水産海洋研究

Bull. Jpn. Soc. Fish. Oceanogr. 74(1) 1–12, 2010

Structure of the marine ecosystem of the Ross Sea, Antarctica—overview and synthesis of the results of a Japanesemultidisciplinary study by Kaiyo-Maru and JARPA—
Mikio NAGANOBU1†, Hiroto MURASE2, Shigetoshi NISHIWAKI2, Hiroki YASUMA3,
Ryuichi MATSUKURA3,Yoshimi TAKAO4, Kenji TAKI5, Tomonari HAYASHI5,
Yuko WATANABE6, Takashi YABUKI7, Yasuo YODA8, Yoshifumi NOIRI3,
Mariko KUGA3, Kohei YOSHIKAWA9, Nobuo KOKUBUN10, Tetsuo IWAMI11,
Kiyoshi ITOH12, Mutsuo GOTO2, Tatsuya ISODA2, Koji MATSUOKA2,
Tsutomu TAMURA2 and Yoshihiro FUJISE2
A multidisciplinary marine ecosystem study in the Ross Sea region of the Antarctic was conducted in austral summer in
2004/05 as a Japanese national program. A fisheries research vessel, Kaiyo-Maru, and JARPA (the Japanese Whale Research Program under Special Permit in the Antarctic) vessels were engaged in the study. The main focus of the study
was to elucidate the ecological interactions among biological organisms, especially between krill and whales. The surveys consisted of (1) oceanographic observations, (2) net sampling of planktons and fishes, (3) hydroacoustic surveys
of krill (4) sighting surveys of top predators (birds and cetaceans) and (5) sampling of Antarctic minke whales. To understand the distribution pattern of various biological organisms in relation to oceanographic conditions, a simple
oceanographic index, Mean TEMperature from the surface to 200 m (MTEM-200) was introduced. Species compositions changed against the north-south gradient of MTEM-200. For instance, Antarctic krill were mainly distributed inwaters between 0 and 1ºC while ice krill were distributed in waters colder than 1ºC. Humpback whales were mainly
distributed in the waters warmer than 0ºC. Antarctic minke whales were mainly distributed in waters around 1ºC in acontinental shelf slope frontal zone. Comparison of stomach contents of minke whales and net samples as well as results of spatial modeling suggested that oceanographic conditions played an important role in defining predator-prey relationships. The data collected through this multidisciplinary study provides vital information to understand the structure of the marine ecosystem in the Ross Sea where data are still sparse.
Key words: Balaenoptera bonaerensis, Euphausia superba, Euphausia crystallorophias, food web, environment-preypredator relationship

Received August 13, 2009; Accepted September 16, 2009.
1
Fisheries Research Agency, 15F Queen’s Tower B, 2–3–3 Minato Mirai,
Nishi-ku, Yokohama, Kanagawa 220–6115, Japan
2
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan
3
Hokkaido University, 3–1–1 Minatocho, Hakodate, Hokkaido 041–
8611, Japan
4
National Research Institute of Fisheries Engineering, 7620–7 Hasaki,
Kamisu, Ibaraki 314–0408, Japan
5
National Research Institute of Far Sea Fisheries, 2–12–4 Fukuura,
Kanazawa-ku, Yokohama, Kanagawa 236–8648, Japan
6
Tokyo University of Marine Science and Technology, 4–5–7 Konan,
Minato-ku, Tokyo 108–8477, Japan
7
Tohoku University, 6–3 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980–
8578, Japan
8
Tokai University, 3–20–1,Orido, Shimizu, Shizuoka, Shizuoka 424–
8610, Japan
9
University of Tokyo, 1–1–1, Yayoi, Bunkyo-ku, Tokyo 113–8657, Japan
10
National Institute of Polar Research, 10–3, Midoricho, Tachikawa,Tokyo 190–8518, Japan
11
Tokyo Kasei Gakuin University, 2600, Aihara, Machida, Tokyo 194–
0292, Japan
12
Environmental Simulation Laboratory, 2–4–1 Arajyukucho, Kawagoe,
Saitama 350–1124, Japan†naganobu@affrc.go.jp

Introduction
The large embayment with a vast area of continental shelf
makes the Ross Sea marine ecosystem unique among theAntarctic waters. The ecosystem on the continental shelf isrelatively known (Smith et al., 2007 for review). However,there is little information on the waters adjacent to the continental shelf. Because the continental shelf and the adjacent waters are generally connected through ocean circulation (Kwok, 2005), it is ideal to conduct a multidisciplinary
survey in these areas simultaneously to understand thestructure of the whole ecosystem of the Ross Sea. The R/V
Kaiyo Maru and the Japanese Whale Research Program
under Special Permit in the Antarctic (JARPA) research
vessels were used to conduct a multidisciplinary ecosystemsurvey jointly in the Ross Sea in the austral summer of
2004/05 as a Japanese national program.
Recently, large-scale, multidisciplinary surveys wereconducted in the Antarctic waters. An Australian national
multidisciplinary survey, BROKE (Baseline Research on—1—

Mikio NAGANOBU et al.

