passive underwater acoustics

150 Pages·1999·3.87 MB·English

TABLE OF CONTENTS
I. Introduction
II. Background Information
Harbor Porpoise Phocoenap hocoena!
Standard Gillnets and the Gillnetting Industry
Underwater Acoustics 10
III. Methods 18
Equipment
Trip Procedures 26
Measurements- Methods and Data Analysis 29
IV. Results 36
Currents
Ambient Noise
Gillnet Noise 51
Modified Gillnet Noise 51
V, Discussion 57
VI. Conclusions 65
VII. Acknowledgements 67
VIII. References
IX. Appendices 72
AppendixA ; AcousticM easuremenDta taT ables 72
Appendix B: Average Acoustic SpectrumL evels 82
Appendix C: Acoustic Mesh Plots 110
Appendix D: Acoustic StandardizedM esh Plots 130
I%I'RODUCTION
Gillnets are widely used by commercial fishermen in the Gulf of Maine, landing
approximatelyt wenty percent of groundfish catch in the area Polachek, 1989!.
Unfortunately, thesen ets have also been responsiblef or entanglinga nd drowning the harbor
porpoise, Phocoena phocoena. Due to the lack of accurate population counts and estimates of
mortality, the actual impact incidental takes have on the harbor porpoise is difficult to
determine. Indirect evidencei ndicatest hat porpoisesi n the Bay of Fundy and Gulf of Maine
form a single population unit Read and Gaskin, 1990!. Further evidencef rom studieso f
summerd istribution patternsa nd life history parameterss uggestt hat this population is in a
stateo f decline Read and Gaskin, 1990!. The primary causef or this decline is incidental
mortality by commercial gillnetting Gaskin, 1984!.
Harbor porpoises are vulnerable to entanglement in gillnets because they are relatively
small, they inhabit near shorew aters, and they feed on commercial fish. The problem is
enhancedb ecauseg illnets are madeo f monofilamentl ine, which is very difficult for the
harbor porpoise to detect. Detection of obstaclesc an occur by vision, echolocation,o r
hearing. Monofilament line is transparent,m aking it difficult to visualize.
The relationshipb etweent he acousticalp ropertieso f the gillnet and harbor porpoise can
be broken down into two parts, active and passivea coustics See Figure 1!. The active
acousticsd eal with the target strengtho f the gillnet. The net's target strengthi s the intensity
of reflection to an incident signal i.e. echolocationc licks producedw hile harbor porpoise
foraging!, If the target strengtho f the net is below a certain thresholdl evel, the porpoise
will not detect the net by echolocation. The active acousticso f the net have been well
studied, including experimentsi n which the net has been modified in hopest o increasei ts
target strength Hembreea nd Harwood, 1987; Ogiwara et.al, 1985; also seer eview by
Dawson, 1990!.
Passive acoustics can be defined as the interaction between a non-echolocating harbor
porpoisea nd sounde mittedb y the gillnet. The acoustics ignalf rom the gillnet resultsm ainly
from current flow through the net, causingi t to strum and emit noise over a certain
frequencyb andwidtha ndi ntensity. !f this signalo ccursa t a frequencyw ithin anda t a
sourcel evel above the auditory sensitivity thresholdo f the porpoise, the harbor porpoise

should be able to detect the gillnet's presence unlessm askedb y more detectabled eterrents!.
Researchers have found that porpoise often travel through familiar areas using passive
navigationt echniques,l istening to noise from the environmento r following one or more lead
swimmers Hatakeyama,1986G; oodsone t al,1990!. It is therefores omewhats urprising that
very few studiesh ave focusedo n this system.
This project concentratedo n the basic passivea coustic interaction betweent he harbor
porpoise and gillnet as describeda bove. Our goal was to analyzep assiveu nderwater
acoustical characteristics of standard and modified gillnets and to determine how these may
relatet o the auditorys enseos f the harborp orpoise. The maino bjectiveo f this reporti s to
presento ur findings and to provide a foundationf or future research. A keen understanding
of this problem must be acquired so that action can be taken to reduce the number of
porpoisesk illed in gillnets in this area.
