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Instructions for use
Title Diving behavior of sei whales Balaenoptera borealis
relative to the vertical distribution of their potential prey
Author(s)Ishii, Midori; Murase, Hiroto; Fukuda, Yoshiaki;
Sawada, Kouichi; Sasakura, Toyoki; Tamura, Tsutomu; Bando,Takeharu;
Matsuoka, Koji; Shinohara, Akira; Nakatsuka, Sayaka; Katsumata,
Nobuhiro; Okazaki, Makoto; Miyashita,Kazushi; Mitani, Yoko
Citation Mammal study, 42(4),
191-199https://doi.org/10.3106/041.042.0403
Issue Date 2017-12
Doc URL http://hdl.handle.net/2115/72274
Type article
File Information 86285D9C-D102-4472-AA04-D2FD2670E3C4.pdf
Hokkaido University Collection of Scholarly and Academic Papers
: HUSCAP
https://eprints.lib.hokudai.ac.jp/dspace/about.en.jsp
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Mammal Study 42: 191–199 (2017) © The Mammal Society of Japan
Original paper
Diving behavior of sei whales Balaenoptera borealis relative to
the vertical distribution of their potential prey
Midori Ishii1,*, Hiroto Murase2, Yoshiaki Fukuda3, Kouichi
Sawada3, Toyoki Sasakura4, Tsutomu Tamura5, Takeharu Bando5, Koji
Matsuoka5, Akira Shinohara1, Sayaka Nakatsuka6, Nobuhiro
Katsumata6, Makoto Okazaki2, Kazushi Miyashita7 and Yoko Mitani71
Graduate School of Environmental Science, Hokkaido University, 20-5
Benten-cho, Hakodate, Hokkaido 040-0051, Japan2 National Research
Institute of Far Seas Fisheries, Japan Fisheries Research and
Education Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa
236-8648, Japan3 National Research Institute of Fisheries
Engineering, Japan Fisheries Research and Education Agency, 7620-7
Hasaki, Kamisu, Ibaraki 314-0408, Japan4 Tokyo University of Marine
Science and Technology, 4-5-7, Konan, Minato-ku, Tokyo 108-8477,
Japan5 The Institute of Cetacean Research, 4-5, Toyomi-cho,
Chuo-ku, Tokyo 104-0055, Japan6 National Research Institute of Far
Seas Fisheries, Japan Fisheries Research and Education Agency,
5-7-1 Orido, Shimizu-ku, Shizuoka-shi, Shizuoka 424-8633, Japan7
Field Science Center for Northern Biosphere, Hokkaido University,
20-5 Benten-cho, Hakodate, Hokkaido 040-0051, Japan
Abstract. In this study, we investigated the diving behavior of
sei whales relative to the vertical
distributionoftheirpotentialpreyinthewesternNorthPacificduringthesummerof2013.Acoustictime-depthtransmitterswereattachedtotwoseiwhalesfor10.2and32.0h,respectively.Theverticaldistributionanddensity(expressedasthevolumebackscatteringstrength,SV)oftheirpotentialpreywere
recorded by an echosounder. Diving behavior was classified into two
shapes: U-shaped
andV-shaped.Forbothindividuals,U-shapeddivingwasassociatedwithhigherSVvaluesthanV-shapeddivingandthefrequencyofU-shapeddivingincreasedfromlateafternoonuntilsunset.Duringthedaytime,densescatteringlayers(presumablyzooplankton)weredistributedatapproximately40mandtheythenmigratedtowardthesurfacearoundsunset.Thedivingdepthofthewhalesfollowedthedielmigrationof
thescattering layersand thedivingwasconcentrated in these
layerswhen
thedensitybecamehigh.Theresultsofthisstudyindicatethatseiwhaleschangetheirdivingdepthandshapesinresponsetothedielverticalmigrationoftheirpotentialprey.
Key words:
baleenwhales,biologging,cetacean,foraging,habitat.
