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Estuarine, Coastal and Shelf Science 86 (2010) 485–490

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Distribution of moon jellyfish Aurelia aurita in relation to summer hypoxia inHiroshima Bay, Seto Inland Sea

Jun Shoji a,*, Takaya Kudoh b, Hideyuki Takatsuji b, Osamu Kawaguchi b, Akihide Kasai c

a Takehara Fisheries Research Station, Hiroshima University, Takehara, Hiroshima 725-0024, Japanb Fisheries and Marine Technology Center, Hiroshima Prefectural Technology Research Institute, Ondo, Kure, Hiroshima 737-1207, Japanc Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-0068, Japan

a r t i c l e i n f o

Article history:Received 14 January 2009Accepted 1 March 2009Available online 10 March 2009

Keywords:moon jellyfishJapanese anchovydissolved oxygenHiroshima BaySeto Inland Sea

* Corresponding author.E-mail address: jshoji@hiroshima-u.ac.jp (J. Shoji)

0272-7714/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.03.001

a b s t r a c t

Biological and physical surveys were conducted in order to investigate the relationship between envi-ronmental conditions and the distribution of moon jellyfish Aurelia aurita in Hiroshima Bay, western SetoInland Sea, Japan. Moon jellyfish and ichthyoplankton were collected at 13 stations in Hiroshima Bayduring monthly surveys from July to September in 2006 and 2007. Surface temperature in 2006 wassignificantly lower during the August and September cruises and surface salinity was lower during allcruises than in 2007. Moon jellyfish was the most dominant gelatinous plankton collected, accounting for89.7% in wet weight. Mean moon jellyfish abundance in 2006 was higher than that in 2007 from Julythrough September, with significant inter-year differences for July and September. Variability inprecipitation and nutritional input from the Ohta River, northernmost part of Hiroshima Bay, weresuggested as possible factors affecting the inter-annual variability in moon jellyfish abundance in thecoastal areas of northern Hiroshima Bay. Moon jellyfish were more abundant in the coastal areas ofnorthern Hiroshima Bay, where the dissolved oxygen (DO) concentration was lower, while low in thecentral part of the bay. Japanese anchovy Engraulis japonicus eggs were most dominant (58.1% in number)among the ichthyoplankton and were abundant in the central area of Hiroshima Bay. Explanatoryanalysis was conducted to detect possible effects of environmental conditions on the abundance of moonjellyfish and Japanese anchovy eggs during the summer months in Hiroshima Bay. Of the environmentalconditions tested (temperature, salinity and DO of surface and bottom layers at each sampling station),bottom DO had the most significant effect on the moon jellyfish abundance: there was a negativecorrelation between the bottom DO and the moon jellyfish abundance in Hiroshima Bay during summer.

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1. Introduction

The moon jellyfish Aurelia aurita is widely distributedthroughout the coastal waters of the world and has been consid-ered as an important predator of zooplankton because of its highconsumption rates (Moller, 1984; Costello and Colin, 1994). Increasein abundance of the moon jellyfish, the same as other large gelat-inous zooplankton such as cnidarians and ctenophores, hasoccurred in estuarine and ocean ecosystems all over the world(Purcell and Arai, 2001; Brodeur et al., 2002), potentially causinga significant impact on smaller zooplankton and their predators inpelagic ecosystems (Uye and Ueta, 2004; Haslob et al., 2007). InJapan, the moon jellyfish is the most common jellyfish species incoastal waters and its biomass has increased over recent decades

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in Tokyo Bay (Toyokawa et al., 2000; Ishii, 2001) and the Seto InlandSea (Uye and Ueta, 2004). Uye et al. (2003) observed moon jellyfishaggregations of up to 250 individuals m�2, which were estimated toconsume nearly 100% of the mesozooplankton biomass duringsummer months in the coastal waters of the western part of theSeto Inland Sea. Clarification of the mechanism of moon jellyfishblooms and effects of environmental conditions on moon jellyfishabundance are indispensable for forecasting and regulation ofmoon jellyfish blooms. However, much of the information availablein the literature is of qualitative evaluations of moon jellyfishdistribution and its dynamics in nature.