BACKGROUND INFOEGIATION
HARBOR PORPOISE Phocoena phocoena!
ASSESSMENT:
The harbor porpoise, Phocoena phocoena, is a fairly small (cid:3)-6 feet long! odontocete,
or toothed whale. It inhabits many places around the world. Research has concentrated on
thosea ppearingi n the near-shorew aters off the northeastc oastso f the United States.
Current estimateso f harbor porpoise abundancein the Bay of Fundy and Gulf of Maine
region range from 8,000 to 16,000 Reada nd Gaskin, 1990; Read and Kraus,1990!.
Approximately 100 porpoisesi n the Bay of Fundy and approximately6 00 porpoisesi n the
Gulf of Maine are killed annually Read and Gaskin,1990;R ead and Gaskin,1988;
Polachek,1989!. An averageo f 5.5 porpoisesp er gillnet fishermanw ere reported to be
caughti n 1986 in this combineda rea Reada nd Gaskin, 1988!. These estimatesa re very
rough for two main reasons. Harbor porpoisesa re very elusive and difficult to observea t
sea, rendering it near impossiblet o perform significant counting methods. Second,
fishermena re reluctant to report incidental takesb ecauset hey fear for their livelihood, which
is becomingm ore threateneda s the problem heatsu p. In order to facilitate the assessmenot f
the population and incidental catch numbers,t he Marine Mammal Protection Act MMPA!
was amendedin 1988 to provide a five year interim during which incidental takes are not
penalizeda nd sightersa re permitted aboardg illnetting vessels.
This estimatedr ate of mortality, however, seemst o be high enough to causea
decreasein harbor porpoised ensity. Reada nd Gaskin (cid:1)988 and 1990! have demonstrateda
correlation betweent he reduction of density with changesi n the parametersi n life history of
the porpoise. Changest hat have occurred since 1969 include femalesr eaching sexual
maturitya t a youngera gea nda n increasein calf length. Theset rendss uggestth e needo f
the speciest o alter its reproductive strategy,a ttemptingt o increasei ts reproductivel ife span
and ensuret he survival of its offspring, respectively.
SONAR:
It hasb eenk nownf or a veryl ong time that the harborp orpoiseis capableo f
echolocation,e mitting sound signalsa nd receiving their echoesf rom objects in the
environment. The actual mechanismsu sed for soundp roduction and receptiona re
controversial. Therefore, it is suffice to say that Phoeoenah as a relatively efficient
acousticals ystemw ith which it can accuratelyp erceive and identify most objects in its
surroundings.
Phocoena emits echolocating "clicks" at both high and low frequencies. Clicks have
been recordedf rom both frontal and ventral anglesf rom the porpoise. High frequencyc licks
are usually beamedf orward through a narrow emissionf ield. These clicks are primarily
usedw hile hunting for prey, typically small fish. Their frequenciesr ange from 110 - 150+
kHz. Low frequency clicks (cid:2) - 8 Khz! are produced at longer durations and are radiated
frontally through a broadere missionf ield. They are thought to be usedf or communication
and navigation, especiallyw hile cruising in unfamiliar territories Amundin, 1990; Johnson,
1966; Goodsone t.al, 1990!. It has beenp ostulatedt hat both low and high frequencyc hcks
can be, and are, produceds imultaneously. This suggeststh at the harbor porpoise can utilize
two acousticals ystemsa t the saine time Norris, 1968!.
