Ishii, Midori; Murase, Hiroto; Fukuda, Yoshiaki; Sawada...

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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), 191-199 https://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

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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

  • 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]

  • 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).

  • 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

  • 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

  • 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.

  • 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

  • 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

  • 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.