A combined acoustic and visual survey of humpback whales off southeast Queensland

17 Pages·2002·5.4 MB·English

by  Michael J Noad

ACOMBINED ACOUSTIC AND VISUAL SURVEY OF HUMPBACK WHALES OFF
SOUTHEAST QUEENSLAND
MICHAELJ. NOADANDDOUGLAS H. CATO
Noad, M.J. & Cato, D.H. 2001 12 31: Acombinedacousticandvisual survey ofhumpback
whales off"southeast Queensland. Memoirs ofthe Queensland Museum 4*7(2): 507-523.
Brisbane. ISSN 0079-8835.
Duringtheirmigrationsbetweenlowlatitudebreedingareasandhigh latitudefeedingareas,
male humpback whales, Megaptera novaeangliae, are frequently heard singing, often
continuously for many hours, and the sounds are audible fortens ofkilometres. The stock
that passes close to the coast of southeast Queensland has been extensively surveyed
visually,butlittleisknownofmovementsofwhalesthatpassoutofsightoflandhere,orin
other areas ofthe world, where the migratory paths ofhumpback whales are often across
open ocean. Acoustic surveyingmaybeusefulinquantifyingwhalemovements inoceanic
watersbeyondtherangeoflandsurveyingandanadditiontovisualmonitoring.Foracoustic
surveystobeofuse,theacousticcuesofthewhalesmustbequantifiedandcalibratedagainst
the numbers ofwhales in an area. In 1997 we performed a combined visual and acoustic
survey of whales migrating close to shore on the coast of southeast Queensland. Song
activity was measured using two indices: number of passing singers and number of
singer-hours observed within a 10km sector, and correlated with the number of whales
passing through the area determined visually. Both were significantly correlated with r =
0.68 and 0.64 forsingers and singer-hours respectively on a dailybasis, and 0.79 and 0.89
respectively on a weekly basis. Linear regressions ofdaily measures ofsong activity with
numbers ofwhales seen lead to estimates ofratios ofsingers with whales seen of0.127 ±
0.027 (95% confidence interval) and singer-hours with whales seen of0.288 ± 0.065. We
O
discuss the possible use ofthese indices for conducting stand-alone acoustic surveys.
Humpback whale, acoustic, song, migration, Australia.
MichaelJ. Noad (e-mail: [email protected]); Douglas H. Cato ; 1, Faculty of
Veterinary Science, University of Sydney 2006; 2, Defence Science and Technology
Organisation, Pyrmont 2009, Australia; 25 October2001.
Traditionally, surveys of whales have been Fristrup, 1997; Norris et al., 1999). While visual
conducted using visual detection from elevated observations are limited to the small proportion
points along coastlines or from ships or aircraft, oftimethatwhalesareatthesurfaceinthefieldof
Visual surveys are limited primarily by the vision,acousticdetectioncan beomnidirectional
cryptic natureofcetaceans which spend much of and possible for as long as the whales are
their time underwater and are available for vocalising. Deployment oi^ automated acoustic
sampling only tor the short proportion of time recording"systemsthatrecordforlongperiods(to
spentatthe surface. Theyarealso limited niitheir be analysed after reCovery) may be less ex-
range of detection (particularly tor ship-based pensivethan^ship-board^surveyssincethevwould
surveys) are highly weather dependent and are
restricted to dayJli°ght hours. wa^s jarueto,£msasted.,rTTdlhey are al,so noni-aiU-nftrtufsainvae,Jan1dS,
Manyspeciesofwhalesproduceintensesounds minimise sampling bias,
mwthhaaeytrabereevuiasseuufdauillbmlieentsthuoorvsduesbysihtnaagvn,eteislapilemcdiiitaseldtlaeynfcifneesccotaninvddeintetishosun.ss 6"A.*c*ou*s}b..lecin^gethyatmitgisail-nsdoir-heacst,i.ti.se..l.icmoiu.tnattiionngs,cut.,ehse
Acoustic surveys have potential advantagesover rather than whales< and so requires careful
visual surveys: large cetaceans in particularmay calibration of the relationship between the
be detectable at many times the range possible occurrence ot sounds and the numbers ofwhales
with visual observations; detection is less (Buckland et al., 1993). It is effective only for
dependent on weather; no restriction to daylight speciesthatvocaliseregularlyand,ofthose, only
hours; and can be automated to varying extents aportionofindividualsinastockmayvocaliseat
(e.g.Thomasetal., 1986;Cummings&Holliday, any time. Also variations in background noise
&
I985;Clarketal., 1986; Gillespie. 1997;Clark levels and local sound propagation characteristics
1
50« MLM-'IRS 01 1 III; QUEENSLAND MUSfiUM
e significant variation in the ((stances Humpback whales offer an opportunity to
detection ofvocalising whales. de\eiop acoustic monitorMU techniques using
populations that can be well surveyed usually
Determining the spatial concentration ci Like many species of baleen whales, they
Sources from the sounds delected, necessary in undertake annual migrations from high latitude
any estimate of abundance, is difficult without feeding areas to low latitude breeding areas.