Perception of these signals is crucial to echolocation, but the porpoise's hearing
mechanism must also be sensitive to a broad range of frequencies at variable source levels in
order to form a true picture of its environment. The minimum power level of sound required
for detectionc an be called the "auditory threshold." Few attemptsh ave been madet o
explain the mechanismsb y which soundi s receiveda nd interpretedb y small cetaceans,
Kellogg (cid:1)952 and 1953! reported that the bottlenosedp orpoise Tursiopst runcatus responded
with an avoidance reaction to short sound bursts between 400 Hz and 80 Khz and with an
attack reaction to soundb etween 100 Hz and 400 Hz. A threshold for sensitivity can not be
determinedf rom Kellogg's data since he did not give sourcel evels for the bursts of sound.
In 1966, Johnsono btaineda n audiogramo f thresholdv alues for the bottlenosed
porpoise over the frequencyr ange from 75 Hz to 150 Khz. The maximum hearing
sensitivity appearedt o be around 50 Khz at a level of -55 decibels Db! re 1 microbar. The
upper limit of hearing was determinedt o be 150 Khz at +35 dB re 1 rnicrobar. Figure 2
showsJ ohnson'sr esulting auditory thresholdc urve for Tursiops. Below 50 kHz, the
thresholdc ontinuouslyi ncreases sensitivity decreasesw! ith decreasingf requency, reaching
+37 dB re 1 ubar at 75 Hz. The thresholds lowly increasesb etween5 0 kHz and 100 kHz,
Figure2: Auditory Threshold- Tursiopst runcatus
Johnson, 1966!
where it occurs at -45 dB re 1 ubar. Sensitivity falls off rapidly above 100 kHz. Johnson
also included a human audiogram as shown Figure 2. This can be used as a reference for
understandingth e rangeso ver which the bottlenosedp orpoise and other small cetaceansc an
hear.
Hatakeyamah as beeni nterestedi n the hearing capabiiitieso f porpoisess ince the early
1980's. Experimentsi nvolving Dali's porpoise indicatedt hat the strongesta voidance
responseo ccurred from soundp ulsesw ith a frequencyo f 115 kHz at >96 dB Hatakeyama,
1983!. Hatakeyama(cid:1)9 86! also found that the Dali's porpoisew as insensitivet o low
frequency(cid:1)- 3 kHz! soundw avesa t pressurel evels up to 70 dB. This insensitivity may
reflect the porpoise's adaptationt o commona mbient, or "background" noise.
The only information availablec oncerningt he harbor porpoise Phocoenap hocoenai s
from Hatakeyamae t al's (cid:1)988! observationso f this animal's behavior to a salmon gillnet.
Their audiogram see Figure 3! showedb est hearing sensitivity ranging from 4 kHz to 40
kHz with a thresholda t 50 dB. The thresholdi ncreasesb y about 15 dB/octaveb elow 4 kHz
and between 40 kHz and 140 kHz and rapidly above 140 kHz.
The auditory capabilitieso f Phocoenas eemt o be worse than both Tursiopsa nd Dali's
porpoise Hatakeyamae t.al, 1988!. Figure 3 comparesa n audiogramf or Tursiopsw ith
Phocoena,d emonstratingt he greater sensitivity of the bottlenosedp orpoise. This difference
is not unusual. Hatakeyamae t.al (cid:1)988! statedt hat the harbor porpoise showsb etter hearing
capabilitiesa t low frequencie sb elow4 0 kHz; up to 20 dB betterb elow 10 kHz! thanD ali's
porpoise. However, Dali's porpoisei s more sensitivea t frequenciesa bove4 0 kHz, with an
auditory thresholda bout 15 dB lower between8 0 kHz and 140 kHz. From theser eports, the
harbor porpoisea ppearst o have a weaker auditory thresholds hifted towards lower
frequencies, We must realize that thesea ssumptionsh ave arisen from only a few studies
only oneo f whichr eferst o the harborp orpoise!. With the presendt ataa vailable,i t is
difficult to determineh ow a harbor porpoisew ill respondt o soundso f certain frequencies
and source levels.
ENTANGLEMENT:
Although Phocoenap ossessehsi ghly sophisticatedm echanismsf or hearing and
perceivingit s environmenti,t still managetso becomee ntangledin gillnetsa ndd rown. The
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