SC/D06/J24

Distribution patterns and biomasses of Antarctic krill(Euphausia superba) and ice krill (E. crystallorophias) withreferece to Antarctic minke whales in the Ross Sea in 2005
using Kaiyo Maru-JARPA joint survey data
HIROTO MURASE1, TSUTOMU TAMURA1, KOJI MATSUOKA, SHIGETOSHI NISHIWAKI1, HIROKI YASUMA2,RYUICHI MATSUOKRA2, TAKASHI. YABUKI3, YOSHIMI TAKAO4, KENJI TAKI5, TOMONARI HAYASHI AND
MIKIO NAGANOBU5
1

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
Field Science Center for the Northern Biosphere, Hokkaido University, 3-1-1 Minato, Hakodate,
Hokkaido, 041-8611 Japan3
Department of Geophysics, Graduate School of Science, Tohoku University, 6-3 Aoba, Aoba-ku,
Sendai 980-8578 Japan
4
National Research Institute of Fisheries Engineering, Ebidai, Hasaki, Kashima,Ibaraki 314-0421,
Japan
5
National Research Institute of Far Seas Fisheries, 2-12-4 Fukuura, Kanazawa, Yokohama,
Kanagawa 236-8648, Japan
2

ABSTRACTDistribution patterns and biomasses of Antarctic and ice krill in the Ross Sea in australsummer in 2005 were studied using a multi-disciplinary survey data set combining cetacean,krill and oceanography data. Two research vessels, KM and KS2, conducted the echosounder survey independently in same area. Distribution patterns and length frequency
information for two species were collected from RMT and stomach contents of Antarctic
minke whale. Ice krill distributed on the continental shelf region (shallower than 1000m
water depth). In contrast, Antarctic krill distributed mainly in the oceanic waters wherewater depth is deeper than 1000m though it distributed on the continental shelf where theintegrated mean water temperatures between 0-200m was higher than -1°C. Distribution
pattern of Antarctic minke whales was related to the distribution patterns of Antarctic krill.
School sizes of Antarctic minke whales were large where the densities of Antarctic krill were
high. Distribution pattern of Antarctic minke whales in the Ross Sea could be regulated by
distribution patterns of Antarctic krill. The Ross Sea was stratified into two strata based on
the distribution patterns of two krill species to estimate their biomasses. Biomass densities
of Antarctic krill using KM and KS2 data were estimated as 5.36±7.45 and 2.64±2.35 g/m2,
respectively. Biomass densities of ice krill using KM and KS2 data were estimated as
3.44±1.96 and 1.56±0.89 g/m2, respectively. Because there was no significant difference
between the biomass density estimates from both vessels, two data sets were combined toestimate the biomass. The biomasses of Antarctic and ice krill in this study were estimatedas 1.46 (CV=0.32) and 0.82 (CV=0.18) million t, respectively. Though KS2 had conductedecho sounder survey without net sampling since 1998, the results of inter-ship comparison of
biomass estimates in this study suggested estimates by KS2 in the past were robust in termsof acoustical units. The study suggested that Antarctic minke whales could be considered asa biological sampler to monitor krill which plays key role in the Antarctic ecosystem
INTRODUCTION
It has long been known that two euphausiid species, Antarctic krill (Euphausia superba) and ice
krill (E. crystallorophias) distribute in the Ross Sea and its adjacent area (Marr, 1962). More resent
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studies suggested that those species showed clear habitat segregation with respect to the bottom
topography (Sala et al, 2002; Azzali et al., 2006). Antarctic and ice krill played important role in theRoss Sea ecosystem as preys. It was reported that many notothenioid fishes relied on two
euphausiid species as food in the Ross Sea shelf waters (Mesa, 2004). No baleen whales except blue
(Balaenoptera musculus) and Antarctic minke (B. bonaerensis) whales distributed in the Ross Sea(Matsuoka et al., 2005). Encounter rates of Antarctic minke whale in the Ross Sea was high incomparison with other Antarctic waters (Kasamatsu et al, 1998). Though blue whale distributed in
the area, the population level is still low (Matsuoka et al., 2005). Antarctic minke whales mainly fed
on ice krill in the continental shelf of the Ross Sea while they fed on Antarctic krill in the waters
north of the continental shelf (Ichii et al., 1998). It was hypothesized that the presence of Antarctic
minke whale near the colonies of Adélie penguin (Pygoscelis adeliae) could be the driving force ofthe prey switching from ice krill to Antarctic silverfish (Pleuragramma antarcticum) of Adélie
penguin because feeding of Antarctic minke whales could cause local depletion of ice krill (Ainley
at al., 2006). Given direct evidence of competition for food between krill predators, concurrent
studies of krill and the predators is necessary to understand their relationship in the Ross Sea marine
ecosystem. To study the magnitude of interactions among krill predators, biomass estimates ofAntarctic and ice krill are critically important. Ichii et al. (1998) pointed out that paradox of highdensity of Antarctic minke whales in low supply in the Ross Sea and they termed it as “Paradox in
the Ross Sea” but the reason is still to be tested. To solve those ecologically important questions,multi-disciplinary study combining surveys on cetacean, krill and oceanography should be
conducted.
The Japanese Whale Research Program under Special Permit in the Antarctic (JARPA) has
been conducted during the austral summer every year since the 1987/1988 season. One of the
primary objectives of the JARPA is “Elucidation of the role of whales in the Antarctic marine
ecosystem through the study of whale feeding ecology”. The JARPA interim review meeting took
place in May, 1997. In the meeting, it was pointed out that concurrent studies on the distribution andabundance of prey species was required to achieve the objective. In addition, it was also pointed outthat process oriented studies would be useful which integrated information from physical andbiological oceanography with zooplankton and predator studies at meso-scale. In response to thosesuggestions, a multi-disciplinary study combining surveys on cetacean, krill and oceanography was
conducted in the Ross Sea in 2005 involving six research vessels. In this study, two vessels
