oyster culture in hokkaido, japan...minister’s secretariat, 2012). although a lot of scallops were...

Oyster Culture in Hokkaido, Japan

NatsukiHASEGAWA＊1,ToshihiroONITSUKA＊2,SatoruTAKEYAMA＊3, andKimihikoMAEKAWA＊4

Abstract:ThePacificoyster,Crassostrea gigas is theoneof themostcommercially importantaquaculturespeciesintheJapanesefisheriesindustryandisculturedinvariousJapanesecoastalareas includingHokkaido.TheSaromaLakeand theAkkeshiBaywithestuary,which faceOhotsukuSeaandPacificOcean,respectively,aremajoroysterproductionareasinHokkaidowithtotalannualproductionofabout700tonsayear.TheseedlingspatsuppliedfromMiyagiknownasMiyagiseedlingsarewidelyusedinJapaneseoysterculturing.Therefore,whenthecatastrophictsunamionMarch11, 2011damaged theMiyagi fisheries,manyoysterculturingareaswereheavilyaffected.Moreover, introductionof seedlings fromgeographically separatedpopulationhaverisksofinvasionofdiseasesandalienorganismsashitchhikingspecies.Usingseedlingsthatoriginatedfromlocalpopulationsineachareaisoneoftheapproachesfordecreasingsomerisks.Forexample,intheAkkeshiarea,theartificialseedlingspatscollectedfromthelocallyprotectedadultsarealsoused foraquaculture,whicharemarketedas thevalue-addedoysterswithshell“Kaki-Emon”andpopularamongconsumersaslocalspecialproducts.

Key words:Pacificoyster(Crassostrea gigas),Miyagiseedlings,SaromaLake,AkkeshiBay

　ThePacific oyster,Crassostrea gigas is theoneof themost commercially important aquaculturespecies in the Japanese fisheries industrywithharvestofaround200, 000metric tonsayear, thesameastheJapanesescallopMizuhopecten yessoensis(Ministry ofAgriculture,Forestry andFisheries,Minister’s Secretariat, 2012). Although a lot ofscallopswereexportedtoothercountries includingthe United States, most oysters are consumeddomestically.Oysterculturing inJapan isreviewedbyInui (2013).TheOyster isculturedusingraftorlong linehangingmethod.HiroshimaandMiyagiprefecturesarethemainproductionareasinJapan,and there are alsomany otherproduction areasalong Japanese coastal area.Most of these areasuseoysterseedlingswhicharenaturallycollectedinMiyagi using scallop shell collectors and areknownasMiyagi seedlings.Therefore,when the

catastrophic tsunamion2011damagedtheMiyagifisheries,manyoysterfarmerswereheavilyaffectedbecauseofuncertaintyofseedlingsupply (Tanabe, 2012)　Hokkaido,anorthernislandofJapanisoneofthepremieroysterculturingareaswithproductionof700metrictonsayear(MarinenetHokkaido;http://www.fishexp.hro.or.jp/marineinfo/internetdb/index.htm), usingprimarilyMiyagi seedlings.Hokkaidohas twomainproductionareas,SaromaLakeandAkkeshiBayandestuary(Fig. 1).SaromaLakeandAkkeshiBayfacetowardstheOhotsukuSeaandthePacificOcean,respectively.TheseareashavesimilarproductionscalesandOyster industryhistory.　Inthe early days inHokkaido natural oyster bedswereveryproductiveintheseareasandtheoysterswereharvestedfromthere(InukaiandNishio, 1937).However,naturaloysterresourceswerelostaround

2015年 1 月30日受理（ReceivedonJanuary30, 2015）＊1 NationalResearchInstituteofAquaculture,FisheriesResearchAgency, 422-1,Nakatsuhamaura,Minamiise,Watarai,Mie, 516-0193,Japan E-mail:hasena@fra.affrc.go.jp＊2 HokkaidoNationalFisheriesResearchInstitute,FisheriesResearchAgency (KushiroLaboratory), 116Katsurakoi,Kushiro,Hokkaido, 085-

0802,Japan＊3 AkkeshiOysterHatchery, 1-1,Wakatake,Akkeshi,Hokkaido, 088-1118,Japan＊4 AquacultureCooperativeofSaromaLake,Sakaeura,Tokoro,Hokkaido, 093-0156,Japan

水研センター研報，第40号，173－177，平成27年Bull.Fish.Res.Agen.No. 40，173－177，2015

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the1930sandsince then theoyster industryhasreliedonMiyagiseedlings.

