DTIC ADA572317: Low-frequency Target Strength and Abundance of Shoaling Atlantic Herring (Clupea harengus) in the Gulf of Maine during the Ocean Acoustic Waveguide Remote Sensing 2006 Experiment

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by  Defense Technical Information Center

Low-frequency target strength and abundance of shoaling
Atlantic herring (Clupea harengus) in the Gulf of Maine
during the Ocean Acoustic Waveguide Remote Sensing
2006 Experiment
Zheng Gong and Mark Andrews
DepartmentofElectricalandComputerEngineering,NortheasternUniversity,Boston,Massachusetts02115
Srinivasan Jagannathan
DepartmentofMechanicalEngineering,MassachusettsInstituteofTechnology,Cambridge,Massachusetts
02139
Ruben Patel
InstituteofMarineResearch,P.O.Box1870,Nordnes,N-5817Bergen,Norway
J. Michael Jech
NortheastFisheriesScienceCenter,166WaterStreet,WoodsHole,Massachusetts02543
Nicholas C. Makris
DepartmentofMechanicalEngineering,MassachusettsInstituteofTechnology,Cambridge,Massachusetts
02139
Purnima Ratilal
DepartmentofElectricalandComputerEngineering,NortheasternUniversity,Boston,Massachusetts02115
(cid:1)Received 30 January 2009; revised 21 October 2009; accepted 26 October 2009(cid:2)
The low-frequency target strength of shoaling Atlantic herring (cid:1)Clupea harengus(cid:2) in the Gulf of
Maine during Autumn 2006 spawning season is estimated from experimental data acquired
simultaneouslyatmultiplefrequenciesinthe300–1200 Hzrangeusing(cid:1)1(cid:2)alow-frequencyocean
acousticwaveguideremotesensing(cid:1)OAWRS(cid:2)system,(cid:1)2(cid:2)arealpopulationdensitycalibrationwith
several conventional fish finding sonar (cid:1)CFFS(cid:2) systems, and (cid:1)3(cid:2) low-frequency transmission loss
measurements. The OAWRS system’s instantaneous imaging diameter of 100 km and regular
updatingenabledunaliasedmonitoringoffishpopulationsoverecosystemscalesincludingshoalsof
Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl
andCFFSsampling.Highspatial-temporalcoregistrationwasfoundbetweenherringshoalsimaged
by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The
mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a
strong,roughly20 dB/octaveroll-offwithdecreasingfrequencyintherangeoftheOAWRSsurvey
over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for
sub-resonance scattering from fish with air-filled swimbladders.
© 2010 Acoustical Society of America. (cid:3)DOI:10.1121/1.3268595(cid:4)
PACS number(cid:1)s(cid:2): 43.30.Sf,43.30.Pc,43.30.Vh (cid:3)RCG(cid:4) Pages: 104–123
I. INTRODUCTION OAWRStoinstantaneouslyimagefishpopulationsoverwide
areas in complex continental-shelf environments with highly
To study the scattering characteristics, abundance and variable bathymetry and oceanography. Shoals imaged by
diurnal behavior of Atlantic herring (cid:1)Clupea harengus(cid:2), the
OAWRS typically comprised tens to hundreds of millions of
mostabundantfishspeciesinandaroundGeorgesBankdur-
individuals and stretched for many kilometers along the
ing their Autumn spawning season,1 an experiment using
northern flank of Georges Bank. Concurrent conventional
ocean acoustic waveguide remote sensing (cid:1)OAWRS(cid:2) was
fish finding sonar (cid:1)CFFS(cid:2) surveys showed high spatio-
conductedintheGulfofMainefromSeptember19toOcto-
temporal coregistration with fish shoals imaged by OAWRS
ber 6, 2006, concentrating on areas where herring shoals
and provided local areal population densities, as well as
weremostlikelytoform(cid:1)Fig.1(cid:2).Theexperiment,knownas
depth and length distributions of the fish populations. Con-
OAWRS 2006, was conducted in conjunction with the U.S.
current trawl sampling, which showedAtlantic herring to be
National Marine Fisheries Service Annual Atlantic Herring
the overwhelmingly predominant species comprising the
Acoustic Survey of the Gulf of Maine and Georges Bank.
Fishpopulationswereinstantaneouslyimagedovera100 km large shoals,2,3 enabled onsite species identification and di-
diameter area by a mobile OAWRS system2 with minute-to- rect biological measurements of parameters such as fish
minute updates to form wide-area movies of fish activity length, swimbladder geometry, stomach content, and sexual
over many diurnal cycles, demonstrating the capacity of development.
104 J.Acoust.Soc.Am.127(cid:1)1(cid:2),January2010 0001-4966/2010/127(cid:1)1(cid:2)/104/20/$25.00 ©2010AcousticalSocietyofAmerica
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1. REPORT DATE 3. DATES COVERED
JAN 2010 2. REPORT TYPE 00-00-2010 to 00-00-2010
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Low-frequency target strength and abundance of shoaling Atlantic
5b. GRANT NUMBER
herring (Clupea harengus) in the Gulf of Maine during the Ocean
Acoustic Waveguide Remote Sensing 2006 Experiment 5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Massachusetts Institute of Technology,Department of Mechanical REPORT NUMBER
Engineering,77 Massachusetts Avenue,Cambridge,MA,02139
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
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12. DISTRIBUTION/AVAILABILITY STATEMENT
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13. SUPPLEMENTARY NOTES
14. ABSTRACT

15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
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a. REPORT b. ABSTRACT c. THIS PAGE Same as 20
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
44°N
Portland 1 5 0 200 Yarmouth
Jeffreys Jordan 50
e Basin
g Bank
ed German
Portsmouth L GULF of MAINE Bank
s
43°N Jeffrey 200 150 CBroawsienll 150 Br1o0w0ns
100 Wilkinson Ro2d0g0ers Georges 201000 Bank
Boston 50 Basin Basin 134 139 Basin 30020C0Nhoarnthneealst
42°N Provincetown 106 137
EntireOAWRS
1 ImagingArea 105
Cape Cod 50 Instantaneous N K
50 OAWRS B A 100 00
Great100 Coverage R G E S 300 10
41°N NaIsnltauncdket CShoaunthnel 50 G E O 2000
50
71°W 70°W 69°W 68°W 67°W 66°W
FIG.1. LocationofOAWRS2006experimentonthenorthernflankofGeorgesBankintheGulfofMaine.PlusindicateslocationofmooredOAWRSsource
arraydeployedonOct1–3at42.2089N,67.6892W,thecoordinateoriginforallOAWRSimagesinthispaper.CircleshowstypicalareaimagedbyOAWRS,
100kmdiameterandwiderthanCapeCod,in70s.GeographiclocationsoftrawlsdeployedbyNOAAFRVDelawareIIareoverlain.Dotsindicatetrawls
whereherringwerepredominantspecies.Incontrast,diamondindicatesatrawlwheresilverhakeandsquidsdominated.Thegraydashedboxboundsthearea
ofOAWRSimagingduringtheOAWRS2006experiment.