conducted acoustic surveys to estimate the biomasses of Antarctic and ice krill in the Ross Sea.
Samples of krill were collected both from net and stomach contents of Antarctic minke whales. This
paper presents the results of krill biomass estimation in the Ross Sea in 2005. In this paper,
following points were examined: 1) comparability of krill biomass estimates using echo sounders
from two independent vessels, 2) utility of krill samples from stomach contents of Antarctic minke
whales in the study of krill ecology and 3) paradox in the Ross Sea
MATERIAL AND METHODS
Survey area, period and vessels
The research area of the Kaiyo Maru-JARPA joint survey was in the Ross Sea region of the
Antarctic. The Ross Sea in this survey was defined as the water south of 69°S and, approximately
between 165°E and 155°W. The joint survey was conducted from 14 January to 15 February 2005.
Two vessels, Kyoshin Maru No. 2 (KS2: 368GT) and Kaiyo Maru (KM: 2630 GT), conductedacoustic survey to estimate krill biomass. Tracklines of KM was set on four longitudinal lines:
175°E, 180°, 175°W, 170°W and 165°25’W. Zigzag trcaklines was set for KS2 in the survey area.
KS2 and KM conducted oceanographic observations. KS2 also conducted cetacean sighting survey
while it steams on the tracklines. KM conducted zooplankton sampling using a Rectangular
Midwater Trawl with 8m2 and 1m2 mouth opening (RMT8+1). Three vessels, Yushin Maru (YS1:
720GT), Yushin Maru No.2 (YS2: 747GT) and Kyo Maru No. 1 (K01: 812GT), were engaged in thewhale survey consisting of sighting and sampling of whales. Those three vessels were terms as
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sighting and sampling vessels (SSVs). Stomach contents of sampled Antarctic minke whales were
examined on the research base ship, Nisshin Maru (NM: 7,659GT).
Oceanographic observations
Oceanographic observations were conducted by KM and KS2 to calculate Integrated TEMperature
Mean water depth from surface to 200m (ITEM-200). Initial idea of ITEM-200 was derived from
Naganobu and Hirano (1982, 1986). They suggested that an integrated water temperature from 0 to
200m ( Q 200 in their term) could be used as an index of distribution patterns of Antarctic krill. The
index expresses not only absolute value but also gradient of temperature pattern reflecting seasonalchange in surface layer. Recently, ITEM-200 was used as an indicator of macorzooplankton
community in the Antarctic (Hosie et al., 2000). ITEM-200 was extrapolated horizontally using
kriging methods. KM and KS2 recorded water temperature profiles using ConductivityTemperature-Depth profiler (CTD, SBE-9-Plus (KM) and SBE-19 (KS2), Seabird, USA) and
expendable CTD (XCTD, Tsurumi Seiki Co., Japan). KM conducted oceanographic observations
south of 60°S so that overall oceanographic conditions surrounding the Ross Sea could be observed.
To see the conditions of sea ice at the time of survey, satellite derived sea ice concentration data,
DMSP SSM/I daily polar gridded sea ice concentrations, were used (Comiso, 1990).
Sampling and analysis methods of krill
To acquire distribution patterns and length frequency data of krill, samples from the stomach
contents of Antarctic minke whales and the RMT8+1 were used. Depth, temperature and salinity
sensors as well as flow meter were equipped with RMT8+1. RMT8+1 was obliquely towed by KM
at the routine sampling stations. Sampling depth was from surface to 1000m water depth. Nominal
towing speed was 2.0konts. If the bottom depth was shallower than 1000m, sampling depth wasfrom surface to 50m above the bottom. Species identification and weight and length measurement
were conducted on KM. Minimum of 100 individuals were examined. If the sampled individuals
were less than 100, all individuals were examined. Individual density of Antarctic was caluculated
at each sampling station.
Antarctic minke whales were sampled randomly by SSVs and their stomach contents were
sampled and preserved in a 10% buffered formalin solution for laboratory analysis on NM. Baleen
whales have a four-chambered stomach system but because the forestomach contents only gives
information about the last feeding event (e.g. Lindstrøm et al. 1997), contents from only theforestomach were used in this study to avoid including prey species of whales which might havebeen consumed outside the survey area. Furthermore, only undigested krill in the stomachs wasused in this analysis. Species identification and length measurement were conducted at thelaboratory. As in KM, minimum of 100 individuals were examined. If the sampled individuals wereless than 100, all individuals were examined.
Total length from the tip of the rostrum to tip of the uropod (Standard 1 as described in Mauchiline,1980) was measured. Length frequency analysis of Antarctic and ice krill was conducted according
to Macdonald and Picher (1979) using a package of R Software (R Development Core Team, 2006),
mixdist, (available from http://www.math.mcmaster.ca/peter/mix/mix.html). istribution maps weredrawn using a GIS, Marine Explorer version 4 (Enviromental Simulation Laboratory Co. Ltd,
Japan).
Estimation of krill biomass
EK500 scientific echo sounders (Simrad, Norway) with operation frequencies of 38 and 120 kHz
were used by KM and KS2 to collect acoustic data. Data were recorded and stored using either
BI500 (KS2: Simrad, Norway) or Echoview (KM: SonarData, Australia). Both ships steamed on
tracklines at the nominal speed of 10 knots. Each ship carried out calibration at least once in thesurvey area. All data were analyzed using Echoview version 3.00. Echo from euphausiid wasdiscriminated from other backscattering by taking the difference between the mean volume
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backscattering strength (ΔMVBS) of 120 and 38 kHz. ΔMVBS falling between 2 and 16 dB wasclassified as euphausiid (Hewitt et al., 2004). Because acoustical discrimination between Antarctic
and ice krill was difficult, species allocation to acoustic data was based on the samples from
RMT8+1 and stomach contents. Mean backscattering area per square nautical mile of sea surface(SA) by species for every 1 n.mile of survey transect over defined depth interval is calculated byfollowing formula;
S A = 4πr021852 2 ∫rr2 sV dr (1

