MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 350: 153–174, 2007
doi: 10.3354/meps07093
Published November 22
INTRODUCTION
Recent concerns that jellyfish populations are
increasing have stimulated speculation about possible
causes including climate change, eutrophication, over
fishing and invasions (e.g. Arai 2001, Mills 2001, Oguz
2005a,b, Purcell 2005, Hay 2006, Graham & Bayha
2007). While speculation is abundant, evidence for sus-
tained increases is lacking. Analyses of several long-
term (8 to 100 yr) trends in jellyfish populations
demonstrate that their abundances vary with climate,
often at decadal scales (reviewed in Purcell 2005).
Some evidence suggests continued upward trends
(Attrill et al. 2007); however, recent time series are still
too short to exclude circa-decadal climate cycles. Even
though blooms of newly introduced species also vary
over time, the invaders may spread into new areas
(reviewed in Graham & Bayha 2007). Reports of human
© Inter-Research 2007 · www.int-res.com*Email: purcellj3@wwu.edu
REVIEW
Anthropogenic causes of jellyfish blooms and their
direct consequences for humans: a review
Jennifer E. Purcell
1, 3,
*
, Shin-ichi Uye
2
, Wen-Tseng Lo
3
1
Western Washington University, Shannon Point Marine Center,1900 Shannon Point Road, Anacortes, Washington 98221, USA
2
Graduate School of Biosphere Science, Hiroshima University,4-4 Kagamiyama 1 Chome, Higashi-Hiroshima 739-8528, Japan
3
Deparment of Marine Biotechnology and Resources, Asian-Pacific Ocean Research Center, Kuroshio Research Group,
National Sun Yat-Sen University, 70 Lienhai Road, Kaohsiung, Taiwan 804, ROC
ABSTRACT: Although recent articles state that jellyfish populations are increasing, most available
evidence shows that jellyfish abundances fluctuate with climatic cycles. Reports of increasing prob-
lems with jellyfish, especially in East Asia, are too recent to exclude decadal climate cycles. Jellyfish
are infamous for their direct negative effects on human enterprise; specifically, they interfere with
tourism by stinging swimmers, fishing by clogging nets, aquaculture by killing fish in net-pens and
power plants by clogging cooling-water intake screens. They also have indirect effects on fisheries by
feeding on zooplankton and ichthyoplankton, and, therefore, are predators and potential competitors
of fish. Ironically, many human activities may contribute to increases in jellyfish populations in coastal
waters. Increased jellyfish and ctenophore populations often are associated with warming caused by
climate changes and possibly power plant thermal effluents. Jellyfish may benefit from eutrophication,
which can increase small-zooplankton abundance, turbidity and hypoxia, all conditions that may favor
jellyfish over fish. Fishing activities can remove predators of jellyfish and zooplanktivorous fish com-
petitors as well as cause large-scale ecosystem changes that improve conditions for jellyfish. Aquacul-
ture releases millions of jellyfish into Asian coastal waters yearly to enhance the jellyfish fishery.
Aquaculture and other marine structures provide favorable habitat for the benthic stages of jellyfish.
Changes in the hydrological regime due to dams and other construction can change the salinity to
favor jellyfish. Accidental introductions of non-native gelatinous species into disturbed ecosystems
have led to blooms with serious consequences. In many coastal areas, most of these environmental
changes occur simultaneously. We summarize cases of problem jellyfish blooms and the evidence for
anthropogenic habitat disruptions that may have caused them. Rapid development in East Asia makes
that region especially vulnerable to escalating problems. We conclude that human effects on coastal
environments are certain to increase, and jellyfish blooms may increase as a consequence.
KEY WORDS: Eutrophication · Fish · Aquaculture · Climate · Warming · Temperature · Zooplankton ·
Salinity · Sting · Powerplant · Introduce
Resale or republication not permitted without written consent of the publisher

Mar Ecol Prog Ser 350: 153–174, 2007
problems with jellyfish have increased and have cap-
tured public attention (e.g. Whiteman 2002, Carpenter
2004, de Pastino 2006, 2007, Owen 2006). Such prob-
lems come mainly from jellyfish stinging swimmers
and interfering with fishing, aquaculture and power
plant operations.
Generally, only a relatively few coastal species of
large scyphomedusan jellyfish are responsible for the
reported problems. These large species are conspicu-
ous; however, they are only a portion of the diverse
gelatinous fauna. Approximately 190 species of scy-
phomedusae (Arai 1997), 20 species of cubomedusae
(Mianzan & Cornelius 1999), 840 species of hydrome-
dusae (Boullion & Boero 2000), 200 species of sipho-
nophores (Pugh 1999) and 150 species of ctenophores
(Mianzan 1999) are now recognized. Therefore, the
potential for further problems with jellyfish may be
very great. In the present review, we discuss only the
zooplanktivorous gelatinous taxa mentioned above.