Features of oyster and scallop culturing in Saroma Lake

　Groundandsuspendedcultureofscallopsarethemainfisheries in theAbashiriSubprefecturewith

SaromaLakeinHokkaidowhereproductionreaches150,000metric tonsayear.MostareasofSaromaLakeareutilized forscallopcultureandunsuitableareas for scallop culture due to shallow depthandhighwater temperaturesduring summerareused foroysterculture (Fig. 1). Inthe lake,scallopculture is amajor fishery (> 77%of total catch),however,most cultureactivitiesmustbe stopped

Fig. 1.MainOysterculturingareasinHokkaido,Japan.

Ohotsuku Sea

Pacific Ocean

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Oyster Culture in Hokkaido, Japan

duringwinterduetolakefreezing.Oysterfisheriescontributealittle(< 1%)toincreasethediversityoffisheriesandreducedependencyonscallopculture,butoysterculturingbringsjobstofarmersinwintersuchasshuckingbecauseoyster’s in thisareaaremainly composedof age1+and so small oystersareshippedasashuckedproduct.Moreover,olderfishermenhavespecial feeling foroysterculturingbecauseoyster fishinghadbeenprospered in thelakeuntil 1928when thecontinuouschannelwasopenedbetweenLakeandcoldOhotsukuSeaandtheoystercouldnotreproduceinstable(Nishihama, 1994).

Features of oyster culturing in Akkeshi Bay and Estuary

　Themost important feature ofAkkeshi is thetopography (Fig. 1),Akkeshi has two differentecosystem types;AkkeshiBay and theAkkeshi-ko estuary.AkkeshiBay faces thePacificOceanwhere a cold subarctic ocean “Oyashio” currentflows,while theEstuary isshallow (averagedepth: 1.5m).The continuousmeasurement of surfacewater temperature shows that the temperaturein thisEstuary increasesearlier than in theBayfrom spring to summer, reaching temperatureshigher than 25 ℃.However,water temperatureintheBayisaround20 ℃inlatesummer(Fig. 2).

Cultureexperimentsusingsingle-seedlingoystersshowedvariations ofgrowth rate andnutritionalcondition indexbetweenoysters fromtheBayandtheEstuary.Theratiooffleshwetweighttowholebodyweightofoyster,whichisoneoftheindicesofoysternutritionalconditionwaslowerintheestuarythan thebay throughout theexperiments (Fig. 3aandb).ThespecificgrowthrateofshellheightwashigherintheEstuarythantheBayinearlysummer(Fig. 3candd).Theearly increase in temperaturefrom spring to summer enhances oyster growthandmaturationandearly spawningoccurs in theEstuary.These featuresofbothcultureareasarerecognizedby farmersandefficientlyutilized.ThefishermenusetheEstuaryas thewarmerarea forenhancingoystergrowthandmaturationwithearlyspawning.Otherwise,theBayhasagreatersurfacearea, water volume, and food availability thantheEstuary,whichcontributes to improvedbodyconditionsofoystersbeforeshippingseasons.　Therefore,fishermenusethesetwoareasproperly,andAkkeshioystershavesomeadvantagestootherculturingareas.Butcarryingcapacity is limitedbythetopographyoftheshallowandsmallestuary.Toavoidthisproblem,single-seedoystersareculturedinAkkeshi,whicharesuitableforfarminginshallowareas.Inthehatchery,larvaearecollectedfromlocaladultspawnsandsettledonsmalloystershellpiecesindividuallyassingle-seedlingoysters.The juvenile

Fig. 2.FluctuationofwatertemperatureatAkkeshiBayandEstuary,Japanduring2012.

