Anthropogenic Causes of Jellyfish Blooms and Their Direct Consequences for Humans: a Review
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Vol. 350: 153–174, 2007 MARINE ECOLOGY PROGRESS SERIES Published November 22 doi: 10.3354/meps07093 Mar Ecol Prog Ser REVIEW Anthropogenic causes of jellyfish blooms and their direct consequences for humans: a review Jennifer E. Purcell1, 3,*, Shin-ichi Uye2, Wen-Tseng Lo3 1Western Washington University, Shannon Point Marine Center,1900 Shannon Point Road, Anacortes, Washington 98221, USA 2Graduate School of Biosphere Science, Hiroshima University,4-4 Kagamiyama 1 Chome, Higashi-Hiroshima 739-8528, Japan 3Deparment 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 INTRODUCTION term (8 to 100 yr) trends in jellyfish populations demonstrate that their abundances vary with climate, Recent concerns that jellyfish populations are often at decadal scales (reviewed in Purcell 2005). increasing have stimulated speculation about possible Some evidence suggests continued upward trends causes including climate change, eutrophication, over (Attrill et al. 2007); however, recent time series are still fishing and invasions (e.g. Arai 2001, Mills 2001, Oguz too short to exclude circa-decadal climate cycles. Even 2005a,b, Purcell 2005, Hay 2006, Graham & Bayha though blooms of newly introduced species also vary 2007). While speculation is abundant, evidence for sus- over time, the invaders may spread into new areas tained increases is lacking. Analyses of several long- (reviewed in Graham & Bayha 2007). Reports of human *Email: [email protected] © Inter-Research 2007 · www.int-res.com 154 Mar Ecol Prog Ser 350: 153–174, 2007 problems with jellyfish have increased and have cap- jellyfish reproduce asexually (Boero et al. 2002). Thus, tured public attention (e.g. Whiteman 2002, Carpenter the ability of pelagic cnidarians and ctenophores to 2004, de Pastino 2006, 2007, Owen 2006). Such prob- bloom in good conditions is intrinsic. lems come mainly from jellyfish stinging swimmers Gelatinous predators have important ecological and interfering with fishing, aquaculture and power effects that are considered to be detrimental to human plant operations. interests. They eat zooplankton and can reduce and Generally, only a relatively few coastal species of change zooplankton populations; therefore, they can large scyphomedusan jellyfish are responsible for the reduce the food available to fish. They eat ichthyo- reported problems. These large species are conspicu- plankton (eggs and larvae of fish) as well as juvenile ous; however, they are only a portion of the diverse fish, and, thus, directly reduce fish populations. These gelatinous fauna. Approximately 190 species of scy- potential effects of jellyfish on fish have been reviewed phomedusae (Arai 1997), 20 species of cubomedusae previously (Purcell 1985, Arai 1988, Bailey & Houde (Mianzan & Cornelius 1999), 840 species of hydrome- 1989, Purcell & Arai 2001) and will not be considered dusae (Boullion & Boero 2000), 200 species of sipho- in detail in this review. Studies showing negative nophores (Pugh 1999) and 150 species of ctenophores effects of jellyfish on fish recruitment are almost non- (Mianzan 1999) are now recognized. Therefore, the existent (but see Lynam et al. 2005). potential for further problems with jellyfish may be In this review, we examine recent cases where jelly- very great. In the present review, we discuss only the fish blooms have increased, describe the problems jel- zooplanktivorous gelatinous taxa mentioned above. lyfish directly cause for humans, and show how a num- We refer to scypho-, cubo-, and hydromedusae as ber of human activities may increase pelagic cnidarian ‘jellyfish’ for the sake of simplicity. and ctenophore populations. We focus on species for The ability of gelatinous species to occur in large which factors contributing to the blooms have been numbers (i.e. to bloom) is due to the cnidarians analyzed or inferred. Concern about blooms of jellyfish (Scyphozoa, Cubozoa, Hydrozoa) having both asexual and ctenophores has stimulated several meetings and and sexual reproduction. Most coastal jellyfish are special volumes that contain more detailed informa- asexually budded from an attached stage in the life tion. The first 2 meetings regarded blooms of Pelagia cycle, a scyphistoma for scyphozoans, and a hydroid noctiluca scyphomedusae in the Mediterranean Sea (often colonial) for hydromedusae. We will refer to (UNEP 1984, 1991). Several meetings focused on the the benthic stages as ‘polyps’. Polyps bud more outbreak and spread of Mnemiopsis leidyi ctenophores polyps, and many jellyfish can be budded from a sin- (e.g. GESAMP 1997, Dumont et al. 2004). Meetings gle polyp; cubozoan polyps are an exception, trans- and publications that were not targeted on any specific forming into individual jellyfish without budding. species include Purcell et al. (2001a), CIESM (2001), Swimming jellyfish reproduce sexually, often have and JMBA (2005). Various authors have suggested great fecundity and may brood the larvae, which set- anthropogenic causes for changes in jellyfish popula- tle to become polyps. Temperate species typically tions (e.g. Mills 1995, 2001, Benovi´c et al. 2000, Arai have an annual cycle, with small jellyfish (1 to 2 mm 2001, Graham 2001, Parsons & Lalli 2002, Oguz 2005b). ephyrae for scyphozoans) being produced in fall or We review these topics on a global scale and present spring, and the jellyfish growing to sexual maturity information that is as comprehensive as possible. over the summer; in the tropics, jellyfish production We present data from East Asia where blooms have can occur all year (reviewed in Lucas 2001). By con- increased recently. trast, siphonophores (also cnidarians) and cteno- phores lack an attached stage in the life cycle (i.e. holoplanktonic). The siphonophores also have asexual ARE JELLYFISH POPULATIONS INCREASING? multiplication of reproductive individuals, followed by sexual reproduction. Most ctenophores are herma- The paucity of long-term data makes it difficult to phroditic, and have direct development and great draw definitive conclusions on the status of jellyfish fecundity. Therefore, siphonophores and ctenophores populations. Most analyzed data sets show variations are not constrained to one1 generation per year, in in jellyfish population size with climatic regime shifts contrast to many species in the other taxa. Although at decadal scales (reviewed in Purcell 2005). Even the this represents a general picture of the life cycles, dramatic increase of Chrysaora melanaster during the cnidarians, especially the hydrozoans, are renowned 1990s in the Bering Sea ended abruptly in 2000, pos- for reproductive variety (Boero et al. 2002). Many sibly because of exceptionally warm conditions after hydromedusae are holoplanktonic (holoplanktonic 2000 (Brodeur et al. in press). Persistent increases of species indicated by an asterisk in our tables), while jellyfish unrelated to climate variation have not yet this is rare in scyphozoans, and some hydromedusan been demonstrated. Purcell et al.: Humans and jellyfish