NOT TO BE CITED WITHOUT PERMISSION BY THE AUTHORS

JA/J05/JR9

Yearly trend of blubber thickness in the Antarctic minke
whale Balaenoptera bonaerensis in Areas IV and V
KENJI KONISHI1), TSUTOMU TAMURA1) AND LARS WALLØW2)
1)The Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan2)Department of Physiology, University of Oslo, 1103, Blindern N-0317 Oslo, Norway
KEYWORDS: ANTARCTIC MINKE WHALE; BODY CONDITION; BALAENOPTERA; BLUBBER THICKNESS;SCIENTIFIC PERMITS

ABSTRACTTo examine the recent trend of body condition and factors that affect the body
condition in Antarctic minke whale, blubber thickness from 1987/88 to 2003/04 in Areas IV
and V were analysed. In the stepwise multiple regression analyses, the blubber thickness at
mid-lateral position was used as dependent variable, and date, diatom film adhesion, sex,
longitude (Area) and year was used as explanatory variables. To see the trend of body
condition in the end of the feeding season, blubber thickness in February were also examinedin each Area and sex. The results of multiple regressions confirmed that blubber thickness of
the Antarctic minke whale has decreased for JARPA survey period, and showed all
explanatory variables were detected as predictor variables. The results in February also
showed decline of blubber thickness over JARPA survey without in pregnant female in Area
V. These results indicated that food availability for Antarctic minke whale in Antarctic has
decreased. This decline possibly caused by both intraspecific and interspecies competitionsfor krill resources.
INTRODUCTION
The Antarctic Ocean consists of simple food web with high productivity and is an
important feeding area for the baleen whale, which is one of the major predator groups,
consuming mostly on Antarctic krill (Euphausia superba). Whales become fat over summer
feeding period and leave for low latitude area for reproduction. Like other baleen whales, theAntarctic minke whale (Balaenoptera bonaerensis) migrates to the Antarctic and spend
austral summer season to feed.
Generally, it is well known that the overexploitation by commercial whaling inSouthern Hemisphere diminished the population of large baleen whales (Laws 1977). Finally,
the composition of whale species in abundance was totally changed when the commercial
whaling was suspended in 1987, comparing to that in pre-whaling period. The Antarctic
minke whale, on the other hand, was not targeted in commercial whaling till 1971, and its
population was not affected largely by the whaling. Thus, the minke whale population started
to grow after the harvest of large baleen whale was started, and then it continued until
recently. This was probably due to the favourable food availability caused by the decline ofother large baleen whale populations, so called “krill surplus” hypothesis (Laws 1977).
Recent sighting surveys of SOWER and JARPA demonstrated the recent recoveryof humpback (Megaptera novaeangliae) and fin whales (B. physalus) (Matsuoka et al. 2005).
Branch et al. (2004) also reported that Antarctic blue whale (B. musculus) population has

