aided by the flat calm sea conditionsand by the depth at which the scans were made. These factors, together with a slight rollangle on the tow-fish, may have exaggerated the detectability of the headline component,which was clearly the strongest C0l11pOnent in all of the images. In any sea state other thanzero, and especially if the dolphin sonar is opera ted much nearer the surface, the detectionof the headline component may be more in question. Wave trough masking effects andthe lack of submerged depth of the headline floats will ensure that negligible echoes returnin the horizontal plane. A diurnal refraction effect can exist due to steep temperature

•

11gradients in upper surfaee layers. This effeet builds up as the suri, \varms the surface diirlngthe day and resuits in strorig refraction of a horizoritally projected sonar signal away fromthe surface. The thermal gradient effect reverses during night time cooUng, and as a resultthe headline components may become rather more deteetable during the night~ In highseastates the headline pulls from the surface, ,bridging ,bctween ,wave crests, and it isprobable that this repeating exit-entry action \vill eritrain fine air bubbles, which tend to bedriven down, making a detectable bubble dOtid in the vichiity of the surface near, the net.However in still rougher \veather, the breaking wave crests aerate the water colurrin toconsiderable depths below the surface layer arid any fishing riet gerieratcid aeration will bemasked..

Acoustic net enhancerrient to make the net \vebbing appear, as a "wall" is needed ifcetacean passage through the large gap apparent between headline and leadline is to, bedeterred. However, acoustic erihancement alone may not be very effident, as the quesÜonof hmv far an ariimal, having detected a barrier, is prepared to be deflected off its coursemust be. considered. It seems reasonable to suppose that the provision of safe andidentifiable passage positions, eHher over the riet (employing sub-sudace headropes) or asgaps between fleets of riets would be useful, although the exact dimensions of a gap \vhichcetaceans will interpret as "significant" require further investigation. In the opinion ofsome \vorking fishermen, gaps can be added ..lt the joints' in existing giIInets withoutdisadvantage to the fishing operation, although this also rcquires further investigation.The advantage of gaps between fleets, as opposed to sub-surface headropes, is that the latter .technique requires many additional buoys which cannot easily be stowed in sinall vessels.

The mechanical problems of attaching the reflectors at regular intervals to a net in away that minirriises the gear handling problems dearly' require further attention.Essentially this is a problem for fishing gear technologists and fishermen to resolve, andwill necessitate experimentation in Cl real fishery at sea., The prototype reflector tested todate is a cheap commerdally available componerit which ineets many (but not all) of thedesign criteria. With experience and feedback froin a commercial fishing operation, someredesign of this device (whilst inaintaining tlie acoustic characteristics) to iinprovemechanical handling and net attachment characteristics, can be considered. Howcver witnonly slight modifications to the existing method of attadunent the techriique can be takento sea for more thorough assessment, particulafly with respect to commerdal catch rates oftarget species.

ConclusiorisThis handling trial demoristrated that' the acoustic modifications under test performedasintended and filled the apparent space between, headrope and footrope effectively.Detection of the supporting lines between eacn reflector \vas predictable, as the large cross,:"section braided lines retained traces of air, and the duratiori of this. trial provided .insufficient soak time to remove this cffect. The buoyant headrope and lead-cored footrope,vere easily detected. The headrope echo compone~t could be easily lost to a sonar near thesurface when the sea state increases. The effect of aeration in the wat<~r, in this caseentrained by the towing ship's propeller, dearly iIIustrated the volume-scattering maskingeffects to be expected iri higher seastates. , .

The marked detectability of the 10m gaps behveen the modified parieis wasantidpated, but it was interesting to note that in some,of the scans the equivalent gapbetweeri unmodified panels was also just perceptible. The panel ends ai each side of thegap were reinforeed with a 4 mm hanging !ine. . ..

. The experimental rriethod of fastening the feflectors to the face of the nct needsfurther refinement, but the problems observed during this first tdal were primarily causcdby inappropriate choiee of braiding material, \vhieh remained too btioyant with trapped air,failed to slide easily oH the headrope and net, and addcid. considcrably to the bulk of ihemodified panels. Alternative materials and attachment techniques are being examinedwhich should overcome these difficulties. .