Observations of the diving behaviors of baleen whales have been
reported widely and have developed in
paral-lelwithadvancesinbiologgingandunderwaterbiotelem-etry (e.g.,
acoustic transmitter) devices. These deviceshave been applied to
several species, including the blue whale (Balaenoptera
musculus;Fiedleretal.1998;Crolletal.2001;Calambokidisetal.2008;Goldbogenetal.2013;Friedlaenderetal.2015;Goldbogenetal.2015),thefinwhale(B.
physalus;Crolletal.2001;Goldbogenetal.2006;Friedlaenderetal.2015),theBryde’swhale(B.
brydei;Alves et al. 2010), theAntarcticminke (B.
bonaerensis;Friedlaenderetal.2014),andthehumpback
whale (Megaptera
novaeangliae;Goldbogenetal.2008;Friedlaenderetal.2009;Nowaceketal.2011).Simulta-neousmonitoringofthedivingprofilesofhumpbackandfin
whales as well as the vertical distributions of
theirpreywasattempted toelucidate their
feedingbehaviorsrelativetopreyavailability(Witteveenetal.2008,2015).Inthelattertwostudies,smallacoustictime-depthtrans-mitters(pingers)wereusedtofollowtheswimmingpathsof
individuals based on their acoustic signals while the vertical
distribution of their prey was recorded by echo-sounders.The food
habits of baleenwhales have been investi-
*To whom correspondence should be addressed. E-mail:
[email protected]
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192 Mammal Study 42(2017)
gated using their stomach contents (e.g.,Konishi et
al.2009),whiletheirpreyselectionshavebeeninvestigatedusingtheproportionsofpreyintheirstomachswiththoseintheenvironment(e.g.,Muraseetal.2007).Thesetradi-tional
methods provide important baseline informationabout the feeding
habits of whales, but they do not con-sider individual
feedingbehavior.Recentdevelopmentsinbiologgingandunderwaterbiotelemetrydevicesnowallowustoobtainsuchdetaileddata.Seiwhale(Balaenoptera
borealis) is the third largest
baleenwhales(typically15minlength)aftertheblue(B.
musculus)andfin(B. physalus)whales,andtheyaredis-tributed in the
temperate waters of both hemispheres,including the North Pacific
(Horwood 2009). Theirfeeding behavior in the North Atlantic was
inferredfrom horizontal movements based on
geographicallocationsobtainedbysatellite-basedbiotelemetry(Olsenetal.2009;Prietoetal.2014),buttheirdivingbehav-iors
relative to the vertical distribution of their prey
havenotbeeninvestigated.Thus,inthisstudy,weinvestigatedthedivingbehavior
of sei whales relative to the vertical distribution of their
potentialpreybyusingacoustic time-depth
transmittersandanechosoundersimultaneously.
Materials and methods
Thesurveywasconductedinthesubarctic-subtropicaltransitionareaofthewesternNorthPacificinthesummerof2013(Fig.1).Thebottomdepthintheareaisaround5000m.Two
surveyvessels calledYushin-Maru (YS1;724 gross tons (GT); cetacean
sighting survey vessel)and Shunyo-maru (SHU;887GT; trawler
typefisheriessurvey vessel) were engaged to deploy transmitters
onAugust13and14,2013,respectively.Thelengthofeachvesselwasroughly70mandtheheightofthebowdeckfromtheseasurfacewasroughly8m.Compoundcross-bowswereusedtoattachtransmitters
to thedorsalsur-facesofseiwhalesfromthebowdecksofthesurveyves-sels.
The transmitters were tethered to small titaniumspearheads (3.8 cm
in length, 1.45 cmmaximum
headwidth,and4ginair)bypolyethylene(Dyneema)fishinglinewithalinearstrengthof55kg.Thespearheadswerelooselyattachedto
titaniumshafts (12.15cmin
length)withtheexpectationthatthetransmittersandspearheadswoulddetachfromtheshaftsuponcontactwithawhale.The
shafts were attached to carbon bolt arrows.
