SC/56/IA10
Possible impact due to variability of sea ice condition on
Antarctic minke whale abundance estimation in the Antarctic
from 1978 to 2002

HIROTO MURASE1 AND HIROYUKI SHIMADA2

1

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan

2

National Research Institute of Far Seas Fisheries, Fisheries Research Agency 5-7-1, Shimizu-Orido, Shizuoka, 424-

8633, Japan

Contact e-mail: murase@cetacean.jp

ABSTRACT
Antarctic sea ice variability in each IWC baleen whale management area from 1978 to 2002 was examined using satellite data. Monthly
anomaly trend from 1978 to 2002 suggested that sea ice extents were significantly decreased in Area I while they were significantly
increased in Area V and Area VI. Sea ice extent showed large year to year changes. Sea ice extent in January and February was generally
larger in CPIII than in CPII except Area II and VI. Greater sea ice extent in CPIII could have negative effect on the abundance estimationof Antarctic minke whale in four ways: 1) high density area of Antarctic minke whale could not be surveyed by the vessels because of sea
ice, 2) krill abundance could be reduced because of see ice extent and the reduction could affect the abundance estimation of Antarctic
minke whale, 3) narrower longitudinal coverage in CPIII than in CPII in single survey could be susceptive to the effect of the sea icechange and 4) larger sea ice extent could reduce the survey area as a whole. In the case of the large sea ice extent in CPIII, the abundance
estimates decreased in Areas I, III and IV. In contrast, the abundance estimate in Area VI increased in CPIII along with decreasing sea ice
extent. The results suggested that large sea ice extent could have negative impact on abundance estimate. Though sea ice extent in Area II
was decreased in CPIII, this was due to the formation of large polynya. Because the polynya region was not covered by the sighting vessel
in CPIII, the abundance must be underestimated. Effect of sea ice configuration on abundance estimates should be investigated further.
Because of large sea ice variability between CPII and CPIII, direct comparison of Antarctic minke whale abundance estimates between
CPII and CPIII is difficult without removing the effect.
KEYWORD: ANTARCTIC MINKE WHALE; SEA ICE; ABUNDANCE ESTIMATE; ANTARCTIC

1INTRODUCTION
The Antarctic minke whale abundance assessment cruises has been conducted in the Antarctic in austral summer by theInternational Whaling Commission (IWC) since 1978/79 (Matsuoka et al., 2003, for review). The names of the cruises
were the International Decade of Cetacean Research programme (IDCR) from 1978/79 to 1995/96 and the Southern
Ocean Whale and Ecosystem Research programme (SOWER) from 1996/97 to 2003/04. The cruises covered threecircumpolar surveys. Abundance estimation was made using each circumpolar data set; 1978/79-1983/84 (first
circumpolar, CPI), 1984/85-1990/91 (second circumpolar, CPII) and 1991/92-2003/04 (third circumpolar, CPIII).
Though the abundance estimate of the third circumpolar set has not been completed, noticeable abundance decline fromCPII (766,000) to CPIII (268,000) (Branch and Butterworth, 2001) brought question whether the decline is true orapparent. It is hypothesized that change in sea ice could have one of the large impacts on the abundance estimations(IWC, 2003). Tynan (2002) advocated that the linkage between the regional sea ice trends and the change in theabundance estimation of Antarctic minke whales was necessary but no analysis was presented to the IWC ScientificCommittee (IWC/SC) hereafter. Shimada et al. (2001) tentatively analyzed the impact of the sea ice change on theabundance estimation of Antarctic minke whales within pack ice but how the change in sea ice related to the open waterabundance estimation was not discussed fully. This paper presents (1) monthly mean anomaly sea ice extent trend 19782002, (2) January and February monthly mean sea ice extent trend 1979-2002, (3) comparison of January and February
monthly mean sea ice extent between CPII and CPIII in each baleen whale management area defined by the IWC and(4) relationship among sea ice extent, krill and Antarctic minke whales in Area IV using JARPA data. Possible
implication of the linkage between the sea ice and abundance estimation is discussed.