Oceanography, Krill and the Environment), was conductedin the Indian Ocean sector of the Antarctic in the australsummer of 1996 (Nicol et al., 2000). CCAMLR (the Commission for the Conservation of Antarctic Marine Living
Resources) conducted a multinational, large scale multidisciplinary survey in the Atlantic sector in the austral summerof 2000 (Watkins et al., 2004). The SO GLOBEC (theSouthern Ocean Global Ocean Ecosystems Dynamics) program also conducted a multinational, large scale multidisciplinary survey in the western Antarctic Peninsula mainly inthe austral autumn and winter of 2001 (Hofmann et al.,2004). Although large scale national ecological surveys
have been conducted (Ackley et al., 2003; Faranda et al.,1999), no large scale multidisciplinary survey including
surveys on cetacean has been conducted in the Ross Sea region in the past. Because ecological information is stillsparse in the Ross Sea, CCAMLR encouraged regional
studies in the Ross Sea (CCAMLR, 2007). The IWC (International Whaling Committee) also recommended that a
multidisciplinary survey in the Antarctic be conducted toelucidate the role of whales in the Antarctic marine ecosystem (IWC, 1998; Fujise et al., 2010).
The Japanese national program took a multidisciplinary approach to clarify the ecological structure of the Ross
Sea marine ecosystem. The main focus of the study was toelucidate the ecological interactions among biological organisms, especially between krill and whales. The surveys
consisted of (1) oceanographic observations, (2) net sampling of planktons and fishes, (3) hydroacoustic surveys on
krill (4) sighting surveys of top predators (birds andcetaceans) and (5) sampling of Antarctic minke whales.
The main objective of this paper is to provide an
overview of the results of the Japanese multidisciplinary
ecological study conducted in the Ross Sea region in the
austral summer of 2004/2005. As a synthesis of the results,
the relationship between distribution patterns of biologicalorganisms and oceanographic conditions are summarized.
A simple oceanographic index, Mean TEMperature fromthe surface to 200 m (MTEM-200), was used in this paper.
Overview of survey methods and results
General survey strategy
The survey was conducted in the Ross Sea region of the
Antarctic (Fig. 1; also see Naganobu et al., 2010). Northernand southern boundaries of the survey area were set at 60°S
and the Ross Sea Ice Shelf (approximately 78°S), respectively. Western and eastern boundaries were set at 160°E
and 160°W, respectively. Western and eastern boundaries of
JARPA were extended to 130°E and 145°W, respectively.
No survey was conducted in the area covered by sea ice because survey vessels were not icebreakers. The survey was
conducted from December 2004 to February 2005.

Figure 1. Transect lines and stations by research items of theR/V Kaiyo Maru during 26 December 2004 and 24 February
2005. The survey area was bounded on the west and east by
160°E and 160°W and the south of 60°S and north of approximately 78°S. The dashed line: 1000 m depth.

A total of six research vessels engaged in the survey.
A stern trawl type fisheries research vessel, Kaiyo Maru
(2630 GT, Fisheries Agency of Japan), conducted the following survey items: (1) oceanographic observations, (2)net sampling of planktons and fishes, (3) hydroacoustic surveys on krill (4) sighting surveys of birds. The JARPA fleet
consisted of five research vessels dedicated to the cetacean
surveys. One dedicated sighting vessel (SV), Kyoshin-maru
No. 2 (372GT), three sighting/sampling vessels (SSVs)
namely Yushin Maru (720GT), Yushin Maru No. 2 (747GT)
and Kyo Maru No. 1 (812.08GT), and one research baseship, Nisshin Maru (NM, 8030GT), were engaged in the
cetacean surveys. These six vessels operated concurrently
in the Ross Sea region so that a variety of data could be obtained for ecological studies. Kaiyo-Maru surveyed on longitudinal tracklines set on 165°E, 175°E, 180°, 175°W,
170°W and 165°W. Tracklines of JARPA were zigzag shapewith starting points selected randomly. Further details of
survey methods of the Kaiyo-Maru and JARPA were described in Anonymous (2006) and Nishiwaki et al. (2005;
2006), respectively.
Oceanographic observation
The oceanographic observations were conducted onboard—2—Structure of marine ecosystem of the Ross Sea