fixing the source positions. This usually requires Humpback whales are often distributed along
threeormorewell spaced receivers with accurately coastlines during part of this annual c\
Ckunomwmninpogssit&ioHnsoltHejdfa.y,Wa1t9k8i5n)a.firbiSfchfesviigln)i,fi1c9a7n2d;y tpoi,bueciiinlasrhlayloluowIhetrnopI.i?ccacldiwma;teprosu(nDdaswbwinIn.ch1966I )
I | uses cost, complexity ofilie wotk at sea ami makingthemrelativelyaccessible ftn surveying.
the amount of analysis required. Under certain
circumstances, simpler methods are cl'leciive in Many techniques have been used for visual
determining tbe distance.- oi the source? which s[uHrevremyasnof&theAsnetwihnioljeas, mc1l9u7d7i;ilBgrayedrieanl. i"119Sv
can be related to source concentration {Cato.
[998) Bannister, [985; Bannister bi al 1 991 ( kirfceron
Ctah, 1994),ship-basedsurveys(C hittleborough.
Despite Ihepotential ofacoustics. few attempts 1965; Herman & Antinoja^ 1977; Whitehead,
have beenmadetocalibrateacoustic cues against lJS2; Stone A Ilamner. 198$; Multila &
l
visual COUttlS, panieulaiiv I .1 .m-..,11•-..-]..-\ I he (laphani. I9K°; Manila et ah. 1994), mark-
most extensive acoustic-visual surveys ipture survev s using phoiograp
1 I
been of bow head wliales {littluomt mysticCtWi) identification of individuals (Whitehead, 19!
during their annual migration offPoint Barrow; Bakes el ah, [985; Darling* iVforowito 1986;
Alaska (Cumminus & HoIUday, 1985; C lark et Baker& Herman, I9S7; riorez-i ion/ale/, I91M;
aRla..ft[e9r86e.t a1l9,961;99C0l;arWiu<r<s.vieEl&lisColna,rk,198199,93;2D/0e0h; dDiarrelcitngl&andMo-rbia.se1d99c3ounts ofI wLh,al1e9s99)a,loanndg
ct al., 1993; Raftcry & Zeh. 1008). Difficult migratory corridors (Brvden, I9S5; Paters©
i\eaihcr Conditions andthe use ofice as a survev Paterson, 19X9, Miyden et al., 1990, |996
platform often severely restricted visual surveys Patecson et aL 1994, 2001; Findlay & Best.
bene whales. Arrays of fixed hydrophones 1996b). 1 * havebeen used toeslimalc
have been used to track vocalising bow'heads absolute population levels, relative abundance
concurrently with visual observations, and and population giowth rates or population
mark-recapture and cfliei statistical techniques densities lorspecie
have been applied to both data sets m an attempt Humpback whale vocalisations are also
to obtain more accurate population estimates comparatively well studied. Male humpback
(Gentleman & Zeh, 191 d aL 1990; whales ptoduce long, complex, vocalisations on
Zebet al., 1993),Clark & Frismipl I997)useda the breeding grounds and during migration
different approach Lo compare and calibrate (KibblewhiteetaL 1967; Payne & McVay,
acoustic and visual detection rates n \\um\ Wuin. 1978;Cato, 1984. I99L). tIhese
.Jules (Hahicnoptfra muscutus) and fin whales songs may be produced continuously for many
(/?. phvsulus) during ship-based line transect h0U|3 v .l\ high source levels. Ihe
visual and acoustic surveyscombined with static combinattlioin ofcoastal distribution and reliable
hydrophone airays. A tf&tfatica] combination of and distinctive vocalisation make humpback
acoustic and visual data attempted to imp whales an ideal model for the development of
&
density estimates (Frrstrup ('lark. [997)t acoustic surveying techniques.
McDonald & Fox (1999) used an acousiic-nnly
iipproach with a single bottom-mounted hydro- Previousacousticsurveysofhumpbackwhales
phone 10 estimate minimum mks o\' fin have been conducted, Winn et al. (1975)
w rules otfHawaii. I i performed ship-based visual and acoustic
Common toall theseacoustic surseys hasbeen toi ideetvesrmoionehuamppobpauclkatiwohnalteostalintiohyethWeesbtreIenddiinegs
ihe use of acoustics either to gather additional n . while Levenson & Leaplej 1 1978) us<
information to support limited visual surveys diflcrent technique to survey the West Indies,
wheretheprobabilityofvisual detection is low oi dropping souobuoys I'mm (he air. Both studies
has noi teen previously determined, or as an made assumptions concerning the ii'iavinniui
almost entirely unealibrated survey tool detectable range of singing humpback whales
;
HUMPBACK WHALE ACOUSTIC SURVEY 509
whales seen. Au et al. (2000) used remote
recording techniques to show variations in
singingactivityacrossthewinteranddiurnally in
25°S Hawaii, but did not attempt to relate measure-
ments ofacoustic activity with singer density or
abundance. They did, however, suggestthatsuch
stand-alone acoustic techniques could be used to
provide either relative abundance estimates of
humpback whales, or, if ground-truthed with
visual and acoustic-positional surveys, absolute
abundance estimates.