ThespearheadsandshaftsweremanufacturedbyKoreisangyo(Yokohama,
Japan).The spearheads were employed to
secure rigid attachment to whales sowe could
obtaindataabouttheirdiurnaldivingbehavior(i.e.,morethan24 h).
Acoustic time-depth transmitters
(FPXG-1040-60P500T30,FusionInc.,Tokyo,Japan)wereused,whichsent
pulses (62.5 kHz) every second and encoded thedepth data (0.01m
resolutionwith ± 0.2% accuracy).The transmittersmeasured43mmin
length,9.5mmindiameter, and 6 g in air.Themaximumduration of
thebatteries was 48 h and the maximum detection rangewas800m.The
transmitted signalswere receivedbyadirectional hydrophone (VH170,
Vemco, Nova
Scotia,Canada).Thehydrophonewashull-mountedonSHUatadepthof4.3mbelowtheseasurface.Four(bow,port,starboard,
and bottom sides) out of the 17 hydrophonechannels were used
because the receiver could only handle
fourchannels.Thereceivedsignalswereprocessedbyareceiver(FRX-4001,FusionInc.,Tokyo,Japan)onboardSHU.
SHU followed the tagged individuals based
ontheirrelativeswimmingdirectionaccordingtothesignalsreceived.Acalibratedquantitativeechosounder(SimradEK60;
Simrad,Horten,Norway),witha120kHztransducerwasused to record the
volume backscattering strength
(SV,dB;anindexofpreydensity)whiletrackingthewhales.Thetransducerwashull-mountedonSHUatadepthof4.3mbelowtheseasurface.Acousticbackscatterdeeperthan7.5mwasprocessedusingEchoview4.9(Myriax,
Fig. 1. Map of the area in the western North Pacific surveyed
inAugust2013.Linesindicatecruisetracksbythesurveyvesselwhiletrackingthefirst(S-1;dottedline)andsecond(S-2;thickline)taggedseiwhales.Circlesindicatethepositionsoftrawlandplanktonnetsam-plingstations(T-1,2,3,and4).Trianglesindicateexpandableconduc-tivity,temperature,anddepthprobe(XCTD)stations(X-1,2,and3).
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Ishii et al., Diving behavior of sei whales 193
Hobart,Australia).Trawl(NBT-2P-SY,NittoSeimoCo.,Tokyo, Japan)
and plankton net, i.e., the North
Pacificstandardnet(NORPAC;mouthopening=45cm,meshsize = 0.33mm)
samplingwere conducted by SHU atfour stations prior to the diving
behavior observations
(August10–12,2013)inordertoobtainqualitativeinfor-mationabout
thepotentialpreyspeciesaround
thearea(Fig.1).Themidwatertrawlnetwas84.9mlongwithamouth opening of
42.4 × 42.4 m and a cod end
of8.0m.Thesamplingdepthandheightofthenet’smouthwere monitored with
the Scanmar system (Scanmar,Åsgårdstrand,Norway).Themouthopeningwas
set to30×30mwhilethenetwastowed.Thetowingspeedofthe trawl net was
4–5 knots.The sampling depth
waseithermidwater(0–90m;T-1inFig.1)orsurface(0–30m;T-2,T-3,andT-4inFig.1).Surfacetrawlswerecon-ductedusingmidwatertrawlswiththefloatsattachedtothebridlesothetrawlcouldbetowedatthesurface.Bothdaytime
and nighttime tows were conducted at T-1 tocompare the species
compositions during the day andnight. It has been well documented
that
myctophidsundergodielverticalmigrationbetweenepipelagiclayer(shallowerthan200m)duringnightandthemesopelagiclayerduringdaytime(Yatsuetal.2005andthereferencestherein).Thepurposeof
thenighttimetowin
thisstudywastoobtainqualitativeinformationonspeciescomposi-tionsinwatercolumn.Becausemyctophidsisnotmaindietofseiwhales(Konishietal.2009),onlyonenight-timetowwasconductedduringthesurvey.Onlydaytimetows
were conducted at the rest of sampling stationsbecause small
pelagic fish, which are main diet of seiwhales in the western North
Pacific (Konishi et
al.2009),aremostlydistributedatshallowdepthsaccord-ing to our
