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Theme Session (0) By-catch of marine mammals: ICES Baltimore September 1997 - File: ices97_2.doc M 1997/Q: 16Goodson et al. ·Set gillnet acoustic deterrents for harbour porpoises, Phocoena phocoena: improving the ec

Set gillrict acoustic detcrrcnts for harbour porpoiscs, Pllocoella pllOcoella:improving thc tcchnology.

Goodson A.D., Newborough D. & Woodward B.

AbstractThe report of the 1996 NMFS/MMC Seattle workshop on Acoustic Deterrence 01Marine Mammal-FisheryInteractions refers to a number of concems about the long term emciency of active acoustic devices. The firstand second generation ofanalogue 'pingers' emit simple pings at regular intervals at relativcly low frequenciesand Source Levels. Rccent Europcan studies with captive harbour porpoises indicated that more complex widerband signals appear more aversive. and when such sounds were testcd with wild porpoises in Scotland theresuIting displacement exceeded 600 m for a SL of around 150 dB re 1 J.1Pa at 1m. Implementing such adeterrent using analogue electronic drcuitry, although possible, appears uneconornic. However, the use ofrnicro-processor digital tcchniques perrnit a variety of complex waveforms to be created using software andthese signals can be transrnitted with pseudo-random delays inserted between them \vith the intention ofreducing the porpoise's rate of habituation to these sounds. The use ofmicro-controller tcchnology has resuItedin an clectro-acoustically efficient and relativcly cheap dciice, that can sense its environment and generate a' ....ide variety of signals. Battery life when using a single alkaline '0' cello is predietcd to approach 1 year in acOInrnercial gillnet fishery and the Source Level does not dccline ,vith falling battery voltage. A furtherdevclopment based on these rnicro-eontroller Beacon Mode Devices is an lnteractive Afode Device where thesound source remains mute until triggered into operation' by an approaching ccholocating anima!. In. theformer case it is possible to incorporate a time-out feature to ensure that pingers lost at sea will deactivate after .apre-set period of several days.

Key words : acoustic, bycatch, harbour porpoise, digital, gillnets, interactive, pinger.

A.D.Goodson, D. Newborough & B. Woodward: Underwater Acoustics Group, Electronic & ElectricalEngineering Department, Loughborough University LE11 3TU, UK.Tel +441509222846, email: a.d.goodson@lboro.ac.uk

IntroductionParameters which need to be considered when designing an emdent acoustic deterrent deviceto minimise harbour porpoise by-catch in bottom set gillnets should i~clude:

• the generation of suitable aversive sounds which will cause a reliable displacement of theunwanted animal away from the net;

• be designed to have a minimum efTect on other species and a zero impact on the fishery' swanted catch;

• measures which make these devices difficult for aporpoise to become habitmited to;• a long reliable operating life with minimum or zero maintenance;• small robust packaging which should not introduce handling difficulties for the fisherman

when shooting or recovering nets.• a lo\v cost per unit.• adequate but not excessive Source Levels. A balance is needed between using a small

number of units widely spaced along the net and unnecessary ensonification of Iarge areas.The combined efTect of several boats working the same area should riot deriy habitat ormigration routes to porpoises by creaiing very large exclusion zones.

Some of these considerations can be seen to be mutually interactive and any final design acompromise between them.

The rate at which the acoustic energy is transmitted, and hence the battery lire, is directlyrelated to the emitted pulse length, its power and the mark/space ratio. However, habituation