the Kaiyo-Maru and the Kyoshin-Maru No. 2 (Yabuki et al.,2010). The conductivity-temperature-depth profilers (CTD)
and the expendable conductivity-temperature-depth profilers (XCTD) were used to investigate the structure of watermass characteristics and circulation features. All of the
CTD data were collected with Sea Bird Electronics Co.
(SBE) CTDs (SBE-9plus) while all of the XCTD data was
sampled with Tsurumi Seiki Co. (TSK) XCTDs. Twentyfour 10-liter Niskin bottles were set for water sampling. Detailed observations of physical, chemical and biological parameters including phytoplankton (Furuya et al., 2010)
were conducted especially along the 175°E trackline.
Various water masses were identified in the survey
area (Fig. 1). Based on data in the literature (e.g., Carmack,
1990; Russo, 2000) the main water masses that occurred in
the present survey were generalized as follows: Seasonal
Summer Water with warm temperature and low salinity
over the summer seasonal thermocline (SSW), Winter
Water (WW) marked by a temperature minimum layer lessthan 0°C, Circumpolar Deep Water (CDW) indicating ahigh temperature and salinity maximum below the Antarctic Surface Water (ASW) and WW, Shelf Water (SW) specially designated as the Antarctic continental shelf such as
the Ross Sea, and Antarctic Bottom Water (ABW) indicating low temperature and high salinity above the bottom.
A simple oceanographic index, the Mean TEMperature structure from the surface to 200 m in depth (MTEM200) was also used to examine the relationships between
oceanographic conditions and distribution patterns of biological organisms. MTEM-200 can be used to summarize
the stratified water masses. Using MTEM-200 for the T-S
diagram, we showed separations of T-S lines with colors(Fig. 1). Black lines (1°C of MTEM-200) distributed in
the Antarctic Circumpolar Current zone. Red color lines (1°CMTEM-2000°C) distributed around the SBACC(Southern Boundary of the Antarctic Circumpolar Current)zone. Green color lines (0°CMTEM-2001°C) werelocated in the Antarctic Surface Water zone (ASW) stratified by SSW, WW and partly CDW. Blue color lines(1°CMTEM-2002°C) were located in continental
SW.
A vertical section of temperature is shown along the
175°E trackline from the surface to near the bottom (Fig.
2). The deepest area (4000 m) extended from 60°S to theRoss Ice Shelf located at around 78°S (Fig. 3). SBACC was
located at around 64°S. The Antarctic slope front (ASF)
was located at around 72°S.
Low salinity water of less than 33.8 and lower density
water of less than 27.2 kg · m3 occurred between 66°S–
70°S probably because of ice melt water. Salinity higher
than 34.2 and density higher than 27.6 kg · m3 occurred in
the surface layer (0–100 m) at the Antarctic Divergence

Figure 2. Temperature-Salinity diagram using all oceanographic
stations by the Kaiyo-Maru survey. Color lines classification:
black 1°C of MTEM-200, 1°Cred0°C, 0°Cgreen1°C and 1°Cblue2°C, respectively.

zone between 71°30S–73°S. High salinity and density
water suggested strong upwelling of CDW. Water in theRoss Sea was also of higher salinity and density suggesting
upwelling to the surface layer between 75°S–76°S. Salinity
higher than 34.7 and density higher than 27.9 kg · m3 occurred on the shelf bottom.
Net sampling of planktons and fishes
Zooplankton, krill, micronekton and fishes were sampled
by a Rectangular Midwater Trawl (RMT) with nominal
mouth opening of 8 and 1 m2 (18) and mesh size of 4.5
and 0.22 mm respectively (Roe and Shale, 1979). Sampling
was conducted at 26 stations along the three longitudinallines of 175°E, 180° and 170°W (Iwami et al., 2010; Taki etal., 2008; Taki et al., 2010; Watanabe et al., 2008: Watanabe et al., 2010). All stations were towed during the daytime and/or nights with the midnight sun except 4 stations
which were towed during night time with the same daytime
positions. The RMT8 nets were obliquely towed at approximately 2 knots and at 0.3 m · s1 hoisting speed. Nets weremainly down to 1000 m or near the sea bottom where thedepth was shallower than 1000 m. In this paper, samples
collected from the upper 200 m water column were considered. Collected samples were kept in 10% formalin—3—

Mikio NAGANOBU et al.