12The addition of these acoustic reflectors distributed as a 2 m x 3 m grid across the net

face does not imply an unacceptable increase in capital net costs when incorporated duringconstruction. However, the additional volume of the modified nets when wet wasunacceptable. Minimising the number of reflectors required will mitigate this problem (aswill different attachment techniques), as weH as reducing capital costs, and appropriateinvestigations are planned.

Whilst it is unlikely that all animals will be deterred from entanglement in fishingnets, regardless of whether acoustic modifications such as those described are employed,this acoustic engineering approach based on detailed consideration of small cetacean sonarbehaviour is designed to avoid the problems identified in earlier attempts. Althoughfurther modifications may be anticipated in order to satisfy operational requirements, thepotential of this technique to mitigate small cetacean by-catch in a commercial fishery nowneeds to be assessed.

Acknowledgements

The support of the Commission of the European Communities through the Eurogroup forAnimal Welfare, The Conservation Foundation, The Cooperative Wholesale Society,Racal Group Services and Racal Recorders Ltd., Sokkisha (UK) Ltd. and of visitors to theUK dolphin centres at Windsor Safari Park and Flamingo Land, is gratefully acknowledged_We also thank our team colleagues and especially the large number of volunteers whoseassistance made the intensive field studies possible. We particularly appreciate thecooperation of Ben Wilson and colleagues at the University of Aberdeen Field Station inCromarty (including use of their boat), as weIl as the help and support of many local peoplein Cromarty and the surrounding area. The support of the sea trial by the Sea Fish IndustryAuthority, Leach and Turner Fishing Gear, and by the master and crew of the MFVBritannia V, as weIl as the help and advice of Julian Swarbrick and the loan of the sidescansonar equipment by the Defence Research Agency (Bincleaves) is also acknowledged withmany thanks.

References

Au, W.W.L. (1980) Echolocation signals of the bottlenose dolphin (Tursiops truncatus) inopen waters. p. 251-282. In: RBusnel and J.F. Fish (Eds) Animal Sonar Systems PlenumPress New York. xxiv + 1135 pp.

Au, W.W.L. and Jones, L. (1991) Acoustic reflectivity of nets: implications concernin'incidental take of dolphins. Marine Mammal Science 7(3): 358-373.

Bloom, P.RS. (1991) The diary of a wild bottlenose dolphin (Tursiops truncatus) residentoff Amble on the North Northumberland coast of England, from April 1987 to January1991. Aquatic Mammals 17(3): 103-119.

Bloom, P.RS. (1992) The 1991 diary of a wild, solitary bottlenose dolphin resident off theNorthumberland coast of England. 20th Annual Symposium of the European Associationfor Aquatic Mammals, Brugges, Belgium.

Bloom, P.RS. (in press) The movelTlents, activity and behaviour observed during threewinter and three summer 24 hr periods of a solitary bottlenose dolphin (Tursiopstruncatus) on the orth Northumberland coast of England. Aquatic Mammals

Canfield and Eaton (1990) Swim bladder acoustic press ure transduction initiatesMouthner-mediated escape. Nature 347: 760-762.

13ColIet, A., Antoine, L. and DaneI, P.(1992) Preliminary report on the incidental catches ofdolphins in the Northeastern Atlantic French tuna fishery. 6th Annual Conference of theEuropean Cetacean Society, San Remo, Italy..

Dawson, S.M. (1991) Modifying gillnets to reduce enianglement of cetaceans. MarineMammal Sciellce 7(3): 274-82.

Goodson, A.D. (1990) Environment, acoustics and biosonar perception. Optimising thedesign of passive acoustic net markers. International Whaling Commission. La JollaSymposium on mortality of cetaceans in passive fishing nets and traps. IWC/SC/090/G17.

Goodson, A.D. ami Datta, S. (1991) Extracting behaviour patterns from the echolocationsounds of a wild bottlenose dolphin (Tllrsiops tnmcatlls). 19th Annual Symposium of theEuropean Assodation for Aquatic Mammals, Riedone, Haly.

Goodson, A.D. and Datta, S. (l992a) Dolphin sonar signal analysis: factors affecting fishingnet detection. 6th Annual Conference of the European Cetacean Society, San Remo, Italy.