previous studies (Murase et al. 2007, 2009,2011, and 2012) aswell
as other studies (Fujino et al.2010). Trawl samplings were
conducted at
predeter-minedsites(approximately93km[≈50nauticalmiles]apart)ratherthantargetingspecificacousticbackscatters.The
NORPAC net was towed vertically from 150 to0 m. Expandable
conductivity, temperature, and depthprobe (XCTD; Tsurumi Seiki Co.,
Yokohama,
Japan)castsweremadeatthreestations(X-1,2,and3;Fig.1)fromAugust13–16,2013inordertomeasuretheoceano-graphicconditionsaroundthetrackingarea.Acontinuousrecording
of the near-surface temperature obtained bySHUwas used to
investigate the oceanographic condi-tions encountered by the tagged
individuals, although the exact geolocations of whales and the
vessel could differbyupto800m(maximumdetectionrangeofthe
transmitters).Diving depth profiles were analyzed using the
Ethographer package version 2.01 (Sakamoto et
al.2009)withIgorProversion6.12(WaveMatrics,Oregon,US).The start and
end points of each diving
behaviorweresetat3mfromthesurface.Individualswerecon-sideredtobe“diving”whentheyremaineddeeperthan5minthewatercolumnforlongerthan5s,consideringthemaximumdivingdepthsoftheseiwhalesobservedin
this study.ThenonparametricMann–WhitneyU test was used to
testwhether themaximum depth in eachdiveand
thedurationofeachdiveshoweddielpattern.Daytimehours(fromsunrise
tosunset)weredefinedas5:00to19:00(localtime),whereasallotherhoursweredefinedasnighttime.Thetimeallocationatdepthindex(TAD)wascalculatedaccording
toFedaketal.
(2001).TADisadimensionlessindexofdiveshape,whichwasdesignedtoutilizerelevantinformationfromdivepro-files
and highlight where the diver centers its activitywith respect to
depth during a dive. TAD varies
fromzerotooneanditrepresentsthedifferencesbetweendiveshapes.Divingbehaviordeeperthan10mwasclassifiedaccordingtotwoshapesbasedonTAD:U-shaped(TAD≥
0.7) andV-shaped (TAD
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194 Mammal Study 42(2017)
of S-2 was observed for 31 h 59 min from 10:37 onAugust 14 to
18:36 onAugust 15, 2013. Due to
datarecordingtechnicalproblems,one(21min)andtwo(1h11minand1h10min)longgapswithoutsignalsexistedfor
S-1 and S-2, respectively, and thesewere excludedfrom the
analysis.The remaining signal gapswere
lessthan4min.ThesteamingdistancesforSHUwhiletrack-ing each
individualwere 101.4 and 259.0 km, and themeanvessel steaming
speedsduring
theseobservationswere10.0and8.1km/h,respectively(Fig.2).Itshouldbenoted
that the distances and speeds of the vessel should not be
translated directly into those of the tagged
indi-viduals.ThetotalnumbersofdivesforS-1andS-2were119
and387,respectively,whiletheoverallmeandivedepthsand standard
deviations
(SD)were17.9±12.6m(deepestdepth=57m)and14.2±9.6m(deepestdepth=48m).Theoverallmeandivedurations
forS-1andS-2were3.2±2.3min(maximumduration=12.2min)and3.0±2.5min
(maximum duration = 11.0min), respectively.The results of
theMann–WhitneyU test revealed that
themeandivedurationsweresignificantlylongerinthedaytime than the
nighttime for both individuals (P
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Ishii et al., Diving behavior of sei whales 195
respectively.The results of trawl sampling were summarized
in
Table 2. Mesopelagic species, especially
myctophids,wereonlysampled in thenighttimetow.Denseschools
were recorded near surface by the echosounder during the
nighttimetowwhilenosuchschoolwasrecordedduringthedaytimetows.Thedenseschoolsobservedduringthenighttimetowwereassumedtobemainlyconsistingof
Fig. 3.
