JA/J05/JR11

Results of the cetacean prey survey using echo sounder in
JARPA from 1998/99 to 2002/2003
HIROTO MURASE1, SHIGETOSHI NISHIWAKI1, HAJIME ISHIKAWA1, KOJI MATSUOKA1, HIROSHIKIWADA1, TAKASHI YOSHIDA2 AND SHIN ITO1.
1
2

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
Kyodo Senpaku Kaiysha, Ltd. 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
ABSTRACT
Krill biomass estimation surveys using quantitative echo sounder have been conducted in JARPA since
1998/99 season to achieve one of the main objectives of JARPA which is elucidation of the role of whales in
the Antarctic marine ecosystem. The survey was conducted concurrently with cetacean survey. Similar biomass
estimates were obtained in Area IV in 1999/2000 (36.4 million t) and 2001/2002 (36.1 million t) season. In
Area V, biomass in 2000/2001 (18.7 million t) and in 2002/2003 (21.0 million t) were similar but biomass in
1998/1999 (32.3 million t) was higher than rest of two years. Higher biomass in 1998/99 could be explained byseasonal effect and area coverage difference. Biomass in Area IV was higher than Area V. Regional krill
distribution pattern difference in response to the southerly shift of the SB-ACC was observed in Area IV.
Because krill biomass survey in whole Area IV and Area V were rarely conducted in the past, krill datacollected by JARPA were quite important to understand krill-baleen whale relationship in the Antarctic.

INTRODUCTION
It is well known that krill is the key species in the Antarctic marine ecosystem because krill is amajor link in the transfer of energy from primary producers to larger organisms such as baleen
whales (Laws, 1985; Murphy et al., 1988). Krill have been known as the major food source ofbaleen whales other than Bryde’s whales (Balaenoptera edeni) in the Antarctic (Kawamura, 1994).
“Krill surplus” caused by intensive commercial harvesting of large whales, blue (B. musculus), fin(B. physalus) and humpback whales (Megaptera novaeangliae), has been central theorem of the
Antarctic ecosystem study. “Krill surplus” resulted in increasing in amount of available food toother krill feeders such as Antarctic minke whales (B. bonaerensis) (Laws, 1977). But it appears
that the situation is changed in recent years. After the ban of commercial whaling of large whales in
1987, it was reported that abundance of those species increased in recent years. For example,
abundance of blue whales increase 8% per year at the circumpolar level (Branch et al., 2004)
though the abundance was still low comparing with pre exploitation population size. Some stocks of
humpback whales in southern hemisphere have been showed remarkable recovery even to near preexploitation level (Jhonston and Butterworth, 2004). To test the magnitude of interaction,
preliminarily baleen whales-krill interaction model was developed but the interpretation was limited
at this moment because of paucity of information including biomass estimation of krill (Mori and
Butterworth, 2004). Krill density showed both short and long term changes. In short term, krill
density showed large year to year fluctuation at decadal scale in response to environmental
variability such as sea ice extent and oceanographic conditions (e.g. Pakhomov, 2000; Hewitt andDemer, 2003). It was reported that krill density have showed statistically significant decreasing
trend in the southwest Atlantic since 1976 (Atkinson et al. 2004) though the magnitude of decrease
should be studied further to draw the conclusion because wide varieties of net types were used in
the analysis. Given the krill density change information, krill biomass survey should be conducted
regularly in the same region. To assess the magnitude of interspecific competition among baleen
whales for krill and the consequences of the competition quantitatively, conducting concurrent
cetacean and krill survey is critical.
The Japanese Whale Research Program under Special Permit in the Antarctic (JARPA)
provided unique opportunity to conduct the concurrent cetacean and krill survey. JARPA has been
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conducted during the austral summer every year since the 1987/1988 season. One of the primaryobjectives of the JARPA is elucidation of the role of whales in the Antarctic marine ecosystemthrough 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 and abundance of
prey species was required to achieve the objective. In response to the comments, echo soundersurvey to examine distribution and abundance of krill has been conducted concurrently withcetacean survey since 1998/99 season. Krill distribution data will be linked with distribution
patterns of baleen whales whereas krill abundance data will allow us to examine the magnitude of
the interspecific relationship among baleen whales for krill. The preliminary report of the results theecho sounder surveys of krill that were conducted in 1998/1999 and in 1999/2000 were reported to
the International Whaling Commission (IWC)/Scientific Committee (SC) as SC/52/E5 (Murase etal., 2000) and published in scientific journal (Murase et al., 2002). This paper presents the echosounder survey methodology and the results of biomass estimation of krill in JARPA from 1998/99
to 2002/2003.
MATERIALS AND METHODS
Survey area
Two baleen whale management area defined by the International Whaling Commission (IWC), Area
IV (70°E-130°E) and Area V (130°E-170°W), were surveyed alternative years. The areas between
south of 60°S and the ice edge line were surveyed. Each area were further divided into two, east andwest, at 100°E in Area IV and at 165°E in Area V. Each sector further divided into two strata, north(between the 60°S latitude line to the line of 45 n.miles from ice edge) and south (between the lineof 45 n.miles from ice edge and ice edge). Exceptions were the Prydz Bay region (south of 66°S
between 70°E and 80°E) and the Ross Sea region (South of 69°S between 165°E and 170°W). The
Ross Sea region was defined as south-east stratum of Area V whereas the Prydz Bay region was
called as it was. In addition to two Areas, eastern part of Area III and western part of Area VI werealso surveyed alternative years. No specific stratification was set in those Areas. Following
acronyms were used to describe strata name;
III-E-F
III-E-S
IV(or V)-NE
IV(or V)-NW
IV(or V)-SW
IV(or V)-SE
IV-PB
VI-W-F
IV-W-S