Goodson, A.D. and Datta, S. (l992b) Classification of underwater sounds from thebottlenose dolphin. 20th Annual Symposium of the European Association for AquaticMammals, Brugges, Belgium.

Goodson, A.D., Datta, S. and Van Hove, M. (1991) Extracting the behaviour patterns fromthe echolocation sounds of a bottlenose dolphin (Tllrsiops trllllCatlls) Aqllatic Mammals17(2): 62.

Goodson; A.D., Klinowska, M. and BIoom, P.RS. (1990) Enharicing the detectability offishing nets. International Whaling Commission. La Jolla Symposium on mortality ofcetaceans in passive fishing nets and traps. IWC/SC/090/G16.

Goodson, A.D., Klinowska, M.. and Morris, R (1988) Interpreting the acoustic pulseemissions of a wild bottlenose dolphin (Tllrsiops trUllCatlls). Aqllatic Mammals 14(1): 1-6.

Goodson, A.D., Klinowska, M., BIoom, P.RS., Datta, S., Mayo, RH., Wilson, B., Prickett, W.and Sturtivant, C. (1992) Wild dolphins and fishing nets: a low-risk technique for assessingaeoustic reflectors. 20th Annual Symposium of the European Association for AquatieMammals, llrugges, llelgium.

Hatakeyama, Y., Ishii, K., Akamatsu, T., Soeda, H., Shimamura, T. and Kojima, T. (1990)Acoustic studies on the reduction of entanglement of Dall's porpoise, Phocoenoides dalli,in the Japanese salmon gillnet. .International Whaling Commission. La Jolla Symposiumon mortality of cetaceans in passive fishing nets and traps. IWC/SC/090/G9.

Hayase, S., Watanabe, Y. and Hatanaka, T. (1990) Preliminary repoft on the Japanesefishing experiments tising sub-surface gillnets in the South and the North Pacific, 1989­1990. International Whaling Commission. La Jolla Symposium on mortality of cetaceansin passive fishing nets and traps. IWC/SC/090/G58.

Hembree, D. and Harwood, M.B. (1987). Pelagic gillnet rnodification trials in northernAustralian seas. Rcp. Int. Wlzal. Commn 37: 369-373.

y IWC (1990) Report of the Workshop on Mortality of Cetaceans in Passive Fishing Nets andTraps, La ]olIa, Ca. IWC/SC/090/Rep.

14

Johnson, es. (1966) Auditory thresholds of the bottlenosed porpoise, Tursiops truncatus .(Montagu). NOTS TP4178.

Klinowska, M. (1990) Review of cetacean non-acoustic sensory abilities. InternationalWhaling Commission. La Jolla Symposium on mortality of cetaceans in passive fishingnets and traps. IWC/SC/909/G18.

Klinowska, M. (1992a) Exploitation of the non-acoustic senses in relation to theentanglement problem. 6th Annual Conference of the European Cetacean Society, SanRemo, Italy.

Klinowska, M. (1992b) Non-acoustic approaches to the entanglement problem. 20thAnnual Symposium of the European Association for Aquatic Mammals, Brugges,Belgium.

Klinowska, M. and Goodson, A.D. (1990) Some non-acoustic approaches to the preventionof entanglement. International Whaling Commission. La Jolla Symposium on mortalityof cetaceans in passive fishing nets and traps. IWC/SC/090/G19.

Mayo, R.H. and Goodson, A.D. (1992) Interaction between wild dolphins and a mooredbarrier: initial results from the 1991 Moray Firth Trial. 6th Annual Conference of theEuropean Cetacean Society, San Remo, Italy.

•Mayo, R.H., Goodson, A.D., Sturtivant, e and Hayers, R.J. (1992) Non-intrusive tracking ofTursiops truncatus: breathing rates and swim speeds. 20th Annual Symposium of theEuropean Association for Aquatic Mammals, Brugges, Belgium.

Silber, G. (1989) Response of free ranging harbor porpoise to potential gill-netmodifications. 8th Biennial Conference on the Biology of Marine Mammals, Pacific Grove,Ca.

