steak for breakfast and calamari for dinner – does 'surf ...€¦ · steak for breakfast and...

IntroductionPredationbyBigg’s (“Transient-type”)killerwhaleshasbeenproposedasoneexplanationforthecontinueddeclineofStellersealionsinthewesternandcentralAleutianIslands.Therearerelativelyfewreportsofkillerwhalepredationonmarinemammalsinthisregion;theseincludeDall’sporpoise,Cuvier’sbeakedwhale,seaotter,andoneStellersealionpup(atKasatochi Islandin2005).However,anacousticrecorderataStellersealion(SSL)rookeryonAgattu Islanddocumentedfrequent(1outofevery3days)occurrenceofBigg’s killerwhales,suggestingpredationofsealionsmayoccurregularly.Alternatively,severallinesofevidence(observedpredation,lownitrogenstableisotoperatios,andsomesatellitetaggingmovements)suggestBigg’s killerwhalesmayalsopreyonsquid.InJune2015,wedeployed3satellite-linkeddivetagsonBigg’s killerwhalesinthecentralAleutians(2inRatIslandPasseastofKiskaIsland,and1atTagIslandintheDelarofs)toinvestigatetheirforagingbehavior.

Delarof Islands WhaleOnJune27,agroupof4Bigg’s killerwhaleswerefoundwithinafewhundredmetersoftheSSLrookeryonTagIslandintheDelarofs.Thegroupwascomposedofanadultmale,asub-adultmale,and2adultfemales.Thesub-adultmalewastagged.Thesewhaleshavenotbeenpreviouslyidentified.

Conclusions InthewesternandcentralAleutianIslands,atleastsomeBigg’s killerwhalesapparentlypreyextensivelyonStellersealions. Atthesametime,many(orperhapsall)Bigg’s killerwhalesinthesameregionapparentlyalsopreyextensivelyonsquid.Ifso,thisraisesthequestionofa“predatorpit”– doestheavailabilityofsquidsustainBigg’s killerwhaleabundanceatlevelssohighthatoccasionalkillerwhalepredationisenoughtopreventtherecoveryofStellersealions?

Figure 1. Locations(Q3-Q0)andtrackforthefirstwhaletagged(tagPTT-102223).Thetagdurationwas28days(June26toJuly23).ThiswhalewasidentifiedasWT318(anadultfemale);shehasbeenpreviouslyseenin2006inRatIslandPass.WT318foragedextensivelyintheRatIslandPassregionfor~2weeks,thenmade2rapidtripswesttotheNearIslands(>400km),oncetonearShemya Is.,andoncetohalfwaybetweenAttuandtheCommanderIslandsinRussia.After17daysthetagwasduty-cycled(tosavebatterylife)toturnonevery5th day–thisexplainsthelonggapsinthewesterntrips.WT318didnotforageroutinelynearSSLrookeries,butdidoccurnearrookerieson4occasions(Ayugudak,Semisopochnoi,andBuldir twice).OnherreturnfromShemya andAttuWT318visitedseveralshallowbankareasonthesouthsideoftheAleutianswestofKiska.

Kiska/Rat Island Pass WhalesAgroupof12Bigg’s killerwhalesweredetectedonJune25inRatIslandPass(justeastofKiskaIsland).Thewhaleswereintwoclearsub-groups,andwereonthesiteofapresumedkillofanunidentifiedmarinemammal(therewasasmalloilslick,manybirdsfeedingonvisiblepreyremainsinthewater,andthewhalesappearedtobefeeding– apreysamplewascollectedandwillbesubmittedforgeneticanalysistoidentifythespecies).Twowhalesweretagged,onefromeachsub-group.

Summaryoftheforagingbehaviorofthesub-adultmaletaggedneartheTagIslandSSLrookery

• Everymorninghewasincloseproximitytoarookery,doingshallowdivesnearthesurface,apparentlyhuntingforStellersealions

• Everyeveninghedidrepetitivedeep-dives(250-350m),apparentlyfeedingonsquid,suggestingthiswhaleprey-switchedbetweensealionsandsquidonadailybasis

Paul R. Wade – NationalMarineMammalLaboratory,AlaskaFisheriesScienceCenter,NMFS,NOAAJohn W. Durban – MarineMammalandTurtleDivision,SouthwestFisheriesScienceCenter,NMFS,NOAARuss D. Andrews – AlaskaSeaLifeCenter/SchoolofFisheriesandOceanSciences,UniversityAlaskaFairbanksTom Gelatt – NationalMarineMammalLaboratory,AlaskaFisheriesScienceCenter,NMFS,NOAA

Steakforbreakfastandcalamarifordinner– does‘Surf&Turf’forkillerwhalesmeanapredatorpitforStellersealions?

