オキアミ類の一種Euphausia nanaの海面浮上群と海岸への打ち上げ

瀬戸内水研報（Bull．Fish．Envion．Inland Sea），No。3：47−51（2001）
Short Communication
S皿face Swarm and Shore Stranding of the Euphausiid
E砂hα〃s’αnαnαin the Seto Inland Sea，Japan
Yukio Hanamura＊1，Masato Kamizono＊2，and Sachie Katayama＊2
Abstract The occurrence of a surface swarm and a subsequent stranding event of E翠hαμs如
nαnαBrinton，1962，is reported for the first time from the westem Seto Inland Sea，Japan．The
euphausii（1swarm consisted of fully mature adults，ranging in bo（1y length from9，0to l35mm
for both sexes．The swarm is considered a breeding population as over60％of females were
fertilized and almost all males ha（1fully developed spermatophores either in the ejaculatory ducts
or on the appendages or branchiae． The sex ratio was unbiased， Foregut analyses showed that
the euphausiids had nearly empty stomachs，indicating low feeding activity during swarming
behaviour at the sea surface，We assume that the swarming individuals of E。nαnαin the
westem Seto Inlan（1Sea were transported from the Bungo Channel with the intmsion of a deeper
water maSS．
Keywords＝ surface swarm，shore stran（1ing，Crustacea，Euphausiacea，E砂h側s彪nαnα，Seto
Inland Sea
Euphausiid c川staceans play a key role in the oce−
Kohno l992，Sawamoto1997）。This species occurs in
anic ecosystem due to their abundance。They are pri−
great abundance in the waters close to the west coast of
marily inhabitants of the meso−and bathypelagic zones，
Ehime Pre￡ecture in the Bungo Channel during spring
but some species occasionally occur in shallow waters，
forming dense swarms at the sea surface．E叩hαμslα
and summer．Euphausiid fisheries，based upon shal−
low water aggregations，have occurred under electric
nαnαBrinton，1962，E．pαcが診oαHansen，1911，and
lights since the1970s or before in Ehime Prefecture and
Th）7sαη06∬α’n67〃乞’s（Krφyer，1846），are the species
the maximum landing was5000metric tons in1988
well known to form surface and subsurface swarms（or
（Hirota and Kohno1992，Nicol and Endo，1999），
aggregations）seasonally in coastal waters of Japan・
Although伽phα戚αnαnαis sometimes￡ound in
Swarming euphausiids have often been commercially
exploited，particularly in northem Japan（Komaki1967，
the Seto Inland Sea（Kohnoαα」．1996，Shibata pers・
com．），euphausii〔ls are not a major component of the
Endo1984，Hanamura窃α」．1989，Hirota＆Kohno
marine community of the Inland Sea（1ue to the shallow
1992，Kuroda1994，Nicol and Endo1999）．
On30th March2000，a surface swarm by numer−
waters（average（1epth38m）。Indeed，no literature has
hitherto reporte（1such a phenomenon，
ousplankt・niccrustaceanswasobservedby1・cal
This paper reports an unusual surface sw飢m and a
fishermen early in the moming in the inshore waters of
subsequent stranding of Eμphα疏s’αnαnαin the Seto
Suo−nada，Seto Inlan（1Sea（Fig，1）。 Subsequently，a
Inland Sea，and includes biological information on
mass stranding of the euphausiid，E塑h側51αnαnα，was
the stran（ied population． The factors relating to this
observed on a beach of Nagai−hama，Fukuoka Pre￡ec−
ture（Fig．2），
particular phenomenon are discusse（1・
Numerous individuals were obtained from eu−
伽phα麗s砂nαnαisdistributedinthesubtropical
phausiids stran（1ed on Nagai−hama beach，Fukuoka
coastal waters of the westem North Pacific，mainly
occurring at depths of200−400m during the daytime
Prefecture， on the afternoon （around 14．00 hrs） of
an〔i migrating to the upper200m at night（Hirota and
euphausiids and those of the surface swarm had the
30th March2000．There is no doubt that the stranded
2000年9月18日受理（Accepted on September18，2000）
瀬戸内海区水産研究所業績A第15号（Contribution No．A15from NatiQnal Research Institute ofFisheries and Ellvironment ofInland Sea）
＊1花村幸生：瀬戸内海区水産研究所〒739−0452 広島県佐伯郡大野町丸石2−17−5（Y。Hanamura：National Research Institute of
Fisheries and Environment of Inland Sea，Ohno−cho，Hiroshima739−0452，Japan）
＊2神園真人・片山幸恵：福岡県水産海洋技術センター豊前海研究所〒828−0022 福岡県豊前市大字宇島76−30（M．Kamizono and S．