Land-basedvisualsurveysalongthemigratory
corridor on the east coast ofAustralia have been
conducted regularly since 1978, mainly from
Point Lookout on North Stradbroke Island with
some from nearby Cape Moreton on Moreton
Island(Fig,1),bytwoindepen&dentsurveygroups
(Paterson, 1984; Paterson Paterson, 1984,
1989; Paterson et al., 1994, 2001; and Bryden,
1985; Bryden&Slade, 1988; Brydenetal., 1990,
1996). Despitesomedifferences in surveydesign
andstatisticalmethodologies,thetwosurveysare
in broad agreement regarding both absolute and
relative abundance, for example, Paterson et al.
Brisbane (1994) and Bryden et al. (1996) reporting
-fa populations of 1900 for 1992 and 1807 for 1993
respectively, with annual population growth rate
estimates of 11.7% and 12.3% respectively.
Humpback whales have also been shown to sing
reliablyinthisareaduringmigration(Cato. 1984,
154 E 1991;Noadetal.,2000;Macknightetal.,2001).
In this study, visual and acoustic surveys were
FIG 1. Southeast Queensland showing study site of performed simultaneously onthis well described
PeregianBeachanditswestwardlocationfromother and surveyedmigratorypopulationofhumpback
study sites ofPoint Lookout and Cape Moreton. whales offsoutheast Queensland to examine the
possible use ofacoustic stand-alone surveys for
(12.8 and 9.3km respectively) without apparently surveying humpback whale populations. Correl-
determining local propagation characteristics ations between the numberofwhales visible and
and with only limited data available concerning those singing are determined, and ratios of
source levels of song. Although Winn et al. whales tomeasurable indices ofacoustic activity
attempted to develop a ratio of 'callers' to aredevelopedforfutureuseinacousticsurveys.
'non-callers7 based on 1 1 visual sightings, ratios MATERIALS AND METHODS
varied widely for different areas leading to the
conclusion that thisratio 'represents the greatest Visual and acoustic observations were con-
weakness in the acoustic method'. ducted at Peregian Beach (26°30'S, 153°07'E)
Several subsequent acoustic surveys of onthesoutherncoastofQueensland(Fig. 1). The
humpback whales have simply relied upon the cshooaasltinhge,resancdoympbreiascehs, tahelnoenagr,essttrhaeiagdhtl,angde6ntklmy
presence or absence of song to determine the
to the south. Data were collected during the
migratory paths or distribution of whales
(Clapham&Mattila, 1990;Dawbin&Gill, 1991 bseotuwtehewnar2d8mAiuggruasttioanndo3f1tOhcetowbhearl.es in 1997,
Gilletal., 1995;Norrisetal., 1999).Frankeletal.
(1995) used an array of hydrophones to VISUAL DATA COLLECTION. Visual
determine the density ofsingers offHawaii but observationsweremade fromthe 73m high peak
did not attempt to relate it to the numbers of ofa nearby hill, Emu Mountain, set 700m back
510 MEMOIRS OF THE QUEENSLAND MUSEUM
18000
from the beach. The viewwas unobstructed in all
directions, coastal features allowing a 145° view
ofthe ocean to the horizon (-30km). Two teams
of3-5 volunteersmadeobservations infourshifts
from 7am to 5pm daily. Position, composition
A
and behaviourofwhale groups were recorded.
theodolite was used to measure horizontal and
vertical anglesto whalepodswith measurements
calibrated by comparison of theodolite-tracked
boat positions with GPS positions determined in
theboatatthesametime. Atrangesofupto 10km
the accuracy was determined to be within the
differential errorofthe GPS and so was taken to
be within 00m.
1
Data were also collected regarding environ-
mental factors that might affect visibility or
sightability including wind speed and direction,
sea-state,cloudcover,glare,precipitationandair
clarity. The number, type and positions ofships
andboatswerealsorecorded. Observations were
abandoned in conditions of poor visibility or
sightability including heavy or steady rain, and
sea-state >4.
Data were entered into a spreadsheet daily -8000
(Excel, Microsoft)whichcalculatedthepositions
of pods; using the theodolite measurements.