Together with long-range transmission loss measure- ments made in situ with sources of rapid power roll-off be-
ments, concurrent CFFS, and trawl data, the OAWRS imag- low 1.7 kHz.7,10,11
ery enabled (cid:1)1(cid:2) estimates of herring target strength to be The population that spawns on the northern flank of
made at low frequencies (cid:1)300–1200 Hz(cid:2) from which physi- GeorgesBank(cid:1)Fig.1(cid:2)isthelargestherringstockintheGulf
cal scattering mechanisms may be inferred, (cid:1)2(cid:2) herring spa- of Maine, and has both ecological and economic
tialdistributionsandabundancetobeestimatedoverecosys- importance.1 It has been surveyed annually by the U.S. Na-
tem scales, and (cid:1)3(cid:2) regular diurnal patterns in herring tional Marine Fisheries Service for roughly 1 decade during
behaviortobedeterminedduringtheAutumnspawningsea- the Autumn spawning season.12–14 Current estimates of the
son on Georges Bank.3,4 Georges Bank herring stock varies from 500000 to 1(cid:1)106
The mean scattering cross-section of an individual tons based on acoustic surveys and other assessment
shoaling herring is found to consistently exhibit a strong, methods,14 respectively. The National Marine Fisheries Ser-
roughly 20 dB/octave roll-off with decreasing frequency in vice acoustic survey employs highly localized CFFS mea-
the range of the OAWRS survey over many measurement surements along widely spaced line-transects, roughly
days, consistent with the steep roll-off expected for sub- 20–30 km apart,14 trawl sampling at selected locations, and
resonance scattering from fish with air-filled swimbladders. takes roughly 1 week to cover the northern flank of Georges
These findings suggest that OAWRS can provide valuable Bank from east to west. As a result, annual stock estimates
evidence for remote species classification over wide areas may be highly aliased in both time and space. One of the
since significant variations in the frequency dependence of primary goals for OAWRS 2006 is to provide images of fish
targetstrengthareexpectedacrossspeciesduetodifferences populationsoverthevastareastheyinhabitthatareunaliased
inresonance.Thisisbecausethedominantsourceofacoustic in both space and time2 so that more reliable abundance es-
scattering at low and mid-frequencies (cid:1)less than 10 kHz(cid:2) is timates may be obtained.
the air-filled swimbladder for fish that have swimbladders.5
Resonance frequencies depend on swimbladder volume,
II. MULTI-SENSOR EXPERIMENT DESIGNAND
shape, ambient pressure, and the effect of surrounding
RESOURCES
tissues.6,7 For many fish species of economic importance in
the size ranging from 10 to 50 cm, resonances are expected TheOAWRS2006experimentwasdesignedtocoincide
torangefromseveralhundredsofhertztoafewkilohertz.6–9 with the National Marine Fisheries Service annual herring
Previous experimental investigation of resonance have been surveyofGeorgesBank.Itwasconductedwithfourresearch
limitedtosmallscaletankmeasurementswithindividualfish vessels(cid:1)RVs(cid:2)thatemployedasuiteofacousticimagingsen-
outoftheirnaturalenvironmentorhighlylocalizedmeasure- sors, several oceanographic monitoring systems, and trawls
J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring 105
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TABLEI. OAWRSreceivingarray3-dBangularresolution(cid:2)(cid:1)(cid:3)(cid:2)atbroad-
side(cid:3)=0andendfire(cid:3)=(cid:4)/2,andaperturelengthLasafunctionofimag- −5 Oct4, 00:16:15EDT DiffuseHerring A
Aggregation 180m
ingfrequencyf .AHanningspatialwindowisappliedinthebeamforming.
c
s
(cid:1)4Hf1cz5(cid:2) 9(cid:1)m4L.5(cid:2) (cid:2)(cid:1)(cid:3)=0(cid:2)3/(cid:2).1(cid:1)5(cid:3)/=31(cid:4).4/2(cid:2)(cid:1)deg(cid:2) Northing−−1150 200m LargeHerrin1g5100m0m fi021s.0h5/m2
735 47.25 3.56/33.3 Shoal 0.02
50m
950 47.25 2.75/29.3 0.001
1125 23.625 4.65/38.1 −10 −5 0 5 10 15
−15
Oct2, 19:33:45EDT B
for species identification. The OAWRS vertical source array
and towed horizontal receiving array were separately de- s 180m
g
ployedfromtwomediumsizedUNOLvessels,RVEndeavor n
hi−20 150m
and RV Oceanus respectively, for bistatic measurement of ort
echo returns. The instantaneous areal coverage of the N 100m
OAWRS system in a single transmission is shown in Fig. 1.
The vertical source array transmitted a suite of individual
Tukey-windowed linear frequency modulated (cid:1)LFM(cid:2) pulses −25
−20 −15 −10 −5
of 1 s duration and 50 Hz bandwidth centered at a suite of
Oct1, 23:11:15EDT C
frequencieswitharepetitionintervalof150 sforeachcenter
frequency.15 Broadband LFM pulses centered at 415 and −10
735 Hzweretransmittedsecondsapart,thenafter75 s those
centeredat950and1125 Hzweretransmittedsecondsapart, gs
n
and the process was repeated. Transmissions radiated with hi
azimuthal symmetry about the OAWRS source array, for ort−15 180m 150m
N
which more information is available in Ref. 16, with source
100m
level continuously monitored with two desensitized hydro-
phones deployed from RV Endeavor. Scattered returns were
acquired with a horizontal receiving array, the ONR five- −20
−10 −5 0 5 10
octave research array, towed by RV Oceanus along desig-
Eastings
nated tracks. The multiple nested sub-apertures of the array
span50–3750 Hzfrequencyrange.Returnsmeasuredwithin FIG. 2. (cid:1)Color online(cid:2) (cid:3)(cid:1)A(cid:2)–(cid:1)C(cid:2)(cid:4) OAWRS images of areal fish density
each linear section of the array are processed by beamform- zoomed-in around massive herring shoals, with densities exceeding
10fishm−2 in population centers. Measured during evening to midnight
ing and matched filtering with angular resolution shown in hours of October 4, 2, and 1, respectively. (cid:1)A(cid:2) The total population of
Table I. The receiving array also contained one desensitized herringinthelargedenseshoalisroughly170(cid:1)106,andthatinthediffuse
hydrophone which was used to measure transmitted signals cloudoutsidethelargeshoalisroughly70(cid:1)106.Imagedshoalpopulations
ofherringareapproximately86(cid:1)106and70(cid:1)106respectivelyfor(cid:1)B(cid:2)and
from the source array for transmission loss and source level
(cid:1)C(cid:2).Uncertaintyintheabundanceestimateis17–20%.Notethatthefigures
calibrations. Two calibrated acoustic targets made of air-
areplottedondifferentscales,andthecoordinateoriginisthesourceloca-
filled rubber hose,17 approximately 30 m long and 7 cm in tionshowninFig.1.