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increased, although its population size is still small. According to Ohsumi et al. (1997), thebody condition of minke whale increased until the end of 1970’s, then decreased with years,
and this yearly trend was caused by environmental change or carrying capacity in the
Antarctic. If this is true, the body condition in the Antarctic minke whale presumably
continues to decrease, because the recent recovery of large baleen whales could cause further
shortage of food availability for minke whales.
Blubber is one of the most important tissues in relation to buoyancy, stream line andenergy storage, and its thickness links directly to feeding habits (Mackintosh and Wheeler
1929). Blubber thickness of baleen whales is known to be changed through season, byreproductive activity and growth, and among populations (Mackintosh and Wheeler 1929),
and its variation is sometimes expressed as "fatness" or "body condition". Monitoring the
body condition of animals is important for assessing energetic health of food availability in agiven population (Beck et al. 1993). However, the research areas cover broad area, and theresearch period is almost three month. The density of krill population is not uniform at entire
Antarctic Ocean (see Atkinson et al. 2004), and migration and pregnancy of whales also
affect their body condition. So, duration of stay in feeding area, geographical and biologicalvariation in blubber thickness should be considered for its analysis.
“The elucidation of role of cetaceans in the Antarctic ecosystem” and “The elucidationof effect of environmental change on whale stocks” are objectives of the JARPA project(Government of Japan 1996). In previous review of JARPA, Ichii (1997) concluded that theminke whale is very important top-predator species in terms of amount of krill consumption
in Areas IV and V. Ohsumi et al. (1997) mentioned that the body condition and its yearlychange of Antarctic minke whale will contribute for the JARPA research objectives.
The objective of this study is to examine the recent trends of blubber thickness of the
Antarctic minke whale to confirm whether this shows trends similar to those reported
previous studies. The second objective is to examine the factors that affect the body conditionand discuss the energetic status for the minke whale in the Antarctic Ocean.
MATERIALS AND METHODS
Sampling and measurement
All Antarctic minke whales were taken by the Japanese Whaling Research Program
under Special Permit in the Antarctic (JARPA). Animals used in this study were limited toAreas IV and V in alternate years from 1989/90 to 2003/04 (Fig. 1). After capturing by threesighting/sampling vessels, the animals were placed aboard a research base vessel where they
were examined. After outer observation was conducted, body length (from snout tip to tailfluke notch in a straight line along the deck) and other morphological parts were measured.
Blubber thickness was measured to the nearest mm, perpendicularly from skin to muscle
without connective tissue and black surface skin in dissection. The blubber thickness at
mid-lateral region (Fig. 2) used in this study have relatively low correlation coefficient withbody length (0 < r < 0.1) and become thick in relation to feeding days (Ohsumi et al. 1997).
Sex and maturity were recorded for each whale from the observation of reproductive organs
routinely in dissection and tissue observation in laboratory.
Statistical analysis
Juvenile animals are a little fatter than adults (Ohsumi et al. 1997), and lactating
females are thinner than other female mature classes in baleen whales (Lockyer 1987). So, weused a total of 3796 mature males and pregnant, but not lactating females to avoid biases.

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To examine recent trend and what factors influence the variation of blubber thickness,
we conducted stepwise multiple regression analyses, and all variables were included in themodel at 5% level. Blubber thickness (in cm) was the independent variable. In the first run,
we used following independent variables; “date”(December 1st= day 1), “diatom adhesion”(0to 4, Table 1), “sex”(0:female, 1:male), “area”(4:Area IV, 5:Area V) and “body length”(in m).
Additionally, year was also included as an independent variable because it reflects the yearly
trend of food availability (87/88=1 89/90=2 91/92=3...). The variables were included in theorder: First date, then diatom film, sex, area, year and at last body length. In a second run,“longitude” (in degrees east) was substituted for “area”. Diatom is sometimes observed on thesurface of the whales like a film and sometimes covers entire bodies. The degree of diatom
adhesion on the skin becomes a rough indicator of time period for the whale to stay in thecold waters in general (Ohsumi et al. 1997).
We also conducted regression I analyses for mean blubber thickness with year in each
Areas IV and V to simply examine the recent trend of blubber thickness. In these analyses, weused data in February (end of feeding season) as same as the last JARPA review (Ohsumi etal. 1997).

RESULTS
Blubber thickness ranged from 1.8 to 7.2 cm in mature males, and from 2.0 to 7.7 cm
in pregnant females in both areas over all research period (Table 2).
Multiple regressions identified all independent variables as a predictor of blubber
thickness in the both runs (Table 4 and 5). The “date” is a best predictor of the blubber
thickness followed by “diatom”, “sex”, “area”, “year” and “body length”. The second run
showed similar result with the first run. The “date” is a best predictor of the blubber thickness
followed by “diatom”, “sex”, “year”, “Longitude (E+W)” and “body length”. The coefficientsof “Year” were -0.024(±0.003SE) in the both runs, indicated the blubber thickness
significantly decreased with years (Fig. 3). Blubber thickness change with date and longitude
were also shown in Figures 4 and 5. These results indicated that the blubber thickness become
thick with feeding dates and toward East. And the blubber in males was thinner than that infemales.
Yearly changes of blubber thicknesses in February are shown in Table 3 and Figure 6.
The overall blubber thickness decreased through JARPA for males (Area V: r = -0.77, p <0.05), although the correlation was not statistically significant in Area IV (r = -0.62, p = 0.08).
The blubber thickness decreased through time for pregnant female in Area IV (r= -0.78, p <0.05), but no consistent trend was detected for pregnant females in Area V (r = 0.05, p =0.91).
DISCUSSION
Yearly trend of body condition
Our results clearly showed that blubber thickness of Antarctic minke whales has
decreased over JARPA in Area IV and V. According to Ohsumi et al. (1997), this decline
started since late 1970’s, indicating these declines has occurred for over twenty years. As
already mentioned, blubber thickness is directly linked to food availability, so this continuing
deterioration of blubber thickness is equal to the decline of food availability in the Antarctic
for each individual. Our results showed blubber in Area IV was thinner than that in Area V,