The Seto Inland Sea is a semi-enclosed basin, which is sur-rounded by heavily urbanized areas with extensive industrial andagricultural development, and receives significant freshwater fromthe surrounding watersheds (Ochi et al., 1978; Okaichi et al., 1996).Eutrophication of coastal waters has been common in HiroshimaBay, the innermost part of the Seto Inland Sea (Okaichi et al., 1996).Nutritional input through the Ohta River, the northernmost part of

Fig. 1. Map showing the 13 sampling stations in Hiroshima Bay, Seto Inland Sea,southwestern Japan. Monthly physical and biological surveys were conducted fromJuly to September in 2006 and 2007.

J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490486

Hiroshima Bay, and estuarine circulation enhance both primary andsecondary production in Hiroshima Bay (Hashimoto et al., 2006).Chlorophyll-a levels and therefore primary production of Hir-oshima Bay are among the highest of the nine bay areas of the SetoInland Sea (from east to west: Kii Channel, Osaka Bay, Sea of Har-ima, Bisan Archipelago, Sea of Hiuchi, Bingo-Geiyo Archipelago, Seaof Aki including Hiroshima Bay, Sea of Iyo, Sea of Suo: Okaichi et al.,1996). During summer hypoxia prevails in the northern coastalareas of Hiroshima Bay, with dissolved oxygen (DO) concentrationsof <2 mg L�1 (Okaichi et al., 1996, present study). Moderatedecreases in DO to near hypoxic conditions (<3 mg L�1), althoughnot lethal during short-term exposure, can reduce the ability oflarval fish to escape and avoid predation and to capture prey(Breitburg et al., 1994). Recent laboratory experiments reportedthat bell contraction rate and predation rate on fish larvae by moonjellyfish under oxygen concentrations< 2 mg L�1 were similar tothose under higher oxygen concentrations (4–6 mg L�1), indicatingthat moon jellyfish are highly tolerant to low oxygen concentra-tions, as are several other jellyfish species (Breitburg et al., 1994;Keister et al., 2000; Shoji et al., 2005; Thuesen et al., 2005). It hasbeen suggested that a shift from size-selective to non-size-selectivepredation prevails in the predator–prey interaction and that rela-tive importance of trophic flow from ichthyoplankton to moonjellyfish increases during summer hypoxia in coastal waters (Shoji,2008). Investigation of the abundance and distribution of moonjellyfish in relation to environmental conditions is thereforeimportant for understanding the effects of hypoxia on trophic flowin plankton communities.

In the present study, biological and physical surveys were con-ducted in the Hiroshima Bay in order to examine the effects ofenvironmental conditions (temperature, salinity, and DO) on moonjellyfish distribution. Pattern of the horizontal distribution andeffects of the environmental conditions on moon jellyfish werecompared with the most dominant ichthyoplankton species, Japa-nese anchovy Engraulis japonicus in summer 2006 and 2007.

2. Materials and methods

2.1. Field survey

Biological and physical surveys were conducted during monthlyresearch cruises from July to September in 2006 and 2007 on the RVAki, Fisheries and Marine Technology Center, Hiroshima PrefecturalTechnology Research Institute (HPTRI), in Hiroshima Bay (Fig. 1).Moon jellyfish were collected with a plankton net (0.45 m indiameter, 0.33 mm in mesh aperture) with a flow-meter at 13stations in Hiroshima Bay. A vertical haul with the plankton netfrom 1 m above the sea bottom to the surface was made at eachstation. Moon jellyfish were sorted from the sample, identified andmeasured for bell diameter (mm) and wet weight (g) on the boat.Ichthyoplankton samples were preserved in 10% formalin. Verticalprofiles of temperature (�C), salinity and dissolved oxygen (DO)concentration (mg L�1) were measured during each samplingevent.