Maximumdepthineachdive(filledcircle)byseiwhales(upperpanel,S-1;lowerpanel,S-2)overlaidontheechogram.Nighttimeisindicatedbyblackhorizontalbars.Grayshadowsindicatehourswithlongsignalgaps(morethan21min).
Table 2.
Speciescompositionsbywetweight(kg)inthecatchoftrawlnetsamplings
Station T-1 T-1 T-2 T-3 T-4Day/night Day Night Day Day DayDate
10Aug2013 10Aug2013 11Aug2013 12Aug2013 12Aug2013Latitude(N) 43°45'
43°45' 44°58' 45°24' 45°33'Longitude(E) 156°38' 156°38' 157°42'
158°33' 158°52'Samplingdepth(m) 0–90 0–90 0–30 0–30
0–30Towduration(miniute) 45 45 30 30 30
Japaneseanchovy(Engraulis japonicus) 0.55[100.0] 226.49[67.4]
0.10[62.5] –Japaneseanchovy(larva)(Engraulis japonicus) – – – –
0.01[14.3]Chubmackerel(Scomber japonicus) – 7.17[2.1] – –
–Spottedmackerel(Scomber australasicus) – – 0.06[37.5] –
–Californiaheadlightfish(Diaphus theta) – 22.32[6.6] – –
–Reinhardt’slanternfish(Hygophum reinhardti) – 39.88[11.9] – –
–Myctophidaespp. – 1.88[0.6] – – –Northernsmoothtongue(Leuroglossus
schmidti) – 20.67[6.1] – – –Unidentifiedfish – 0.43[0.1] – –
–Japaneseflyingsquid(Todarodes pacificus) – – – 0.23[100.0]
0.06[85.7]Borealclubhooksquid(Onychoteuthis borealijaponica) –
1.25[0.4] – – –Fireflysquid(Watasenia scintillans) – 16.08[4.8] – –
–
Total 0.55[100.0] 336.17[100.0] 0.16[100.0] 0.23[100.0]
0.07[100.0]
Compositionsinpercentageareshowninsquarebrackets.
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196 Mammal Study 42(2017)
theJapaneseanchovy(Engraulis
japonicus)basedontheresultsofthetrawlsamplingaswellaspastsurveyscon-ductedinthewesternNorthPacificasshowninFujinoetal.(2010)andMuraseetal.(2012).Presenceandabsenceof
dense schools during sampling were reflected
inamountofsampledJapaneseanchovies.Wetweightpro-portions of
Neocalanusspp.intheplanktonnetsamplesat T-1, T-2, T-3, and T-4 were
64.0, 16.4, 53.2, and49.2%, respectively.Neocalanus spp. were
dominatedinthesesamplesexceptT-2.U-shapeddivingwasassociatedwithahigherSVvalue
than V-shaped diving for both individuals (P
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Ishii et al., Diving behavior of sei whales 197
Discussion
Bothindividualsdoverepeatedlytotheshallowscat-tering layers in
thedaytime,althoughthedivingdepthsvariedgreatly (from50
to30m).However,both
indi-vidualsdovesequentiallyoncethedensitybecamehighand the diving
depths were concentrated in the dense scattering layers, especially
in the late
afternoon.Aftersunset,theswimmingdepthbecameshalloweratapproxi-mately10m.Neocalanusspp.arerecognizedasoneoftheimportantpreyspeciesforseiwhalesinthewesternNorthPacific(Konishietal.2009).Ithasbeenreportedthat
Neocalanus spp. occurs in such shallow
scatteringlayers(e.g.,Muraseetal.2009).ItcanbeassumedthatNeocalanusspp.occurredintheshallowscatteringlayersobserved
in our study because Neocalanus spp. weredominated in the plankton
net samples. However, theinference was limited because exact
distribution depthcould not be investigated by the net. It has also
beenreported that Neocalanusspp.aredistributedatapproxi-mately 30 m
in the daytime and that they migrate
toapproximately10minthenighttime(SekiandShimizu1998). It has
beenobserved thatNeocalanus spp. formdispersed small-dense patches
at dawn, whereas
theyformlargepatchesfromduskuntilmidnight(Tsudaetal.1993).Therefore,similar