Eastern half of Area III, first period
Eastern half of Area III, second period
North-East stratum in Area IV (or V)
North-West stratum in Area IV (or V)
South-West stratum in Area IV (or V)
North-East stratum in Area IV (or V)
Prydz Bay region in Area IV
Western half of Area VI, first period
Western half of Area VI, second period

Timing of surveys
Surveys were conducted during austral summer (December-March). Detailed survey dates weresummarized in Table 1 and 2.
Trackline designs
Sawtooth type zigzag lines were used in each survey. Details of the trackline designs were reviewed
in Nishiwaki et al. (2005).

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JA/J05/JR11
Survey vessel
The cetacean sighting vessel, Kyoshinmaru No.2 (368 GT) was engaged in the sighting survey ofcetaceans as well as echo sounder and oceanographic surveys. The nominal steaming speed of SV
on the track line was 10.5 knots.
Data acquisition and storage system
An EK500 scientific echo sounder (Simrad, Norway) with software version 5.30 operating
frequency at 38 and 120 kHz on board Kyoshin-maru No.2 was used to collect data for the acoustic
survey from 1998/99 to 2003/04. The transducers were hull-mounted at the depth of 4.3 m from thesurface. Each transducer was covered with a 40 mm polycarbonate acoustic window to minimize
the damage on the transducer surface from contacting sea ice. The hydraulic oil filled the space
between the transducer surfaces and the acoustic windows. Data were stored and interpreted withthe aid of a BI500 post processing system (Simrad, Norway) from 1998/99 to 2003/04. BI500 was
software operated under the UNIX operation system. BI500 had three major function; 1) recording
echo sounder data, 2) analyzing echogram data and 3) outputting the analysis results.
Data analysis system
BI500 was used in 1998/99 and in 1999/2000. Echoview version 3.00.74.01 (SonarData Pty Ltd,
Australia) was used from 2000/01 to 2003/04. Major function of Echoview was same as BI500 but
it allowed more detailed analysis than BI500. Echoview was software operated under the Microsoft
Windows. Echoview was capable to analyze BI500 data file.
Calibrations
The copper sphere technique that described in EK 500 operation manual (Simrad, 1997) wasapplied for the calibrations. Calibrations were conducted in Antarctic water every year. In 2002/03
and 2003/04, calibrations were attempted but the results weren’t applied to the analysis because
those were conducted in unsuitable conditions for calibration. For those years, the result of thecalibration in 2001/02 was applied for analysis.
Data recording methodology
The survey was conduced during diurnal hours from an hour after sunrise to an hour before sunset.
Maximum survey time per day was 12 hours. Data were recorded continuously while the vessel
steamed on predetermined trackline. The data were not used in the analysis when the vessels
deviated from the trackline such as during cetacean species confirmation.
Data analysis methodology
We applied the acoustic data analysis described by Hewitt and Demer (1993) and Demer and Hewitt(1995). The following procedures came from those papers. Mean backscattering area per square n.
mile of survey transect (sA) attributed to krill for every 1 n. mile of survey transect over 10 to 250 m
depth was calculated by following formula;s A = 4πr021852 2