diameter with known scattering properties,18 were vertically
deployed at selected locations to enable accurate charting of ambiguity about the horizontal line-array’s axis in the
scattered returns in both range and azimuth. One of the tar- OAWRS images were resolved mainly by varying receiver
getswasmooredwithlowerend5 mofftheseafloorandthe ship heading, sometimes only slightly for several transmis-
other was centered at 140 m in waters 200 m deep. sions by what we call a “Crazy Ivan” for immediate results,
Over the course of the OAWRS 2006 experiment, more as well as ship position. These approaches for ambiguity
than 3000 wide-area images of the ocean environment were resolution are described in Refs. 19–21.
acquired by the OAWRS system for each of the four LFM Examplesofthemassivefishshoalsinstantaneouslyim-
center frequencies leading to more than 12000 images in aged by OAWRS near Georges Bank are shown in Fig. 2.
total.Similarly,morethan12000transmissionlossmeasure- ThemassiveshoalimagedduringmidnighthoursofOctober
mentsweremadeoverthesurveyareatocalibrateourtrans- 4 within the 150–180 m bathymetric contour in Fig. 2(cid:1)A(cid:2),
mission loss model. The length of each RV Oceanus towed for example, extends 15(cid:1)5 km2 and comprises roughly
array track was typically 15 km. With a nominal tow speed 170(cid:1)106 fish distributed about several population centers.
of2 ms−1 forthereceivership,atotalofroughly75images The area occupied by this shoal is approximately equal to
of the ocean environment were generated per frequency thatofManhattanIslandinNewYork.Thefishpopulationin
along each track. Minute-to-minute updates of the OAWRS the diffuse cloud region to the north is comprised of over
imagery made it possible to closely monitor herring activity 70(cid:1)106 individuals.
over wide areas and observe patterns of spatial distribution Concurrent localized imaging of fish aggregations at
evolve over the course of each day. The inherent left-right OAWRS-directed locations was conducted by two other re-
106 J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring
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TABLE II. Conventional fish finding sonars, SIMRAD EK60 and EK500 0
specifications.Theangular3-dBbeamwidthisdenotedby(cid:2),thepulsedu-
ration by PD, and repetition rate by RR. The resolution diameter, Res, is −20
calculatedfor200mwaterdepth.
−40
f (cid:2) Res PD RR
Sensor (cid:1)kHz(cid:2) (cid:1)deg(cid:2) (cid:1)m(cid:2) (cid:1)ms(cid:2) (cid:1)s−1(cid:2)
−60
EK60 38
120 7 24 1 1 −80 Bank
200 m)
(
h−100
EK500 18 11 39 2 pt
e
38 12 42 1 0.5 D
−120
120 7 24 1 Basin
−140
−160
search vessels, the RV Hugh Sharp and the NOAA FRV
Delaware II, using two downward-directed CFFS systems,
−180
the SIMRAD EK60 and EK500 echosounders, respectively.
Both the EK60 and EK500 echosounders insonify the water
−200
column directly beneath the survey vessel simultaneously at 1470 1480 1490 1500 1510
SoundSpeed(m/s)
three frequencies to provide the local depth dependence of
dominant fish layers within their instantaneous resolution FIG.3. Profilesofwater-columnsoundspeedfromXBTandCTDmeasure-
footprints, of between 24–50 m diameter, and estimates of mentsmadefromallfourresearchvesselsontheNorthernFlankofGeorges
BankandGeorgesBasinduringOAWRS2006.
volumetricandarealfishpopulationdensities.Specifications
of these two echosounders appear inTable II.AReson 7125
Seabat multi-beam sonar (cid:1)400 kHz(cid:2) system was also de-
III. DATAPROCESSINGANDANALYSIS
ployedfromRVHughSharpwithanangularswathof128°.
It was useful in providing detailed three-dimensional mor- A. Generating instantaneous wide-area OAWRS
phology of smaller fish groups located in the mid-water images of the ocean environment
column.22 A high-speed rope trawl23 deployed by NOAA
Wide-area images of instantaneous scattered intensity
FRV Delaware II enabled species identification14 at
spanning 100 km in diameter were generated in near real-
OAWRS-directed locations.
time for every broadband transmission centered at each of
Physical oceanography was monitored by sampling
the four frequencies, f =415, 735, 950, and 1125 Hz. For
c
water-column temperature and salinity with expendable
each transmission, the pressure data on the receiving array
bathythermographs (cid:1)XBTs(cid:2) and conductivity-temperature-
werefirstbeamformedtodeterminetheazimuthofthearriv-
depth(cid:1)CTD(cid:2)sensorsatregularhourlyintervalsfromallfour als,thenmatchedfilteredwiththesourcesignal,andcharted
research vessels. The water-column sound speed profile was in range using two-way travel time.19–21 Each image was
found to be relatively constant in space and time over the then mapped onto geographic space using the GPS latitude
2006 OAWRS survey, as shown by the compilation of over and longitude information of the source and receiving array.
roughly 200 samples taken during the experiment in Fig. 3. Anominalsoundspeedof1475 m/sthatminimizescharting
No mesoscale oceanographic features such as eddies were errors was used to convert the travel-time of the signal to
found or expected. The small fluctuations about the mean range.21,28Therangeresolution(cid:5)(cid:6)oftheOAWRSsystemis
profile are due to mild internal wave activity that causes approximately 15 m after matched-filtering, and the azi-
well-understood short-term Gaussian field fluctuations in muthal resolution (cid:2)(cid:1)(cid:3),f (cid:2) associated with each frequency
c
acoustic transmission that have an intensity standard devia- band both at broadside and endfire is tabulated inTable I.A
tion that can be reduced to a small fraction of the mean by Hanning spatial window was applied in the beamforming to
stationary averaging.24–26 An instrumented tow cable was reduce sidelobe levels by more than 30 dB from the main
also deployed from the RV Hugh Sharp to provide continu- lobe.Adetailed explanation of the image formation process
ous measurement of temperature. This oceanographic infor- is provided in Refs. 2, 20, and 21.