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and increased from east to west, suggesting there was a geographical difference of foodavailability for minke whales.
The elucidation of role of cetaceans in the Antarctic ecosystem
In the last JARPA review meeting in 1997, it was great concern which hypothesis“Whale over-hunting - krill surplus” or “Environmental changes” is better to explain thedeterioration of body condition in the Antarctic minke whale over few decades. However bothhypotheses were not excluded because of little information (see Ichii 1997). The
environmental change is important factor which affect the krill abundance, and the abundancearound Antarctic Peninsula decreased since the 1970’, because of the sea-ice extension
induced by high temperature (Loeb et al. 1997, Atkinson et al. 2004). However, no suchenvironmental trend was observed in JARPA (Watanabe et al. 2005).
Our results indicated the decline of food availability for the minke whale in Area IV
has occurred for over two decades, because the decline of body condition started in the end of
1970’s (Ohsumi et al. 1997). Meanwhile, the peaks of the recruitment in the minke whale
were estimated from 1970 to the mid-1980’s by Butterworth et al. (1999). If the trend ofphysical environment has been stable, it is conceivable the total biomass of consumers was
already near carrying capacity more than twenty years ago. As a reason for the deteriorationof food availability for the minke whale, its intraspecific and interspecies competitions forfood among krill consumers were both important factors.
Lately the recovery of large baleen whales such as fin and humpback whales(Matsuoka et al. 2005) were reported. These large whales feed on the Antarctic krill and sharesame food niche with the minke whale (Nemoto 1962, Kawamura 1980), so their recovery
could cause the further deterioration of food availability for the minke whale.
Conclusion and Future Studies
We conclude, again, the food availability of the Antarctic minke whale has decreasedin Area IV and V, so “food surplus period” had already shifted to “competition period”.
Therefore, we need to focus on interactions among consumers for food to examine population
dynamics of whales. To understand the propositional content, the study of body condition isnecessary as a part of ecosystem monitoring in the Antarctic Ocean. In addition, thecoincidental monitoring of the physical and prey environment is also important (Ichii 1997).
The comprehensive work will provide important information of “what is going to happen in
Antarctic Ocean ecosystem near future” for sustainable management of whales.
ACKNOWLEDGEMENT
We would like to thank the all captain, crews and researchers, who were involved in
JARPA survey, and Mrs. R. Zenitani, Mr. T. Bando and Dr. Y. Fujise for providing us
maturity and fetus length data in JARPA, and Mrs. Y. Hosone helping us for organizing data
set. We also thank people who made useful comment on this paper.
PREFERENCES
Atkinson, A., Siegel, V., Pakhomov, E. and Rothery, P. 2004. Long-term decline I krill stock andincrease in salps within the Southern Ocean. Nature. 432: 100-103.
Beck, G. G., Smith, T. G. and Hammill, M. O. 1993. Evaluation of body condition in the Northwest
Atlantic harp seal (Phoca groenlandica). Canadian Journal of Fisheries and Aquatic Science 50:
1372-1381.