r2 = 250m2∫r =10 sV dr ( n.mi 2 )1where, r is depth from the surface, r0 = 1m representing the reference range for backscatteringstrength and sV = 0 if 10 log (sV) ≦ -80dB, because threshold backscattering was set at –80dB.
Because direct sampling method (e.g. net sampling) to identify species was not available, thedifference between the mean volume backscattering strength (ΔMVBS) of 120 and 38 kHz fell
between 2 and 12 dB is classified as krill (Madureira et al., 1993). Because BI500 didn’t havecapability to calculate ΔMVBS, the differences were visually identified using the echogram in
1998/99 and 1999/2000. For the lest of year, ΔMVBS was calculated quantitatively using Echoview.
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JA/J05/JR11
Krill backscattering cross section area (σ) was calculated with the following formula based on krill
target strength described by Greene et al. (1991):
σ = 4πro210 −12.745 l 3.485

where, l was standard length of krill. Krill wet weight (w) was calculated with the following
formula based on Siegel (1986):
w = 0.00193l 3.325 .
Average area krill biomass density ( p ) was calculated as follows;ρ = SAw

σ= 0.249l −0.16 S A .

Then, frequency distribution of euphausiids standard length ( f i ) was applied to the following
formula;ρ = 0.249

n∑ fi (li )−0.16 S A .
i =1

Because minor variation in the frequency distribution of krill length did not affect the krill biomass
estimate, a combined distribution data based on Loeb and Siegel (1992) was used (Demer and
Hewitt, 1995) as follows;n∑ f i (li ) −0.16 ≅ 0.562 .
i =1With this formula mean krill biomass of each transect in each stratum was calculated. 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 p using above motioned formula. Biomass was estimated as;
Bk = Ak ρ kwhere, Bk is density biomass in kth block and Ak is area of kth block. Variance of Bk was calculatedwith following formula;var( Bk ) = Ak 2 var( ρ k ) .
Coefficient of variation of Bk was calculated as;
BB

var(Bk )CV ( Bk ) =Bk.

Biomass (B0) in each Area was calculated as;
B0 =N∑ Bk .
k =1

Overall survey variance of B0 was;Var ( B0 ) =N∑ var(Bk ) .
k =1

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JA/J05/JR11
Coefficient of variation of B0 was calculated as;
B

Var ( B0 )CV ( B0 ) =B0.

