SC / 60 / IA 12

Influence of sea ice concentration in the research area on IDCR-SOWER
abundance estimation
KOJI MATSUOKA, SHIGETOSHI NISHIWAKI, HIROTO MURASE, NAOHISA KANDA, SAEKO KUMAGAIAND HIROSHI HATANAKAThe Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan
Contact e-mail:matsuoka@cetacean.jp

ABSTRACT
We examined the relationship between the sea ice condition using “sea ice concentration” as an index thatobserved during IDCR-SOWER surveys, and the abundance of Antarctic minke whales estimated using themodel of Okamura and Kitakado (2008). An operation of sighting survey and abundance of Antarctic minke
whales appears to be influenced by the changes in the sea ice condition because the sighting survey cannot
cover all of water in research areas due to higher sea ice concentration and formation of a polynya. We thus
hypothesized that, if the abundance estimates from CPII and CPIII from a given research area were
substantially different, the sea ice condition should have been also quite different between the two. In fact,
this relationship was clearly seen in Area II (Weddell Sea) and Area V (Ross Sea) where the shape of ice
field varies substantially year by year due to changes in the sea ice condition. Abundance estimate was highat the year when the survey was able to approach and cover waters adjacent to the continental shelf andcontinental slope and was low when the survey was not able to do so due to the high ice concentration.
Recent sighting survey conducted on the Japanese, German and Australian ice breaker and aerial survey
reported existence of Antarctic minke whales in such areas. These observations strongly support thishypothesis and further qualitative analysis is required to accommodate the discrepancy.
KEY WORDS: ANTARCTIC, SURVEY-VESSEL, ABUNDANCE ESTIMATE, SEA ICE, ANTARCTICMINKE WHALE

INTRODUCTION
In recent years, SC/IA sub-committee recommended that the issue of sea ice condition in research areas thatappeared to influence abundance estimates of Antarctic minke whales should be examined using sea ice
condition indices such as sea ice concentration (“Changes in the location of the ice-edge and the proportionof animals south of the ice-edge”; IWC,2003, 2005; Branch, 2007). The sub-committee also concerns about
estimation of the Antarctic minke whales abundance inside sea ice. Some preliminary studies for thisincluding spatial analysis have been presented (IWC, 2006, 2007).
It has been observed that Antarctic minke whales are concentrated along the ice edge. Similarly,
Kasamatsu et al. (2000) reported a significant negative correlation between Antarctic minke whale encounterrates and distance from the sea ice edge. In addition to that, recent sighting and aerial surveys conducted bythe Japanese, German, and Australian ice breakers and airplanes that covered the areas where SOWER
vessels were not able to enter due to sea ice reported existence of many Antarctic minke whales in such areas

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(Shimada and Murase, 2006, Shimada and Kato, 2007, Scheidat, 2007). Therefore, abundance of Antarctic
minke whales estimated from the data that was unable to cover these areas should be underestimated.
In this document, in order to illustrate influence of sea ice condition on abundance of Antarctic
minke whales estimated from the historical IDCR-SOWER survey data, we examined the relationshipbetween the sea ice condition, “sea ice concentration” observed during IDCR-SOWER surveys as indices,
and abundance of Antarctic minke whales estimated using the model described by Okamura and Kitakado,(2008).

MATERIAL AND METHODSTo see the effect of change in sea ice concentration on abundance estimation of Antarctic minke whales by
the IWC baleen whale management area (Areas I-VI), January sea ice concentration anomaly by each
latitudinal sector from 60°S to the Antarctic continent from 1979 to 2006 was calculated using satellite
derived monthly sea ice data, Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I(Comiso, 1999). Resolution of original data was 25 * 25 km grid in polar stereographic projection and sea
ice concentration in each grid was available in the original data. Firstly mean sea ice concentration by each
latitudinal sector from 60°S to the Antarctic continent in each year was calculated using original data.
Secondary, anomaly from mean sea ice concentration values from 1979 to 2006 for respective latitudinal
sector was calculated. The anomaly data were then plotted on time series contour plot.
January mean sea ice concentration by each IWC baleen whale management Area (I-VI) at theyear of IDCR/SOWER survey was also calculated as an index of sea condition. The calculated January meansea ice concentration by each Area was plotted against estimated abundance of Antarctic minke whale basedon the OK method.
To clarify the effect of sea ice of abundance estimation of Antarctic minke whale, past papers(Murase and Shimada, 2004; Murase, et al., 2005; Shimada and Murase, 2006) were revisited in this paper.