These calculations included the measured height
Eastings (m)
ofthe theodolite above the peak ofthe hill, the
tide height and a refraction coefficient (k= 0.08, FIG.2.PeregianBeachstudysiteshowingobservation
see Appendix). Pod identities and continuity of areas and grid system used (grid north lies between
sightings determined by the observers were true and magnetic north). Visual observations were
checkedagainstmeasuredpositionsforconsistency. made from 73m high Emu Mt while acoustic
Aerial surveys outto 60km from shore in good recordings were made using three offshore
visibilitybyBryden(1985) indicatedthat<5%of hydrophones (crosses). The small shoal south ofthe
array caused sudden and profound attenuation of
humpback whales passed beyond 10km of the
song soundsofsingersinthesouthern portion ofthe
headlands of North Stradbroke and Moreton
study area.
Islands, where most visual surveys have been
conducted. He considered that 10km was the southwards although some whale groups did not
useful limit ofvisibility from shore under good have enough sightings to determine direction of
conditions. Peregian Beach is -100km north of movement. These were assigned a direction
PointLookout(onStradbroke1.)and~45kmwest according to the ratio of north-south whales
(Fig. 1). Whilewesawmanywhalesatranges far observed during that day.
greaterthan 10km, we have limited this analysis
towhales seenwithin 10km ofshore. Theclosest ACOUSTICDATACOLLECTION. Anarrayof
approach of whales was only a few hundred three custom-designed hydrophone-buoys (A, B
metres offshore and so ourobservation area was and C) was deployed 1,500m offshore in 20m of
considered to extend from shore to 10km sea- water (Figs 2,3). The hydrophone-buoys were
ward, and limited north and south between spaced in a line ~750m apart, giving an array
bearings 10° and 160° on the study grid (Fig. 2). base-line of -1,500m, with the central buoy B
The visual survey area was not centred on Emu slightlyoffsettothewest. Eachbuoywasmoored
Mt as it was inland from the coast and so would by a 40kg concrete and steel clump attached to
have included a significant area not available to 6mofchainanda4.5kgploughanchor. Thebody
migrating whales, and would have been less of each buoy was a hollow tube of PVC pipe
directlycomparablewiththeacousticsurveyarea. supportedbya fibreglassfoam-filled 'torus' float.
Whaleswereseentravellingboth northwardsand Each contained a sonobuoy VHF transmitter
HI iMPBACK WHALE AGO!JSTIC SIJRVI V
6000
real-time spectrographie monitoring and
tl
other for computation of singer positions a
tour-track analog tape recorder (Tascam 424
Portastudiu) and a stereo DAT recorder (S
TCD-D7 Walkman). The audio signal was
monitored continuous!) during the hours of
\ isual observations When a singer was delected
withasignal-to-noiseratiosufficienttoallow the
pattern of the song to be determined, recording
was initiated. Some tracking ofsingers occurred
in real-rime in the field while the majority
occurred at a later time using the multi-track
owordings.
Appropriate sounds in Ihe song (rapidly
frequency-modulating tonal sounds) were
manually selected and sampled miorhecomputer
Eastings (m) using Cool Edit 96 (Syntrillium). Matlab
PIG. 3. Accoutre positioning ol a singing humpback (Maihworks) customised software performed
whale. Differences inthearrival timeofsoundsfrom waveform cross-correlations ofthe same sound
the singer lo cacti ot the hydrophone;, were used to received on each of the three pairs of hydro-
:rratc a byperbolfl fercy lino) lor each pair o\' phones (buoy-pairs AB, BC and (A) to
buoys. The painl ol midsection ot ihe lhiee determine the time-of-arrival-differenees
dhtiyaspkceerrnebpoaalsnaceyt(hifenroipnmotsebirutsoeiyco-tnpiaoniosrisre(Ashluell.tseBdt<aignearan.dtrAiAatngslIme,waalmls (reTsOuAlDtisn)gfoTrOeaAcDhspaiwraofsbuuosyes.dEtaochgoefntehreatthereea
which ease Ihe BUlgcr W»fl taken 10 be at the centre. hyperbola along which the source of the sound
couldlie.Theintersectionofthethreehyperbolae
(rSepcahratragneabBllee"ebattrtcemrykspacakns12sVo. 30A-4hI)Ba)ndanwderea wWahislteaktheenreasistahmebpiogsuiittiyoninohfetrheentsiinngtehris(mFiegt.ho3)d.
( (since the hyperbolae intersect at two points, one
designed to follow (lie rise and foil ol (he sea each side ol ihe line of hydrophones), in oui
itUfaceand remain upright, thus optimising lin-
experiment the westerly solution was usually
orientation ofthe transmitter antenna. This was
inland and could be discaided. Sequeuiinl
achieved by attaching themooring line at a point
calculation ofpositions allowed the singer to be
in relationship to the distribution of mass that
minimised rotation in the vertical plane A40dB (racked (Fig. I).
gain pie-amplifier (cu.stom-butlt) was contained Caiibratfon and Hanging Error, The array
in a separate underwater housing attached to the ground-truthed using two methods. The First was
mooring clump and connected !o the buoy by comparison oi acoustically calculated posir
standard RG58 coaxial cable (single ton with theodolite positions of visually idem.