mation was used to carefully update horizontal locations Astandard procedure of averaging three consecutive in-
and depths of the OAWRS source (cid:1)typically centered at stantaneous OAWRS images and two adjacent range cells is
60–70 m(cid:2)andreceivingarrays(cid:1)centeredat105 m(cid:2)(cid:1)Ref.15(cid:2) usedforallOAWRSimagespresentedhere.Thisleadstoan
tooptimizeOAWRSimagingoffishgroups.Decisionswere experimentally determined standard deviation in log-
often based on the outputs of the range-dependent acoustic intensityofroughly1.5 dB,3consistentbothwiththeoryand
model (cid:1)RAM(cid:2), based on the parabolic equation, for multi- previous experiments.2,3,16,20,21 This standard deviation is
modal waveguide transmission loss in the range-dependent negligiblecomparedtothedynamicrangesoffeaturesinthe
GeorgesBankenvironmentwiththehourlysoundspeedpro- OAWRS images and the variations in herring target strength
file updates and known bathymetry.27 measured across frequency in the OAWRS range.
J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring 107
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B. Estimating areal fish population density from (cid:6) (cid:5)(cid:6)(cid:2)(cid:1)(cid:3),f (cid:2) is the range and azimuth-dependent spatial
m c
instantaneous OAWRS imagery resolution of the OAWRS imaging system.21
InsertingEq.(cid:1)3(cid:2)intoEq.(cid:1)2(cid:2)andtaking10log ofboth
Herewedescribehowarealfishpopulationdensityover 10
sides, we obtain the scattered intensity level in decibels;
wideareasmaybeestimatedfromOAWRSintensityimages
of the ocean environment.At typical OAWRS operating fre- L(cid:1)(cid:1) ,f (cid:2)(cid:11)SL(cid:1)f (cid:2)+TTL(cid:1)(cid:1) ,f (cid:2)+SS (cid:1)(cid:1) ,f (cid:2)
m c c m c OAWRS m c
quenciesfromhundredsofhertztoafewkilohertz,mostfish
+10log (cid:1)A(cid:1)(cid:1) (cid:7)(cid:5)(cid:6),f (cid:2)(cid:2), (cid:1)4(cid:2)
are acoustically compact scatterers, with swimbladder sizes 10 m c
that are much smaller than the wavelength. The sonar equa- where L(cid:1)(cid:1) ,f (cid:2)=10log (cid:5)I (cid:1)(cid:1) ,f (cid:2)(cid:6), TTL(cid:1)(cid:1) ,f (cid:2)=
m c 10 s m c m c
tionapproachisthenvalidforanalyzingscatteringfromfish 10log (cid:7)(cid:1)(cid:1) ,f (cid:2) describes the expected second moment of
10 m c
since the scattered field from each individual is omni- depth averaged propagation to and from the fish layer aver-
directional, making propagation and scattering factorable aged over the resolution footprint of the OAWRS system,
eveninawaveguide.29Theexpectedscatteredintensityfrom SL(cid:1)f (cid:2)=10log (cid:5)(cid:7)Q(cid:1)f (cid:2)(cid:7)2(cid:6) is the spectral source level, and
c 10 c
fish aggregations after matched-filtering is dominated by the SS (cid:1)(cid:1) ,f (cid:2) is the scattering strength.
OAWRS m c
incoherent intensity or variance of the scattered field and From Eqs. (cid:1)2(cid:2)–(cid:1)4(cid:2), OAWRS scattering strength can be
multiple scattering effects are negligible for the densities
expressed as
foundhere,asshowninRefs.30and31.Asaresult,givena
SS (cid:1)(cid:1) ,f (cid:2)=TS (cid:1)f (cid:2)
source at r transmitting a broadband signal with bandwidth OAWRS m c OAWRS c
0
centered at fc and a receiver at r, the expected scattered +10log10(cid:5)nA,OAWRS(cid:1)(cid:1)m(cid:2)(cid:6), (cid:1)5(cid:2)
intensity,(cid:5)I (cid:1)(cid:1) ,f (cid:2)(cid:6),withintheOAWRSresolutionfootprint
of area A(cid:1)(cid:1)s (cid:7)(cid:5)m(cid:6),cf (cid:2) centered at horizontal location (cid:1) can whereTSOAWRS(cid:1)fc(cid:2)=10log10¯(cid:8)(cid:1)(cid:1)m,fc(cid:2)inunitsofdBre1 m2
m c m
is the target strength corresponding to the average scattering
be expressed as
cross-section of an individual fish over the OAWRS reso-
M(cid:8)(cid:1)(cid:1)m(cid:2) lution footprint and depth layer within the bandwidth cen-
(cid:5)I (cid:1)(cid:1) ,f (cid:2)(cid:6)= (cid:5)(cid:7)Q(cid:1)f (cid:2)(cid:7)2(cid:6)
s m c c tered at f .
c
i=1 ThetermsinEq.(cid:1)4(cid:2)areevaluatedseparatelyforeachof
(cid:1)(cid:1)(cid:9)4(cid:4)(cid:2)4(cid:5)(cid:7)G(cid:1)r(cid:7)(cid:10)r ,f (cid:2)G(cid:1)r(cid:7)r,f (cid:2)(cid:7)2(cid:6) the four OAWRS LFM waveforms with different center fre-
i 0 c i c
(cid:7)S(cid:1)r,f (cid:2)(cid:7)2 quencies fc. A calibrated stochastic transmission loss model
(cid:1) i c , (cid:1)1(cid:2) basedontheparabolicequation27forarange-dependentfluc-
k2
tuating ocean waveguide is used to estimate the random
whereM(cid:1)(cid:1)m(cid:2)isthenumberoffishwithintheresolutioncell, Green’s functions and determine TTL(cid:1)(cid:1)m,fc(cid:2) following the
r is the location of the ith fish, (cid:7)Q(cid:1)f (cid:2)(cid:7)2 is the source inten- approachdescribedintheAppendixEandRef.16.Expected
i c
sity, G(cid:1)r (cid:7)r ,f (cid:2) and G(cid:1)r(cid:7)r ,f (cid:2) are the waveguide Green’s source level is estimated from one-way propagated signals
i 0 c i c
functions from the source to each scatterer and from each received by a desensitized hydrophone on the moving re-
scatterertothereceiver,respectively,S(cid:1)r ,f (cid:2)isthefishscat- ceiver array using the approach of Ref. 16. The two moni-
i c
ter function, and k is the wavenumber. toring hydrophones on the source ship were used to verify
the source level estimates. Our analysis indicates the source
The expected intensity in a fluctuating waveguide from
transmitted a stable output over the course of each day.
uniformly distributed targets within the resolution footprint
TheapplicationofEq.(cid:1)4(cid:2)toestimatescatteringstrength
can be approximated as (cid:9) (cid:10)
from OAWRS imagery is illustrated in Figs. 2 and 3 of Ref.