m2
n.mi 2)where r is depth from the sea surface, r0 = 1m representing the reference range for backscatteringstrength. Depth interval was set 10 to 250m for KS2. Though the depth interval of KM was set from15 to 500m, effective SA values attributed to euphausiid came from upper 250m depth. Krillbackscattering cross section area (σ) was calculated with the following formula based on Antarctic
krill target strength (TS) described by Greene et al. (1991):
σ = 4πro2 10 −12.745 l 3.485

where, l was standard length of Antarctic krill. Because no backscattering cross section area wasavailable for ice krill, same formula as Antarctic krill was used. Average area biomass density ( ρ )for each species is calculated as follows;
SA

ρ=∑

σ

f iWi

where fi is frequency distribution of ith length class. Krill wet weight (w)-length(l) relationship ofkrill was calculated based on RMT samples. Following procedures were adopted from Jolly and
Hampton (1990). Weighted mean of SA of each block was;Nk

∑ S Aki (nk i )S Ak = i =1 N

∑ nk ik

i =1where, S Ak = mean SA in kth block, N k = number of transects in kth block, S Aki = mean SA on the ithtransect in kth block and nk i = number of 1 n. mile averaging intervals on the ith transect in kth
block. In this formula, each transect was regarded as a single biomass density sample. Thenvariance of S Ak was calculated with the formula (Jolly and Hampton, 1990);
Nk

Var ( S Ak ) =∑ (S Ak i − S Ak )2 nk i 2

N k i =1
Nk −1

⎞
⎛ Nk
⎜ n ⎟
ki ⎟
⎜
⎠
⎝ i =1

2∑

S A was converted to ρ using above motioned formula. Biomass was estimated as;
Bk = Ak ρ kwhere, Bk is density biomass in k block and Ak is area of k block. Variance of Bk was calculated withfollowing formula;var(Bk ) = Ak 2 var(ρ k ) .
Coefficient of variation of Bk was calculated as;
BB

var(Bk )CV ( Bk ) =Bk.

Overall mean density of euphausiid (Antarctic and ice krill) in the survey area was calculated as;N∑ Ak ρ kρ = k =1N

∑ Ak

k =1

Variance of ρ was calculated as;

4.

SC/D06/J24
N

Var ( ρ ) =2∑ Ak Var ( ρ k )

k =1

⎛N
⎞
⎜⎜ ∑ Ak ⎟⎟⎝ k =1 ⎠2.

Overall biomass was calculated as;N

B = ∑ Ak ρ k
k =1

Variance of B was;N

Var ( ρ ) = ∑ Ak2Var ( ρ k ) .
k =1

CV of B was;CV B. =

Var ( B)
B.