50Ohna) running along (he anchor rope, A singers (based on the riming ofsurface intervals
CiEC-Marconi SHIOLX hydrophone was predicted acoustically by features ofthe song)
connected to ihe preamplifier by Idm ol RG58 (Fig, 4). The second was experimental and
cable and was suspended from a small float involved the implosion of light bulbs under Uic
attached Lo the anchor, approximately 1m above research vessel at various locations in the study
the substrate. The hydrophone cable was wound area. These bulbs, smashed at depth, produced a
with string to help prevent low frequency VWI single brief popping sound audible at several
shedding noise in conditionsot signtficuntcurrent kilometres range that could be acoustically
or groimdswell. positioned for comparison with GPS positions.
Signals mmthebuoyswereicccivcdby aVagi Bulbs were enclosed in a fine net so that broken
I debris could be recovered.
antenna mounted as high as possible (- 10m
-
above sen-level) ai the base station located fjOrlfl As three hydrophones were necessary to
behindthebeach. Theantennawasconnected toa calculate rhe location of the singer. acoiMic
fottf channel. U>w noise, VHF receiver. Signals tracking was not possible ifone or more of the
from the receiver wen. split and passed lo rWfl buoys was not operating. Tunc lost due to
desktop computers (IBM PC clones) - one for technical problems was minimal although the
512 MEMOIRS OF THE QUEENSLAND MUSEUM
9000 - visual observations were curtailed due to high
^L sea-state and rain, and similar conditions also
8000 - I I 16-27 reduced singer detectability due to increased
16:20*1614
7000 - ,' ambient noise, this ensured that visual and
16:07?h558
-g 6000 - i 'I, 1539&©1515:2518 aucnoduesrtifcavoobusrearbvlaetidoentsecwteiornecdoinrdeicttiloynsc.omparable
£c, 5000 - arraV, 15:27*Tgi502 Two methods of quantifying singing activity
E 4000 BASE i + \ Q 14:02 wereused: 'numberofsingers1whichwasacount
Z 3000 - STAT. i + \13:58 ^13:52 of the number of individual singers passing
21000000nuooo-- EMMoooTU1A ooo•*1 ifooo,I1 +ooo1M01 moooi ooo?<\*?i oooiooo\V1 12o060o9.57o ttin1hhn0uedr-meohdbxuaoe'gybrh.swethorhfievcasshtieincwogtneaorrsdsapyedinreatnt1ehdremhsioauunrmreesmad,ienbaanygcdhtch'ohesuoirnunetgrsiuenlrogt-fshottfuhhoreer
o o o
fo-*r*n(or^coo»o*- no no ^o The 'number of singers1 was the acoustic
*— f- T- T- *— analogy of the visual count of number of
Eastings (m) individualspassingthroughper lOh. Singingwas
FIG.4. Anexampleofacombinedvisualandacoustic consideredtobe fromthe samewhale ifthe song
track of a singing humpback whale {70904s1) was heardcontinuously with onlyshortgapsofa
including times oflocations. Acoustic positions are few minutes between song cycles, and no
black circles and theodolite-generated visual significant change in source position occurred.
positions are grey crosses. Wherethegapwaslonger,weassumedthatitwas
from the same whale ifthe position ofthe new
buoyswereremovedfromthewaterinthemiddle song-session was close to that of the original
ofthe survey period for scheduled maintenance. song-session, ifthe songcontained idiosyncrasies
ofpatternconsistentwiththeoriginal singer, orif
Quantification of Singing Activity. Although the singer was tracked visually between singing
some singers passed very close to thearray, only locations. This method provided a direct
onesingerwent insidethearrayandthen byonly measurement ofthe true number of individuals
a few tens of metres. The area used for the singing as they passed through the sector in the
acoustic survey was therefore taken as being a
10-h observation day, but required substantial
10km sectorouttoseafromthearraybetweenthe effort, since all singers had to be tracked
bearings 10°and 130° (Fig. 2)wheretheacoustic acoustically throughout the full observation
tracking was found to give reliable results.
period.