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Theme Session (Q) By-catch of marine mammals: ICES Baltimore September 1997 - File: ices97_2.doc CM 1997/Q: 16Goodson et al. ·Set gillnet acoustic deterrents for harbour porpoises, Phocoena phocoena: improving the technology •

and desensitisation of the animal' s response may be deferred if the signal waveforrns presenteddiffer from ping-to-ping and also if irregular timings intervals are introduced between thesesignals. The habituation rate may actually increase if these pings are transmitted at too high arate or at very regular intervals. The use of pseudo-random inter-pulse time intervals requiresset minimum and maximum limits, the latter to ensure that an approaching animal cannotapproach a net zone without hearing several transmissions. Maximum swim speed data for theporpoise must therefore be considered a parameter for consideration. The continuousensonification of the fishing net's environment in the absence of cetaceans may be counter­productive as other animals such as pinnipeds may leam to associate these sounds with the netand increase their predation on the fish in the net. This factor may be reduced by choosingsignal characteristics that are less audible, or actually aversive, to these species but thedevelopment of an interactive deterrent, i.e. a device which remains silent or operates at avery low duty cycle, until triggered by porpoise echolocation behaviour, is seen as a practicalsolution that can now be exploited.

Many of these requirements would not be economic propOSItIOns if implemented usingconventional analogue electronics as the circuitry would be expensive and too bulky for this •application. Hardware costs can be reduced significantly, and the electro-acoustic efficiencyincreased, by changing the technology to exploit digital techniques and in particu1ar smart-card controller devices, e.g. ultra low power digital micro-controllers. The prograrnmableapproach perrnitted by these devices enables most of the features mentioned above to beimplemented with a minimum component count and this can be constructed using surfacemount technology into a miniature circuit assembly. A.s micro-controllers are prograrnmeddevices considerable flexibility remains available through the software for the refinement of thetechnique and for the development of alternative applications.

Solid Micro-Controller PiezoEncapsulation Circuitry Transducers

1------150 rnrn -----~

MountingHole

HARDWARE CONSTRAINTSThe construction of a deterrent device designed to operate for long period in this very harshenvironment must be robust. Devices must survive the sudden impact loads likely to occurwhilst shooting nets and then be repeatedly pressure cYcled with the static water pressure.Multiple device failures cannot be tolerated as these would leave acoustic holes in the net thataporpoise, attempting to avoid the sounds, might perceive as a safe passage. The distributionpattern of devices spaced along the net should therefore be planned to allow sufficient overlapbetween the zones influenced by each to accommodate a single unit failure. The earlydetection of any failed device must be possible, preferably whilst still at sea so thatreplacements can be instalied without delay. Planned replacement, before battery exhaustioncauses eventual failure, is aided ifan indication of the approach ofthis condition can be provided. Theprototype digital devices recentlydeveloped at LoughboroughUniversity have been moulded intoa sealedjor-life module using atough plastic encapulation as theworking life in a commercialfishery is expected to be aroundone year, (Figure 1). The economic Fig.l Cross-seclion through the prototype digital pinger

costs of re-working these devices on an exchange basis requires further study, but there aresome c1ear advantages to this approach. Changing batteries at the quay side is a task that most

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Theme Session (Q) By-catch of marine mammals: ICES Baltimore September 1997 - File: ices97_2.doc CM 19971Q; 16Goodson et al. ·Set gillnet acoustic deterrents for harbour porpoises, Phocoena phocoena: improving the technology •

fishennan would prefer to avoid and there are some environmental constraints concerning thedisposal of batteries which need to be considered.

.:.:.;

POWERSPECTRUM

B

AMPUTUDE

As these deviees operatefrom a regulated voltagesupply the acoustic signalcharacteristies do notdecline with falling batteryvoltage. The PIC micro­controller monitors thebattery supply voltage and Figure 2 - Example pulse spectrum - Frequency Sweep Signal, 300 msas this approaches a pulse sweplfrom 20 kHz Lo 160 kHz.

preselected lower limit the programme changes the transmit pattern and pulse rate to acharaeteristie double pulse that indieates the impending failure whilst still providing anaversive signal to the porpoise. The transmitted acoustie signals are expeeted to be detectableon most standard echosounders and this mode change warning of impending failure should beeasy to detect. The use of the ship' s echosounder to obtain arecord of the activity of eachdevice, whilst these are attached to the net on the seabed, has several other advantages. Lostnets should be easier to detect should they lose their surface markers or be dragged away fromtheir proper position. A unique code ean be ineorporated into the sounds transmitted so thatidentification ofthe net's owner or ship becomes possible.