tootherbaleenwhalespecies,suchasfin(Friedlaenderetal.2015)andBryde’swhales(Alvesetal.2010),seiwhalesmaychangetheirdivingdepth
in response to changes in the distribution depth of
theirpreyinordertomaximizetheirfeedingefficiency.Theresultsof
trawlsamplingindicatedthat theJapa-
nese anchovy was distributed near the surface in the
day-time,whilebothanchovyandmyctophidsweredistrib-utedinthesampledwatercolumnalthoughtheinferencewasqualitative.Nevertheless,itisreasonabletoassumethatthedeepscatteringlayersobservedinthenighttimemainlycomprisedmyctophids.Our
results indicate
thattheobservedseiwhalesswamnearthesurfaceanddoverarely to the
depth of the deep scattering layers, which
weredistributedaround30matnightwherethemycto-phidswerelocated.Myctophidsarenotrecognizedasamaindietarysourceforseiwhales(Konishietal.2009).However,
theseobservationscannotpreclude thepossi-bility that sei whales
feed near the surface at night because the behavior and prey
distribution near the surface could not be recorded by the acoustic
devices (acoustic
trans-mitterandechosounder)usedinthisstudy.Inthesouth-westernGulfofMaine,thecallingratesofseiwhalesarereducedatnighttime(BaumgartnerandFratantoni2008).
Calls are probably used for communication with otherindividuals,
so it is hypothesized that the reduced
callratesinthenighttimemayindicatethatseiwhalesallo-catemoretimetoforagingandfeedingoncopepods,i.e.,Calanus
finmarchicus, near the surface rather than social
behavior.Smallpelagicfish,suchastheJapaneseanchovyand
mackerels(Scomberspp.)arealsoimportantpreyofseiwhalesinthewesternNorthPacific(Konishietal.2009).However,
relationship between diving behavior of seiwhales and their
vertical distributions could not be inves-tigated in this study.
These species tend to occur asschools rather than layers. However,
few schools wererecorded by the echosounder during diving behavior
observations.The relationship could be investigated
infutureifsufficientnumberofschoolsarerecordedduringbehaviorobservations.Thesameistrueforkrillwhichisalsoknownasdietofseiwhales.AlthoughPacificsaury(Cololabis
saira)isalsofedbyseiwhales,investigationon the relationship is
difficult because Pacific saury
ismainlydistributedjustbeneathofseasurfacethatcannotbedetectedbyechosounder.Thewatertemperaturesencounteredbytheindividuals
inthisstudywerestablealongthehorizontalmovementpaths.Previousstudies
suggest that thespatialdistribu-tionofseiwhales in
thewesternNorthPacificisdeter-mined largely by oceanographic
conditions such as
theseasurfacetemperature(SST)andoceanicfronts(Sasakiet al. 2013;
Murase et al. 2014). Previous results andthose obtained in the
present study indicate that the spa-tial distribution of seiwhales
at themeso-scale (> 100km) is determined largely by
oceanographic
conditionssuchastheSSTratherthanpreyavailability.Seiwhalesthen
search for their prey within the optimal oceano-graphic conditions
at the micro-scale (> 10
km).Thishypothesisshouldbetestedinafuturestudy.Themeanswimmingspeedofseiwhalesestimatedby
satellite trackingdata is in
therangeof3.7to7.4km/h(Olsenetal.2009;Prietoetal.2014).Itappearsthattheswimming
speeds of seiwhales inferred based on thevessel steaming speeds in
the present study (10.0 and8.1 km/h) were fast in the feeding area.