M(cid:8)(cid:1)(cid:1)m(cid:2) (cid:7)S(cid:1)r,f (cid:2)(cid:7)2 32 and in Ref. 33. Scattering strength is a useful parameter
(cid:5)I (cid:1)(cid:1) ,f (cid:2)(cid:6)(cid:11)(cid:5)(cid:7)Q(cid:1)f (cid:2)(cid:7)2(cid:6)(cid:7)(cid:1)(cid:1) ,f (cid:2) i c , (cid:1)2(cid:2)
s m c c m c k2 for characterizing submerged objects, both distant and
i=1
nearby, because it is independent of the spatially varying
where (cid:7)(cid:1)(cid:1) ,f (cid:2)=(cid:5)(cid:7)(cid:1)4(cid:4)(cid:2)2G(cid:1)r (cid:7)r ,f (cid:2)G(cid:1)r(cid:7)r ,f (cid:2)(cid:7)2(cid:6) for suffi- transmission loss and areal resolution footprint of the imag-
m c m 0 c i c
ciently narrow depth layers H and areal footprints over ing system. Once the target strength expected of an indi-
which (cid:7)(cid:1)(cid:1) ,f (cid:2) becomes effectively constant, as shown for vidual fish is known, an areal fish population density image
m c
the OAWRS 2006 fish shoal imaging in Ref. 4. The last canbeobtainedfromascatteringstrengthimage33usingEq.
factor of E(cid:9)q. (cid:1)2(cid:2) can(cid:10)be written as (cid:1)5(cid:2).The target strength corresponding to the average scatter-
M(cid:8)(cid:1)(cid:1)m(cid:2) (cid:7)S(cid:1)ri,fc(cid:2)(cid:7)2 =M(cid:8)(cid:1)(cid:1)m(cid:2) (cid:12)(cid:12)(cid:12) (cid:7)S(cid:1)ri,fc(cid:2)(cid:7)2P(cid:1)r(cid:2)dr3 ifnregqucreonscsi-essecistioenstiomfaatnedinbdyivmidautaclhifinsghbaettwOeAeWn ROSAWopReSratainndg
k2 k2 i i CFFS areal fish population density measurements where si-
i=1 i=1
multaneous sampling through stationary fish populations is
=M(cid:1)(cid:1) (cid:2)¯(cid:8)(cid:1)(cid:1) ,f (cid:2), (cid:1)3(cid:2)
m m c available.
where P(cid:1)r(cid:2) is the probability density of finding the ith
i
fishatlocationr,and P(cid:1)r(cid:2)=1/A(cid:1)(cid:1) (cid:7)(cid:5)(cid:6),f (cid:2)H foruniformly
i i m c
distributed fish shoals, ¯(cid:8)(cid:1)(cid:1) ,f (cid:2) is the average scattering C. Estimates of areal fish population density from
m c CFFS
cross-section of an individual fish over the OAWRS reso-
lution footprint and the depth layer, n (cid:1)(cid:1) (cid:2)= The CFFS measurements at 38 kHz are used to provide
A,OAWRS m
M(cid:1)(cid:1) (cid:2)/A(cid:1)(cid:1) (cid:7)(cid:5)(cid:6),f (cid:2) is the mean areal fish population den- local estimates of areal fish population density.14,34,35The 7°
m m c
sity within the resolution footprint, and A(cid:1)(cid:1) (cid:7)(cid:5)(cid:6),f (cid:2)(cid:11) 3-dBbeamwidthyieldsaninstantaneouscircularsurveyarea
m c
108 J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring
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TABLEIII. Physicalparametersofmodeledfishspeciesandtheirmeasured low-frequency target strength near resonance varies signifi-
targetstrengthat38kHzwithaCFFS. cantly,asdiscussedinSec.IVC.ThesemodeledTS val-
CFFS
ues are in good agreement with those obtained by experi-
Species Atlanticherring Acadianredfish Silverhake
mentally analyzing the CFFS backscattered field from
L (cid:1)cm(cid:2) 19–30 15–39 2–35 individual fish distinguishable in the periphery of various
FL
LTL(cid:1)cm(cid:2) 20–34 16–41 2–35 aggregations consistent with 0.1 dB mean squared errors re-
Depth(cid:1)m(cid:2) 120–190 120–190 10–75 ported in Ref. 37.
(cid:5)TSCFFS(cid:6)(cid:1)dB(cid:2) −39.7a −38.9b N/A Figures 4(cid:1)D(cid:2) and 4(cid:1)E(cid:2) illustrate the application of Eq.
(cid:8)CFFS(cid:1)dB(cid:2) 1.3c 2.4 N/A (cid:1)6(cid:2) to estimate areal fish density for herring aggregations in
(cid:6)d(cid:1)kgm−3(cid:2) 1071 1080 1050
f the 120–180 m water depth range.
(cid:9)e(cid:1)Pas(cid:2) 50 50 20
(cid:10)f 5–10 6 8
(cid:11) g 0.05 0.05 0.03
nb
(cid:11)h xi 0.05 0.03 D. Estimating low-frequency target strength by
z
matching OAWRS and CFFS population densities
aMeantargetstrengthofherringcalculatedusingEq.(cid:1)7(cid:2).
bMeantargetstrengthofredfishcalculatedusingequationinRef.71. Here we describe our procedure for estimating the low-
cStandarddeviationofderivedherringtargetstrengthat38kHzincorporat- frequency target strength corresponding to the average scat-
ingfishlengthanddepthdistributionfromCFFSandtrawlsurveys. tering cross-section of an individual shoaling herring over
dFishfleshdensity. theresolutionfootprintoftheOAWRSsystembycorrelating
eViscosityoffishflesh.