ELECTRONICSThe PIC-microcontroller used to control, compute and then transmit these complex acousticwavefonns uses a software programme contained in an internal PROM. This offersconsiderable flexibility and the sounds may be changed to suit different target species andadditional features can be added without involving additional hardware costs. Several differentwideband waveforms, based on aversive sounds determined during tests using captive and wildharbour porpoises, have been programmed to be transmitted at pseudo-random intervals. Theexisting package is provided with capacitive sensing electrodes to determine when the deviceis submerged and not simply sandwiched in layers of wet net in the pound. When out of thewater the device is deactivated and held in a sleep mode but is programmed to wake briefly totest its environment for immersion at regular intervals (Life in storage is estimated from testsand from manufacturers data as a Half-life == 6 years).Onee aetivated by being submerged underwater this Beacon Mode device emits sequences of 8(or more) different ping wavefonns with varying time intervals between these, (Figure.2). Theselected signals have allbeen shown containaverSlve characteristicsand by presenting avariety of different soundsin this way the rate atwhich the animalhabituates to them shouldbe rninimised.

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Theme Session (0) By-catch of marine mammals: ICES Baltimore September 1997 - File: ices97_2.doc CM 1997/Q: 16Goodson et al. "Set gillnet acoustic deterrents for harbour porpoises, Phocoena phocoena: improving the technology •

FUTURE DEVELOPMENTSThese existing Beacon Mode devices have been supplied for use in an observed commercialgillnet fishery study planned for late 1997. Development is now continuing with tests of anInteractive Deterrent (Figure 3). These new devices, based on the same microcontrollertechnology and signal synthesis software as the Beacons, also incorporate a listening facility tosense the presence of echolocating animals. These devices may remain mute, or may functionas an acoustic light-house by transmitting warning signals at very low repetition rates. Whenan approaching echolocating porpoise is detected this characteristic alters and the device then

:~~:,IY ;~sS~~pi;o::~an;~:~~:: r:ll~§-~.·;:·.I.:~m,~m:'.~:;$~.m~~~t.)~!jii~.~~:~~~~:=feedback to the approachinganimal as the transmitted ping ratewill slow and then stop once thestimulus of the animal' s sonar isremoved as it turns away. Thisping-on-demand approach is seenas offering several advantages as itprovides the least encouragementto pinnipeds whilst it is reasonableto expect that it will be moredifficult for the porpoise tobecome desensitised or habituatedto this activity. As the ping rates

are demand driven then Figure 3. Block diagram to illustrate the /unction 0/anunnecessary ensonification of large lnteractive Mode deterrent.

areas around the net will also beavoided. It is anticipated that the ship's echosounder may also be used to stimulate a responsewhich will maintain most of the operational advantages discussed. One additional feature thathas been designed into this interactive mode device allows them to 'ripple-fire' sequentiallyalong the length of the net. A short dead-time is programmed after each firing to ensure thatthe devices do not enter a continuous retriggering mode, but once a device is activated by anapproaching animal the signal will then be echoed by its immediate neighbour and thesequence repeated along the length of the net. This approach was initially designed for a •pelagic trawl application where it seems desirable to be able to acoustically mark the surfacegeometry or envelope of the trawl and, by triggering a number of devices almostsimultaneously, drive cetaceans that have entered the net back towards the mouth.

CONCLUSIONMuch has already been learned from the analogue pinger trials carried out in North Americaand from cIoser studies of harbour porpoise reactions to sounds in captivity and in the wild.This new generation of digital Beacon Mode and Interactive Mode Devices, incorporate anumber of engineering improvements which attempt to address some of the known problemsand which are expected to improve the efficiency of these by-catch reduction acoustic devices.The flexibility obtained using digital programmable techniques also makes these devicesapplicable to a number of alternative applications. These new active acoustic deterrents cantransmit complex signal waveforms which have been shown to be aversive at very low SoundPressure Levels during tests with captive and wild porpoises. The operational life has beenappreciably extended and the unit costs minimised by employing computer aidedmanufacturing techniques. If adopted for commercial mass-production then the costs should

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Theme Session (0) By-catch of marine mammals: ICES Baltimore September 1997 - File: ices97_2.doc CM 1997/Q: 16Goodson et al. ·Set giltnet acoustic deterrents tor harbour porpoises, Phocoena phocoena: improving the technology·

be further reduced, however the production cost of the prototypes produced for the currentfishery tests were competitive with an available analogue producl.