Sighting survey of cetaceans
KS2 was engaged in the sighting of the minke whales as well as other large baleen whales over theentire area. Principally the KS2 conducted the survey 8 hours per day by passing mode and 4 hours
per day by limited closing mode. The sighting survey was conduced during diurnal hours.
RESULTS
Oceanographic observations
CTD casts were conducted at 15 and 34 stations (total=49 stations) by KS2 and KM, respectively.
XCTD casts were conducted at 21 and 71 stations (total=92 stations) by KS2 and KM, respectively.
A map of ITEM-200 was shown in Fig. 1. Sharp north-south gradient of ITEM-200 was observed.
In the Ross Sea, ITEM-200 was lower in eastern part than it in western part.
Distribution patterns and length frequencies of Antarctic and ice krill
RMT sampling was conducted at 14 stations. Krill samples in stomach contents of Antarctic minke
whales were collected at 27 locations. Sampling of stomach contents had wider geographicalcoverage than RMT. The samples from RMT8+1 and stomach contents of Antarctic minke whales
indicated that Antarctic and ice krill showed different distribution patterns in the Ross Sea (waters
south of 69°S) with respect to the bottom topography (Fig. 2 and 3). Ice krill distributed on thecontinental shelf region (shallower than 1000m water depth). In contrast, Antarctic krill distributedin the oceanic waters where water depth is deeper than 1000m though it distributed on thecontinental shelf where ITEM-200 was higher than -1°C. Based on their distribution patterns, thesurvey area was divided into two strata for the acoustical biomass estimation for two species.
Because length frequency pattern at each RMT and stomach content sampling point was
highly heterogeneous, all sampled krill individuals were pooled to construct representative length
frequency for each euphausiid species in the survey area. Age group compositions of Antarctic andice krill were summarized in Table 1. Length frequency histograms of Antarctic and ice krill wereshown in Fig. 3 and 4, respectively. Antarctic krill consisted of 2 age groups in both RMT andstomach content samples although proportion of older animals (4+) was high in stomach content.
Ice krill consisted of 3 age groups though proportion of older animals (3+) was high in the stomach
content samples. Length(l)-weight(w) relationships of Antarctic and ice krill were estimated as
w=0.0153l+0.0261 and w=0.0018l+0.0448, respectively. Mean length of Antarctic and ice krill
using stomach content samples were 45.0±3.8(SD) (N=1526 individuals from 17 stomachs) and
22.7±6.0(SD) mm (N=1100 individuals from 11 stomachs), respectively. Corresponding wetweights of Antarctic and ice krill were estimated as 0.175 and 0.086 g, respectively.
Estimated biomass of Antarctic and ice krill
Distribution patterns of Antarctic and ice krill using acoustic data collected by KM and KS2 were5