2.2. Laboratory procedure

Jellyfish and ichthyoplankton were enumerated as kg m�2 andnumber m�2 according to the flow-meter counts. Of the totaljellyfish catch for the six cruises, the moon jellyfish Aurelia auritawas most dominant, accounting for 89.7% of the total wet weight(see the Results). Other jellyfish species was excluded from thefurther analysis due to the small sample size. Ichthyoplankton weresorted and identified to the lowest taxa possible. Eggs of Japaneseanchovy Engraulis japonicus, the most dominant ichthyoplankton

collected during the six cruises (58.1% in number of the total ich-thyoplankton, see the Results) were processed for analysis ofseasonal change in abundance and horizontal distribution in Hir-oshima Bay. Other ichthyoplankton were excluded from the furtheranalysis due to the small sample size.

Exploratory analysis was conducted in order to detect possibleeffects of the environmental factors on variability in the moonjellyfish and Japanese anchovy egg abundance. Temperature,salinity and DO of surface and bottom layers at each samplingstation were used as explanatory variables and abundance of moonjellyfish and Japanese anchovy eggs as dependent variables. Theenvironmental conditions (temperature, salinity and DO) andmoon jellyfish and Japanese anchovy egg abundance werecompared between the two years by the use of Wilcoxon test.

3. Results

3.1. Environmental conditions

Mean sea surface temperature ranged between 24.0 �C (July)and 26.8 �C (August) in 2006 and between 23.0 �C (July) and 28.9 �C(August) in 2007 (Fig. 2). There was a significant inter-annualdifference in the surface temperature in all months (Wilcoxon test,d.f.¼1, July: P¼ 0.004; August: P¼ 0.0006; September:P< 0.0001). The surface temperature was lower in northern Hir-oshima Bay during all cruises (Figs. 3 and 4).

Mean surface salinity increased from 20.8 in July to 28.1 inSeptember in 2006 and from 27.8 in July to 31.8 in September in2007 (Fig. 2). In all months, surface salinity in 2007 was signifi-cantly higher than that in 2006 (Wilcoxon test, d.f.¼1, July andSeptember: P< 0.0001; August: P¼ 0.005). The surface salinity waslower in the northern part of Hiroshima Bay.

The mean bottom DO for all 13 stations ranged between3.3 mg L�1 (September) and 5.0 mg L�1 (August) in 2006 and3.3 mg L�1 (August) and 4.7 mg L�1 (July) in 2007 (Fig. 2). In August,

Fig. 2. Seasonal changes in physical and biological conditions in Hiroshima Bay fromJuly to September in 2006 (open circles) and 2007 (closed circles). Mean sea surfacetemperature (WT), salinity (SA), bottom dissolved oxygen concentration (DO), abun-dance of moon jellyfish (MJ) and Japanese anchovy egg (AE) are shown with verticalbars as standard deviation. Asterisks indicate significant difference between the years(Wilcoxon test, *P< 0.05; **P< 0.01; ***P< 0.001; ****P< 0.0001).

J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490 487

there was a significant inter-annual difference in the DO (Wilcoxontest, d.f.¼1, P¼ 0.003). The bottom DO was lower in the northernand/or eastern parts of Hiroshima Bay throughout the cruises. InSeptember 2006 and August and September 2007, some stationsindicated a bottom DO of <2.0 mg L�1 (Figs. 3 and 4).

3.2. Occurrence and distribution of moon jellyfish

Of the total jellyfish collected throughout the cruises, moonjellyfish was most abundant, accounting for 89.7% in wet weight.Bolinopsis mikado and unidentified jellyfish species composed only10.3%. The mean moon jellyfish abundance was highest in July inboth 2006 (4.68 kg m�2) and 2007 (1.24 kg m�2: Fig. 2). The inter-year difference in the moon jellyfish abundance was significant inJuly (P¼ 0.014) and September (P¼ 0.013). There was no significantdifference in the bell diameter of moon jellyfish between the twoyears (2006: 177.6� 32.2 mm; 2007: 174.0�19.6 mm).

Moon jellyfish were more abundant in the northern part ofHiroshima Bay (Figs. 3 and 4). The areas of high abundance corre-sponded with those of low bottom DO. In July 2006 and 2007, thehighest moon jellyfish abundance was observed in the eastern partof the bay, where bottom DO was lower than 2.0 mg L�1. Moonjellyfish were sparsely distributed in the central area of the baythroughout the cruises.

