SC/56/IA14
Alternative estimation of Antarctic minke whale abundance
taking account of possible animals in the unsurveyed large
polynya: A case study in Area II in 1997/98.

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

ABSTRACTAn Antarctic baleen whale management area, Area II (0°-60°W), was surveyed in 1986/87 as second circumpolar survey (CPII) and, in
1996/97 and 1997/98 as third circumpolar survey (CPIII). Whole area of Area II was covered in 1986/87. In CPIII, the area was separated
into two 30 degree sectors at 30°W. The sector between 0° and 30°W was surveyed in 1996/97 and the rest (30°W-60°W) was surveyed in
1997/98. Sea ice extent in the southeastern Weddell Sea was unusually large in the summer of 1996/97. Unusual extensive sea ice free area,polynya, adjacent to southeastern side of Antarctic Peninsula was observed in the 1997/98. No such a large polynya was observed during
satellite observation period (1979-2002). The survey vessels could not conduct survey in the polynya because sea ice prevented the accessto there. The area of the polynya was estimated as 78,615 n.mile2 using satellite derived data. It was 25.6% of the total survey area in
1997/98. Abundance estimates in Area II (closing mode) in CPII and CPIII were 142,837 and 46,414 individuals, respectively (Branch andButterworth, 2001). Abundance estimates in CPIII increased to 86,264 individuals after adding estimates in the polynya. Abundance
estimate in the 1997/98 would be underestimated because of presence of the large polynya. Although it is difficult to estimate the impact of
sea ice configuration change on the abundance estimation exactly, the effect of the large polynya on the abundance estimate should be
evaluated further.
KEYWORD: ANTARCTIC MINKE WHALE; SEA ICE; POLYNYA; ABUNDANCE ESTIMATE; ANTARCTIC

1INTRODUCTION
The International Whaling Commission (IWC) has conducted the Antarctic minke whale abundance assessment cruises
since 1978/79 in the Antarctic in austral summer (Matsuoka et al. 2003, for review). The name of the cruises werecalled as the International Decade of Cetacean Research programme (IDCR) from 1978/79 to 1995/96 and then changed
to the Southern Ocean Whale and Ecosystem Research programme (SOWER) from 1996/97 2003/04. The cruises
covered three circumpolar 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 1991/92-1997/98 (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. Several hypotheses that might affect the apparent abundance change were pointed out (IWC 2003(b)). One of
the factors is proportion of Antarctic minke whales population south of the ice edge where survey vessels can not enter.
The 1996/97 SOWER was conducted in eastern half (0°-30°W) of Area II. It was reported that amount of sea
ice in the southeastern Weddell Sea was unusually large (Hegseth and Von Quillfeldt 2001). The 1997/98 SOWER was
conducted in western half (30°W-60°W) of Area II. Though the large sea ice free area, polynya, was observed adjacent
to southeastern side of Antarctic Peninsula during the cruise, the vessels could not conducted survey in the area because
sea ice prevented the vessels accessing (Ensor et al. 1998). The polynya was called as the Ronne Polynya and it hadlargest extent in 25 years satellite observation periods (Ackley et al. 2001). Shimda et al. (2001) tentatively estimate theabundance of Antarctic minke whales in the pack ice in this region but the study was not considered the polynya
specifically. In this paper, abundance estimation of Antarctic minke whale in the polynya was carried out. The
implications of unusual sea ice conditions in CPIII for the interpretation of the abundance difference between CPII and
CPIII in Area II were discussed.