Katayama：Buzen−kaiLab・rat・ry，Fuku・kaFisheriesandM諭neTechn・1・gyCenter，Un・shima・Buzen−shi・Fuku・ka828−0022」apan）
49
Surface swarm and stranding of E叩h側3’αnαnα
100
50
40
80
30
ω
ぷ
20
＞ 60
0
邸
5
−
＝
①
＝
10
び
＞
Φ
℃
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』
o
89101112｛31415
20
ρ
40
Z
30
E5
L
0
⊆
や
o
』 40
0
ll
llI
20
IV
Gut Fu”ness
10
Fig．4。 Furegut fullness of E叩hα騰∫αnαnα：1，empty to1／4
ful1；II，half full；III，3／4fu11；IV，completely fu1L
0
891011て2131415
Body Length（mm）
Fig。3． Size frequency distribution of E麗phα麗s如ηαnα（N＝
150）。Filled colu甲ns indicate females with a sper−
matophore on their thelycum。
than half full guts． Stomach contents of E．nαnα
mainly consisted of small quantities of detritus and
crustacean setae． Unidentified egg−1ike materials were
infrequently fOun（1．
Our understanding of the mechanisms which
induce surface swarms of euphausii（is are far from com−
significantlydif￡erentげtestニ1。98，df＝llP＞0．05）．
The sex ratio of the surface swarms of翫ph砿s’α即一
plete． En〔lo（1984）stated that s皿face swarms of
E砂hαμs’αραo静ocαin S＆nriku coastal waters corre−
〇弟ocαin Sanriku coastal waters且uctuates around50％，
spond with breeding behaviouL In contrast，Kodama
with a slight male predominance（En（101984），an（1
an（11zumi（1994）suggested that surface swarms of this
those on the west coast of Hokkaido consists of30−
50％males（Kotoriε‘α」．1995）。Incontrast，surface
swarms of T勿5αno召s5α伽r履s off the west coast of
euphausii（l are strongly related with the southward
intrusion of the cold Oyashio Current． Similarly，
several authors have discussed the formation of surface
Hokkaido exhibit a sex ratio greatly biased towards
swarms in E．∫，αo昴cα，giving more weight to physical
females（65．2−96．9％females）（HanamuraααZ．1989）。
rather than physiological factors as causes（Komaki
Epibiont infestation sometimes has negative effects
to crustacean host，including impair gas exchange，inter−
1967，NishikawaααZ。1995，KotoriααZ。1995）。
Daily meteorological data from the Buzen−kai
fere with mobile activity an（l as a cause of stress。In
laboratory indicated that westerly winds intensifie（l
the E叩hα麗s如nαnαsample，no extemal parasites were
from28th March2000（Kamizono pers．obs。）．In
observe（1．
Stomach analyses revealed that approximately70％
association with this，the near bottom water temperature
in the coastal area of Buzen−kai noticeably decreased，
of翫ρhα麗s’αnαnαhad completely and／omearly empty
reaching a minimum on30th March，when the su㎡ace
stomachs（Fig．4）；no clear（1ifference between the sexes
swarm of E塑hα麗3如nαηαoccurred（Fig。5）． There−
was observed（記2test＝0．07，（lfニ11p＞0．05）． This
fore，it is unlikely that the euphausiids inva（led the
in〔licates that the swarming E．ηαnαdi（l not engage in
Inland Sea with the warm Tsushima Current via the
feeding activity． Previous observations in Japanese
Kanmon Strait．The westerly wind may drive water
waters have shown similarly low gut fullness in E。Pα一
from the coast offshore，combined with the intrusion
α耽αand Thysαno召s3αZηα雁5from daytime surface
swarms，suggesting low feeding activities during
swarming behaviour at the sea surface（En（101984，
Hanamura6∫α1．1989，Kotoriαα1．1995）． Suhαα」．
（1998）alsoobservedthat重ndividualsofE．ρα即り6α
of a〔leeper water mass from the Bungo ChanneL
Moreover，E。nαnαaggregates in the Bungo Channel in
this season（Hirota and Kohno l992）． The observe〔l
swarming phenomenon，however，can not be explained
by the water movement alone，since similar hydro−
from daytime swarms in the south−west Sea of Japan
graphic events frequently occur in the area，
possesse（l nearly empty stomachs，in contrast to
indivi（luals from nighttime swarms which had more
Saito （1996） reported that shore strandings of
Eμρhα那3’αs∫溺’1∫3Sars，1883，in Suruga Bay，central
50
Hanamura，Kamizono and Katayama
11．0
9 10，5
了
』
量
お
ユ10．O
Eo
←
＄
睾9・5
9．0
25 26 27
March2000
Fig．5。
28
29 30 31 1
Apri1
Changes in the neal bottom water temperature off the Buzen−kai Laboratory from25