3.3. Occurrence and distribution of Japanese anchovy egg

A total of 654 fish eggs and 88 larvae were collected during thesix cruises. Japanese anchovy eggs were the most dominant,accounting for 65.9% in number of the total fish eggs. Japaneseanchovy egg abundance was highest in August in 2006 (276.3 m�2)and in July in 2007 (39.4 m�2: Fig. 2). There was a significantdifference in the Japanese anchovy egg abundance in August(P¼ 0.022) between the two years.

Japanese anchovy eggs were more abundant in the central orsouthernparts of Hiroshima Bay from July through September in 2006and in July in 2007 while sparsely distributed in the coastal areas ofthe northern part of the bay throughout the six cruises (Figs. 3 and 4).

3.4. Effect of environmental conditions on moon jellyfishdistribution

Of the environmental conditions tested, only bottom DO hada significant effect on the moon jellyfish abundance (F¼ 4.4758,P¼ 0.0379: all data combined). Moon jellyfish were more abundantin the areas with lower levels of bottom DO (<3.0 mg L�1). Anexponential model was fitted to the plots of moon jellyfish abun-dance to bottom DO in July and September 2006 (Fig. 5).

4. Discussion

4.1. Effect of dissolved oxygen concentration on moon jellyfish

Correspondence of high abundance of moon jellyfish with theareas of moderate bottom-layer hypoxia (<3 mg L�1) wasobserved in Hiroshima Bay during summer 2006 and 2007.Previous laboratory experiments showed a high tolerance to lowDO conditions (<2.0 mg L�1) in moon jellyfish: bell contract rate(an index of feeding activity) of the moon jellyfish was constantover the DO levels tested (1.0–5.8 mg L�1: Shoji et al., 2005).Strong tolerance to low DO levels has also been observed inAurelia labiata and sea nettle scyphomedusa Chrysaora quinque-cirrha under laboratory conditions (Breitburg et al., 1994;Thuesen et al., 2005) and high densities of the ctenophore

Fig. 3. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellyfish (MJ) and Japanese anchovy eggs (AE)from July to September in 2006 in Hiroshima Bay.

Fig. 4. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellyfish (MJ) and Japanese anchovy eggs (AE)from July to September in 2007 in Hiroshima Bay.

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Fig. 5. Plots of moon jellyfish abundance to bottom dissolved oxygen concentration (DO) in each month. The effect of DO on moon jellyfish abundance was significant in July andSeptember in 2006.

J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490 489

Mnemiopsis leidyi in low DO concentrations in Chesapeake Bay(Keister et al., 2000).

Contrastingly, ichthyoplankton were less abundant in thenorthern part of Hiroshima Bay during the summer months.Swimming and/or feeding performance of major fish predatorshave been observed to significantly decrease at DO lev-els< 2.0 mg L�1 in coastal waters (juvenile striped bass Moronesaxatilis, adult naked goby Gobiosoma bosc and juvenile Spanishmackerel Scomberomorus niphonius: Breitburg et al., 1994; Shojiet al., 2005). Therefore, competition for prey and space betweenmoon jellyfish and fish weakens as DO decreases in coastalhabitats. As a result, the relative importance of trophic flow fromplankton to jellyfish is considered to increase in coastal waterswhere summer hypoxia prevails (Breitburg et al., 1994; Shoji et al.,2005). We suggest that low DO conditions in the coastal waters ofHiroshima Bay during summer were favorable for moon jellyfishfeeding, growth and survival and resulted in the high abundancein the northern area.

4.2. Distribution of moon jellyfish in Hiroshima Bay

In addition to the strong tolerance to low DO level, supply ofyoung stages of moon jellyfish and retention of them within thecoastal areas of northern Hiroshima Bay should be included inthe possible factors that explain the high moon jellyfish

abundance in this area. Hiroshima Bay is located at the mostinner part and is the most enclosed area in the Seto Inland Sea(Okaichi et al., 1996). Moon jellyfish polyps use artificialsubstrate such as concrete blocks, plastic floats and oyster beds(Yasuda, 2003). In Hiroshima Bay, the natural shoreline hasdecreased to less than 40% of its original existence (Okaichiet al., 1996), with much of it replaced by concrete seawallswhich are favorable for jellyfish polyps. Intensive oyster culturein Hiroshima Bay (ca. 50% of the total landings of oystercultured in Japan) can provide suitable substrate for the moonjellyfish polyps. We conclude that a combination of environ-mental factors such as available substrate for the polyps andhigh retention within the northern bay, in addition to theirbiological features (strong tolerance to low DO conditions),enhances the moon jellyfish population in the northern part ofHiroshima Bay.