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oysters are cultured in thebay and the estuaryusingbaskets.Byusingbasketrearing,moreoysterscanbeculturedinshallowareassuchasAkkeshi-koestuary.Andbaskethangingkeepsoysters in thewatercolumnaway frompollutionon theestuarybottom.Moreover, throughbaskethanging, single-seedoystershavedeepershellbreadththanoysterson scallop shells.These oysters aremarketed asvalue-addedoysterswithshell“Kaki-Emon”andarepopularamongconsumersas localspecialproducts(Fig. 4).Akkeshi town is aiming tobeknownas“Oystertown”usingsingle-seedoysterstopromotetourism and sightseeing, and to develop brandstrengthofother fisheriesproducts.However, theyield of single-seed oysterswas lower than thatofMiyagi-seedlingoystersat thebeginningof thisculturing,because thestandardculturingmethodsforMiyagiseedlingswerenotsuitableforsingle-seedoysters.Therefore, the culture of single-seedlingoystersdoesnot increaseasmuchas theresearchandpromotionalorganizationsexpected,eventoday.

Fig. 4.PRposterof localpremiumoyster inAkkeshi “Kaki-Emon”.

Fig. 3.Temporalandspatialvariation inratioof freshwetweight to totalweight towholeweight(aandb)andspecificgrowthrateofshellheight(candd)ofCrassostrea gigasinAkkeshiBayandEstuary,Japan.

10

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Problems facing oyster culturing in Hokkaido

　Miyagi-seedlinghasbeenwidelyusedinJapaneselocalproductionareas includingHokkaidobecausemost of the seedingarenaturally collectedusingscallop shellswith lowcosts anda stable sourceprovided by the specialized farmers inMiyagiprefecture. These Miyagi oysters have fastergrowthpotential, and thusarepreferredbymanyoysterfarmers.Eventhoughthereiswide-scaleuseofMiyagi-seedlings throughout the area, geneticdifferences can be detected among some local/wildpopulations inJapanese localproductionareas(Usukiet al., 2002).Thetsunamiin2011exposedtherisksof excessivedependenceonMiyagi seedlingsupplies.Moreover, introductionof seedlings fromgeographically separated populations have risksof introducing diseases and alien organisms ashitchhikingspecies.Forexample, invasiveascidianswerealsorecorded insomebivalveculturingareasin Japan. Inparticular,Ascidiella aspersa stronglyaffectthescallopculturing inFunkaBay,Hokkaido(Nishikawaet al., 2014).Theparamyxeanparasite,Marteilioides chungmuensis,hasnegative impactsontheoysterindustryinsomeJapaneseproductionareas with heavily infected oyster’s showingabnormal tissue (Ito et al., 2002).Moreover, thepotential transportofharmfulalgae togetherwithbivalveshasalsobeenreported (Matsuyamaet al., 2010).Usinglocalseedlingsisoneoftheapproachestodecreasingtheserisks.AftertheMiyagiseedlingcrisis, theuseof localpopulations isbeginning insomeproduction areas.However, similar to theexperience inAkkeshi, it isnecessary to improveculturing techniquesandmanagement forcultureof local seedlings because they have differentcharacteristics.Moreover, salesstrategiesof “localoyster”withpremiumpricearealso important topromotinguseof localseedlingstocompensate foradditionalcostandeffort.

Acknowledgements

　This researchwaspartly supportedbyannualresearchgrants “Studyofcoastalshellfishfisheriesto support the recovery from tsunamidamage ineasternHokkaido” andgrants-in-aid forScientific

Research“Analysisandfuturepredictionofcoastalecosystem” provided by theHokkaidoNationalFisheriesResearch Institute,FisheriesResearchAgencyofJapan.

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