MATERIALS AND METHODS
Satellite derived daily sea ice data, Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I(Comiso, 1999) from October 1978 to September 2002 was used in the analysis. The original data was in the polarstereographic projection and it was converted to the Zenithal equal area projection. Sea ice extent (km2) was defined as
the sum of the image pixels that contained more than 15% sea ice concentrations. Sea ice concentrations more than 15%
was commonly used in the sea ice trend analysis (e.g. Bjøgo et al., 1997; Hanna 2001; Zwally et al., 2002). The
monthly anomaly (deviation) trend of sea ice extent was estimated using the simple least squares regression in each
IWC baleen whale management area. January and February monthly mean sea ice extent trend in each area was also
estimated using the simple least squares regression. The IWC baleen whale management area was listed in Table 1.
Differences of sea ice extent between CPII and CPIII in each management area were calculated using January andFebruary monthly mean data.

2

Abundance estimates listed in Table 1 of Branch and Butterworth (2001) was used from 1984/85 to 1997/98.
Abundance estimates in the 1998/99, 1999/2000 and 2000/01 cruises were extracted from Burt and Stahl (2001), Burtand Hughes (2002) and Burt (2002), respectively.
Bootstrap sea ice data were not available from 2003 to 2004. Abundance estimates of Antarctic minke whale in
Area V in CPIII (2001/02-2003/04) were not completed. For those reasons, comparison between CPII and CPIII in Area
V was not considered in this paper.
To see the relationship among the sea ice extent, krill density and Antarctic minke whale abundance, krill
densities described in Pauly et al. (2000) and Murase et al. (2002), and Antarctic minke whale abundance estimates in
JARPA described in Hakamada et al. (2001) were used. Pauly et al. (2000) conducted the krill survey off East
Antarctica (80-150°E) from January to March, 1996. Murase et al. (2002) conducted the krill survey in Area IV (70130E°) from January to March, 2000. Because the areal coverage of those two surveys was overlap each other, they
were considered as comparative surveys. JARPA was conducted in Area IV in 1995/96 and 1999/2000 and theabundance estimates in corresponding year described in Hakamada et al. (2001) were used.

RESULTSMonthly anomaly sea ice extent trend significantly decreased in Area I while it was significantly increased in Area V
and VI (Table 2, Fig. 1). January and February monthly mean sea ice extent significantly decreased in Area I (Table 3
and 4, Fig. 2). Year to year variations were observed in each data set in each area. Images of January monthly mean ice
extent in each management area in CPI, CPII and CPIII were shown in Fig. 3-8.
Differences of January and February monthly mean sea ice extent (km2), survey area (km2) and abundance
estimates between CPII and CPIII were summarized in Table 5. Sea ice extent in January and February was generally
greater in CPIII than in CPII except Area II and Area VI. Crude abundance estimates in CPII were larger than in CPIII
except Area VI. Sea ice extent in Area II was smaller in CPIII than CPII because unusual large polynya observed in
1997/98.
January and February monthly mean sea ice extent in Area IV in 1996 were 558,484 km2 and 322,609 km2,
respectively. Those in 2000 were 466,461 km2 and 278,184 km2, respectively. Sea ice extents in Area IV were lower in2000 in both months. Krill density in 1996 and 2000 was 5.5g/m2 (CV=17%) and 13.6g/m2 (CV=11%) (recalculated
based on Table 7 of Murase et al. 2002), respectively. First transects in IV-NW and IV-NE were not included in the2000 estimate as described in Murase et al. (2002). Abundance estimates of Antarctic minke whales in the 1995/96 and1999/2000 JARPA in Area IV was 25,660 (CV=18.3%) and 46,145 (CV=14.1%) individuals, respectively. Sighting

3

survey area in the 1995/96 and 1999/2000 JARPA were 1,840,902 km2 1,923,617 km2, respectively. Those results weresummarised in Table 6.