Although this did not correspond exactly to the
10km sector from the beach used for the visual The 'singer-hour index' was determined by
survey, the two sectors overlapped substantially counting the number ofsingers detected within
and could be considered to provide comparable the sectoronceperhourandsummingthe results
visual and acoustic samples of the migrating for all hours of the observation day. To avoid
whales. missing a singer during the pause between song
Within this sector, the signal-to noise ratio cycles, a 10min period was monitored every
(SNR)ofthesounds usedinthecross-correlation hour, from5minbeforethehourto 5min afterthe
analysiswasmorethanadequateforthepurpose. hour. This method provided a relative index of
TheSNRof'modulatedbellows',thesoundmost singing activity related to the number ofsingers
and the duration ofsinging, since an individual
frequentlyused, werecalculated for20singersat
92 points within 13km ofthe array by measuring singer would be counted for each hour that it is
the relative levels of the signal and the back- audible.Thepurposeofmeasuringthisindexwas
ground noise in the 1/3 octave band containing to test its effectiveness as a relative indicator of
the centre frequency ofthe signal (210-400Hz). the number ofwhales passing, since it was less
averagedoverthe duration ofthe sound (approx. time consumingto measure thanthe actual num-
212.2dsBec)u.ndTehreamveeraangeSNobRseartvaatiroannaglecoofnd1i0tikomnsw.as biendriovfidsuianlgesrisn.gIetrsdiodrnothteirreqluoicraetiiodnesn,tiafpiacrattiofnroomf
an estimateoftheirranges. In this test, the range
Only song recorded during hours of visual was determined using the three-hydrophone
survey were included in this analysis. As most localisationmethoddescribedabove,butsimpler
HUMPBACK WHALE ACOUSTIC SURVEY 513
30.0 Whalestotal
Singers
Singer-hours!
25.0
1997 Week beginning (1997)
FIG. 5. Fluctuationsofhumpbackwhalevisual andacousticcountsduringthestudyperiod;alldataaveragedfor
10-h sampling periods. A, daily fluctuations; B, weekly fluctuations. All visual data are shown for daily
fluctuations whereas weekly averages are based only on days where both visual and acoustic data were
collected.Theweekbeginning2Octoberisexcludedastherewereonly5hofacousticobservationsduringthis
period.
methods of estimating range are possible. For werenotincludedintheanalysis).Itwasassumed
example, ifthesource level ofthesoundsandthe that the numbers of humpback whales passing
propagationconditionsforthesiteareknown,the Peregian Beach were unaffected by whether it
rangecanbeestimatedfromthelevelreceivedon was day or night and that the numbers passing
onehydrophone. Whilethis index islikelytobea daybydayvariedinarandommanner,apartfrom
less effective indicator of numbers of whales the broad rise and fall over the full period of
passing than the number of singers, it can be migration ofseveral months. Then our sampling
measured with a simpler system and with less could be considered to be a reasonable approx-
effortandsomaybesuitableforuseinautomated imation of random sampling (Cochran, 1963).
systems. Each sample was drawn out of a population of
156 10-hunitsoverthe65daysofobservation(65
ESTIMATEOFPOPULATIONPARAMETERS. x 24/10= 156).
Four parameters of the population were
estimated for the period ofobservation: (i) total Because of the long term rise and fall in
number ofsouth-bound whales passing through numbers during migration (Fig. 5A,B), there are
the study area, based on visual observations, (ii) advantages in using stratified random sampling
total numberofwhalespassingthroughthestudy theory (Cochran, 1963). Application of this
area regardless ofmigratory direction, based on technique to surveys of this humpback whale
visual observations, (iii) total number ofsingers population is discussed in Paterson et al. (1994,
passingthroughthestudyarea,basedonacoustic 2001). The following three strata were used for
observations, and (iv) total number of singer- all estimates of acoustic and visual data: 28
hours generated in the study area. Ratios of August- 1 October (days 1-35), 2 October- 15
singersandsinger-hourswithwhalesseen,across October (days 36-49), 16 October - 31 October
the entire study period, were calculated using (days 50-65).
these estimates. A total of 55 visual sample units and 46
Data for each day with 5 or more hours of acoustic sample units were obtained out of a
observation were normalised to the equivalent possible 156 units over the 65 day observation
for a 10-h day which was considered to be a period. This includesacoustic dataonly forthose
sampleunit(dayswithlessthan5hofobservation days thathad corresponding visual data (i.e. >5h
514 MEMOIRS OF THE QUEENSLAND MUSEUM
ofvisualobservations). Nineofthedaysofvisual 16.0 -i
observations did not have acoustic positions
becausethearraywasdown(includingascheduled
maintenance period from 30 September to 8
October).
