Establishing a Conceptual Design for Jellyfish Blooms in the Seto Inland Sea
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Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,
Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 65–71.
© by TERRAPUB, 2011.
Establishing a Conceptual Design for Jellyfish Blooms
in the Seto Inland Sea
Naoki FUJII1, Atsushi KANEDA2, Shinya MAGOME3 and Hidetaka T AKEOKA1
1
Center for Marine Environmental Studies, Ehime University,
2-5, Bunkyo-cho, Matsuyama 790-8577, Japan
2
Department of Marine Bioscience, Fukui Prefectural University,
1-1, Gakuen-cho, Obama 917-0003, Japan
3
Sanyo Techno Marine Inc.,
1-3-17, Nihonbashi Horidome, Chuo-ku, Tokyo 103-0012, Japan
(Received 8 January 2011; accepted 13 January 2011)
Abstract—The increase in moon jellyfish (Aurelia aurita) populations in the
Seto Inland Sea of Japan have caused damage in fisheries and many significant
social and economic problems. To fully understand the population dynamics
(e.g., abundance) of moon jellyfish in this region, analysis of patterns in
jellyfish temporal abundance in relation to environmental fluctuation is essential.
However, an effective monitoring method of jellyfish abundance appears to be
lacking and there remains no quantitative data on jellyfish abundance in the
past. Some attempts to identify patterns in temporal abundance of jellyfish
have recently been carried out. Uye and Ueta (Bull. Jpn. Soc. Fish. Oceanogr.,
68, 9–19) reported that A. aurita populations have shown apparent increase
since the 1980s, most dramatically in the last 10 years. Through a questionnaire
targeting commercial fishers in the Seto Inland Sea, it was found that 65%
believed that jellyfish populations have increased since 1982. This report
suggests the long-term increasing trend of jellyfish population in the region. In
another study, Nagai (Mar. Pollut. Bull., 47, 126–131) reported similar results
by using other survey data. In this paper, we discuss the factors that contribute
to jellyfish biomass increase in the Seto Inland Sea and present the ecological
point of view of “jellyfish bloom dynamics.”
Keywords: Aurelia, jellyfish, blooms, Seto Inland Sea
INTRODUCTION
In recent years, the expanding influence of anthropogenic activities has caused
many changes in coastal sea ecosystems, such as harmful algal blooms (red tide),
hypoxia, and loss of biodiversity. In addition, it is currently argued that jellyfish
(Cnidaria and Ctenophora) populations are increasing in a variety of coastal
regions worldwide. Previous reports have shown scientific evidence of an
increase in jellyfish populations in some water bodies such as the Black Sea
(Shiganova and Bulgakova, 2000), Bering Sea (Brodeur et al., 1999, 2002), and
65
66 N. FUJII et al.
Fig. 1. Schematic representation of the jellyfish spiral theory.
northern Gulf of Mexico (Graham, 2001). Although scientific evidence is
unavailable in many other coastal seas, some scientists and fisherpersons have
noticed changes in the occurrence of jellyfish in their neighboring waters. For
example, in the Seto Inland Sea of Japan, fisherpersons have perceived the
increase in population biomass of the common jellyfish Aurelia aurita, the
scyphozoan Chrysaora melanaster, and the ctenophore Bolinopsis mikado. They
also have observed a consistent decrease in fish catch. In recent decades, the
ecological importance of jellyfish has been increasingly recognized in biological
oceanography (reviewed by Schneider and Behrends, 1998; Arai, 1988, 2001;
Purcell et al., 2001, 2007; Parsons and Lalli, 2002).
Some scientists have provided meaningful discussions on why jellyfishEstablishing a Conceptual Design for Jellyfish Blooms 67
Fig. 2. Circumstantial diagram of the jellyfish spiral theory.
blooms occur. The most popular explanation is the “jellyfish spiral” theory, the
mechanism of which was proposed by Uye and Ueta (2004). However, to predict
the variations in jellyfish populations, the concept of jellyfish blooms should be
more easily understood. Therefore, in this study, we reexamined the mechanism
of the jellyfish spiral theory. Here, we describe recent jellyfish blooms, mainly
A. aurita, in the Seto Inland Sea and propose the new “jellyfish bloom dynamics”
theory.
THE JELLYFISH SPIRAL THEORY
Uye and Ueta (2004) proposed the mechanism of the jellyfish spiral theory
as shown in Fig. 1. Although the causes for the recent increase of A. aurita
population in many coastal seas are still speculative, they may be associated with
1) Overfishing of planktivorous fish;
2) Increase in overwintering populations due to warming of seawater
temperature;
3) Increase in polyp attachment area due to waterfront constructions;
4) Increase in jellyfish food due to eutrophication or modification of
nutrient composition. Once an ecosystem falls into this spiral, it would
have more jellyfish and less finfish.68 N. FUJII et al.
Fig. 3. Diagram showing the effects of jellyfish blooms.
NEW THEORY: JELLYFISH BLOOM DYNAMICS
To predict the variations in jellyfish populations, the concept of jellyfish
blooms should be more easily understood. In this paper, we reexamine the
mechanism of the jellyfish spiral theory.
Figure 2 shows a diagram of the modified jellyfish spiral theory. This figure
offers a detailed description of the “jellyfish spiral.” Figure 3 shows a diagram
used to represent words, ideas, tasks, or other items linked to and arranged around
a central key factor of the previous slide. We explain some part, because this
figure is too complicated.
1) The top left part indicates a causal relationship between the variation in
jellyfish population and nutrient dynamics.
2) The top right part indicates a causal relationship between the variation in
jellyfish population and changes in waterfront constructions. Because
waterfront constructions provide increased polyp attachment area, A.
aurita polyps can increase in number, which may eventually lead to
increase in the medusa population.