Figure 3. Vertical section of temperature along 175°E from thesurface to near bottom between 60°S and the Ross Ice Shelf.
The upper graph shows the mean of integrated temperature
between the surface and 200 m as an environmental index(MTEM-200; a dotted line frame in the bottom graph). SB:
the Southern Boundary of the Antarctic Circumpolar Current. SF: Shelf Front.

buffered seawater. In the laboratory, the samples were measured for wet weight, sorted into 17 taxonomic groups andenumerated. Gelatinous forms, salps and jellyfish, whichwere measured separately, were removed before measuring
the wet weight. The biomass and abundance were expressedin numbers per 1000 m3 of water filtered.
Chaethognaths (17%), Copepoda (62%), Euphausiaccea (9%) and Pteropoda (7%) were dominant taxa groups
by number in the survey area. A total of 33 fish species belonging to 14 families, Nemichthyidae, Bathylagidae,
Gonostomatidae, Stomiidae, Paralepididae, Scopelarchidae,
Myctophidae, Muraenolepididae, Macrouridae, Melamphaidae, Nototheniidae, Artedidraconidae, Bathydraconidae
and Channichthyidae, were identified. Bathylagidae andParalepididae were grouped into “mesopelagic fishes” andthe latter four families into Notothenioidei (Iwami et al.,2010).

Hydroacoustic survey
A multifrequency quantitative echosounder (EK 500, Simrad, Norway) was used to investigate the distribution andabundance of Antarctic krill (Euphausia superba and ice
krill (E. crystallorophias) (Murase et al., 2008; Yasuma etal., 2010). EK 500 a quantitative echosounder with operation frequencies of 38, 120 and 200 kHz were used by the
Kaiyo Maru survey. The same echosounder was also used
onboard the SV to record acoustic data. Data were recorded
and stored using Echoview (SonarData, Australia). The
ship steamed on track lines at the nominal speed of
10 knots. The ship carried out calibrations in the survey
area. All data were analyzed using Echoview. Echoes fromeuphausiids were discriminated from other backscattering
by taking the difference between the mean volume
backscattering strength (D MVBS) of 120 and 38 kHz.
D MVBS falling between 2 and 16 dB were classified as euphausiid (Hewitt et al., 2004). Although the acoustic properties of ice krill have not been well studied, it has been reported that the mean of D MVBS ice krill in the Ross Sea
was 16.33 dB (Azzali et al., 2006). Therefore it was considered that D MVBS values reported by Hewitt et al. (2004)
were appropriate to identify echoes from euphausiid. Because acoustical discrimination between Antarctic and ice
krill was difficult, species allocation of acoustic data wasbased on the samples from a RMT. Though the depth interval for the Kaiyo Maru survey were set from 15 to 500 m,
effective Nautical-area-backscattering coefficients (sA) values attributed to euphausiids came from the upper 250 m
depth. Nautical-area-backscattering coefficient (sA) by
species for every 1 n.mile of survey transect over the defined depth interval was calculated. Nautical-area-backscattering coefficients (sA) were converted to indivisuals densities by using target strength (TS). Biomasses were then calculated by using wet weight of an individual. Preliminary
estimates of the biomasses of Antarctic and ice krill southof 69°S were 1.40 (CV0.32) and 0.60 (CV0.18) milliont, respectively (Murase et al., 2008).
Seabird sighting survey
The seabird sighting survey was conducted onboard the
Kaiyo-Maru (Kokubun et al., 2010). The survey was conducted every hour in daytime while the vessel was steamingon the trackline. The duration of each observation was 15
minutes. The number of seabird by species within 300 m
strip width on one side of the vessel was counted in eachobservation period. A total of 5,861 individuals consistingof 21 species were observed. The most dominant seabirds
were Pachyptila spp., (24%) followed by Antarctic petrels(Thalassoica antarctica, 21%), slender-billed prions(Pachyptila belcheri, 19%) and snow petrels (Pagodroma
nivea, 7%). Distribution patterns differed between species.