For the future, the adoption of any deterrent device designed to mltlgate small cetaceanbycatch must not be assumed to provide a permanent solution to the problem. Deterrentswhich work in one fishery with one species of cetacean may not With another. Aninials doadapt to new and unusual stimuli with time and the bycatch reduction benefits obtained maynot be permanent. Continuing studies are necessary to determine if deterrent systems niaintain .their performance and the teclmology trap, which may be caused by introducing regulations toapply a specific tool to solve the problem must be watched for and avoided. Such legislationmay effectively remove support for continuing research and hence stagnate futuredevelopment work. This is an area where we still have much to leam and where continuinginter-disciplinary research is essential if we are to first achieve and then maintain a significantreduction in the incidental mortality ofthe harbour porpoise in commercial set gillnets.

ACKNO'VLEDGEMENTSThis work has been supported by the European Commission DGXIV, the UK Ministry ofAgriculture, Fisheries and Food, the UK Department of the Environment and by the DanishInstitute for Fisheries Research.' Technical assistance from Mr A. Webb and Professor T.Curtis (DERA-\Vinfrith) and Mr J. Hurford (Dayford Designs) is gratefully acknowledged.

REFERENCESKraus, S. Reael, A, Anderson, E., Bald\\in, K., Spradlin, T. and Williamson, J. (1996). A field test ofthe useof acoustic alarms to reduce incidental mortality of harbour porpoises in gillnets. (Report to NMFS ­unpublished)Lien J., Hood K., Pittman D., Ruel P., Borggaard D., Chisholm C., Wiesner L., Mahon T. and Milchell D.(1995). In: Sensory systems 0/aquatic mammals. RKastelein, J.Thornas and P. Nachtigall (eds). pp349-364.Kastelein, RA, Goodson, AD. Lien 1. and De Haan, D. (1995). The efTects of acoustic alarms on harbourporpoise (Phocoena phocoena) behaviour. In: llarbour porpoises laboratory studies 10 reduce bycatch.P.Nachtigall, J.Lien, W.W.L.Au and AJ.Read. (eds) ISBN 90-72743-06-7. ppI57-167.Kastelein, RA, De Haan, D., Goodson, AD. and Vaughan, N. (1997)The efTects of various sounds on a harbour porpoise (Phocoena phocoena). pp367-383. In: AJ.Reael,P.RWeipkerna, P.E.Nachtigall (cds), The biology ofthe harbour porpoise. pub. De Spil, Woerdcn, NL.Goodson AD., Amundin M., Mayo RH., Newborough D., Lepper P.A. Lockyer, C.,Larsen F. & Blomqvist C. (1997) Aversive sounds and sound pressure levels for the harbour porpoise(Phocoena phocoena): an initial field study. ICES CM 1997/Q: 17.Frady T., Northridge S. and Srnith T.S. (1994) Identifying potential modifications to sink gillnet gear to reduceharbour porpoise by-catch. NOAAINMFSINEFSC Report: Woods Hols, MA. NEFSC (Northeast FisheriesScience Center) Re] Document 93-25.Woodward, B. and Goodson AD. (December 1994). Prevention ofthe by-catch of cctaceans by exploiting theiracoustic capability). Report for the European Commission DGXIV. Special Study Contract PEMl93/04.Reevcs RR, Horman RJ., Silber G.K. and Wilkinson D. Acoustic deterrence of harmful manne mammal­fishery interactions, (1996) Proceedings of the Seattle Workshop held in March 1996. National MarineFisheries ScrvicelMarinc Mammal Commission, USA.

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