DISCUSSION
The greater sea ice extent in CPIII except Area II and VI could have negative impact on the abundance estimation of
Antarctic minke whale in four ways as follows;
1) High density area of minke whale could not be surveyed by the vessels because of sea ice.
2) Krill abundance could be reduced because of see ice extent and the reduction could affect the abundance estimationof Antarctic minke whale.
3) Narrower longitudinal coverage in CPIII than CPII could be susceptive to the effect of the sea ice change.
4) Greater sea ice extent could reduce the survey area as a whole.

1) High density area of minke whale could not be surveyed by the vessels because of sea ice
As pointed out by Shimada and Murase (2003), changes in sea ice extent and configuration affect the distribution andabundance of Antarctic minke whales because the extensive ice covered the continental slope zone. Krill was abundant
continental slope zone that coincided with ice edge (Murase et al., 2002). Such area is important feeding ground of
Antarctic minke whales (Ichii, 1990). Thus large sea ice extent change could have negative impact on the abundance
estimates.

2) Krill abundance could be reduced because of see ice extent and the reduction could affect the abundance
estimation of Antarctic minke whale
The krill density in 1999/2000 (low sea ice extent year) was larger than in 1995/96 (large sea ice extent year) though theinterpretation of the results needed caution because the areal coverage and survey design was slightly different fromeach other. The krill density in 1995/96 was lower end while it was in 1999/2000 was larger end of the density in thisarea based on past survey results in the Prydz Bay region which were summarized in Pauly et al. (2000). In response to
the change in the density of krill, Antarctic minke whale could be more abundant in 1999/2000 (large krill density) thanin 1995/96 (low krill density). It was reported that poor body fat condition of minke whales were observed when thecontinental slope region was covered by sea ice during the austral summer months (Ichii et al., 1998). The result of Ichii
et al. (1998) suggested that the krill biomass could be low when the sea ice extent was large. Primary production was
low when the sea ice cover was large (Hegseth and Von Quillfeldt, 2001; Arrgo et al., 2002). The low primary4

production in the large see ice cover season could decrease the abundance of krill though further study is required tounderstand the mechanism of krill-summer sea ice extent relationship.

3) Narrower longitudinal coverage in CPIII than CPII could be susceptive to the effect of the sea ice change
Each baleen whale management area was surveyed in single year in CPII while the area was longitudinally separated
into 10 to 50 degree sectors and they were surveyed in different year in CPIII. It took two to four years to complete
survey in a baleen whale management area in CPIII. Narrower longitudinal coverage in CPIII than CPII could besusceptive to the effect of the sea ice change. As shown in Fig 5, western part of Area III in 1994/95 (CPIII) was low
sea ice extent as in 1987/88 (CPII) but only eastern part where the sea ice extent was large was surveyed in this year. If
wider longitude had been covered in 1994/95, the risk of surveying in large sea ice extent would have been hedged. To
test the effect, it could be worth to have Antarctic minke whale abundance estimates in each management area in CPIII(for example, combing 1996/97 and 1997/98 into one to get abundance estimates in Area II) instead of estimating
abundance by year.

4) Greater sea ice extent could reduce the survey area as a whole
It should be noted that abundance estimate in Area VI in CPIII was larger than in CPII. The result indicated that
reduction of sea ice extent in CPIII could be one of the most important factors of the change in abundance estimates in
Area VI because survey design was not substantially changed between CPII and CPIII. Increasing in sea ice extent in
Areas I, III, and IV was coincided with decreasing in the Antarctic minke whale abundance estimates in CPIII. The
result suggested that the large sea ice extent could affect negatively on the abundance estimate though the simple
inference could not be made because survey design was substantially changed in these areas. The effect of greater sea
ice extent could be large in Area V where see ice extent is generally large. It was reported that survey area wassubstantially small in the 2003/04 cruise conducted in Area V (Ensor et al., 2004). Though the abundance estimates in
the 2003/04 is not available, this factor should not be ignored. Though sea ice extent in Area II was decreased in CPIII,
this was due to the formation of large polynya. The effect of the polynya was discussed in Murase and Shimada (2004).

