Bulletin Of the National Salmon
Resources Center, N0. 1: 35‑38

さけ・ます資源肯埋センター
研究報告 第 @号 35‑38 頁
l998 年 I2 月December 1998

RESEARCHNOTEA Status 0f Lepeophtheirussalmonis (COpepoda: Caligidae) on
Seawater‑cultured COho Salmon {Qncorhynchus kisutch)
and Rainbow Trout (Q. mykiss) ;n Japan
Shigehiko Urawa*', Teiichi Kato*2,and Akira Kumagai*3*' Research Division, National Salmon Resources Center, Fisheries Agency 0/ Japan,
2‑2 Nakanoshima,
TOyohira‑k@, Sapporo 062‑0922, Japan*2 National Research Institute ofAquaculture, Fisheries Agency Or Japan,Nansei‑ch0, Watarai‑gun, Mie 516‑0/08, Japan*3 Fisheries Development Division, Miyagi Prefectural GOvernment,
HOncho 3, AOba‑ku, Sendai 980‑0014, Japan
Abstract‑

Infection levels 0f the salmonlouse Lepeophtheirus

salmonis

wereexaminedfor coho

salmon (Oncorhynchus kisutch) and rainbow trout (0. mykiss) cultured tn seawater net pens fn
northern Japan. The prevalence Of L. salmonis 0n coho salmon gradually increased from 59.2% fn
late Junet0 84.6%

tn Augustwhen fish were harvested, but the mean

intensity remained less than3.2 parasites per fish. The prevalence Of parasite 0n rainbow trout increased
withto 92%

in NOvember

intensity Of 4.1 parasites per fish. The low infection levels Of L. salmonis amongsea‑water cultured salmonids tn Japan may be largely attributable to the complete single year class cu1‑
ture system harvested within one year, and t0 relatively low susceptibility 0r host fish to the para‑site.
a mean

Key words: parasitic COpepoda, Lepeophtheirus salmonis, prevalence, coho salmon, rainbow troutIntroduction
The salmonlouse Lepeophtheirussalmonis(Kroyer, 1837) is a marine ectoparasitic copepod
infecting wild and cultured salmonids in [he northernHemisphere.
The heavy parasite infections cause
serious problems ;n marine salmonid farms fn theAtlantic coasts Of Scotland, NOrway, Ireland, andCanada (see BOxshall and Defaye 1993). In addi‑tion, the parasite has impact 0n wild populations of
sea trout (Salmotrutta) and sockeye
salmon(Oncorhynchusnerka) (Birkeland 1996; JOhnson et.
a1. 1996).

In Japan,marine farms annuallyproduce approxi‑
mately twenty thousand tons Of coho salmon (0.
kisutch) (Mahnken1991). MOst salmon farms arelocated along the northeast coast Of HOnshu Island inthe Sanriku District. In addition, several farms
attempt rainbow trout (0. mykiss)culturesfn seawa‑
ter net pens fn Japan. The salmon louse was record‑
ed from these farmed coho salmon and rainbow trout
COntribution A N0. 4 from the National Salmon
Center.
@ 1998 National Salmon Resources CenterResources(Nagasawa and Sakamoto
1993) aS well aS from wildfishes (Nagasawaand Yanagisawa
1992; Nagasawa
et a1. 1993), but the impact Of L. salmonis on
salmonid rnariculture have not been well known.
The aim 0f the present study tS to assess the infectionlevel Of L. salmonis On seawater‑reared coho salmon
and rainbow trout fn Japan.

Materials and Methods
COho salmon
COho salmon
werereared in net pensin
Shizugawa Bay along the Pacific coast 0f northeast‑ern HOnshu(Fig. 1). Underyearlingcoho salmon
smolts (body weight about 150 g) were stocked inseawaternet pens fn the middle 0r October, 1991,
and farmed by the early August Of the next year (seeMahnken1991). Fish harvested fn the Shizugawa

Market were bi‑weekly examined for parasites
between June and August fn 1992.
Rainbow trout
A marine rearing experiment was conducted forrainbow trout tn Katsurakoi Harbor near Kushiro
along the Pacific coast Of eastern HOkkaido (Fig. 1)

36ぽsCen

さけ・ます資源管理センター
所報 Bul1.Natl.SalmonResour

忙 r， NO ． I，

1998

and adult stages. The fork length and bodyweight Of each fish were also measured. SOme lice
were preserved fn 10% formalin for species identifi‑cations. The terms indicating the level Of parasiteyoung

応Kushiro

infections(prevalence, meanintensity,and abun‑dance)

those proposed

wereaccordance

withbyMargoliseta1.(1982).