We refer to scypho-, cubo-, and hydromedusae as
‘jellyfish’ for the sake of simplicity.
The ability of gelatinous species to occur in large
numbers (i.e. to bloom) is due to the cnidarians
(Scyphozoa, Cubozoa, Hydrozoa) having both asexual
and sexual reproduction. Most coastal jellyfish are
asexually budded from an attached stage in the life
cycle, a scyphistoma for scyphozoans, and a hydroid
(often colonial) for hydromedusae. We will refer to
the benthic stages as ‘polyps’. Polyps bud more
polyps, and many jellyfish can be budded from a sin-
gle polyp; cubozoan polyps are an exception, trans-
forming into individual jellyfish without budding.
Swimming jellyfish reproduce sexually, often have
great fecundity and may brood the larvae, which set-
tle to become polyps. Temperate species typically
have an annual cycle, with small jellyfish (1 to 2 mm
ephyrae for scyphozoans) being produced in fall or
spring, and the jellyfish growing to sexual maturity
over the summer; in the tropics, jellyfish production
can occur all year (reviewed in Lucas 2001). By con-
trast, siphonophores (also cnidarians) and cteno-
phores lack an attached stage in the life cycle (i.e.
holoplanktonic). The siphonophores also have asexual
multiplication of reproductive individuals, followed
by sexual reproduction. Most ctenophores are herma-
phroditic, and have direct development and great
fecundity. Therefore, siphonophores and ctenophores
are not constrained to one1 generation per year, in
contrast to many species in the other taxa. Although
this represents a general picture of the life cycles,
cnidarians, especially the hydrozoans, are renowned
for reproductive variety (Boero et al. 2002). Many
hydromedusae are holoplanktonic (holoplanktonic
species indicated by an asterisk in our tables), while
this is rare in scyphozoans, and some hydromedusan
jellyfish reproduce asexually (Boero et al. 2002). Thus,
the ability of pelagic cnidarians and ctenophores to
bloom in good conditions is intrinsic.
Gelatinous predators have important ecological
effects that are considered to be detrimental to human
interests. They eat zooplankton and can reduce and
change zooplankton populations; therefore, they can
reduce the food available to fish. They eat ichthyo-
plankton (eggs and larvae of fish) as well as juvenile
fish, and, thus, directly reduce fish populations. These
potential effects of jellyfish on fish have been reviewed
previously (Purcell 1985, Arai 1988, Bailey & Houde
1989, Purcell & Arai 2001) and will not be considered
in detail in this review. Studies showing negative
effects of jellyfish on fish recruitment are almost non-
existent (but see Lynam et al. 2005).
In this review, we examine recent cases where jelly-
fish blooms have increased, describe the problems jel-
lyfish directly cause for humans, and show how a num-
ber of human activities may increase pelagic cnidarian
and ctenophore populations. We focus on species for
which factors contributing to the blooms have been
analyzed or inferred. Concern about blooms of jellyfish
and ctenophores has stimulated several meetings and
special volumes that contain more detailed informa-
tion. The first 2 meetings regarded blooms of Pelagia
noctiluca scyphomedusae in the Mediterranean Sea
(UNEP 1984, 1991). Several meetings focused on the
outbreak and spread of Mnemiopsis leidyi ctenophores
(e.g. GESAMP 1997, Dumont et al. 2004). Meetings
and publications that were not targeted on any specific
species include Purcell et al. (2001a), CIESM (2001),
and JMBA (2005). Various authors have suggested
anthropogenic causes for changes in jellyfish popula-
tions (e.g. Mills 1995, 2001, Benovic´ et al. 2000, Arai
2001, Graham 2001, Parsons & Lalli 2002, Oguz 2005b).
We review these topics on a global scale and present
information that is as comprehensive as possible.
We present data from East Asia where blooms have
increased recently.
ARE JELLYFISH POPULATIONS INCREASING?
The paucity of long-term data makes it difficult to
draw definitive conclusions on the status of jellyfish
populations. Most analyzed data sets show variations
in jellyfish population size with climatic regime shifts
at decadal scales (reviewed in Purcell 2005). Even the
dramatic increase of Chrysaora melanaster during the
1990s in the Bering Sea ended abruptly in 2000, pos-
sibly because of exceptionally warm conditions after
2000 (Brodeur et al. in press). Persistent increases of
jellyfish unrelated to climate variation have not yet
been demonstrated.
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