MATERIALS AND METHODS

Cetacean survey data
IWC divided southern ocean into six areas for the management of baleen whales (Donovan, 1992). Area II is set
between 0° and 60°W (Fig. 1). Sighting survey for the abundance estimation of Antarctic minke whale was conductedin austral summer. CPII survey was conducted from December 28, 1986 to February 4, 1987. Full longitudinal coverage
was made in 1986/87 cruise. In CPIII, Area II was divided into two at 30°W. Eastern half (0°-30°W) was surveyed in
1996/97 cruise (January 16-February 14, 1997) whereas western half (30°W-60°W) was survey in 1997/98 cruise(January 18-February 14, 1998). Latitudinal coverage was south of 60°S to ice edge except 0-°20°W of CPII where the2northern limit was set at 61°S-63°S. Sighting trackline data, positions of sighted Antarctic minke whale and ice edge
data stored in the IWC Database-Estimation Software System (DESS) version 3.0 (Strindberg and Burt, 2000) wereused. The ice edge is defined by a level of ice cover that prevents the survey from being conducted at normal survey
speed (11.5 knots) (IWC, 2003(a)). The ice edge was obtained by the dedicated ice edge survey vessel in CPII whereas
it was estimated using the sea ice chart published by the National Ice Center (US) except around the waypoints.
Because the sea ice chart was published weekly, the estimated ice edge using it lacked precision of CPII though the data
were comparable (Ensor et al., 1997; Ensor et al., 1998). To estimate Antarctic minke whale abundance in the polynya,
estimated density of whale (per n.mile2, Dw) in Branch and Butterworth (2001) was used. The extrapolation method
which was applied to unsurveyed northern areas in Brach and Butterworth (2001) was used. The both closing and IO
mode density data were used. Survey area (n.mile2) described in the paper was also referred.

Satellite sea ice data
Satellite derived daily sea ice data, Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I(Comiso, 1999) from 1978 to 2002 was used to map the see ice concentration and extent south of the ice edge. Sea ice
observation using the satellite passive microwave sensors was started with the launch of Scanning Multichannel
Microwave Radiometer (SMMR) on Nimbus-7 in 1978. The sensor was changed to Special Sensor Microwave/Imager(SSM/I) in 1987 and the data collection is still on going. The daily satellite data corresponding to midday of thesurveyed period in each 5-10 latitudinal and longitudinal degree sectors were integrated into a map using geographic
information system (GIS), Marine Explorer (Environmental Simulation Laboratory Co. LTD, Japan). Because the mostof the ice edge was estimated using the weekly charts in CPIII, they were referred to determine the appropriate date of
the satellite data. The area of the polnya was calculated using the map. The polynya was defined as 0% sea ice
concentration area south of the ice edge in this paper. Monthly mean sea ice satellite data in January from 1979 to 2002
were used to investigate long term sea ice configuration change in Area II.

Satellite chlorophyll-a concentration data
To see the chlorophyll-a concentration in Area II in January 1998, The SeaWiFS (Sea-viewing Wide Field-of-view
Sensor) Level-3 monthly averaged chlorophyll-a concentration image (provided by the SeaWiFS Project,
NASA/Goddard Space Flight Center and ORBIMAGE) was used. Because the SeaWiFS image is available after
September, 1997, there is no satellite chlorophyll-a concentration image for the 1986/87 and 1996/97 cruise.

3

RESULTS
Monthly averaged sea ice extent image in the Area II in January from 1979 to 2002 was shown in Fig. 2. No large
polynya was observed during the satellite observation period except 1998.
Sighting survey trackline, sighting positions of Antarctic minke whale, ice edge line defined by the survey
vessels and satellite derived sea ice concentration data in CPII and CPIII were overlaid in Fig. 3 and Fig. 4, respectively.
Positions of ice edge defined by the satellite and the vessel had good agreement though there was some discrepancy
between them. The sea ice extent in the southern part of the 1996/97 cruise was larger than that in the 1986/87 cruise. In
1997/98 cruise there was a large polynya which were not observed in the 1986/87 cruise. The area of the polynya wasestimated as 78,615 n.mile2. The total survey area in the 1997/98 was 306,981 n.mile2. The area of the polynya was
25.6% of the total survey area. The abundance estimates in the polynya using the density estimates from adjacentsouthern stratum (ES1 and ES2 in Fig. 5) were shown in Table 1. The range of the estimates were from 5,417 to 39,850
individuals. Abundance estimates of CPII and CPIII in Area II in Branch and Butterworth (2001) was summarized inTable 2. CPIII abundance in closing and IO mode was 32% and 20% of CPII, respectively. After including theabundance estimates in the polynya in the estimates in Table 2, the ratio was reduced (Table 3).
SeaWiFS image suggested that higher chlorophyll-a concentration was occurred in the polynya region in
January 1998 (Fig. 5). The chlorophyll-a concentration in the survey area was low relative to the polynya.