OAWRS data with simultaneous measurements made along
fmajor-to-minor-axisratiooffishswimbladder.
gFish swimbladder volume to fish body volume ratio at neutral buoyancy CFFS transects. The target strength of herring at 950 and
depth. 1125 Hz is found to be significantly higher than at 415 and
hFishswimbladdervolumetofishbodyvolumeratioatdepth. 735 Hz, making much lower herring densities observable at
ix is a linear function of ambient pressure at depth given by x thesehigherfrequencies.Atthelowerfrequenciesof415and
=(cid:11) (cid:1)P /P(cid:2),whereP istheambientpressureatneutralbuoyancydepth
nb nb x nb 735 Hz, the herring target strength is weaker causing the
z ,andP istheambientpressureatanydepthz.
nb z
scatteredreturnstobebackgroundsaturatedatmoderatefish
densities. Due to the receiving array’s sub-aperture design,
of 24 m diameter directly under the survey vessel at 200 m
OAWRS images at 950 Hz have the best cross-range reso-
waterdepth.Volumetricscatteringfromalltargetswithinthe
lution, making this an optimal frequency for wide-area sens-
conical beam were measured. The localized areal fish popu-
ing. An alternative approach for target strength estimation,
lation density in fishm−2, denoted by n , can be esti-
A,CFFS based on differencing pairs of OAWRS wide-area scattering
mated using
(cid:12) strengthimagesattwodistinctfrequencies,isappliedinSec.
4(cid:4) z2 IIIE to determine target strength at the lower frequencies.
n = s dz, (cid:1)6(cid:2)
A,CFFS ¯(cid:8) v The target strength estimates are summarized in Table IV.
bs z1 Calibrated acoustic targets were deployed on October
wheres isthevolumebackscatteringcoefficient36inm−1,z 2–3enablingindependentandprecisegeographicchartingof
v 1
and z delimit the depth bounds for fish aggregations, and OAWRS images. By making small adjustments to the chart-
2
¯(cid:8) =4(cid:4)10(cid:5)TSCFFS(cid:6)/10 is the mean backscattering cross-section ing speed and array orientation, scattered returns from cali-
bs
of an individual at 38 kHz in units of m2, where (cid:5)TS (cid:6) is brated targets were accurately charted to the correct range-
CFFS
the corresponding mean target strength at ultrasonic fre- azimuth resolution cell relative to the source and receiver.
quency. This ensures that scattered returns from all other targets, in-
The expected target strength for an individual fish at cluding the fish aggregations, have been accurately charted
38 kHz varies with species, depth, and total fish length. aswell.InthissectionwefocusondataacquiredonOctober
Here, the expected TS in dB of an individual herring of 2–3 when calibrated target data were available and present
CFFS
total length L in centimeters at depth z in m is obtained target strength estimates for other days in theAppendix B.
TL
from Eq. (cid:1)5(cid:2) of Ref. 37, Close to midnight on October 2, both OAWRS and
CFFS systems simultaneously co-registered a massive her-
TS =20log L −2.3log(cid:1)1+z/10(cid:2)−65.4, (cid:1)7(cid:2)
CFFS 10 TL ringshoalbetweenthe150and180 m isobathsonthenorth-
and then converted to (cid:8) . The mean backscattering cross- ernflankofGeorgesBank,asshowninFigs.4(cid:1)A(cid:2)–4(cid:1)C(cid:2).The
bs
section¯(cid:8) isobtainedastheweightedaverageoverthetotal observations were made continuously over a 90-min period
bs
length and depth distribution of the fish aggregations. From between 23:30 Eastern Daylight Time (cid:1)EDT(cid:2) on October 2
trawl surveys of the imaged fish populations in OAWRS, and 01:00 EDT on October 3. Measurements from the two
herring was the overwhelmingly dominant species compris- systemsarehighlycorrelatedduringthecourseoftheobser-
ingthelargeshoals,whichhadsmallfractionsofredfishand vationsbecauseofthestatisticalstationarityofthefishpopu-
silverhake.EstimatesofthemeanTS forindividualher- lations even though their resolution footprints are signifi-
CFFS
ringandredfishbasedonourtrawlmeasurements(cid:1)Appendix cantlydifferent.TheOAWRSsystemmonitoredandsampled
A(cid:2) of the length distribution are provided in Table III. The the temporal and spatial evolution of the shoal’s horizontal
expected target strength of herring and redfish over similar morphology at intervals of 75 s without aliasing. Concur-
depth extent at 38 kHz are close, varying at most by 1 dB, rently, the CFFS system crossed the same shoal twice along
albeit their different length distributions. In contrast, their aU-shapedtransectwithtwoparalleltransects1.5 kmapart.
J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring 109
Downloaded 21 Dec 2011 to 18.38.0.166. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp
−18
23:36:15EDT 180m 00:16:15EDT 00:56:15EDT
α α α
m) Ω Ω Ω
k
(
s −21
g 150m
n
hi
rt
o 100m
N −24
A B C
2
−18 −15 −12 −9 ScatteringStrength(dB) Fish/m
Eastings(km)
−63 −56 −47 −40 −33 0.01 0.05 0.4 2 10
100 α Ω D Fish/m3
m) 120 0.5
h( 140 0.1
pt 0.02
e 160 0.004
D
180
0
10 3
E
nsity(dB) −100 12 Density2Fish/m)
e (
D
−20 0
B) −35 F
d
( −40
S
S
−45
S
R
W −50
A
O −55
2) −31 G
m
−37
1
1125Hz
e
r −43
B 950Hz
d −49
(
S
T −55
60
on s) H
pulati million 40 NnnAo>>th00r..e12shold
po ( 20 nA>0.5
A
0
23:20 23:30 23:40 23:50 00:00 00:10 00:20 00:30 00:40 00:50 01:00 01:10
EasternDaylightTime
FIG. 4. (cid:1)Color online(cid:2) Herring target strength at 950 and 1125Hz estimated by matching areal fish density in OAWRS and CFFS data acquired during
midnighthoursofOctober2.(cid:3)(cid:1)A(cid:2)–(cid:1)C(cid:2)(cid:4)AsequenceofinstantaneousOAWRSscatteringstrengthimageszoomedintotheregioncontainingamassiveherring
shoalwithoverlainCFFSline-transect(cid:1)solidline(cid:2)madeatnominaltow-speedof2.5ms−1.(cid:1)D(cid:2)CFFStime-depthechogramprovideslocaldepthdistributions
offishaggregations.Dashedlinesat23:30EDTand01:00EDTcorrespondtotransectstartandendpoints(cid:12)and(cid:13),respectively.(cid:1)E(cid:2)Thearealfishpopulation
densitiesinferredfromCFFSmeasurementsfollowingEq.(cid:1)6(cid:2)areplottedasafunctionoftimeinblack,andthecorrespondingarealfishpopulationdensities
indB,10log (cid:1)n (cid:2),areplottedingray.(cid:1)F(cid:2)TheOAWRSscatteringstrengthmeasurementsand(cid:1)G(cid:2)instantaneoustargetstrengthestimatesalongCFFS
10 A,CFFS
line-transectsat950and1125Hz.Targetstrengthestimatesneartheedgeofshoalsarenotaccuratebecauseofnonstationarity.(cid:1)H(cid:2)Populationofherring
withintheareashownin(cid:1)A(cid:2)–(cid:1)C(cid:2)determinedwithvariousOAWRSfishdensityn thresholds.Solidlinegivespopulationabovethethresholdanddottedline
A
givespopulationbelowthethreshold.