—4—Structure of marine ecosystem of the Ross Sea

For example, slender-billed prions and snow petrel were
distributed in the northern and southern parts of survey
area, repetitively while Antarctic petrels were distributed in
the middle part.
Cetacean sighting survey
Four cetacean sighting survey vessels were operated in thesurvey area (Nishiwaki et al., 2005; Matsuoka et al., 2010).
The survey was conducted in daytime. The standard sighting survey method used for the IWC Antarctic cruises(Matsuoka et al., 2003) was adopted. Principally, the survey
was conducted in closing mode. The SV conducted passing
mode survey in addition to closing mode survey. When thesightings during Closing Mode (primary sightings) were
thought to be Antarctic minke whales, the vessels approached to primary sightings to confirm species as well as
the number of individuals in the schools. All sightings werepassed in Passing Mode and treated as primary sightings.
All sightings within 3.0 n.mile strip width on both sides ofthe vessels were recorded. Antarctic minke whale (Balaenoptera bonaerensis) was dominant over other baleen
whales: blue (B. musculus), fin (B. physalus) and humpback(Megaptera novaeangiae) whales. Killer whale (Orcinusorca) was dominant over other toothed whales: sperm (Physeter macrocephalus), Arnoux’s beaked (Barardius arnuxii)
and aouthern bottlenose (Hyperoodon planifrons) whales.
Two dolphins, specacled porpoise (Australophocaena dioptrica) and hourglass dolphin (Lagenorhynchus cruciger),were sighted though numbers were few. Antarctic minke
and killer whales were mainly distributed in waters south of
69ºS though they were found throughout the survey area.
Other cetaceans were mainly distributed in the northernpart of the survey area.
Preys of Antarctic minke whales
Antarctic minke whales were sampled randomly by SSVs
and their stomach contents were sampled and preserved forlaboratory analysis on NM (Tamura et al., 2010). Species
identification and the length measurement of the stomach
contents were conducted in the laboratory. Antarctic minke
whales fed on ice krill in the continental shelf area while
they fed on Antarctic krill north of continental break as previously reported by Ichii et al. (1998). The geographicaldistribution patterns of two krill species based on the stomach contents coincided well with the samples from the
RMT. Stomach content mass was high around the shelf
break (1000–3000 m bottom depth) in waters west of 180°.
Geographical distribution patterns of Antarctic and ice krill
and length distribution patterns of the two krill species inthe stomach contents were similar to those in the samples
from the RMT.

Synthesis: distributional ecology with reference tooceanographic conditions
To investigate the effect of the oceanographic structure on
distribution patterns of various biological organisms, anoceanographic index, Mean TEMperature from the surface
to 200 m in depth (MTEM-200), was used (Murase et al.,2010; Naganobu and Murase, 2010). In general, sea surface
temperature (SST) is used as an index to describe distribution patterns of biological organisms worldwide. However,
it is difficult to use SST as an environmental indicator in the
Antartic because the gradient of SST is limited to a narrow
range. Because krill fisheries are conducted mainly in the
0–200 m water column, MTEM-200 could be an appropriate environmental index to describe krill distribution. The
relationship between distribution patterns of krill and
MTEM-200 was initially considered by Naganobu and Hirano (1982, 1986). In fact, variation of macrozooplankton
community structure in the Indian Ocean sector of Antarctic (Hosie et al., 2000) and distribution of krill in the entire
Antarctic Ocean (Naganobu et al., 2008) were described by
MTEM-200.
Distributions and the abundance and composition of
zooplankton at each station were compared with MTEM200 (Fig. 4). Overall, two groups were separated by the1°C isopleth. One group located offshore in water
warmer than 1°C was characterized by high abundance(1000 individuals per 1000 m3) and the dominance ofcopepods (52–93%; mean 68%) and chaetognaths (5–42%;
mean 22%), while the other group located in the Ross Sea
in water colder than 1°C was characterized by low abundance (500 individuals per 1000 m3) and the dominance

Figure 4. Distribution of the integrated abundance of zooplankton (individuals per 1000 m3) in the water column (0–200 m)
and percent composition of major taxa with MTEM-200.

—5—Mikio NAGANOBU et al.

Figure 5. Distribution of Antarctic krill in routine trawls n thewater column (0–200 m) with MTEM-200. Abundance circles are proportional in area to density (individuals per1000 m3). sampling sites where no specimens were caught.

of pteropods (11–93%; mean 35.5%) and euphausiids(0–66%; mean 25%). At the stations near the edge of the
Ice Shelf, pteropods were dominated (93%) with a very
high abundance at the southernmost station along the
175°E trackline. Pteropods and other taxa occurred withlow abundance at southernmost station along the 170°W
trackline.
The results of routine oblique trawls of RMT8 (0–200m) suggested that Antarctic krill was distributed in the waters between the MTEM-200 of 0°C and 1°C (the ASWzone) except a station in the waters of 1.5°C on the Ross
Sea continental shelf (Fig. 5). On the other hand, geographical distribution of ice krill was limited to waters lower than
1°C on the continental shelf (the SW zone) (Fig. 6). No
occurrences were found in the warmer waters north of the1°C isopleth. Maximum density appeared near the Ross
Ice Shelf at 78°30S in the southeast part.
Distributions, abundance and composition of fishes at
each station are shown with MTEM-200 on a map (Fig. 7).
Horizontal distribution patterns of many species seem to beclearly correlated to hydrographic features and different
water masses, although the quantitative analysis has notbeen completed yet. Three groups were separated by
MTEM-200 of 0 and 1°C. Occurrences of notothenioid
fishes were limited to the waters lower than 1°C on theRoss Sea continental shelf except for one station in the waters between 0 and 1°C. Myctophidae were distributed in

Figure 6. Distribution of ice krill in routine trawls in the watercolumn (0–200 m) with MTEM-200. Abundance circles areproportional in area to density (individuals per 1000 m3).
sampling sites no specimens where were caught.