Conclusions
Direct comparison of abundance estimates between CPII and CPIII is not valid unless abovementioned four effects aretaken account. Because Antarctic sea ice has highly variable nature, the effect on the abundance estimates of Antarctic
minke whale could be substantial. Careful treatment is necessary to elucidate the relationship between sea ice andAntarctic minke whales.

5ACNOWLEGEMENT
Comments from Drs. Hiroshi Htanaka and Hidehiro Kato greatly improve the manuscripts. We thank Atsushi Wada forthe sea ice data management. We are grateful to the researchers, captains and crews of IDCR-SWOER for their
dedication colletcting cetacean sighting data under hash environmental conditions in the Antarctic.

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Sekiguchi, K. 2004. 2003-2004 International Whaling Commission-Southern Ocean Whale and Ecosystem
Research (IWC-SOWER) Circumpolar Cruise, Area V. Paper SC/56/IA13 Paper SC/56/IA14 presented to the IWC
Scientific Committee, June 2004 (unpublished). 54pp. [Paper available from the Office of this Journal]
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61595-1598.
Hakamada, T., Matsuoka, K., Nishiwaki, S., Murase, H. and Tanaka, S. 2001. Abundance trend of Southern Hemisphereminke whales in Area IV and V obtained from JARPA data. Paper SC/53/IA12 presented to the ScientificCommittee of the International Whaling Commission, July 2001 (unpublished). 35pp.
Hegseth, E. N. and Von Quillfeldt, C. H. 2001. Low phytoplankton biomass and ice algal blooms in the Weddell Sea
during the ice-filled summer of 1997. Antact. Sci. 14:231-243.
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NIPR Symp. Polar Biol. 3: 34-45.
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(Suppl.) 5: 248-268.
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Rea and Manage. 5:173-201.
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Murase, H. and Shimada, H. 2004. Alternative estimation of Antarctic minke whale abundance taking account ofpossible animals in the unsurveyed large polynya: A case study in Area II in 1997/98. Paper SC/56/IA14 presented
to the IWC Scientific Committee, June 2004 (unpublished). 13pp. [Paper available from the Office of this Journal]
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superba) off East Antarctic(80-150°E) during the Austral summer of 1995/1996. Deep Sea Res. PartII 47:24652488.
Shimada, H, Segawa, K. and Murase, H. 2001. Tentative trial for estimation of Antarctic minke whale abundance
within pack ice region incorporating IDCR/SOWER data with meteorological satellite data. Paper SC/53/IA14
presented to the IWC Scientific Committee, July 2001 (unpublished). 6pp. [Paper available from the Office of this
Journal] 6pp.
Shimada, H. and Murase, H. 2003. Further examination of sea ice condition in relation to changes in the Antarctic
minke whale distribution pattern in the Antarctic Area IV. Paper SC/55/IA7 presented to the IWC ScientificCommittee, May 2003 (unpublished). 6pp. [Paper available from the Office of this Journal]

7

Tynan, C. T. 2002. Need for updated evaluation of climate change and impacts on cetaceans; decrease in Antarctic
minke whale abundance, case in point. Paper SC/54/IA7 presented to the IWC Scientific Committee, 2002
(unpublished). 8pp. [Paper available from the Office of this Journal]
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Geophys. Res. 107(C5):10.1029/2000JC000733.

8

Table 1. IWC baleen whale management areas.