Overall ResultsThetwowhalestaggedinRatIslandPassnearKiskaIs.(whileonakillofamarinemammal)continuedtoforageinthatareainitially.ThewhaletaggedatTagIs.foragedextensivelyintheDelarofs.Twoofthewhalesshowedrepetitivedivingtodepthsof250-350monanearlydailybasis,consistentwithforagingonsquid.Thethirdwhaledidnotprovidemuchdivedata,butalsoshowedoneboutofrepeateddivesto350m,soallthreewhalesshowedsomeevidenceofdeepdivingbehavior.Additionally,twoofthewhalesalsospentconsiderabletimeatshallowdepthsaroundStellersealionrookeries.

Figure 2. Divetime-series forWT318(PTT-102223).MostofWT318’sdiveswere100morless,butshedidboutsofdeepdiving(200-350m)onanearlydailybasis(12of16dayswithdivedata).Therewasnopatterntothetimeofdayofherdivingbehavior.

Figure 3. LocationsandtrackfortagPTT-102224.Thetagdurationwas5days.ThiswhalewasidentifiedasWT313(alsoanadultfemale).Thiswhalewaspreviouslyseentwicein2006inRatIslandPass(onetimewithWT318).ThedeploymentofthetagonWT313waslow(atthebaseofthedorsalfin),soasexpected,locationdataweresparseandofpoorerquality.Over6days(25Juneto1July)thewhalestartedinRatIslandPass(wheretagged),movedtowardthesouthernendofKiskaIsland,andthenspentthenext5daysaroundKiska.WT313spentalotoftimeclosetoshore,apparentlyforagingneartheSobaka,Lief,orCapeSt.StevensSSLrookerieson5occasionsoverthe6days.

Figure 4. Divetime-series forWT313(PTT-102224).Duetothelocationoftaglowonthedorsalfin,littledivedatawastransmitted.However,mostofWT313’sdiveswere100morless,butshedidoneboutofdeepdivingto~350m

Figure 5. Locations(Q3-Q0)andtrackforthesub-adultmale(tagPTT-87624).Thetagdurationwas17days(June27toJuly13).Theentiretrackofthewhaleisplottedasthethinline,butforclarityonlymorning(4-10am,yellowcircles)andevening(4-10pm,bluecircles)positionswereplotted.Thiswhalestayedwithina60by80kmareaintheDelarof Islandstheentiretime,extendingeasttothewesternsideofTanaga Island.ThemajorityofthemorningshewasclosetoeithertheTagIslandSSLrookeryortheGramp IslandSSLrookery(about20kmsoutheastofTagIsland).Onthemajorityofthedaysthewhalewouldthenmovetodeeperwaterintheeveningandnighttime,usuallyinthewidechannelsouthofGareloi IslandandnorthoftheotherDelarofIslands(calledhereGareloi Channel).OnonemorningthewhaleoccurredclosetothecoastofTanaga Islandinanareawhereseveralharborsealhaul-outsarelocated.

Figure 6. Geographiclocationbytimeofdayforthesub-adultmaletaggedattheTagIslandSSLrookery(thewhalecouldbeatmorethanonelocationduringasingletimeperiod).On13of14morningsthatthetagreportedalocation,thewhalewasateithertheTagorGramp SSLrookery;the14thmorningitwasalongthecoastofTanaga Is.During12oftheafternoonsthewhalewasstillatthesamerookeryasthemorningorhadmovedtotheotherrookery(3occasions).On10ofthenightsthewhalewasalsoatoneoftheSSLrookeries(meaninghearrivedpriorto4am).During10oftheeveningsand8ofthenightsthewhalewasindeeperwaterinGareloi Channel.

Figure 7. Divetime-series forthesub-adultmaletaggedattheTagIslandSSLrookery(PTT-87624).Thewhalespentsubstantialtimedoingrepetitivedeep-dives(300-400m)on12ofthe15daysthathadafulldiverecord.Thesedeep-divingboutsoccurredinlateafternoon(lightblue),evening(blue),andthenight(black),whenthewhalewasusuallyinGareloi Channel,andareconsistentwithforagingonsquid.ThewhaledidonlyshallowdivesinthemorningwhenatSSLrookeries,consistentwithhuntingofsealions.

Figure 8. Boxplotofthedepthdistributionofdivesplottedbyhouroftheday(inlocaltime)forthesub-adultmaletaggedattheTagIslandSSLrookery.Notethatfrom3-7amthewhalewasstayingwithin30mofthesurface,presumablywhilehuntingStellersealionsorresting.Thewhalestarteddoingdeepdivesafter10am,butthemediandivedepthwasstillshallowerthan100muntil5pm.From8-11pmthemediandivedepthwasgreaterthan250m.After11pm,thedives,onaverage,becameshallowerasthenightprogressed.Duringthistimeperiod,sunrisewas~6:45amandsunsetwas~11pm.

Acknowledgements: TheauthorsthankPCCRCandNOAAforprojectfunding,thecaptainandcrewoftheUSFWSR/VTiglax,andtheresttheStellersealionfieldworkteam,formakingthistaggingworkpossible.