Figure 7. Distribution of the integrated abundance of fishes (individuals per 1000 m3) in the water column (0–200 m) andpercent composition of major taxa with MTEM-200.

—6—

the oceanic waters higher than 0°C and mesopelagic fishgroups were mostly limited to the waters between 0 and1°C in the ASW zone. Among these, Pleuragramma
antarcticum of Nototheniidae was dominant both in termsof abundance and frequency. In the waters colder than
1°C on the shelf, P. antarcticum was characterized by ahigh abundance (max 100%, mean 66%). The distribution
pattern of the fish taxa generally corresponds with MTEM200.

Structure of marine ecosystem of the Ross Sea

Figure 8. Distribution and density (g/m2) of krill; Antarctic
krill (Euphausia superba) and ice krill (E. crystallorophias)
by the acoustic survey by Kaiyo Maru with MTEM-200.

We compared the results of krill distribution by
acoustic measurement with MTEM-200 (Fig. 8). High density of Antarctic krill was observed in the northwestern waters in the survey area. The high density areas of Antarctic
krill approximately coincided with the sharp gradient waters of MTEM-200 around 0.5°C, which indicated afrontal zone between southward warmer advection andnorthward colder waters. On the other hand, ice krill wasobserved in the shelf waters of MTEM-200 less than 1°C
on the continental shelf. The high density areas of ice krill
tended to distribute in small clockwise eddies and near theice shelf along 175°W.
Figure 9 shows the distribution of Antarctic minke
whales (number of schools sighted per 100 n. miles) with
MTEM-200. The distribution of Antarctic minke whales
ranged through south of 1°C. Relatively high density index
of Antarctic minke whales was concentrated at the steep
gradient around 0°C and south of 1°C. In addition, thedensity index was higher in the tongue-shaped southward
advection along 175°W between 0 and 1°C and the
tongue-shaped northward advection along 170°W. The relative high density occurred in the same area as the tongueshaped advection with a north-south meander. High density
indexes were especially concentrated in the waters with eddies around 75°S along 170°W. Figure 10 shows the distribution humpback whales (number of schools sighted per 100
n. miles) with MTEM-200. The distribution of humpback
whale mainly ranged through north of 0°C. The high density indexes were concentrated at the frontal zone around
0°C.

Figure 9. Distribution of Antarctic minke whale with MTEM200. Abundance circles are proportional in area to density(number of schools sighted per 100 n. miles). A black mark( · ) indicates no sighting.

Figure 10. Distribution of humpback whale with MTEM-200.
Abundance circles are proportional in area to density (number of schools sighted per 100 n. miles). A black mark ( · ) indicates no sighting.

Krill-cetacean distributional relationship with reference to oceanographic conditions in the waters south of
69ºS was investigated by using a generalized additivemodel based spatial model (Murase et al., 2010). MTEM200 was selected as a covariate in the model to describe the
distribution pattern of Antarctic minke whales. The results
indicated that the density of Antarctic minke whales related—7—

Mikio NAGANOBU et al.

to MTEM-200 positively within the range of MTEM-200
between 1.5 and 1°C. Latitude, longitude, mean salinityfrom 0–200 m in depth (MSAM-200) and density of two
krill species were also selected to model the number ofschools of Antarctic minke whales. The results showed thatpredicted school counts of Antarctic minke whales were
low where ice krill was distributed while they were highwhere Antarctic krill was distributed.
Discussion
The Japanese multidisciplinary ecosystem survey in theRoss Sea in 2004/2005 collected numerous data sets to understand the components of the ecosystem (e.g. oceanographic conditions and distribution patterns of biologicalorganisms). The findings will deepen the understanding ofeach field of science especially in the data sparse area, theRoss Sea. In addition, the multidisciplinary data set allowed
us to understand the distributional ecology in the Ross Sea.
The findings were summarized in a conceptual map (Fig.
11). The oceanographic conditions generally indicated threewater types; the ACC zone with MTEM-200 warmer than0°C, the ASW zone between 0 and 1°C and the SW zone
below 1°C. The eastward arrow indicates a strong eastward flow in the ACC. A north-south meandering shape
was indicated as shown with MTEM-200 0 and 1°C isopleths. Southward arrows indicate putative flows in the
ASW. Clockwise and anticlockwise arrows suggest regionaleddies with upwelling and downwelling in the SW. Recent
oceanographic results of Picco et al. (2000) and Russo(2000) for current patterns in the SW near the ice shelfwere inferred in the figure.
Figure 11 shows that Antarctic krill were mainly distributed in the waters with MTEM-200 between 0 to 1°C,
which approximated with the area of the ASW zone, andslightly extended into the waters of less than 1°C, Shelf
Water in the SW zone. Ice krill were mainly distributed in
the waters less than 1°C (SW) but not more than 1°C
(ASW). Distributions of other zooplankton and fishes also
showed segregated patterns with MTEM-200. Humpback
whales were mainly distributed in the waters more than0ºC, approximating with the ACC zone, with high density
around 0ºC near the SBACC (Orsi et al., 1995). Antarctic
minke whales were mainly distributed in the ASW and SW
zones with a high density around 1°C in the continental
shelf slope frontal zone. The relationship between distributions of Antarctic and ice krill, and Antarctic minke whales
with MTEM-200 was also detected in the spatial model(Murase et al., 2010). The results suggested that MTEM200 can be used as an environmental index along with traditional water mass/front classification to describe thestructure of the Antarctic marine ecosystem.
As sophisticated survey equipment has been devel-