During most of the cruises, on the contrary, highest abun-dances of Japanese anchovy eggs were observed in the centralpart of Hiroshima Bay, where bottom DO was higher. The spatialdifference in the Japanese anchovy abundance between thenorthern and central parts of the bay indicates that: (1) Japaneseanchovy spawning was less intensive in and around the northernpart of Hiroshima Bay, and/or (2) egg mortality was higher innorthern part of Hiroshima Bay. It is likely that Japanese anchovyeggs were more vulnerable to predation by moon jellyfish in the

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northern part of Hiroshima Bay where moon jellyfish wereabundant during summer 2006 and 2007. Survival rates of bayanchovy Anchoa mitchilli eggs have been reported to markedlydecrease at DO< 3.0 mg L�1 (Chesney and Houde, 1989) althoughthere is no information on how low DO affects survival of Japa-nese anchovy eggs. In Chesapeake Bay, distribution of bayanchovy spawners was confined to the southern (near bay mouth)region when summer hypoxia prevailed in the northern (innerbay) region (Jung and Houde, 2004). We conclude that thenorthern part of Hiroshima Bay was less favorable for Japaneseanchovy spawning and/or egg survival due to low DO duringsummer 2006 and 2007.

4.3. Inter-annual difference in moon jellyfish abundance

Global warming and increase in temperature of shallowwaters have been suggested to potentially enhance moon jelly-fish blooms in coastal waters of Japan (Uye and Ueta, 2004). Inthe present study, however, moon jellyfish abundance washigher in 2006, when the temperature was lower during latesummer in Hiroshima Bay. Therefore, the inter-annual differencein moon jellyfish abundance cannot be explained by tempera-ture condition alone, and other composite factors may betterexplain the higher moon jellyfish abundance in 2006 in Hir-oshima Bay.

Among the environmental factors tested, difference in salinitywas most significant between the two years from July throughSeptember. The lower salinity in 2006 reflects higher precipita-tion during summer, which might be favorable for growth andsurvival of moon jellyfish. The total precipitation in HiroshimaCity from January through July (1120.0 mm) and from Julythrough September (689.0 mm) in 2006 was double the values in2007 (January through July: 550.0 mm; July through September:345.5 mm, Japan Meteorological Agency: http://www.jma.go.jp/jma/). Higher freshwater flow through the Ohta River, thenorthernmost part of Hiroshima Bay, increases primary andsecondary production within the bay by enhancing the nutri-tional input and estuarine circulation (Hashimoto et al., 2006). In2006, chlorophyll-a levels in the northern part of Hiroshima Baywere significantly higher than in 2007 (Kawaguchi et al.,unpublished data). Previous stomach contents analysis (Uyeet al., 2003) and recent stable isotope analysis (Shoji et al.,submitted for publication) showed copepods are the major preysource of moon jellyfish in the Seto Inland Sea. Increase infreshwater flow through the Ohta River should enhance growthand survival of moon jellyfish in the northern part of HiroshimaBay by increasing copepod production. We still lack ecologicalinformation on younger pelagic (ephyra) and benthic stages(polyp and strobila) of moon jellyfish in nature. Further inves-tigation on the effect of variability in freshwater flow throughthe Ohta River on primary and secondary production in Hir-oshima Bay in relation with growth and survival of moon jelly-fish ephyra, polyp and strobila stages would help to clarify themechanism of moon jellyfish bloom, distribution and inter-yearfluctuation in Hiroshima Bay.

Acknowledgements

We are grateful for the crews of RV Aki, HPTRI, for their helpduring the field sampling.

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