However, theswimmingspeedcalculatedbasedonthedistancebetweentwo
consecutive positions obtained by satellite tags can
beconsideredastheminimumspeed.Thisisbecausetheswimming path between
these pointsmight not be theactual path as the two consecutive
points obtained by
sat-ellitetagsgenerallyhavelongtimedurations.Thelineardistance
calculated between two points separated by an
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198 Mammal Study 42(2017)
hour during our behavior observations rather than the actual
steamingdistances gavemean swimming
speedsforS-1andS-2of5.0and3.6km/h,respectively.Thus,caution is
necessary when comparing the
swimmingspeedsobtainedusingdifferentdevises.Basedonvisualobservationsinoursurvey,thetagged
individuals exhibited no obvious behavioral reactions
duringtransmitterdeploymentorinsubsequentobserva-tions.Witteveenetal.(2008)trackedhumpbackwhalesusing
acoustic transmitterswithin close proximity
(lessthan1km)ofthesurveyvesselbutitappearedthattheyhad no effect on
their behavior. Watkins et al.
(1981)reportedthatatankerpassingwithin800mdidnotdis-rupt
thefeedingofhumpbackwhales.Thisrangecorre-spondstothemaximumdetectionrangefortheacoustictransmitters
used in our survey. Nevertheless, it wasdifficult topreclude
thepossibilityof effectsonwhalesdue to thecloseproximityof
thevessel inour study.An
independentsurveyusingbiologgingdeviceswithacareful design might be
necessary to assess this effectquantitatively.This study provides
new insights into the foraging
behaviors of sei whales in the western North Pacific.However,
general conclusions cannot be made at
thisstagebecauseofthelimitedsamplesize(twoindividuals)and
surveymethods. Itmay be assumed thatU-shapeddives are related to
underwater foraging behavior, whereas V-shaped dives are related to
other behaviorssuch as traveling or searching for prey. However,
thedistinctionisnotclear-cut.Forexample,bluewhalesex-hibitforagingbehaviorwhileascending(Goldbogenetal.
2015). Recordingmore detailed data using
archivaltagswithaccelerometersmaybenecessary
toelucidatethebehaviorof seiwhalesmore fully. Inourstudy,
theresultsobtainedbynetsamplingwereusedasqualitativeinformation to
understand the distributions of potentialprey species. Species
identification based on a
multi-frequencyechosounderdataandsamplingusingmultipleopeningandclosingnetsystemmayhelptounderstandtheprey-predatorrelationshipatafinescale.
Acknowledgments: The authors thank the crews andresearchers who
participated in the survey conducted to
collectallofthedata.Dr.ShingoMinamikawaandMr.Shigetoshi Nishiwaki
provided useful advice regardingthe development of the pinger
deployment system.WethankDr.KenjiMinamiandDr.HokutoShirakawa
fortheir supportwithanalyses, andDr.GenkiSahashi
andMr.KeizoItofortheiradvice.Thisstudywassupported
by the FisheriesAgency of Japan, the Japan FisheriesResearch and
Education Agency, and the Institute ofCetacean Research. The field
surveywas conducted
inaccordancewithsection“4.5.Attachmentofequipment”ofthe“GuidelinesfortheTreatmentofMarineMammalsinFieldResearch”bytheSocietyforMarineMammalogy.
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Received 11 November 2016. Accepted 22 June 2017.Editor was
Masao Amano.