Anthropogenic causes of jellyfish blooms and their
direct consequences for humans: a review
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 sustained
increases is lacking. Analyses of several longterm
(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 problems with jellyfish have increased and have captured
public attention (e.g. Whiteman 2002, Carpenter
2004, de Pastino 2006, 2007, Owen 2006). Such problems
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 conspicuous;
however, they are only a portion of the diverse
gelatinous fauna. Approximately 190 species of scyphomedusae
(Arai 1997), 20 species of cubomedusae
(Mianzan & Cornelius 1999), 840 species of hydromedusae
(Boullion & Boero 2000), 200 species of siphonophores
(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 single
polyp; cubozoan polyps are an exception, transforming
into individual jellyfish without budding.
Swimming jellyfish reproduce sexually, often have
great fecundity and may brood the larvae, which settle
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 contrast,
siphonophores (also cnidarians) and ctenophores
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 hermaphroditic,
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 ichthyoplankton
(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 nonexistent
(but see Lynam et al. 2005).
In this review, we examine recent cases where jellyfish
blooms have increased, describe the problems jellyfish
directly cause for humans, and show how a number
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 information.
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 populations
(e.g. Mills 1995, 2001, Benovi´c 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, possibly
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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Purcell et al.: Humans and jellyfish blooms
Perhaps the best indications that jellyfish blooms
have increased are from the reports of problems
caused by jellyfish. In the Seto Inland Sea, Japan, Aurelia
aurita (sensu lato, s. l.) populations have apparently
increased since the 1980s, and most dramatically
in the last 10 yr; 65% of fishermen surveyed believed
that jellyfish had increased during the last 20 yr, while
35% thought they had not (Uye & Ueta 2004). Blooms
of the giant Nomura’s jellyfish Nemopilema nomurai
(up to 2 m diameter and 200 kg biomass) have created
serious problems in Japanese waters. Before 2000,
blooms of N. nomurai had been reported only in 1920,
1958 and 1995 in Asian waters; however, beginning
in 2002, blooms have occurred every year, except for
2004 (Uye in press). Problems reported with jellyfish
(stinging people and interfering with fishing and
power plants) in Japan certainly have increased in
recent years (Fig. 1, Tables 1 to 4).
Reports of blooms and resulting problems have increased
elsewhere in the world as well (Tables 1 to 4).
Since the mid-1990s jellyfish have become a problem for
fisheries in the East China and Yellow seas, and are associated
with decreased fish catches (Cheng et al. 2005).
Yan et al. (2004) showed inverse correlations between
fish and jellyfish, with greater biomass of Cyanea spp.
and Stomolophus spp. in 1998 to 2003 than in 1990 to
1997. In the northern Benguela Current off Namibia,
catches of anchovy and sardines have been reduced dramatically
after 1988 compared with those during 1975 to
1988, and large populations of 2 jellyfishes, Chrysaora
hysoscella and Aequorea forskalea, now predominate
(Lynam et al. 2006). In addition to interfering in fisheries
there, jellyfish have blocked water intakes of power
plants and diamond mining operations (Lynam et al.
2006). Jellyfish populations have bloomed in coastal
lagoons along the Mediterranean coasts of Spain and
France since the 1990s (Pagés 2001, Anonymous 2006).
Pelagia noctiluca periodically blooms in the Mediterranean
Sea (UNEP 1981, 1984, Goy et al. 1989) and
caused problems there in the 1990s (Molinero et al.
2005). In coastal waters of the Middle East, blooms of the
rhizostome scyphozoan Crambionella orsini have been
associated with power- and desalination-plant interruptions
since 2000 (Daryanabard & Dawson in press).
It is too soon to know whether these recent jellyfish
increases will be sustained or the populations will fluctuate
with climate as seen for other species. If environmental
deterioration, including ocean warming, is
responsible for the blooms, high jellyfish populations
may persist.