March to2Apri12000．Arrow indicates the occurrence of surface swarming and
strεしn（ling events of E配phα麗s’αnαnα。
Japan， are seasonal and relatively regular events・
（in Japanese）
He also found that E， 3Z配’1’5 strandings ＆re often
Kohno，Y．，H．Sakaguchi，and Y，Hirota，1996． Euphausiids
accompanied by a small number of other euphausiids，
collected from eastem part of Iyo−mda，Seto Inland Sea．
Thysαnopo4α nlonαcαn言hα Ortmann， 1893 and
N佛αオ03副’s4塀’o’JsHansen，1911，aswellassome
3αon撚α’S惣θηK鱒oKen蜘一たα置oたoた麗，2，25−30・
deep−water mysid and hyperid cmstaceans．0’Brienα
αZ，（1986）noted that strandings ofハケcだphαn6sαμs〃α」’s
Sars，1883，0n the Tasmanian coast occur synchro−
（inJapanese）
Ko〔1ama，J．and Y．Izumi，1994．Hydrographic con（1itions
c・ntributingcatchesoftheeりphausiidE励α配3εαραc伽α
and its importance as food for demersal fish， Kα砂o
ルでon言h∼y，26，228−235．（in Japanese）
nously with matting behaviouL Unfortunately，no
Komaki，Y．，1967．On the s皿face swarming of euphausiid
evidence is available about this particular behaviour
crustaceans． Pαo顔どc So∫．，21，433−448．
with regard to the E叩h側5如nαnαstranding， Further
Kotori，M．，M．Tamura，and Y，Hanamura，1995，An inci−
studies should be done to better understand the combi−
dence of surface swarming of E叩hα麗8諭pαcε舜’ωoff the
nation of factors mediating such events．
coast of westem Hokkaido，Japan． βμ1」．PJαn㍑on Soo・
海即n，42，80−84，
Kuroda，K．，1994． Review of the euphausii〔1fisher孟es and
Ack皿owledgements
We thank Dr S．Sawamoto ofthe Institute of Oce−
anic Research and Development，Tokai University，
Shimizu，for confirming our identification of the eu−
fisheriesbiologicalstudiesinJapan。κ碕oM・n助，26，
203−209．（in Japanese）
Nico1，S．and Y．Endo，1999． Krill fisheries：development，
management and ecosystem implications，Ag己燃”v加8
R召so尻君s．，12，105−120．
phausii（ls．Thanks are extend to Dr K Iseki of the Na−
Nishikawa，J．，A．Tsuda，T，Ishigaki，and M．Terazaki，1995，
tional Research Institute of Fisheries and Enveironment
Distribution of euphausiids in the Kuroshio front and
of Inland Sea，Hiroshima，for reading the manuscript．
warm water tongue with special reference to the surface
Ms R．Shibata of the same institute has infomed us of
aggregationof伽ρhα房s如ραo旗α，／，P伽㍑oηRθs，，17，
an unusual instance of E．nαnαbeing￡ed upon by a re（l
611−629．
sea bream Pαg濯5灘のor in June in Hiuchi−nada，the
central Seto Inland Sea．
0’Brien，D．P．，D．A。Ritz，and R．J．Kirkwoo（1，1986，
Stranding and matting behaviour in IVyc珈hαn6s侃s枷」ごs
（Euphausiidae：Crustacea）．ル歪α孔B’oJ．，93，465−473，
References
Endo，Y．，1984，Daytime surface swarming of E叩h側s∫α
ραo哲oα（Crustacea：Euphausiacea）in the Sanriku coastal
waters off northeastem Japan．ル勉r，B’oJ．，79，269−276，
Hanamura，Y。，M．Kotori，and S．Hamaoka，1989，Daytime
surface swarms of the euphausiid T勿sαπ06s5如n8r禰s off
the west coast of Hokkaido，northem Japan． Mαr．β’oム，
102，369−376．
Hirota，Y．and Y．Kohno，1992． Euphausiid fisheries in
Uwajima Bay，Ehime Prefecture（southwest Japan）。
Fズ5h，β’oJ，Oc8αno87。So配言hw63義V巳α尼孔／αραπ，8，89−95．
Saito，N．，1996．Note on E叩h側s如3〃n‘Z‘s and some other
small crustaceans stranded on Miho Beach，Sumga Bay，
middle Honshu．五〇，P，αv加81〉6ws，7，2−6、
Sawamoto，S．，1997，0rder Euphausiacea，In：Chihara，M．
and M．Murano（eds）An∫伽3枷‘648卿のoη2副n6
μαη々on’n／αPαn。 Tokai Univers重ty Press，Tokyo，P・
1204−1226，（in Japanese）
Suh，H．一L，J．一H、Lim，and H，Y．Soh，1998， Population
structure of surface swarms of the euphausii（1E叩hαじ‘3♂α
ραα算oαcaught by drum screens of Uljin Nuclear
Power Plant in the east coast of Korea．／．κ07εαn So6．
Oc6侃097，，33，35−40，
Surface swarm and stranding of伽1アhα配3如ηαnα
オキアミ類の一種E叩h側s∫αn侃αの海面浮上群と海岸への打ち上げ
花村幸生・神薗真人・片山幸恵
オキァミ類の一種E塑h側5如nαnρの海面浮上群と，それに続く海岸への打ち上げ現象を初めて瀬戸内海で
確認した。オキアミ浮上群は9．0∼13．5mmの成熟個体で構成され、雄は十分に発達した精包を持ち，また
雌の60％は交尾していたことから，、この群れは産卵群と考えられた。浮上群の性比はほぼ等しかった。オキ
ァミ個体の殆どは空胃であり、海表面で索餌活動を行っていた形跡は認められなかった。環境データの時間
変動とそのパターンから，瀬戸内海で確認されたオキアミ浮上群は豊後水道を起源とするものであると考え
られた。
51

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