NOrth Pacific Ocean

Results
COho salmon
gradually fn‑
The prevalence Of L.salmonis
creased from 59.2% fn late June t0 84.6% in August
intensity also increased to 3.2(Table 1). The meanparasites per fish in late July, but slightly decreased
became close to 20oC (Fig.
when water temperature2). There were no significant relations between the巳 25*m@20

NOrth Pacific Ocean

E山Fig. 1. Maps

showing the sampling locations

Of seawater‑culturedcoho

veyfor parasite‑M亘sur‑

山salmon and rainbow trout

in northern Japan.
―for 6 months in 1988. One thousand rainbow trout(mean weight 215 g) previously reared tn a freshwa‑
ter hatchery for two years were held ;n a seawater net
pen On June 14, 1988. Fish anaesthetized after cap‑
ture using MS222
wereexamined for parasites 0n

三0上 20p,g

the rearing period.

June

July

@ @‑asondchanges ln prevalence

August(columns ln %) andintensity (closed circles) 0f Lepeophtheirus salmonis oncoho salmon, and surface water temperature(closedsquares) in Shizugawa Bay, northeastern HOnshu

Parasite survey
were二一‑一山二September 2, October 6, and NOvember 22,1988.Notreatmentwas conducted for salmon licethroughout

FishRO羊―@i 60

examinedfor L. salmonis including

Table 1. Prevalence, intensity and abundance 0f Lepeophtheirus salmonis 0n coho salmon reared in Shizugawa Bay. northeastern HOnshu,andDate0nrainbow trout reared ln Katsurakoi, eastern HOkkaido

Fish size0fFOrk length(cm)surveyPrevalence

Numberweight (g)examined

Mean

intensity

Abundance

Maximum

infected

COho salmon in Shizugawa in 1992

3， 260 士

June 2957

July54+July'2856+

820

August 857@Rainbow5*

trout in Katsurakoi

4559

12876

680

52

4484

301 + 119

137
60

50

74

27.0+

October

30478

@ 188

120

NOvember 22

33.7 @ 3.6694 @ 227123@ SD.69

三 0*.47

1.22!.28

1.05.24*.85.64

2． 62 土 2． 4l'.58

in 1988

September 2*Mean

2.876
167

720'.48.23

4 ． l3 士 2． 28

24.52'.46I8

Urawaet a@ ‑ Sea lice Oncoho

一 Q ︒ ‑C42

卜コ‑ 山 ‑ 山 oE+‑ノ二の‑一てノ[Qの ‑@のつ山 ‑ 巾き20

September
Fig 3. Seasonal

Oct0beNOvember「changes in prevalence(columns @n %) andintensity(closed circles) Or Lepeophtheirus salmonis On

rainbow

trout,

andsurface

watertemperature(closedsquares) in Katsurakoi Harbor, eastern HOkkaido

parasite intensity and fish size (fork length, weight Or
condition factor; p > 0.05)
R ば而 hnwtrouア

The prevalence was only 2.2% tn September, but
despite Of decreased
increased to 92% fn NOvember
water temperature
below 5oC (Fig. 3). The intensity
also increased to average 4.1 parasites per fish with amaximum
Or t8 parasites per fish tn NOvember
There were no significant relations between the para‑site intensity and fish size (fork length, weight Or
condition factor; p>0,05)
throughout the parasite
surveys

Discussion
Salmon louse causes serious problems to salmon
culture industry fn NOrway,Scotland, Iceland andNOrth America. In Japan, however, serious diseasecaused by sea lice has not been reported, except thatCaligus orientalis caused heavy mortalities among
pen‑reared rainbow trout along the coast Of Okhotsk
Sea tn eastern HOkkaido(Urawa and Kato 1991)
The present study confirmed that the infection Of L.
salmonis ts low levels On cultured coho and rainbow
trout tn northern Japan. Hemorrahages
were Ob‑served fn the perianal region Of infected fish, butthese might

notcause37

salmon and rainbow trout in Japan

severedisease

In the NOrth PacificOcean the infectionlevelOfL. salmonis

was different amongsix species Of
Pacific salmon: pink salmon (0. gorbuscha)hadhighest infection levels, followed by steelhead trout(0. mykiss), chino0k (0. tshawytscha) and cohosalmon (Nagasawaet a1. 1994). Salmonlouse ls
prevalent On pen‑reared Atlantic salmon and chino0ksalmon (JOhnson 1993), but coho salmon tS moreresistant to infection than these fishes (JOhnson andAlbright 1992a, 1992b). The low susceptibility of
host fish against L. salmonis may result tn the lowlevel Ot the parasite infections On cultured cohosalmon fn Japan.
The Sanriku District Of northeastern HOnshu has
an annual seawater surface temperature
range from 9
to 22oC (Mahnken1991). Then cultured salmon arecompletely harvested before early August to avoid
the high summer
water temperature
exceeding theupper lethal limit for coho salmon.
By comparison,seawater temperaturesapproach the lower lethal lim‑
tt for salmonids ;n winter months along the Pacificcoast Or HOkkaido
where fish are harvested by theend Of NOvember.
Thus Japanese marine salmon
culture fs characterized by a fact that fish are rearedtn single year class sites for less than one year. Bron
et a1. (1994) found a different pattern Of population
dynamics Of L. salmonis On farmed Atlantic salmon
betweensingle and multiple year class sites in
Scotland. On the single year class site,mean intensi‑
ty Of L. salmonis gradually increased from one totwo lice fn early August to over 45 lice per fish in
October Of the next year. On the multiple year classsite,however, numbers rose far more rapidly than On
single year class sites with mean intensity reaching40 by August and peaking at over 100 lice per fish inOctober Of the first year. The low level Of L. salm0‑
nis infections On farmed salmonids tn Japan may be
attributable t0 a single year culture that salmon