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shown in Fig. 6. Distribution patterns were similar for both vessels. High density of Antarctic krill
was observed in the northwestern part of the survey area. Ice krill tended to distribute near the ice
shelf. Mean TS of Antarctic and ice krill were -69.7 dB and -79.0 dB, respectively. Biomasses were
estimated using those TS values. Results of biomass estimation were summarized in Table 2.
Biomass densities of Antarctic krill with 95% CI using KM and KS2 data were 5.36±7.45 and2.64±2.35 g/m2, respectively. Biomass densities of ice krill with 95% CI using KM and KS2 data
were 3.44±1.96 and 1.56±0.89 g/m2, respectively. Because there was no significant difference
between the biomass density estimates from both vessels, two data sets were combined to estimate
the biomass. The biomasses of Antarctic and ice krill in this study were estimated as 1.46
(CV=0.32) and 0.82 (CV=0.18) million t, respectively. Total biomass of euphausiid in the survey
area was estimated as 2.28 million t (CV=0.22).
Distribution patterns of Antarctic mink whales
Total cetacean searching effort of KS2 was 1995 n.miles. Numbers of sightings of Antarctic minke
whales were 601 individuals in 300 schools. Other baleen whales, Blue and humpback (Megaptera
novaeangliae) were sighted in the survey area but numbers were small (2 individuals in 1 school foreach species). Distribution pattern of Antarctic minke whales was shown in Fig. 7. Distribution
pattern of Antarctic krill and Antarctic minke whales were overlapped. School sizes of Antarctic
minke whales were large where the densities of Antarctic krill were high.
DISCUSSION
As previous surveys (e.g. Azzali et al., 2006) suggested, the samples from RMT and stomach
contents of Antarctic minke whales indicated that two euphausiid species, Antarctic and ice krill
showed different distribution patterns in the Ross Sea (waters south of 69°S) with respect to thebottom topography. In this study, distribution area of Antarctic and ice krill was separated by the
combination line of 1000 m bottom depth counter and -1°C isotherm line of ITEM-200 thoughsome overlap was found around the line. Haul catches of Antarctic krill was almost coincident with
the intrusion of surface waters coming from the open ocean onto the self area in the west of 175°W
until 73-74°S (Sala et al., 2002). Their observation was confirmed in our study but our results
showed that oceanographic condition could not be a controlling factor of distribution of Antarctic
krill. Stomach contents of Antarctic minke whales in the east of 175°W suggested that Antarctic
krill distributed in low ITEM-200 where the bottom depth was deeper than 1000m. Ichii et al.
(1998) also found that Antarctic krill distributed near the continental slope in the east of 175°W.
Those findings suggested that distribution patterns of Antarctic krill could be regulated by the
combination of oceanographic conditions and bottom topography.
Azzali et al. (2006) postulated that the distribution pattern of Antarctic and ice krill shiftedto north as the ice edge moved to northward in austral summer but the magnitude of movement wasmore significant for Antarctic krill. As the results, density of Antarctic krill in January 2000 was
low by comparison with November in 1994 and December in 1994 and 1997 though one needcaution of the interpretation because of methodological differences among those surveys (Azzali etal., 2006). Our survey in the Ross Sea were mainly conducted in January. It seemed that there was
general agreement between our results and the hypothesis proposed by Azzali et al. (2006).
Densities of Antarctic and ice krill were similar to the value in January in Azzali et al. (2006) whilebiomass estimates were in the comparative range regardless of month (Table 3). Even if densities
were low in January, surveyed area in January was generally large as the result of sea ice retreat.
Dispersal response of euphausiid distribution to seasonal sea ice retreat in the Ross Sea should bestudied further in the future cruises.
There is no reported values of TS for ice krill at the presentation of this paper. Because TS
of Antarctic krill was applied to ice krill in this analysis, interpretation of biomass estimate of ice
krill need caution. Measurement of TS of ice krill should be conducted in the near future. It should
be noted that Demer and Conti (2005) proposed the new TS for Antarctic krill using Stochastic
Distorted-Wave Born-Approximation (SDWBA) model. If the new TS applied to our results, krill
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biomass would be 2.5 times higher than the current estimates.