Swarbrick. J. and Goodson, A.D. (in preparation) Initial trials to increase acousticdetectability of drift nets used in the albacore tuna fishery. Seafish Report No. Sea FishIndustry Authority, Hull.

Wilson, B., Thompson, P. and Hammond, P. (1992) The ecology of bottlenose dOlphin~(Tursiops truncatus) in the Moray Firth. 6th Annual Conference of the European CetaceanSociety, San Remo, Italy.

Tablc 1

Eqllipmcnt for Moray Firth 1991 Trial

Radio Equipmcnt .Wide band sonobuoy (UEL 30059) l110dified for extended hfeMarine band communications by hand held transceivers (4)Tclemetry receivers: Yaesu Fr9600 (2); kom R1; AR 2002

Audio Recording Eqllipnwl1tRacal Store 4 DS - high speed instrumentation recorderAiwa HO-51 R·DAT digital audio cassette recorderNagra IV SJ reel to reel audio recorder

TimecodeYam EBU timecode generator ami reader

Video EquipmentSony Broadcast Hi-8VHS camcorderJVC portable recorder

TheodoliteSokkisha Set 5, EDM prism and data logger

ComputerWalters 386 Notebook (IBM compatible)

VehiclesFord camper (base)Shogun 4 x 4 (transport)

Boats7 m hard chine double hull motor boatZodiac inflatable with outboard motor

Tesl Barrier200m headline made up 10 the same specifications as the headline of the current EasternAtlantic tuna gilJ-nets, uscd 10 support the reflectors (see Table 2)

Tab'" 2

Nel Specification for 1992 Sca Trial

Mesh

Twine size: 210/18 1 (420 lex) red l1ylon l11ultifilamcntMesh sizc: 168 111111 slrelched (6.625 inches)

PanelMesh long: 588Mesh deep: 125.5Slretched panel lenglh: 100 111

RiggingHanging ratio (E) 0.55Staple settings: 2 full meshes onto the staple lengthStaple length: 197 ml11 (7.375 inches)Set depth: 17.8 mSet length: 55 m

Flotation: one polyurethane 350g buoyant float every 1.1 m (44 inches)Leadline: No. 4 rein{orced, runnage 11 kg/l00 m

Prototype Acoustic ReflectorsTarget strength: nominal -35dB (reE. 2m radius sphere)Rigged in a 2 x 3 m grid across the face of lhe net

Reflectors: plastic, elliptical, air-filled, 20 g weight in air, 20 g lift in seawater (nominal);length 67 mm, maximum diameter 33.5 mm, axial hole 10 mm internal diameterAttachment sheath: braided polyethylene/polypropylene/worsted twine composition;runnage 35.3 g/m

Reflector vertical spacings ((rom headrope downwards): 3 111, 6 m, 9 m, 12 m, 15 mReflector slring horizontal spacings: every 2 m along the net

1 210/18 is a Denier notation for twines.

HEAD UNE - FJoaIs 8144" spaci~

REFl.ECTORS

--- SAN08AGS--

2 metregrid

()( lEADUNE

•Figure 1. Design of the experimental headline barrier.

!.....-.•

PPBJ1CTED PATH Of DOlPHlNS

~,---- ----- --~\,

llDALFLOW

~_._---------. '. '.'.'

Barred with reflector grtdSupported fram headline

Contral - Headline only

II

IIII.

Figure 2. Diagram of deployment plan for the headline barrier.

t I I , ,~

I II,

I ~A"'....'i

'.'.~

..••." ...

\ ,.. ........Y'-,L • So fVOI UO

"'X ....~ I,~ ~. i ! i!- ". , ,

200

100

eoo700

eoo500

400

300

.1~tm I tE3mH]\../. .-

.200 !!I "" I!'111 1111 . '.

300 100 ·100 -300 -600 ·700 ·900 .1100200 0 -200 -400 -600 -800 ·1000

Northlngs (metrss)

eFigure 3. The passage of the leading animal (direction shown by arrow, surfacings bytriangles, dotted line represents minimum distance between surfacings) of agroup of five bottlenose dolphins (including one accompanied by a caIO be[oredeployrnent of the headline barrier. Deployrnent of the sonobuoy is cIearly notassociated with any deviation in the line of trave!. 0 - denotes position ofobserver on shore; x - position of buoys marking crab pots.