RESULTSArea IV and eastern part of Area III
Surveys were conducted in Area IV and eastern part of Area III in 1999/2000 and 2001/2002.
Density and biomass in each stratum in each year were summarized in Table 1. Distribution patterns
were shown in Fig. 1. Timings of surveys were almost same for two years. Density and biomass in
IV-SW in 2001/2002 were remarkably higher than 1999/2000. It should be noted that survey effort
in Northern strata in Area IV in 2001/2002 was lower than that of in 1999/2000 because of poor
weather conditions for the echo sounder survey.
Area V and western part of Area VI
Surveys were conducted in Area V and western part of Area VI in 1998/1999, 2000/2001 and2002/2003. Density and biomass in each stratum in each year were summarized in Table 2.
Distribution patterns were shown in Fig. 2. Timing of survey in 1998/1999 was different from otheryears. Because the mouth of the Ross Sea was closed in 1998/1999, the Ross Sea region was notsurveyed. In addition, V-NE was not surveyed.
Biomass estimation by Area
Results of biomass estimation of krill by area by year were summarized in Table 3. Overall, year toyear differences of biomass estimations and CVs were small except 1998/1999, though each stratumin each area showed year to year biomass and CV variations. It should be noted that the survey
timing and coverage in Area V in 1998/1999 were different from other years.
DISCUSSION
The position of the Southern Boundary of the Antarctic Circumpolar Current (SB-ACC) in
2001/2002 shifted to south in comparison with that in 1999/2000 (Watanabe et al., 2005). The SBACC was corresponded to high primary production and hence to distribution of whales (Tynan,1998). It was postulated that krill would be coastally constrained when the position of the SB-ACC
shows southerly shift (Nicol et al., 2000(a)). Higher density and biomass in VI-SW in 2001/02 thanin 1999/2000 could be related to the southerly shift in position of the SB-ACC. This is the first time
to confirm the hypothesis proposed by Nicol et al. (2000(a)) in this region. Few echo soundersurveys were conducted between 35°E to 170°W in the past except the Prydz Bay region. Survey of
distribution and abundance between 80°E and 150°E from south of 63°S to ice edge was conductedby Australian research vessel in 1995/96 austral summer (Pauly et al. 1997). They reported krill
densities of 5.5 g/m2, 6.7 g/m2, 4.2 g/m2 and 9.2 g/m2, in whole survey area, west area (80°E-115°E),
east area (115°E-150°E) and shelf break area, respectively. Those densities were significantly lower
than those reported in this paper, though it was difficult to compare the two results directly because
survey coverage, design and timing were totally different from each other. Quite significant interannual variability of biomass densities caused by environmental factors were reported in the
Cooperation Sea (50°E-85°E) (Pakhomov, 2000) and in the South Shetland Islands region (Hewitt
and Demer, 2003). Such inter-annual variability could explain the differences but frequent surveys
in this area should be conducted in future to clarify the existence of the inter-annual variability andthe causes of variability. Acoustic survey in the Ross Sea region was conducted in December toJanuary in 1989/1990 austral summer (Azzali and Kalinowski, 1997). They reported krill biomass
densities as 9 g/m2 (≈ 32 t/n.mile2) in the end of December and at beginning of January, and 11 g/m2
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JA/J05/JR11(≈ 39 t/n.mile2) in January. Those densities were similar to values reported in this paper except
2000/2001 austral summer (6.8g/m2). It was reported that primary productivity in the mid
December in 2000 was mach lower than previous years based on the satellite observation, because
presence of the large ice berg inhibited normal drift of pack ice resulting in heavier spring/summer
pack ice cover (Arigo, et al., 2002). Lower primary production in the Ross Sea in 2000/2001 could
reduce the biomass density of krill.
It should be noted that highest biomass recorded in Area V in 1998/1999 especially in V-NW
and V-SW. In 1998/99, those two strata were surveyed earlier about a moth than latter two years(2000/2001 and 2002/2003). V-NW and V-SW were surveyed in mid January to mid February
which in 1998/1999 while they were surveyed in mid February to at beginning of March in latter
two years. Intra-annual variability in krill abundance was reported around South Georgia (Brierley,
et al., 2002). They reported that krill abundance late (March) austral summer was significantly
lower than that in January. Such intra-annual variability of krill abundance could explain theabundance difference among three survey years in V-NW and in V-SW. Effect of intra-annual
variability of krill abundance should be considered in data from III-E and VI-W where surveys wereconducted in either December or March. The reasons of the seasonal differences of krill biomass
could be either or combination of 1) onshore migration of krill in autumn to winter (Siegel et al.,1997) and 2) depletion of krill by predators (Brierley et al. 2002).
There were two main categories of limitations to interpret the result of this analysis. First
category of limitation was applied to acoustic survey of krill in general. Those general limitations
were 1) background krill which could not be detected by echo sounder because of low density, 2)
krill refuge which meant that krill could distribute where the survey vessel could not enter such as
under sea ice and 3) surface krill which meant that krill could distribute shallower than thetransducers (Nicol et al., 2000(b)). Those three general limitations of abundance estimation of krill
using echo sounder would contribute to underestimation of krill biomass. The other category oflimitation was applied specifically to this analysis. Though krill identification using ΔMVBS
between 120kHz and 38kHz have been well established, there was some uncertainty associatedspecies identification of mark on echogram because no net sampling was conducted in the analysis
presented here. Along with this, there was no in situ length frequency and weight data of krill toconvert acoustic backscattering to biomass. This was also the source of bias. Poor sea condition forecho sounder survey in Northern Stratum of Area IV in 2000/2001 resulted in poor survey coverage.
Calibration of echo sounder is very important because subtle parameter setting change will resultsin large effect on biomass estimation of krill. Because there was no calibration result in 2002/03,
interpretation of the result in that year may need caution.
Over all, the results presented here provided general distribution and biomass patterns in thesurvey area. Following points should be considered to improve the echo sounder survey in future: 1)
survey should be conducted in peak abundance season of krill (January and February) to minimize
seasonal effect on abundance estimation, 2) survey should be conducted in same area in same
survey timing to interpret yearly changes, 3) target net sampling should be conducted to identify
species compositions of marks detected by the echo sounder so that appropriate amount ofbackscattering can allocate to krill and 4) calibration must be conducted every year to set
appropriate parameters in the echo sounder.
ACKNOWLEDGEMENT
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. Shoko Ohkawa who
supported the handling of huge amount of echo sounder data. We also thank Drs. Hiroshi Hatanaka,
Seiji Ohsumi and other colleagues at the Institute of Cetacean Research who made criticalcomments to improve this manuscript.