The results of comparison of biomass between KM and KS2 suggested that data from two
vessels were comparable. KS2 had conducted echo sounder survey as a part of JARPA without net
sampling since 1998. The results of inter-ship comparison of biomass estimates in this study
suggested estimates by KS2 in the past were robust in terms of acoustical units.
Differences of proportions of age groups of Antarctic and ice krill were observed between
samples from RMT and stomach contents of Antarctic minke whales though estimated mean lengthof each age group was almost same. The proportions of age groups of RMT8+1 samples weresimilar to the reported values of Antarctic and ice krill in the Ross Sea (Table 1, Sala et al., 2002).
Antarctic minke whales fed on larger krill than those in the net hauls. Antarctic minke whales areagile engulfing feeder. Though there was no measurement for Antarctic minke whales, it wasreported that the average of foraging lunge speed of blue whales was 2.4m/s (≈4.7konts) (Croll et al,2001). The speed of foraging lunge of Antarctic minke whales would exceed the speed of towing
speed of RMT8+1 (2knots). While the course of the vessel could not change during towing of
RMT8+1, Antarctic minke whales could change their swimming direction in response to the
behavior of krill. It was known that direct sampling of euphausiid using net has problems such as
net avoidance to construct length frequency data (Watkins, 2000 for review). Considering thosepoints, length frequency of krill using the stomach contents Antarctic minke whales could be morereliable than using the RMT8+1 samples. However, it was reported that smaller size classes wererepresented in the diet of fur seals because of either active or passive (lack of availability) preyselection (Murphy and Reid, 2001). Our results might indicate that Antarctic minke whales show
prey selectivity toward large size euphausiid. Because this was the firsrt attempt to compare thelength frequency of euphausiid between samples from RMT8+1 and stomach contents of Antarctic
minke whales, further concurrent study of baleen whales and euphausiid should be conducted toconclude whether Antarctic minke whales have prey selectivity or not. Because JARPA has longterm stomach contents data series, euphausiid population dynamics could be construct if theappropriate selectivity is estimated (Reid et al., 2004). This study suggested that Antarctic minke
whales could be used as biosampler to study the Antarctic marine ecosystem.
Present study suggested that distribution of Antarctic minke whales in the Ross Sea could be
regulated not by ice krill but by Antarctic krill. Because “Paradox in the Ross Sea” washypothesized thoroughly based on information from stomach contents of Antarctic minke whales(Ichii et al, 1998), this hypothesis should be reevaluated using both concurrent cetacean andeupusaiid survey data. Distributional relationship of euphausiid and Antarctic minke whales should
be examined quantitatively using statistalcal model such as applied to the Western Antarctic
Peninsula Region (Friendlaender et al., 2006).
Overall, a multi-disciplinary study combining surveys on cetacean, krill and oceanography
gives new insight for the study of the Ross Sea ecosystem. Continuation of this kind of study isessential of management and conservation of the Antarctic marine living resources.
ACKNOWLEDGMENT
Authors express thank you to the crews and the researchers who dedicated to collect data in harshenvironmental condition in the Antarctic. Special thanks were given to Ms. Tomoko Hasegawa who
supported the handling of echo sounder and sampled krill data. We also thank Dr. Hiroshi Hatanaka
and other colleagues at the Institute of Cetacean Research who made critical comments to improve
this manuscript.
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Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL
http://www.R-project.org.
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Reid, K., Jessopp, M. J., Barrett, M. S., Kawaguchi, S., Siegel, V. and Goebel, M. E. 2004.
Widening the net: spatio-temporal variability in the krill population structure across the Scotia
Sea. Deep Sea Res. II. 51:1275-1287.
Sala, A., Azzali, M., and Russo, A.,2002. Krill of the Ross Sea: distribution, abundance anddemography of Euphausia superba and Euphausia crystallorophias during the Italian
Antarctic Expedition (January-February 2000). Scientia Marina 66:123-133.
Watkins, J. L. 2000. Smapling krill. In: Everson, I. (ed.), Krill, Biology, Ecology and Fisheries, Fish
and Aquatic resources, Series 6. Blackwell Science, Oxford University Press, Oxford, pp.8-39.