Figure 11. A summarized image relating water mass and circulation pattern of the oceanic surface layer, the distribution
and abundance of krill and baleen whales. Each line on the
map indicates the Southern boundary (of Antarctic Circumpolar Current) and MTEM-2000 and 1 (°C). The 1000 m
depth contour is shown. Arrows show the flow pattern. ACC:
Antarctic Circumpolar Current zone. ASW: Antarctic Surface Water zone. SW: Shelf Water zone. SBACC: Southern
Boundary of ACC (Orsi et al., 1995). ANK: Antarctic Krill.
ICK: Ice Krill. MIW: Antarctic Minke Whale. HUW: Humpback Whale.

—8—

oped, it seems that use of a simple oceanographic index
such as MTEM-200 is not necessary because more detailed
data can be obtained using such equipment. However, as in
the case of Hosie et al. (2000) in the BROKE survey,
MTEM-200 is useful to understand the distribution patternof biological organisms qualitatively in relation to watertemperature gradients in both longitudinal and latitudinal
directions. MTEM-200 can also be used as an environmental index in quantitative analyses. Murase et al. (2008) used
MTEM 200 to identify the boundary of the distribution of
Antarctic krill and ice krill for biomass estimations using
acoustic data in our cooperative surveys.
Though some modeling work was conducted by using
the data obtained through our survey (e.g. Murase et al.,2010), the ecological interactions among biological organisms in the Ross Sea have not been fully investigated quantitatively at this stage. The results of this study suggested

Structure of marine ecosystem of the Ross Sea

that habitats of predators overlapped each other. It has beenreported that the presence of Antarctic minke whale near
the colonies of Adélie penguins (Pygoscelis adeliae) could
be the driving force of the prey switching from ice krill to P.
antarcticum of Adélie penguin because feeding of Antarctic
minke whales could cause local depletion of ice krill (Ainley et al., 2006). Many notothenioid fishes in the Ross Sea
relied on Antarctic and ice krill as food in the Ross Sea
shelf waters (Mesa, 2004). However, the magnitude of theseinteractions with prey has not been investigated quantitatively. The concept of Ecosystem approach to fisheries(EAF) has attracted interest from the international community in recent years. The term, EAF, was adopted by the
FAO Technical Consultation on Ecosystem-based Fisheries
Management held in Reykjavik in 2002 (FAO, 2003).
Ecosystem models play an important role as operating
models to test the management strategies of EAF. The modeling of the southern ocean ecosystem has a long history(Hill et al., 2006 for review). The CCAMLR has tried to develop ecosystem models for the purpose of krill fisheries
management in the in the Scotia Sea-Antarctic Peninsula
region though they have not yet been applied to management (Hill et al., 2007). No ecosystem model for fisheries
management is available in other regions of the Antarctic at
this stage largely because of lack of data. Ecosystem models require species interaction sub-models such as the functional responses of predators (amount of prey consumed perunit area per unit time by predators). To estimate functional
response, biomass of prey and amount of prey consumed by
predators must be obtained simultaneously. Data obtained
through our survey allows us to construct ecosystem models and their sub-models. Therefore efforts in future research should focus in this direction. The Japanese multidisciplinary study by Kaiyo-Maru and JARPA serves as astarting point toward EAF for the Antarctic. Continuationof the program is crucial to collect data for ecosystem models.
Changes in oceanographic/climatic conditions in the
Antarctic have been reported in recent years. There aremany oceanographic papers on the variability and/or warming of the surface layer of the Indian Ocean sector (Aoki etal., 2005), the surface layer of the Antarctic Peninsula waters (Marshall et al., 2006) and the whole scale of the
Antarctic/Southern Ocean (Gille, 2002). Freshening of the
Antarctic Bottom Water in the Ross Sea (Jacobs et al.,2002) and the Indian and Pacific Ocean (Rintoul, 2007;
Ozaki et al., 2009) has also been reported. Although thelong term relationship between krill and oceanographic/climatic conditions was investigated (e.g. Atkinson et al.,
2004; Brandon et al., 2004; Kawaguchi et al., 2006; Loeb etal., 1997; Murphy et al., 2007; Naganobu et al., 1999), theeffort was concentrated mainly in the data rich region, theAntarctic Peninsula region. To assess the impact of oceanographic/climatic conditions on the Ross Sea ecosystem inlong time scales for monitoring purposes, multidisciplinary
surveys should be conducted on a regular basis.
Acknowledgements
The authors would like to thank Captain J. Doi and all
the crew of the R/V Kaiyo Maru and captain D. Tohyama
and all the crew of the Nisshin Maru fleet for supporting
the surveys in the Ross Sea waters. The authors express
their thanks to Drs. K. Miyashita, K. Kuma, K. Hanawa, K.
Furuya, T. Ishimaru, A. Takahashi, S. Sawamoto and K.
Kutsuwada who support the work of their students. We also
appreciate Ms. A. Matsumoto, Mr. Y. Matsuo and Mr. J.
Kondo for assisting with the manuscript preparation. The
work was supported in part by the Ministry of Agriculture,
Forestry and Fisheries of Japan, the Fisheries Agency ofJapan, the Fisheries Research Agency of Japan, the Instituteof Cetacean Research and Kyodo Senpaku Kaisha, Ltd. We
thank these institutions for their support. This manuscript isbased on the outcome of a symposium titled “The Ross Sea
marine ecosystem-frontline of field survey and the prospect
to the future” held in Tokyo, 5 April 2009. The symposium
was sponsored by the Oceanographic Society of Japan, theJapanese Society of Fisheries Science and Japanese Societyof Fisheries Oceanography. We thank these societies forproviding such an opportunity.
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Mikio NAGANOBU et al.