DISCUSSION
Southern portion of the 1996/97 survey area (0°-30°W) had larger amount of sea ice than the 1986/87 survey. Because
the heavy sea ice condition in this area limited the growth season of the phytoplankton, biomass of the phytoplankton
was low in this area in the summer of 1997 (Hegseth and Von Quillfeldt 2001). Antarctic krill (Euphausia superba), thedominant prey species of Antarctic minke whale (Ichiii and Kato 1991), feed predominantly on the phytoplankton(Miller and Hampton 1989). If the biomass of the phytoplankton was low, growth and sexual development ofAnarchistic krill was negatively affected. In the case of the food scarcity, Antarctic minke whales could change thefeeding area in response to it. The western Weddell Sea was also covered with high concentration of sea ice in the
1996/97 survey. Because Antarctic minke whales were move across the IWC baleen whale management area boundary(Wada 1984), some proportion of the population in Area II could move to adjacent area, such as Area III. Migration
behavior in response to the food availability could affect the change in apparent abundance estimation between CPII
and CPIII.
In contrast, there was a large polynya in the 1997/98 survey. SeeWiFS satellite image suggested that the
primary production in the polynya was high. The size of the polynya increased from November and reached maximum4

(300,000 km2 = 87,466 n.mile2) about on 1 March (Ackley et al. 2001). The prolonged open water period in the polynya
could enhance the primary production. Coastal polynya is observed in the Ross Sea region during spring (Zwally et al.
2001). High density of Antarctic minke whales was also observed in this region (Kasamatsu et al. 1998). SeaWiFS data
indicated that high values of chlorophyll were occurred in the Ross Sea region (Moore and Abbott, 2000). In the heavy
sea ice coverage year because of the presence of ice berg, the primary production in this area decreased (Arrigo et al.
2002).
Abundance estimate in the 1997/98 would be underestimated because of presence of the large polynya. The
effect of the polynya on the CPIII abundance estimates in Area II must be taken account though it is difficult to estimate
Antarctic minke whale in the south of the ice edge where the sighting vessels can not conduct the survey (IWC 2002).
Without considering the effect of the large polynya, the direct comparison of abundance estimates between CPII and
CPIII is invalid.

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.

5

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Antarctic iceberg. Geophys. Res. Letter. 29(7):10.1029/201GL014160.
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Table 1. Abundance estimates in the polynya observed in 1997/98 cruise. Density estimates in Table 1 of Branch andButterworth (2001) was used.

Surveymode
Closing
Passing

Stratum
ES1
ES2
ES1
ES2

Area of
Polynya(ind/n.mile2) (n.mile2)
0.06890.506978,6150.12920.0819DensityP (ind.)5,417
39,850
10,157
6,439

Table 2. Abundance difference between CPII and CPIII reported in Table 6 of Branch and Butterworth (2001).

Surveymode
Closing
IO

CPII
142,837
159,596

P (ind)(Year)
CPIII
46,414
1986/87
32,259(Year)
1996/97 &1997/98

Ratio(CPIII/CPII)
32%
20%

Table 3. Abundance difference between CPII and CPIII after including abundance estimates in the polynya in 1997/98.

Surveymode

CPII

Closing

110,984

IO

128,680

P (ind)CPIII(corrected)
51,831
86,26442,416
38,698

Ratio(Stratum) (CPIII/CPII)
ES1
47%
ES2
78%
ES1
33%
ES2
30%

8

Fig. 1. Schematic map of survey area, Area II.

9

Fig. 2. January monthly mean sea ice concentration images in Area II from 1979 to 2002.

10

Fig. 2. (Continued)

11

Fig. 3. Sighting survey trackline (black line), sighting positions of Antarctic minke whale (pink open circle), ice edge
line (red line) defined by the survey vessels and satellite derived sea ice concentration in the 1986/87 CPII Area II
cruise.

Fig. 4. Sighting survey trackline (black line), sighting positions of Antarctic minke whale (pink open circle), ice edge
line (red line) defined by the survey vessels and satellite derived sea ice concentration in the 1996/97 (0°-30°W) and1997/98 (30°W-60°W) CPIII Area II cruise. Pink hatched area was defined as the Ronne Polynya in this study.

12

Fig. 5. Strata surveyed in the 1997/98 cruises (from Branch and Butterworth, 2001).

Fig. 6. Chlorophyll-a concentration in Area II in January, 1998.

13