The population size with 95% confidence
interval is (Cochran, 1963)
Ny ±tNs(yJ
u
where N is the total number ofunits, y is the
xl
weighted mean, t is the value ofthe Student's t
distribution for a two-tailed value of0.05, and
s(y )i$ the weighted estimate of standard
deviaxttion. The value of/ was determined forthe
effective numbers of degrees of freedom (for
small sample sizes, Cochran. 1963. based on
Satterthwaite, 1946)
&g*st)'
=
tf,
ghs
t
C<eCn C^CEOrl ti/—5 CaO. 1Lsn0 OCuN OCn O(uO O OoO
where /?/, is the numberofsamples forstratum k, CNJ 04
and s'i, the variance of the samples in each Week beginning (1997)
stratum. The final termgi, is givenby
NAN -n FIG. 6. Weekly numbers of visually observed
h h) humpback whales passing within 10km ofPeregian
Beach. Includesall visual dataregardless ofwhether
where Nh is the total possible number ofsample it is matchedby acoustic data.
units in each stratum. clear migratory direction, mean pod sizes were
Daily and weekly numbers of singers and 1.73 north-bound (s.d. = 0.69, n - 52) and 1.97
singer-hours were correlated against each other south-bound (s.d. = 0.83, n = 172).
to test the strength and significance of song
activity as an indicator ofthe number ofsingers The number of south-bound whales passing
within 0km of Peregian Beach between 28
using linear regression analysis (Excel, Micro- 1
csoofrtr)e.lSaitnegderwnituhmbneursmbaenrdssionfgerw-hhaoluersswseereen.alIson A1u,1g4u8st±and17301 O(c9t5o%becro1n9f9i7dewnacsecianltceurlvaalt,edutsoibneg
addition to using population estimates to techniques described by Cochran, 1963). This
calculate ratios ofsong activity and whales seen can be expected to significantly underestimate
thestocksizesince: wehavesampledonlypartof
across the entire study period, linear regression
analysis was used to calculate regression thesouthwardmigration;asignificantproportion
coefficients with confidence limits for daily and ofwhalespassesbeyond 10kmatPeregianBeach;
weekly data. and, an unknown proportion may have been
missed. Anestimateoftotalstockpassingduring
RESULTS our limited period of observation can be made
withreferencetothedataofPatersonetal. (1994:
VISUAL CENSUS. During the 65 day survey fig. 4) for 1992 oil Point Lookout. These data
period, 529 hours of observations were made show thatthenumberofsouth-bound humpback
including39full 10-hdays, 16dayswith5-1Oh,5 whales seen passing Point Lookout between 28
dayswith someobservationsbut<5h, and 5 days August and 31 October (the period of our
with no observations. A total of 279 pods of observations) amount to -78% of those seen
whales were observed travelling in both north-bound between 5 Juneand 31 October, the
directions containing 501 whales including 43 period over which their population estimate of
calves (Fig. 6). Pods were tracked with 1,792 1,900 was made. At the annual rate ofincrease
theodolite-measured positions. For pods with determined by Paterson et al. (11.7%), the
HUMPBACK WHALE ACOUSTIC SURVEY
515
180O0
population in the northern migration of 1997 is
estimated to be 3,300, so that the numbers
passing between 28 August and 3 1 October
would be 2,580. Thus, ifthe ratio is similar off
Peregian Beach, we would expect 2,580 whales
to pass during our period of observation, 2.25
times the number estimated from our observ-
ations. Our estimate is therefore 45% of the
numbersexpected from thedataofPatersonetal.
The most likely explanation forthis discrepancy
is that approximately half the whales passed
beyond the 10-km limit ofthe survey.
Theestimatednumberofwhales (regardlessof
migratory direction) passing through the study
area between 28 August and 31 October 1997
was 1,424 ± 186 (95% confidence interval).
ACOUSTIC SURVEY. The full array was
operational for 44 full-days, 7 part-days and not
operational on 14 days due to the loss ofone or
more hydrophones. At leastonehydrophonewas
operational for all but 48 hours of the survey
period. Approximately 380 hours ofrecordings
were made during the observation period
although some ofthese were made out ofvisual
survey hours including at night. Four hundred
and thirty-two hours of array monitoring
Eastings (m)
coincidedwith visualobservationsyielding 24h
1
poafsrseecdorwdiitnhgisnf1r0okmmaonfetshteiamrartaeyd. 48 singers that FIacGous7.ticDi(sbtlraicbkutdiootn)poofsiatlliovnissfuarlom(tghreeystsudqyu.arSei)ngaenrds
Concurrent acoustic and visual observations could not be tracked through the southern portion of
occurred across the survey period except for the the study area due to severe attenuation of song
week starting 2 October when the array was sounds received at the array. Although whales were
undergoing maintenance. During this week only seen traversing the study area in less than 10m of
water, singerswerenotrecordedsinging inwatersof
5h ofconcurrent observations were made and so <20m depth.
analyses using average weekly data exclude this
period. Cummings & Holliday, 1985; Frankel et al.,
&
The accuracy ofdetermining the range ofthe 1995; Clark Ellison, 2000).