Management areas
Area I
Area II
Area IIIArea IV
Area V
Area VI

Longitude range
120-60W
60W-0
0-70E
70E-130E
130E-170W
170W-120W

Table 2. Monthly anomaly sea ice extent trend in each baleen whale management area from 1978 to 2002.

Management areas
Area I
Area II
Area IIIArea IV
Area V
Area VI

Extent trend
3
2 -1(10 km yr )-9.68
± 3.29-4.41± 5.460.01± 4.08-1.72± 2.649.88± 2.8610.80± 4.57

P-value
<0.05
0.11
1.00
0.20
<0.05
<0.05

Table 3. January monthly mean sea ice extent trend in each baleen whale management area from 1979 to 2002.

Management areas
Area I
Area II
Area IIIArea IV
Area V
Area VI

Extent trend
3
2 -1(10 km yr )±-14.91
7.35-7.15± 23.96-0.54± 7.260.08± 3.6311.50± 12.01
4.27± 18.90P-value
<0.05
0.54
0.88
0.96
0.06
0.64

Table 4. February monthly mean sea ice extent trend in each baleen whale management area from 1979 to 2002.

Management areas
Area I
Area II
Area IIIArea IV
Area V
Area VI

Extent trend
3
2 -1(10 km yr )± 7.15-20.42
4.54± 13.36-0.60± 5.730.05± 4.287.19± 8.137.36± 13.94

P-value
<0.05
0.49
0.83
0.98
0.08
0.29

9

Table 5. January and February monthly mean sea ice extent (km2), survey area (km2) and abundance estimates difference between CPII and CPIII of IWC-IDCR/SOWER.

Sea ice extent (km^2)

Abundance estimates (individuals)

Survey year

Survey area (km^2)
January monthly mean

Area
CPII
1989/90

CPIII

CPII

1993/94

Area I
1986/87

1996/97

CPIII
663,905-1%

798,94219%

1997/98
1987/88

1992/93

Area III

1,510,092-20%

1,648,656-13%

1988/89

1994/95 *3

Area IV

1985/86*4

1990/91

1995/96
2000/01*510%
10%

1,213,222

12%-7%

418,51076%

403,34970%

252,093119%

254,914

121%

507,713

4%

333,5396%

13%402,29228%

1,360,952-7%

638,872-26%

1,011,899-31%

846,893-2%

115,293

313,442
552,490
In analysis

1,469,193

536,271535,2141,001,675

489,026
1998/99

CPIII

Diif CPII &CPIII (%)

1,079,947

237,638
1994/95*2

CPII

Closing mode

486,910

1,892,640

Area II

Area VI

Diif CPII &CPIII (%)

669,193
2000/01*1

Area V

February monthly mean

In analysis
866,637*1

Abundance estimates in eastern sector (110W-120W) was included in CPIII estimates.

*2

Abundance estimate from 70E-80E (Area IV) secotor north to the Prydz Bay was included in CPIII estimates*3

Abundance estimate from Prydz Bay in 94/95 was included in CPIII estimates*4

Abundance estimation in CPIII (2001/02, 2002/03, 2003/04) was not completated.

*5

Abundance estimates in Weastern sector (120W-140W) was included in CPIII estimates.

IO modeSurvey desgin change

CPIII

Diif CPII &CPIII (%)

CPII

CPIII

Diif CPII &CPIII (%)

CPII

CPIII

Diif CPII &CPIII (%)

1,473,185

2,828,453

92%

66,52935,656-46%

61,16841,394-32%

Exntend survey area to 60S

1,698,516

2,581,675

52%

110,984

50,386-55%

128,680

39,852-69%

Exntend survey area to 60S

1,645,428

2,924,967

78%

96,04342,522-56%

145,600

52,134-64%

Exntend survey area to 60S

1,622,080

1,401,723-14%

78,660

9,246

-88%

54,305

15,320-72%

Trackline desgin (ratice tozigzag line)

In analysis

3,304,603
1,911,911

2,546,402

10

CPII

229,175
33%

53,091

In analysis
74,704321,20741%

53,541

In analysis
61,558Stratifcation in western sector
15%

No significant change

Table 6. Sea ice extent, krill density and Antarctic minke whale abundance estimates between the 1995/96 and1999/2000 JARPA in Area IV.