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Table 1. Mean length, standard deviation (SD) of mean length and proportions of Antarctic and ice
krill in the Stomach contents of Antarctic whales and RMT samples. Results in the Ross Sea in2000 (Sala et al., 2002) was also shown for comparison purpose.

Speciese

Antarctic krill

Ice krill

Age
Group
2+
3+
4+
1+
2+
3+

Stomach Contents
Length
(mm)
40.9
46.4
12.1
21.0
26.4

SD %
2.9
3.2
1.3
2.0
3.3

10

34
66
18
28
54

RMT
Length
(mm)
43.2
47.4
12.3
21.9
25.8

Sala et al. (2002)

SD %
2.6
1.8
1.4
1.8
4.3

74
26
48
17
35

Length
(mm)
37.4
42.8
47.2
13.5
21.5
25.5

SD %
2.5
2.4
2.6
1.9
1.9
3.4

6
52
42
36
32
32

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Table 2. Density and biomass of Antarctic and ice krill using quantitative echo sounder in the Ross
Sea in 2005.
Antarctic krill

2

Weighted mean ρ (g/m )Ice krill

KM

KS2

Total

KM

KS2

Total

Gross Total

5.4

2.6

3.9

3.4

1.6

2.2

3.1

2

110792

Surveyed area (n.mile )106800217592

Biomass (million t)

2.04

1.00

1.46

1.26

0.57

0.82

2.28

CV (million t)

0.44

0.36

0.32

0.21

0.26

0.18

0.22

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Table 3. Summary of biomasses of Antarctic and ice krill in the Ross Sea from 1994 to 2005.
Year Month
1994
1994
1997
2000

Nov
Dec
Dec
Jan

2005

Jan

Antarctic krill
Density
Biomass(g /m^2) (million t)
21.9
2.38
21.4
2.78
16.3
2.21
11.0
1.23
3.9