RESULTS AND DISCUSSIONS
Abundance estimates (with confidential intervals) of Antarctic minke whales during CPII and CPIII wereplotted for each of the six research areas covered by IDCR/SOWER sighting survey (Fig. 1). The abundance
estimates at Areas II and V substantially differed between CPII and CPIII, especially in Area V. At the bothAreas, the abundance estimates were high during CPII and low during CPIII. This could indicate a decreaseof Antarctic minke whales over time or it could simply be due to operational difficulties on sea ice field. At
this time, we go for the latter explanation because it is known that shape of ice field usually varies
substantially year by year due to sea ice condition especially at Areas II and V, resulting in changes incoverage area for sighting survey followed by subsequent underestimation of Antarctic minke whales
abundance. Antarctic minke whale abundance should be underestimated if sighting survey cannot cover allof the water in the research areas due to the higher sea ice concentration and the formation of polynya thatprevent the research vessels (but not Antarctic minke whales) from approaching the waters adjacent to shoreline. We thus hypothesize that, if abundance estimates from CPII and CPIII from the same research area
were substantially different, the sea ice condition should have been also quite different between the two.
We carefully compared the sea ice condition during CPII to that during CPIII at the research area
(Fig. 2, 3a and 3b). During CPIII, a huge polynya was formed at Area II (Weddell Sea) and the ice edge wasextended southwestward at Area V (Ross Sea). This ice condition should have resulted in substantial
reduction in coverage for sighting survey (Figs. 3a and 3b) followed by low abundance estimates during
CPIII compared to CPII. Fig. 3a shows an unusually extensive sea ice free area, polynia, adjacent to the2southeastern side of the Antarctic Peninsula observed in the CPIII-Area IIW. Such large polynia were notobserved via satellite during the rest of the observation period (1979-2002). The vessels could not survey
within the polynia because the sea ice prevented access to the polynia (IWC, 2006). Similarly, Fig. 3b showsa narrower area of coverage by the sighting survey during CPIII due to extension of ice edge in the Ross
Sea.
We then looked into the level of sea ice concentration as an index for sea ice condition. We
depicted sea ice concentration anomalies in January from 1976 to 2006 (Fig. 4). Under the high level of ice
concentration, research vessels are restricted to narrow water. At Area V (Ross Sea), the sea ice
concentration anomaly during CPII was the lowest in the past 30 years, indicating that during CPII thesighting survey were able to cover the research area thoroughly. In contrast, the sea ice concentration
anomaly was high during CPIII, indicating the sighting survey was unable to cover most, if not all, of theresearch area. At Area II (Weddell Sea), the sea ice concentration anomaly was high during CPIII. All of theinformation indicated that at Areas II and V the areas covered by the sighting survey quite differed between
CPII and CPIII.
Influence of the sea ice concentration was thus clearly seen on abundance estimates of Antarctic
minke whales during CPII and CPIII (Fig. 5). As we hypothesized, at Area V, a negative relationship wasdetected as the abundance estimate was lower with higher sea ice concentration. Magnitude negative
influence was observed in Area V. Contrast to Area V, no such relationship between the ice condition andabundance estimates was observed at the Areas III, IV, and VI where Antarctica land shape and ice shape
was not variable. Such relationship was not also detected in Area II. We think, however, that in Area II
abundance estimate during CPIII was mainly influenced by the formation of the huge polynya.
Resent sighting survey conducted on the Japanese, German, and Australian ice breakers andIDCR-SOWER aerial survey in fact reported existence of many Antarctic minke whales in such areas(Shimada and Murase, 2006, Shimada and Kato, 2007, Scheidat, 2007). Spatial analysis also showed similarresults (Fig. 5; Murase et al., 2005; IWC, 2006). These observations support our hypothesis.
Further studies are necessary to investigate large impact of sea ice conditions on Antarctic minke
whale abundance estimation especially in Areas II (Weddell Sea) and Area V (Ross Sea) such as case studyof analysis by 10 degree longitudinal slices for small scale study area which already started in IA e-mail sea
ice working group.

ACKNOWLEDGEMENTS
We wish to express our gratitude to Hiroyuki Shimada, Seiji Ohsumi, Yoshihiro Fujise, Hiroshi Hakamada
and Mitsuyo Mori for their constructive suggestions to improve a previous version of this paper. We also
thank Keisuke Kimura for providing sea ice concentration data and Tomoko Hasegawa for her help to draw
figures.

REFERENCES
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whales from the IDCR/SOWER surveys between the second and third circumpolar sets of cruises. 9pp.
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Boulder, CO, USA: National Snow and Ice Data Center. Digital media.
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this Journal].
Murase, H., Shimada, H. and Kitakado, T., 2005. Alternative estimation of Antarctic minke whaleabundance taking account of possible animals in the unsurveyed large polynya using GAM-based spatialanalysis: A case study in Area II in 1997/98 IWC/SOWER. Paper SC/57/IA6. 20pp.
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4Area I

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Figure 1. Abundance estimates of Antarctic minke whales between CPII and CPIII with 95 % confidence
intervals by OK method with CNB (Okamura and Kitakado, 2008). In Area I, triangle indicated the “surveyonce (1994+2000+2001)” and black circle indicated abundance combined “1994+2001”.

5Figure 2. Top and middle figure shows Ice edge estimated by SOWER vessels and ice concentration fromsatellite information in CPII and CPIII, respectively. Bottom figure shows ice edge anomaly between CPIIand CPIII. Pink color shows extent sea ice north CPII than CPIII and blue color shows it extent north CPIII
than CPII (Shimada and Murase, 2006). We focused huge polynia in Area II (middle figure, red circle) andlarge unsurveyed areas in Area V (bottom figure, black circle).

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Fig. 3a. Relationship between sea ice extent and Antarctic minke whales in Area II (Weddell Sea). Maps ofsighting survey track lines, boundary lines of strata, sighting positions of Antarctic minke whale (opencircle), and satellite derived sea ice concentrations in CPII (left) and CPIII (middle and right) in Area II.
Hatched area in CPIII was the Ronne Polynia (Murase et al., 2005).

Fig. 3b. Relationship between sea ice extent (without sea ice concentrations) and Antarctic minke whales inArea V (Ross Sea). Maps of sighting survey track lines, boundary lines of strata, primary sighting positionsof Antarctic minke whale (open circle), and observed sea ice edge line by research vessels in CPII (left) andCPIII (left and right) in Area V. Data were obtained from the IWC/DESS.

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Figure 4. Sea ice concentration anomaly (south of 60S) in January by longitude (from 1979 to 2006) with
IDCR/SOWER survey year (colored squares). Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR andDMSP SSM/I (Comiso, 1999).

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Area I

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Figure 5. Sea ice concentration of south of 60S in January from 1979-2006 with IDCR-SOWER survey year.
Data set was from National Snow and Ice Data Center (NSIDC). IDCR/SOWER abundance estimates wereobtained from Okamura and Kitakado (2008). Square (blue) in Area I: 1994+2001.

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