Category

1995/96

1999/2000

2

558,484

466,461

Difference between
95/96 and 99/00 (%)-16%

322,609278,184-14%*2

Sea ice extent (km , January)
2

Sea ice extent (km , February)
2*1

Krill density(g/m )*3

Antarctic minke whale abundance (ind.)2 *3

Sighting survey area (km )5.5
25,66013.6
46,145147%

1,840,902

1,923,617

4%

11

80%

-1.0

0.0

0.5

1980
0.5

1985

1985
-0.6

-1.0

0.0

0.5

1990

1990
1995

Year
1995
2000

Area III

12
-0.2

0.0

0.2

0.4

1.0

1995

-0.4

Sea Ice Extent Anomaly (10^6km^2)-0.5

Sea Ice Extent Anomaly(10^6km^2)

1990

0.0

1980
1985

-0.5

Sea Ice Extent Anomaly (10^6km^2)-0.5

Sea Ice Extent Anomaly(10^6km^2)

1980
Year
2000
1980

2000
1980

1980

1985

Area IYear
1985

Area II

1985

1990
Year

Area IV

1990
Year

Area V

1990

1995
2000

1995
2000

Year
1995
2000

Area VI

Fig. 1. Monthly anomaly sea ice extent trends in each baleen whale management area from 1978 to 2002.
-0.4

-0.4

0.0

0.2

-0.2

0.0

0.2

Sea Ice Extent Anomaly (10^6km^2)-0.2

Sea Ice Extent Anomaly (10^6 km^2)

0.4

0.4

0.40

0.45

Extent(10^6km^2)

0.50

0.55

1.0
0.9
0.8

Extent(10^6km^2)

0.7

0.35

0.6
0.5

1980

1985

1990

1995

2000

1980

1985

Year

1990

1995

2000

1995

2000

1995

2000

Year

Area IV

0.4

0.6

0.8

Extent(10^6km^2)

2.0
1.5

Extent(10^6km^2)

1.0

2.5

Area I

1980

1985

1990

1995

2000

1980

1985

Year

1990
Year

Area V

1.4
1.0

1.2

Extent(10^6km^2)

0.5
0.4
0.2

0.8

0.3

Extent(10^6km^2)

0.6

1.6

0.7

Area II

1980

1985

1990

1995

2000

1980

Year

1985

1990
Year

Area IIIArea VI

Fig. 2. January monthly mean sea ice extent trend in each baleen whale management area from 1979 to 2002. Black
filled circles indicate the years when the IDCR-SOWER surveys were conducted.

13

Fig. 3. January monthly mean sea ice extent image in Area I (CPI=1979; CPII=1990, CPIII=1994, 2000 and 2001). Red
shadows indicate surveyed area.

Fig. 4. January monthly mean sea ice extent image in Area II (CPI=1982; CPII=1987, CPIII=1997 and 1998). Red
shadows indicate surveyed area.

Fig. 5. January monthly mean sea ice extent image in Area III (CPI=1980; CPII=1988, CPIII=1993 and 1995). Red
shadows indicate surveyed area.

14

Fig. 6. January monthly mean sea ice extent image in Area IV (CPI=1979; CPII=1989, CPIII=1995 and 1999). Red
shadows indicate surveyed area.

Fig. 7. January monthly mean sea ice extent image in Area V (CPI=1981; CPII=1986, CPIII=2002). Red shadows
indicate surveyed area.

Fig. 8. January monthly mean sea ice extent image in Area VI (CPI=1984; CPII=1991, CPIII=1996 and 2001). Red
shadows indicate surveyed area.

15