1.46*

Ice krill
Density
Biomass(g /m^2) (million t)
1.8
0.02
1.6
0.02
1.4
0.02
0.7
0.01
2.2

0.82**

Surveyarea(nm^2)
31800
37800
39600
60600
110792*106800**12

Areal coverage

Reference

69-30S - 78-06S
164-30E - 175-30W

Azzali et al . (2006)

69-00S - 78-40S
163-50E - 154-00W

This sutdy

SC/D06/J24

Fig.1. Map of Integrated TEMperature Mean water depth from surface to 200m (ITEM-200) basedon CTD and XCTD casts in the Ross Sea and adjacent region. CTD and XCTD stations used in theanalysis were shown. Pink thick line (▬) represented -1oC isothermal line of ITEM-200. Sea ice
concentration data, DMSP SSM/I Daily and Monthly Polar Gridded Sea Ice Concentrations, at thetime of the survey were overlaid on the map.

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SC/D06/J24

(a)

(b)

Fig.2. Distribution patterns and length frequencies of Antarctic krill: (a) sampled by RMT8+1 and(b) samples from stomach contents of Antarctic minke whales. Notes that units of histograms werediffrencent (ind/m2 for RMT8+1 and % for stomach contents). “X” denoted sampling locations
without Antarctic krill. Thick blue line (▬) represented survey area boundary. Thick black solidline (▬) and dotted line (- -) represented bottom depth contour lines at 1000 m and -1oC isothermalline of ITEM-200, respectively. Sea ice concentration data, DMSP SSM/I Daily and Monthly Polar
Gridded Sea Ice Concentrations, at the time of the survey were overlaid on the map.

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SC/D06/J24

(a)

(b)

Fig.3. Distribution patterns and length frequencies of ice krill: (a) sampled by RMT8+1 and (b)samples from stomach contents of Antarctic minke whales. Notes that units of histograms werediffrencent (ind/m2 for RMT8+1 and % for stomach contents). “X” denoted sampling locations
without ice krill. Thick blue line (▬) represented survey area boundary. Thick black solid line (▬)
and dotted line (- -) represented bottom depth contour lines at 1000 m and -1oC isothermal line of
ITEM-200, respectively. Sea ice concentration data, DMSP SSM/I Daily and Monthly Polar
Gridded Sea Ice Concentrations, at the time of the survey were overlaid on the map.

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SC/D06/J24

(a)

(b)

Fig.4. Length frequencies of Antarctic krill using stomach contents of Antarctic minke whales (a)and RMT8+1 samples. Green line represented overall fitting curve while red lines represented
fitting curve of each age group. Red triangle represented mean length of each age group.

16SC/D06/J24

(a)

(b)

Fig.5. Length frequencies of ice krill using stomach contents of Antarctic minke whales (a) andRMT8+1 (b) samples. Green line represented overall fitting curve while red lines represented fitting
curve of each age group. Red triangle represented mean length of each age group.

17

SC/D06/J24

(a)

(b)

Fig. 6. Distribution patterns of Antarctic and ice krill based on quantitative echo sounder data: (a)Kaiyo Maru and (b) Kyoshin Maru No.2. Thick blue line (▬) represented survey area boundary.
Thick black solid line (▬) and dotted line (- -) represented bottom depth contour lines at 1000 m
and -1oC isothermal line of ITEM-200, respectively. Sea ice concentration data, DMSP SSM/I Daily
and Monthly Polar Gridded Sea Ice Concentrations, at the time of the survey were overlaid on the
map.

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SC/D06/J24

Fig. 7. Primary sighting positions and school sizes of Antarctic minke whales. Thin black line (-)
represented sighting survey tracklines. Thick blue line (▬) represented survey area boundary. Thickblack solid line (▬) and dotted line (- -) represented bottom depth contour lines at 1000 m and -1oC
isothermal line of ITEM-200, respectively. Sea ice concentration data, DMSP SSM/I Daily andMonthly Polar Gridded Sea Ice Concentrations, at the time of the survey were overlaid on the map.

19