·100 -300 -500 ·700 -000 ·1100o ·200 -400 -600 -800 ·1 000

Northings (metms)

I I I I . I ·1I

i I I II ,.I I Ii II

I I II I I

"'1\ ~.-~ r ..... I:'-I-Il

.,c '% RD It-J'NE

I X4" \. II \,. I I ~ ~ I ,

I !- ! ,Lc ;. • I . .I

,.

~ l I.ii ~".

-200300 100

200

~ 800~'cn 100~

600

i 500

~ 400l..iJ300

200

100

0

·100

Figure 4. The assage of the leading animal of a group of 8-10 (including 2 ju~eni1es) afterde l~ ment of the headline barrier. Although the initial approach ~s exactly ~nth~li~e shown in Figure 3, there is a clear deviation inshore to aVOld the barner.(For further explanation, see Figure 3.)

·700-300-200

'"'R~ ,

,\ C, RECilON, \,, \

,

'\..*_50~OBUOY\

\\, \,

,

~.~ ~p •••n{

.... ,

/,,,

- ·1,

HEADUNE ~..'. n

~...._- G

·rp·,·····

h" '" ,. "

J!J·200

-100

·100

400

o

100

Northing (metres)

Figure 5. The passage of a pair of anim~l~l~ading--~h;fi;~t'~ub-groupof the second main •group on 30 September 1992, after final deployment of the headline barrier closerinshore. (For further explanation, see Figure 3. Crab pot buoys and observerposition not shown, surfacing positions shown here by squares.)

·700-800-300 -400 -600

Northlng (metres)

fJ. I I Ii

"--j--""",'.,.

:

4. ~

~\, [ lRECTlON\ ,

\'.

]\,\.....\:-5C NOBUOY

~.....................•...

~/--4HEAOUNE i

~ "-alt.f$ ./'~

'e::

"

·100

·200·100 ·200

500

100

o

600

400

lCI) 300~,~i 200lij

Figure 6. Passage of a single animal, a trailing member of the second main group on30 September 1992, first sighted 55 In from the headline barrier. Note that in thisFigure the arrow only indicates the general travcl direction. (See text, Figure 3,and Figures 7 and 8 for further explanation.)

'.r.....a......- .J.:6m/8 S'v\ IMSPE EDS" ----................... --..." ..........: --200m ."" '. 1.3m/sHEADUNE V ~"-

~."-- ~ i

\\2.0mls3.6mls~

i\

~·~1m/Sm/a \ \4':: mI ,.......1iS t: ~.:4 \3.1. ,'. 2.S'm/s

2.6m1a I ~, .4\ ~ \3.7m/a~~1 ....

~.8m/s, .... f',.\..........>1.4m1s

1.8 mla,

I-28:16

I

Figure 7.1

27:09 -""'

26:02

14:25:33 -4~

!!. .A...... \

-,--+---4---"-

31:25--~i:.; I I~....:. i TIMeO TRACK

• ~-t3....;.1::...;.;02:""-_"-r--_-_··_··.+./....;:.....:::::....;:;.:,....-+:...,::::-..--+I-~k--l,~~ ...~...,... ''''Ä-- -29:35

.,,'" l,./. A

. ~ - :-29:03

Time between surfacings of the animal from Figure 6. Figure B. lI

Minimum calculated swimming speed, and direction of travel, betweensurfacings of the animal from Figure 6.

r\~.. ,.~.. ~ :.~= "'.'.==\ I;, . , . , , , 1! ' t' ",

. ~ ~ ; ; ~ 1 i··:;·:,

\ ~ i I , • ,! . i , • : • , •

\ ' !;" _, ;, ! - ! ! . ! ' , i ; i

. . . - ; .. -, .,::

MOOIFIED PANEL GAP MODlFIED PANS.-.--------•. - ---'--1Ofn -._..

UN-MODIRED GAP UN-MOOIFIEO--·--··-···----1Om- -....-------.-

220 metres Long x 18 malres Deep

Figure 9. Arrangement of the net panels für June 1992 sea trial. .