Blooms of the giant jellyfish Nemopilema nomurai: a threat
to the fisheries sustainability of the East Asian Marginal
Seas
1. Introduction: brief history of mass occurrences of
jellyfish in Japan
In Japanese coastal waters, like many other coastal waters,
the moon jellyfish Aurelia aurita Linnaeus 1758 (see
Dawson & Martin 2001, Dawson 2003, for sibling species
of the genus Aurelia) is the most common and abundant
scyphozoan species. The seasonal aggregation of its
medusa stage has been a well-known phenomenon even before
modern times. The oldest Japanese history book, the
Kojiki published in 712 AD, described that in the beginning
of the Japanese islands were like aggregated medusae on
the sea surface. Until recently, jellyfish have received little
scientific attention, particularly concerning their roles in
marine ecosystems. There is great concern that the occurrence
of jellyfish aggregations has intensified or that the jellyfish
biomass has increased in the last few decades, since
the effects of jellyfish on fisheries and coastal power plant
operations have become seriously in many cases (Purcell et
al. in press). The first noticeable increase in the A. aurita
population took place in Tokyo Bay during the 1960s, when
the bay was heavily eutrophicated by increased industrial
and civil sewage discharge (Unoki & Kishino 1977). At the
same time, aggregated medusae often clogged the cooling
water intakes of coastal power plants (Kuwabara et al.
1969). The A. aurita medusa population has since become
one of the predominant zooplankton components in Tokyo
Bay (Omori et al. 1995, Toyokawa et al. 2000, Ishii 2001).
After the 1980s, the A. aurita population significantly increased
in the Inland Sea of Japan (Seto Inland Sea), due
probably to a combination of factors such as increases in
water temperature, food supply and marine construction, as
well as decreased zooplanktivorous fish stocks (Uye & Ueta
2004). The largest recorded bloom occurred during the
summer of 2000, with estimated biomass of ca. 9.4104
tons of A. aurita medusa wet weight (WW) extending along
approximately 100 km of coastline in the Uwa Sea, western
Shikoku (Uye et al. 2003).
From the turn of this century, the population of the giant
jellyfish, Nemopilema nomurai Kishinouye 1922, began to
explode in the Japan Sea. This species is one of the largest
jellyfish in the world, attaining a bell diameter of ca. 2 m
and a body weight of ca. 200 kg WW (Kishinouye 1922,
Omori & Kitamura 2004). The medusae are transported from the main habitat, i.e. the Bohai, Yellow and East
China Seas, by the Tsushima Current to the Japan Sea, usually
in small numbers, but sometimes in extreme abundances
sufficient to seriously damage local fisheries. Such
a mass occurrence took place in 1920, 1958 and 1995,
about once per 40 years (Kishinouye 1922, Shimomura
1959, Yasuda 2004). Since 2002, however, damaging blooms
have occurred almost every year.
In this article, I summarize the current knowledge of the
ecological characteristics of N. nomurai and the known impact
of this species to fisheries and marine food chain dynamics.
2. Life cycle and transportation route of N. nomurai
In the fall of 2003, we collected N. nomurai medusae
with fully mature gonads, obtained its scyphistomae
(polyps) by artificial fertilization, and reared the budded
ephyrae to the young medusa stage (Kawahara et al. 2006).
This rearing experiment enabled us to reconstruct the life
cycle of this species as depicted in Fig. 1. At 20°C, the fertilized
eggs develop into planula larvae ca. 1 day after fertilization.
After swimming for 4 to 8 days, they settle onto
hard substrates and metamorphose into scyphistomae. Developed
scyphistomae with 16 tentacles asexually reproduce
by podocyst formation. An incubation of the scyphistomae
at 23°C from pre-incubation at 13°C induces strobilation,
followed by liberation of 3 to 7 ephyrae from each
strobila. After releasing ephyrae, the basal part of the strobila
develops into a normal scyphistoma to resume asexual
reproduction; the life span of the benthic scyphistoma stage
may be several years.
Field collections plus laboratory rearing enabled a description
of the annual population dynamics of N. nomurai.
Based on seasonal temperatures in the area where the
medusae originate, the above results suggest that ephyrae
are released into the plankton in early summer. In the laboratory
at 23°C and with abundant food, the new ephyrae
grow through metephyrae to young medusae of ca. 10 cm
bell diameter in approximately 50 days following liberation.
In the southern Japan Sea, the medusae reach ca. 0.5 m bell
diameter in summer and larger than 1 m by fall. After
gonad maturation and subsequent sexual reproduction, they
die during winter due to low temperatures and/or genetically
determined senescence. The medusa life span is less
than one year.
In addition to Chinese literature on the occurrence of
N. nomurai medusae (cf. Hon et al. 1978, Zhang & Li 1988,
Cheng et al. 2004), recent information on the occurrence of
larval/young medusae (Kawahara et al. 2006, Uye unpublished)
and on the probable transportation routes of medusae
as estimated by a numerical model of water circulation
(Reizen & Isobe 2006) demonstrate that the large bay area
surrounded by the Korean Peninsula and the mainland of
China (i.e. the Bohai, Yellow and East China Seas) is the
geographical origin or the main habitat of N. nomurai (Fig.
2). However, its scyphistomae have not yet been found in
the field. The ephyrae may be released into the plankton
from the benthic scyphistomae during April–June (Kawahara
et al. 2006). Due to heavy rainfall in June and July in
temperate East Asia, the Chanjiang (Yangtzu) low salinity
water mass (LSWM) is formed in the Yellow Sea and
northern East China Sea, with its front extending to near
Cheju Island (Chang & Isobe 2003). The larval/young
medusae are entrained offshore by this LSWM, and are
then transported by the southerly Taiwan-Tsushima Current
(Isobe 1999). Hence, the medusae first begin to appear in
the Tsushima Strait in July/August, and spread into the
northern Japan Sea. In October, they pass through the Tsugaru Strait to the Pacific Ocean, and are transported by
a coastal current as far south as the Boso Peninsula, until
they die during the winter months (Fig. 2). Hence, the
medusae entrained into the Tsushima Current are expatriated
from their home waters