source decreased with distance, small errors in
Empirical observations and calibration studies
determiningthebearingsfrom each pairofbuoys
showedthatpropagationofsoundthroughoutthe
resultinginprogressivelylargererrorsinrangeas
range increased. Calibration results indicated that area was not uniform (Fig. 7). Sounds from
acoustic positions suffered mean range errors tracked singers suffered sudden and severe
increasing from approximately 5% of range at attenuationwhenenteringthesouthernpartofthe
2km to 10% at 10km and 18% at 20km. These study area, particularly on the southernmost
errors were for single positions using a single buoy, and so were not able to be tracked further
cue. When positions were calculated forsingers, and were often soon lostaltogetheron all hydro-
however, impossible positions based on the phones. This acoustic shadow was confirmed by
courseandspeedofthesingercouldbediscarded the bulb-imploding calibration experiments and
allowing some improvement in accuracy. These appeared to be due to the presence of a shoal
results are consistent with other studies using south of the array. Another array 'blind spot1
similar methods that have found reasonably existed to the north ofthe array. Here the singers
accurate results at ranges of4-10 times the array could be heard but range determination was
dimensions (Watkins & Schevill, 1972; prone to large errors within -10° ofthe end-fire
h
1
516 MEMOIRS OF THE QUEENSLAND MUSEUM
axisofthearray (linethrough
the hydrophones) due to the
increased sensitivity of the
estimatesofbearingstosmall
eiTors in the measured time-
of-arrival-differences, as
well as the increasingly acute
angles of intersection of the
hyperbolae. Thearcofeffect-
ive array function, therefore,
extended from 10° to 130°
(Fig. 2). 0.0 2.0 4.0 6.0 0.0 1.0 2.0 3.0
Total singers and singer- Daily singers Weekly singers
hours in the useable portion per 10 h per 10
of the study area during the
study period were 180 ± 50 FIG. 8. Linear regression ofdaily and weekly numbers of singers against
and 418 ± 106 (95% numbers ofsinger-hours of song activity. A, daily counts ofsingers and
confidence interval) respec- singer-hours normalised for a 10-h day; B, weekly averages of daily
tively. Dailyandmeanweekly normalisedcounts.Onlydayswithmorethan5hofmatchingvisualdataare
song activity fluctuated included.
throughoutthe migration ina averages0.79 and0.89, respectively (P<0.01, n
manner that reflected the numbers of singers =
tracked through the study area (Fig. 8A,B). 10).
Correlation ofdaily singer-hours against singers Ratios of singers and singer-hours to whales
gaveacorrelationcoefficientr=0.86(P<0.01,n seen for daily and weekly averages were
= 44) while correlation of mean weekly determined by linear regression analysis (Fig.
singer-hours against singers gave a correlation 9A-D, Table 1). In all cases, regression coeffic-
coefficientr=0.94(P<0.01, n=9). The singer- ients were calculatedforboth regression lines of
hour index was therefore a reliable and accurate best-fit and for regression lines passing through
indicator of the number of singers passing theorigin(asthereshouldhavebeennosingersor
through the area. song ifthere were no whales). Ratios ofsingers
and singer-hours to whales seen were also
Although whales were seen traversing the
calculated using the calculated full-survey
nsottudryecaorreadeidnsliesnsgitnhganin1w0amteorfswoaft<er2,0smindgeeprtshw(etrhee population parameters (Table 1).
depth at the array) (Fig. 7). These results effectively give arange ofratios
calculat—ed from data averaged over three time
COMPARISON OF VISUALANDACOUSTIC frames daily, weekly, and the entire 65-day
RESULTS. Numbersofsingers andsinger-hours study period. Daily results, with their greater
also fluctuated throughout the migration in a spread and sample size, probably provide the
manner similar to the numbers of whales seen most accurate measure of the relationships
(Fig. 5A,B). Correlation analysis for daily between acoustic activity and whales seen,
averages gave con-elation coefficients r = 0.68 reflected in their narrower confidence intervals.
and 0.64 for singers and singer-hours, Also daily coefficients of regression are less
respectively (P < 0.01, n = 46), and for weekly affected by regression through the origin than
TABLE 1.Ratiosandregressioncoefficients(h)with95%confidenceintervalsofnumbersofsingerstrackedand
numbers ofsinger-hours to numbers ofwhales seen overthree differenttime scales. No confidence intervals
werecalculated forthe hill survey ratios.
Regressionofdailyresults Regressionofweeklymeanresults
Best-fit Throughorigin Best-fit 1hroughorigin Fullrastuirovey
b ft&4< b 95%CI b 95%CI b 95%CI
i
nNoo..ooffwshianlgeersssvesen 0.137 ± 0.045 0.127 ± 0.027 0.13& ± 0.087 0.13 ± 0.035 0.126
nNoo..ooffwshianlgeesr-sheoeurnsvs 0.299 ± 0.110 0.288 ± 0.065 0.467 ± 0.199 0.345 ± 0.093 0.294