The depth distribution of the fish population, within roughly tems. We derive threshold values for CFFS population
40–60 m of the seafloor, is relatively consistent across the densityandOAWRSscatteringstrength.Thesegmenteddata
two CFFS transects, as shown in Fig. 4(cid:1)D(cid:2). above these thresholds are used for target strength estima-
Toaccuratelyestimatelow-frequencytargetstrength,we tion. The CFFS threshold is set at 0.2 fishm−2, as shown
confine our present analysis to contiguous space-time seg- in Fig. 4(cid:1)E(cid:2). For OAWRS, two square areas of dimension
mentsthatconsistentlyregistersignificant,stationaryscatter- 6.2(cid:1)3 and 1.57(cid:1)5.58 km2 that continuously register sig-
ing from fish aggregations in both OAWRS and CFFS sys- nificantfishscatteringanddiffusebackgroundreverberation,
110 J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring
Downloaded 21 Dec 2011 to 18.38.0.166. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp
TABLE IV. Mean low-frequency target strength estimates.The TˆS esti- Similar statistical analyses have been conducted for
cor
mates are obtained by correlating OAWRS with CFFS data along CFFS OAWRS data at 1125 Hz, with estimated target strength ap-
transect.ThisapproachisonlyappliedtoOAWRSdataat950and1125Hz. pearing in Fig. 4(cid:1)G(cid:2). This approach is also applied to infer
Fortheotherfrequencies,theTˆS estimatesareobtainedbytheapproachof
sc herring target strength at 950 and 1125 Hz using OAWRS
differencingOAWRSimages.TheDiffistheexpectedtargetstrengthdif-
and CFFS data on October 3, where two contiguous shoal
ferencebetweenthegivenfrequencyand950Hz.
segments are imaged. Estimated target strengths for these
f TˆS Diff TˆS ¯(cid:8)ˆ two segments are provided in Table IV. The target strength
c cor sc TS
Date (cid:1)Hz(cid:2) (cid:1)dBre1m2(cid:2) (cid:1)dB(cid:2) (cid:1)dBre1m2(cid:2) (cid:1)dB(cid:2) estimates at 950 Hz for the three data sets are consistent,
with a standard deviation of roughly 1 dB.
Oct2 415 N/A (cid:11)−17.5 (cid:11)−60.5 2
The approach of this section is not used to estimate tar-
735 N/A (cid:14)−8 (cid:14)−51.0 (cid:15)3
950 −43.0 N/A −43.0a 0.7 getstrengthat415and735 Hzbecausetheherringaremuch
1125 −40.3 (cid:11)7 (cid:11)−36.0 1.7 weaker scatterers at these frequencies, as seen in Fig. 5(cid:1)A(cid:2)
where only the densest shoal population centers stand above
Oct3 415 N/A (cid:11)−17 (cid:11)−57.9 2 background scattering levels.An alternative approach to es-
transect1
timate herring target strength at these lower frequencies is
735 N/A (cid:11)−8.5 (cid:11)−49.4 1.5
developed and applied in Sec. IIIE.
950 −40.9 N/A −40.9 0.8
1125 −35.6 (cid:11)8 (cid:11)−32.9 1.5
E. Frequency dependence of target strength
Oct3 415 N/A (cid:14)−17 (cid:14)−58.7 (cid:15)3 estimated by differencing OAWRS scattering strength
transect2 images over wide areas
735 N/A (cid:11)−7.5 (cid:11)−49.2 1.5
950 −41.7 N/A −41.7 0.5 Here, we develop an alternative approach to estimate
1125 −37.0 (cid:11)7.5 (cid:11)−34.2 1 target strength expected of an individual shoaling herring by
differencing pairs of OAWRS scattering strength images ac-
Sep27 415 N/A (cid:14)−15 (cid:14)−57 (cid:15)3
quired at two distinct frequencies over the entire area of the
735 N/A (cid:11)−12.5 (cid:11)−54.5 1.7
shoal. We apply this to data at 415 and 735 Hz. The ap-
950 −42 N/A −42 0.7
1125 N/A (cid:11)5 (cid:11)−37 1.5 proachisillustratedbytheconceptualdiagramshowninFig.
6. From Eq. (cid:1)5(cid:2), we observe that scattering strength in areas
Sep29 415 N/A (cid:14)−10 (cid:14)−54.4 (cid:15)3 containing fish increases logarithmically with areal density
735 N/A (cid:14)−7 (cid:14)−51.4 (cid:15)3 n . Here, f represents a low frequency, such as 415 Hz,
A 1
950 −44.4 N/A −44.4 0.8
where the target strength for fish is lower than at another
1125 N/A (cid:11)10 (cid:11)−34.4 2
frequency f , such as 950 Hz. The background scattering
2
aBasetargetstrengthat950Hzusedinthedifferencingapproach. strengthfromsourcesotherthanfishisexpectedtobestatis-
tically stationary and can be identified by its mean level
which is frequency dependent. The total scattering strength
respectively, throughout the course of observation are first measured at any given pixel is a sum of the contribution
examined.Thehistogramofscatteringstrengthvalueswithin from fish and other background effects. The difference be-
these areas, averaged over multiple OAWRS images, are tweenthetotalscatteringstrengthacrossvariouspixelsatthe
plotted in Fig. 5(cid:1)D(cid:2). The histograms are approximately two frequencies then follows the trend illustrated in Fig.
Gaussian. The OAWRS threshold is then set at −50 dB for 6(cid:1)B(cid:2),whereatverylowfishdensities,thescatteringstrength
950 Hz to distinguish fish scattering from the background. is dominated by the background reverberation, and at very
This threshold is roughly 2 standard deviations below the high fish densities by fish scattering. The difference in the
mean for the fish histogram and roughly 2 standard devia- scattering strength at low fish densities therefore provides a
tions above the background mean. measure of the difference in background reverberation. The
Employing Eq. (cid:1)5(cid:2), and assuming local stationarity of difference in the scattering strength at high fish densities is
fish population, we set the areal fish density within the equal to the target strength difference for fish at these fre-
OAWRS resolution footprint to that simultaneously sampled quencies. If the target strength at one of the frequencies is
by CFFS transect through the OAWRS footprint, n known accurately, then the target strength at the other fre-
A,OAWRS
(cid:11)n . The resulting target strength estimates for fish in quency can be obtained.