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Table 1. Density and biomass in each stratum in Area IV and eastern part of Area III in 1999/2000
and 2001/2002
Surveyed
Number of
distancestransects(n.miles)
1999/2000 12/5/99-12/26/99
13
1,749
2001/2002 11/29/01-12/24/01
8
1,116
1999/2000 12/27/99-1/11/00
5
1,117
2001/2002 12/25/01-1/8/02
2
165
1999/2000 1/11/00-1/26/00
5
1,257
2001/2002 1/9/02-1/25/02
5
582
1999/2000 1/28/00-2/18/00
15
1,061
2001/2002 1/26/02-2/8/02
17
1,092
2/18/00-2/29/00
1999/2000
17926
3/6/00-3/9/00
2001/2002 2/10/02-2/21-02
14
975
1999/2000
3/1/00-3/6/00
4
428
2001/2002 2/22/02-2/27-02
3
432
1999/2000
2001/2002 2/28/02-3/8-02
3300

Stratum Year
III-E-F
IV-NW
IV-NE
IV-SE
IV-SW
IV-PB
III-E-S

Date

9Surveyed Mean
densityarea
2
2(n.mile ) (g/m )356,046
13.232354,965
34.519236,30719.014
200,738
18.283
229,576
20.285
223,108
14.191
33,12916.28866,79020.275

BiomassCV (%)(Million t)
16.16042.02715.411
12.58815.97310.860
1.851
4.645

29.9
30.2
54.1
49.3
33.7
50.9
28.9
27.7

34,82513.711

1.638

21.8

61,51727,00029,155525,71541.01616.73613.73856.754

8.654
1.550
1.374
102.337

15.9
22.6
44.0
57.8

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Table 2. Density and biomass in each stratum in Area V and western part of Area VI from1998/1999 to 2002/2003.
Surveyed
Number of
distances
Stratum Year
Datetransects(n.miles)
1998/1999
VI-W-F 2000/2001 12/11/00-12/31/00
5658
2002/2003 12/3/02-12/30/02
14
1,810
1998/1999
V-NE 2000/2001
1/1/01-1/23/01
5896
2002/2003
1/5/03-1/25/03
9
1,329
1998/1999
1/14/99-2/2/99
5
1,405
2000/2001
2/10/01-2/23/01
4
565
V-NW
2/11/03-2/20/03
2002/2003
5805
3/5/03-3/7/03
1998/1999
2/3/99-2/21/99
17
1,152
V-SW 2000/2001
2/25/01-3/19/01
13
857
2002/2003
2/21/03-3/5/03
5377
1998/1999
2/22/99-3/13/99
13
541
V-SE 2000/2001
1/24/01-2/9/01
15
1,401
2002/2003
1/27/03-2/9/03
10775
1998/1999
3/14/99-3/28/99
12
590
VI-E-S 2000/2001
2002/2003-

10

Surveyedarea
2(n.mile )290,908
309,998
334,377
338,026
321,375
249,712Mean
density2(g/m )
3.386
1.514
11.6175.717
21.713
1.273

257,084
45,45564,854
65,671
52,553
105,458
58,424
316,727-

BiomassCV (%)(Million t)
3.379
1.610
13.3246.628
23.934
1.090

60.6
34.2
27.4
27.3
20.0
19.4

10.146

8.947

25.4

25.326
8.557
16.84424.679
6.498
8.337
0.210
-

3.948
1.903
3.794
4.448
2.350
1.671
0.229
-

33.5
49.4
20.0
36.0
30.5
27.8
67.0
-

JA/J05/JR11
Table 2. Densities of krill in Area IV and V from 1998/1999 to 2002/2003.

Area
IV
V

Surveyed
Biomassarea
CV (%)
Year(Million t)
2(n.mile )1999/2000
560,837
36.422
27.3
2001/2002
581,308
38.121
22.4
1998/1999
419,38332.331
16.1
2000/2001
754,40118.668
20.6
2002/2003
719,205
21.03914.5

11

JA/J05/JR11
Fig. 1. Distributions and densities of krill in Area IV and eastern part of Area III in 1999/2000 and
2001/2002. Densities were shown as mean backscattering area per square n. mile of survey transect(SA) in square root scale. Bottom depth contour lines:▬ (500m), ▬ (1000m), ▬ (1500m) and ▬(2000m). Ice edge line: ▬.
1999/2000

2001/2002

12

JA/J05/JR11
Fig. 2. Distributions and densities of krill in Area V and western part of Area VI from 1998/1999 to
2002/2003. Densities were shown as mean backscattering area per square n. mile of survey transect(SA) in square root scale. Bottom depth contour lines:▬ (500m), ▬ (1000m), ▬ (1500m) and ▬(2000m). Ice edge line: ▬.
1998/1999

2000/2001

2002/2003

13