3) The bottom-left part indicates a causal relationship between jellyfish
(medusa stage) and other organisms. Since A. aurita preys primarily on
mesozooplanktons such as copepods, cladocerans, 1arvaceans, and larvae
of various benthic animals and fish eggs, it is a competitor as well as a
predator of zooplanktivorous fish. If there are significant changes in the
secondary production processes of zooplanktons, there would be a
change in the relationship between jellyfish and zooplanktivorous fishEstablishing a Conceptual Design for Jellyfish Blooms 69
production (in this study, we consider only jellyfish and finfish).
4) The bottom-right part indicates a causal relationship between the variation
in jellyfish abundance and water temperature. For example, an increase
in water temperature during winter may lead to an increase in the
overwintering population of A. aurita.
Although Fig. 3 is too complicated, this figure encompasses numerous
influences of variation in jellyfish population.
THE SETO INLAND SEA
There is an absence of scientific monitoring data on long-term variation in
the biomass of A. aurita population in the Seto Inland Sea. Therefore, 2
investigations (Nagai, 2003; Uye and Ueta, 2004) were carried out to obtain such
information.
Uye and Ueta (2004) conducted a poll to survey the recent increase in A.
aurita population and its negative effects on fisheries in the Seto Inland Sea of
Japan. A total of 1152 respondents, with >20 years experience as a net fisherperson
(85%), angling fisherperson (7%), and others (8%), were included in the study.
Their results show variations in A. aurita populations in the Seto Inland Sea over
2 decades (1982–2002). On average, 65% of total respondents indicate that A.
aurita population has increased every 4 years from 1982 to 2002. In addition,
there is a clear regional difference in the increasing pattern of A. aurita abundance.
For example, the increase is most remarkable, particularly during the 10 years
from 1992 to 2002, in the western Seto Inland Sea. On the other hand, the A. aurita
biomass level is largely the same as before 1982 in the central Seto Inland Sea,
where the seasonal occurrence pattern has also been more or less constant. An
increase in the time spent in the medusa stage was obvious in jellyfish in both the
eastern and western Seto Inland Sea, which may be due to the recent increase in
annual minimum water temperature.
Nagai (2003) compiled the monthly fishing reports of the prefectural fisheries
experimental stations in the Seto Inland Sea and counted the frequency of
jellyfish blooms. Their study showed that jellyfish (Cyanea nozakii, A. aurita,
Pelagia panopyra, Dactylometra pacifica, Beroe cucumis, and others) occurred,
up until the first half of the 1990s, only in and near the entrance areas of the Seto
Inland Sea, such as the Kii Channel, Bungo Channel, Suo-Nada, and Iyo-Nada.
In the latter half of the 1990s, however, jellyfish became more common over the
entire area of the Seto Inland Sea. The number of cases of jellyfish blooms
reached a maximum in 1997.
We considered factors of jellyfish blooms in the Seto Inland Sea on the basis
of the jellyfish bloom dynamics theory. In the Seto Inland Sea, eutrophication
significantly progressed during the 1960s. However, Uye and Ueta (2004)
reported that A. aurita populations have shown apparent increase only since the
1980s. Thus, eutrophication was not a major factor, but a change in the low-
trophic food web due to eutrophication may be a principal factor in the jellyfish
blooms. Several districts around the Seto Inland Sea have been reclaimed, and the
coastal area has dramatically increased to about 225 km2 from 1950 to 197370 N. FUJII et al.
(EMECS, 1997). Although, Uye and Ueta (2004) reported that A. aurita populations
have shown apparent increased since the 1980s, the change in waterfront
constructions was not a major factor. The annual catch of zooplanktivorous fish
from the Seto Inland Sea of Japan was highest (>350,000 metric tons) in the mid-
1980s, and then declined to ca. 130,000 metric tons in the late 1990s. Under such
conditions, an increase in jellyfish population leads to a decrease in the finfish
population; in other words, the ecosystem shifts from a finfish-dominated
community to jellyfish-dominated community. The connection between the
populations of jellyfish and other organisms requires further research. The
seawater temperature around the Seto Inland Sea is increasing, particularly the
winter water temperature in the entire area of the Seto Inland Sea (Bungo-
Channel). Considering that Uye and Ueta (2004) and Nagai (2003) reported that
A. aurita populations have increased in the entrance areas of the Seto Inland Sea,
there may be a relationship between jellyfish abundance and water temperature
in winter. However, we do not yet understand the nature of this relationship.
To more fully understand the population dynamics of moon jellyfish in this
region, analysis of patterns in jellyfish temporal abundance in relation to
environmental fluctuation is essential. However, it is important to obtain scientific
monitoring data on long-term variations in A. aurita population. Thus, in a future
study, we will attempt to create a dataset of jellyfish abundance by using the
following novel methods:
1) Jellyfish Aggregation Monitoring System in Uwa Sea (JAMSUS; 2003–
2010);
2) Jellyfish landings from the Ikata Nuclear Power Plant (1998–2010).
Using the JAMSUS method, we had set up a video monitoring system on a
hill with a full view of Hokezu Bay (part of the Bungo-Channel) during summer
and autumn from 2003 to 2010. We observed temporal shifts in dense aggregations
of moon jellyfish, the frequency of occurrence of which varied widely in Hokezu
Bay. From jellyfish landings in the Ikata Nuclear Power Plant, which takes in
cooling water from the Iyo-Nada (part of the Seto Inland Sea), we maintained
records of the jellyfish clogging biomass cleaned daily from the intake screens,
providing a decade-long record of jellyfish abundance. Using data from these 2
methods, we will attempt to understand the factors influencing jellyfish populations
in the Seto Inland Sea.
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N. Fujii (e-mail: medusae@sci.ehime-u.ac.jp)You can also read