A,CFFS
these contiguous shoaling regions at 950 Hz are shown in This approach is implemented for pairs of OAWRS im-
Fig. 4(cid:1)G(cid:2). The differences in target strength estimates along ages using 950 Hz as the base frequency. The difference in
the transect are due to the fact that the OAWRS and CFFS scatteringstrengthiscalculatedandplottedforOAWRSdata
systems have different resolution footprint sizes, and so the acquired between 22:00 and 22:45 EDT on October 3 in
true mean fish areal densities within the OAWRS resolution Figs. 7(cid:1)A(cid:2)–7(cid:1)C(cid:2) for various frequency pairs.We observe the
cell may be overestimated or underestimated by the CFFS scattering strength difference in the background is roughly
system given nonstationary spatial distributions, as occurred 1 dB between 1125 and 950 Hz, but the fish target strength
atshoalboundaries.Thecombinationofmeasurementsfrom difference is larger, roughly 7.5 dB. Between 950 and
many space-time locations from both systems should yield 415 Hz, the background scattering strength difference is
mean target strength estimates with small variance by virtue roughly1.5 dB,butthefishtargetstrengthdifferenceismore
of the law of large numbers as discussed inAppendix C. than 17 dB. Between 950 and 735 Hz, no conclusion can be
J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring 111
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415Hz 180m
Fish
Background
150m
100m
A B
950Hz 180m
150m
100m
C D
−18 0.2 m
1125Hz 180m a
m) gr
0.15 o
k t
( s
ngs −21 150m 0.1 dhi
hi ze
Nort −24 100m E F 0.05 rmali
o
0 N
−18 −15 −12 −9 −6 −3 −60 −50 −40 −30
Eastings(km) Scatteringstrength(dB)
FIG.5. (cid:1)Coloronline(cid:2)Theintensityofscatteredreturnsfromshoalsishighlyfrequency-dependent.Thehistogramsillustratethatitiseasiertodetectshoals
overbackgroundregionsathigherfrequencies.Simultaneoustrawlsshowshoalsareoverwhelminglycomprisedofherringwhilebackgroundregionsyield
negligibleherring(cid:1)TableIV,Fig.12(cid:2).(cid:3)(cid:1)A(cid:2),(cid:1)C(cid:2),and(cid:1)E(cid:2)(cid:4)OAWRSimagesofherringshoalacquiredsimultaneouslyatthreedistinctfrequencybandscentered
at415,950,and1125Hzat00:41:15EDTonOctober2.Thecolorscaleusedin(cid:1)A(cid:2),(cid:1)C(cid:2),and(cid:1)E(cid:2)isthesameasinFigs.4(cid:1)A(cid:2)–4(cid:1)C(cid:2).(cid:3)(cid:1)B(cid:2),(cid:1)D(cid:2),and(cid:1)F(cid:2)(cid:4)
Histogramsofscatteringstrengthvaluesatlocationswithintheshoal(cid:1)areasinsidethedashedbox(cid:2)andinabackgroundregion(cid:1)areasinsidethesolidbox(cid:2)
plottedforcomparison.The735Hzdataareambientnoiselimitedinbackgroundareasduetoweaksourcelevelandisnotshown.
drawn about background scattering because the 735 Hz data IV. RESULTSAND DISCUSSION
were dominated by ambient and nearby shipping noise since
A. Measured abundance
thesourcelevelforthisfrequencywaslower.Thefishtarget
strength difference between 950 and 735 Hz is roughly Herring areal population densities and abundances are
7.5 dB. These results are tabulated in Table IV. estimated by subtracting the estimated target strength ex-
As the OAWRS imaging frequency increases from pected of an individual herring from OAWRS scattering
415 to 1125 Hz, the target strengths of both fish populations strengthimagesasexplainedinSec.IIIB.Arealfishdensity
andbackgroundlevelsalsoincrease.Fromthehistogramsof is first calculated by applying Eq. (cid:1)5(cid:2) at each pixel in an
Fig. 5, the scattering strength increase with frequency is OAWRS image. Integrating over the area of a shoal then
greater for the fish shoals than background levels, making it provides an estimate of its abundance. We illustrate abun-
easier to detect fish aggregations at the higher frequency.At dance estimation with OAWRS images generated at 950 Hz
415 Hz, only densely populated fish regions are distinguish- since these have the best spatial resolution.
ablefromthebackground.Fishdensitiesatshoalperipheries Figures 4(cid:1)A(cid:2)–4(cid:1)C(cid:2) show the areal fish density for a se-
are typically too low to be detectable. quence of instantaneous OAWRS images close to the mid-
ThebackgroundlevelsinFigs.7(cid:1)A(cid:2)–7(cid:1)C(cid:2)canbeusedto night hours of October 2 where average density within the
derive the minimum detectable fish densities in the OAWRS shoal often exceeds 10 fishm−2. Figure 4(cid:1)H(cid:2) shows the
system at various frequencies. From Fig. 5(cid:1)D(cid:2), if we require population over time within the area shown in Figs.
thatfishreturnsstandatleast1standarddeviationabovethe 4(cid:1)A(cid:2)–4(cid:1)C(cid:2)forconsecutiveOAWRSimagesfrom23:30EDT
background mean to be detectable, then scattering strength onOctober2to01:00EDTonOctober3.Whennothreshold
levels that are above roughly −52 dB at 950 Hz would be is applied, we simply integrate the densities throughout the
detectable. This corresponds to a minimum detectable areal area. The total population of roughly 40(cid:1)106 fish includes
density of roughly 0.1 fishm−2. The minimum detectable fish in the shoaling region as well as diffuse fish clouds out-
densities at the other frequencies are tabulated in Table V. side the shoal. Contributions from outside the fish shoal,
These are based on scaling the fish densities up or down which may include background from the seafloor, are esti-
depending on the background mean scattering strength level matedtoaccountforlessthan10%ofthetotalpopulationin
at the other frequencies (cid:1)Fig. 7(cid:2) and also accounting for the the area shown.
target strength differences. These results are consistent with To exclude background reverberation, a density thresh-
those obtained from analyzing the histograms in Fig. 5. old is selected to segment shoaling regions where fish scat-
112 J.Acoust.Soc.Am.,Vol.127,No.1,January2010 Gongetal.:Targetstrengthandabundanceofshoalingherring
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