arecompletely reared tn single year class sites for lessthan one year. A main source Of the initialinfection
may be salmon lice infecting wild chumsalmon (0.
keta), but their migration to the coastal waters ofnorthern Japan is limited from the late August toDecember.
In conclusion, the infection level 0f L. salmonis isquite low amongseawater cultured coho salmon andrainbow trout @n Japan. tt may be attributable to asingle year class culture system harvested within one
year and relatively low susceptiblity Of host fish tothe parasite.
Re 止renCeS
Birkeland, K. 1996. COnsequences
Of premature
return by sea trout (Salmo trutta)
infestedwith thesalmon louse (Lepeophtheirus salmonis Kroyer):migration, growth, and mortality. Can. J. Fish.
Aquat.Sci., 53: 2808‑2813.
BOxshal1, G. A., and D. Defaye. 1993. Pathogens ofwild and farmed fish: sea lice. Ellis HOrwo0d,New YOrk. 378 p.

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Rae. 1994. Influence Of treatment with dichlorvos
On the epidemiology
Of Lepeophtheirus
salmonis(Kroyer,1837) and Caligus elongatusNOrdmann,
1832 On Scottishsalmon farms. f@ Pathogens ofwild and farmed fish: sea lice (edited by G. A.
BOxshall and D. Defaye). Ellis HOrwo0d, LOndon.

pp.263‑274.
JOhnson, S. C. 1993. A comparison Of development
and growth rates Of Lepeophtheirus
salmonis(COpepoda: Caligidae) On native Atlantic salmon(Salmo salar) and chino0k salmon (Oncorhynchus
tshawytscha). f@ Pathogens Of wild and farmed
fish: sea lice (edited by G. A. BOxshall and DDefaye). Ellis HOrwo0d, LOndon, Pp. 68‑80
JOhnson, S. C., and L. J. Albright. 1992a. COm‑
parative susceptibility and histopathology Of thehost response Of native Atlantic, chino0k, and coho
salmon to experimental
infection with Lepe0‑
phtheirus salmonis (COpepoda: Caligidae). Dis.
Aquat. Org, I4: 179‑193.
JOhnson, S. C., and L. J. Albright. 1992b. Effects ofcortisol implants 0n the susceptibility and theOf native coho
histopathology
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kisutch to experimental
salmon Oncorhynchus
salmonis (COpe‑infection with Lepeophtheirus． I， 1998

Bering Sea. r@ Pathogens 0f wild and farmed fish:
sea lice (edited by G. A. BOxshall and D. Defaye).
Ellis HOrwo0d, LOndon, PP. 166‑178.
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the salmon louse Lepeophtheirus salmonis(COpepoda:Caligidae) On seawater‑farmedsalmonids fn northern HOnshu,
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Patho1.,28: 143‑144.
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Lepeophtheirus salmonis (COpepoda: Caligidae)
from white‑spotted chair (Salvelinus leucomae‑nis), juvenilechum salmon (Oncorhynchus keta),

and Japanese dace [Tribolodonhakonensis) from
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Nagasawa,K., and F. Yanagisawa.
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tshawytscha,a chino0k salmon, Oncorhynchus
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shore waters Or the NOrth Pacific Ocean and

日本で海面養殖されたギンザ ケ と二ジマスにおける榛脚類 Lepew ね theiruLssalm。乃 is の寄生状況

浦和茂彦 ・加藤禎―・熊谷門

北口本 沿岸で海面上貴養殖されたギンザ ケおよび 二ジマスにおける サケジラミぬ peophtheirussalmonis の寄生レべルを調査した．宮城県志津川湾で 養殖されたギンザケにおける寄生率ほりX 穫が終了する

8 月には84.6% に増加したが ，平均寄生数は 3． 2 生体以下と低かった．北海道東部の 桂恋港で養殖された 二ジマスにおける本虫の 寄生率は飼育が 終了する 口 月までに92%となったが，平均寄生数は 4 ． l 主体以下でやはり低かった．欧米では 養殖サケマス 類における サケジラミの大量寄生が大きな 問題となっているが ，日本における本虫の低い 寄生レべルは 海面における 飼育期間がI 年以内であ ることと宿主の 比較的低い感受性によると推定される．