南極ロス海の海洋生態系構造―開洋丸／ JARPA 共同による生態系総合調査の概要と結果総括―

瀧永延幹男 1†，村瀬弘人 2，西脇茂利 2，安間洋樹 3，松倉隆一 3，高尾芳三 4，
憲司 5，林 倫成 5，渡邉侑子 6，矢吹 崇 7，依田康男 8，野入善史 3，久我真理子 3，
吉川康平 9，國分亙彦 10，岩見哲夫 11，伊藤喜代志 12，後藤睦夫 2，磯田辰也 2，
松岡耕二 2，田村 力 2，藤瀬良弘 2

2004/05 年南半球夏季に南極ロス海域において，特にオキアミと鯨類の関係解明を目的とした，生態系総合調査を実施した．水産庁開洋丸と南極海鯨類捕獲調査（JARPA）船団が共同して調査を行った．（1）海洋観測，（2）RMT によるプランクトンと魚類採集，（3）計量魚探によるオキア
ミ現存量調査，（4）高次捕食者（鳥類と鯨類）の目視調査，
及び（5）クロミンククジラ標本採集の 5 項目を調査対象とした．多様な生物群集の分布と海洋条件との関係を理解するために，海洋学的指数として表面から 200 m までの水温平均値（MTEM-200）を用いた．南北の水温傾度に沿って，プランクトン，魚類，鳥類及び鯨類それぞれの種組成が変化することが明らかとなった．生態系総合調査を通して得られたデータは，未知が多いロス海の海洋生態系の構造を理解するための基礎となる．

1

水産総合研究センター：〒 220–6115 神奈川県横浜市西区みなとみらい 2–3–3
2日本鯨類研究所：〒 104–0055 東京都中央区豊海町 4–53北海道大学：〒 041–8611 北海道函館市港町 3–1–1
4
水産工学研究所：〒 314–0408 茨城県神栖市波崎 7620–75
遠 洋 水 産 研 究 所 ： 〒 236–8648 神 奈 川 県 横 浜 市 金 沢 区 福 浦
2–12–46東京海洋大学：〒 108–8477 東京都港区港南 4–5–7
7
東北大学：〒 980–8578 宮城県仙台市青葉区荒巻青葉 6–3
8
東海大学海洋学部：〒 424–861 静岡県静岡市清水区折戸 3–20–1
9
東京大学：〒 113–8657 東京都文京区弥生 1–1–1
10
国立極地研究所：〒 190–8518 東京都立川市緑町 10–3
11東京家政学院大学：〒 194–0292 東京都町田市相原 2600
12
環境シミュレーション研究所：〒 350–1124 埼玉県川越市新宿町 2–4–1†
E-mail: naganobu@affrc.go.jp— 12 —