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Branch et al. 2004. Evidence for increases in Antarctic blue whales based on Bayesian modelling.
Marine Mammal Science 20:726-754.
Butterworth, D.S., Punt, A.E., Geromont, H.F., Kato, H. and Fujise,Y. 1999. Inferences on the
dynamics of Southern Hemisphere minke whales from ADAPT analyses of catch-at-age information.
J. Cetacean Res. Manage. 1(1): 11-32.
Ichii, T. 1997. A review of the Marine Ecosystem Studies under the JARPA. Paper SC/M97/4
presented to the JARPA review meeting, May 1997 (unpublished). 9p
Ichii, T., Shinohara, N., Fujise, Y., Nishiwaki, S. and Matsuoka, K. 1998. Interannual changes in body
fat condition index of minke whales in the Antarctic. Mar. Biol. Prog. Ser. 175: 1-12.
Kawamura A. 1980. A review of food of Balaenopterid whales. Sci. Rep. Whales Res. Inst. 32:
155-197.
Konishi, K. (in preparation) Characteristics of the blubber and body condition indicator for the
Antarctic minke whales (Balaenoptera bonaerensis)
Laws, R. M. 1977. Seals and whales of the southern ocean. Philos. Trans. R. Soc. Lond. B. 279: 81-96.
Lockyer, C. 1987. The relationship between body fat, food resource and reproductive energy costs innorth Atlantic fin whales (Balaenoptera physalus). Symposia of the Zoological Society of London
57: 343-361.
Loeb, V., Siegel, V., Holm-Hansen, O., Hewitt, R., Fraser, W., Trivelpiece, W. and Trivelpiece, S.
1997. Effects of sea-ice extent and krill or salp dominance on the Antarctic food web. Nature 387:
897-900.
Mackintosh, N. A. and Wheeler, S. F. C. 1929. Southern blue and fin whales. Discovery Report
1:257-540.
Matsuoka et al. 2005. Distribution and abundance trend in large baleen whale species in Antarctic
Areas III-VI. Paper JA/J05/PJR15 presented to the pre JARPA review meeting, January 2005.
Víkingsson, G. A. 1995. Body condition of fin whales during summer off Iceland. In (A. S. Blix, L.
Walløe and. Ø. Ulltang, eds.) Whales, seals, fish and man. Pp. 361-369. Elsevier, Amsterdam.
Nemoto, T. 1962. Food of baleen whales collected in recent Japanese Antarctic whaling expeditions.
Sci. Rep. Whales Res. Inst. 16: 89-103.
Ohsumi, S., Fujise, Y., Ishikawa, H., Hatanaka T., Zenitani, R. and Matsuoka, K. 1997. The fattynessof the Antarctic Minke Whale and its yearly change. Paper SC/M97/18 presented to the JARPA
review meeting, May 1997 (unpublished). 21p
Watanabe, T., Yabuki, T., Suga, T., Hanawa, K., Matsuoka, K. and Kiwada, H. Results ofoceanographic analyses conducted under JARPA and possible evidence of environmental changes.
Paper JA/J05/PJR15 presented to the pre JARPA review meeting, January 2005.

Table 1 Scale of diatom film adhesion
Level
Scale
0
1
2
3
4

no adhesion
adhesion is slite
adheredlargely ahdered
coverd entire body

Table2 Blubber thickness at mid-lateral region in JARPA from 1987/88 to 2003/04.
N
Mean(cm)
SD
Max
Area
IV
1202
3.42
0.82
6.5
Mature male
V
1099
3.90
0.91
7.2
IV
677
3.85
0.92
7.0
Pregnant female
V
818
4.28
0.94
7.7

5

Min
1.8
2.0
2.0
2.3

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Table3 Blubber thickness at mid-lateral region in Februaly from 1987/88 to 2003/04.
N
Mean(cm)
SD
Max
Min
Area
IV
321
3.70
0.83
6.5
1.8
Mature male
V
439
4.20
0.83
6.4
2.0
IV
388
4.01
0.91
7.0
2.1
Pregnant female
V
393
4.45
0.82
7.7
2.4

Table4

The final regression coefficients etc were:

Constant
Date
Diatom Film
Sex
Area
Year
BL (m)

Coefficient
B
SE
0.375
0.301
0.015
0.001
0.239
0.010
-0.322
0.029
0.263
0.025
-0.024
0.003
0.152
0.029

P-value
0.213
0.000
0.000
0.000
0.000
0.000
0.000

95% confidence interval of B
Lower bound Upper bound-0.216
0.965
0.014
0.016
0.219
0.259
-0.379-0.2650.213
0.313-0.029-0.0190.095
0.210

Table5 In a second run “longitude” (in degrees east) was substituted for “area”.
Again all independent variables were included at the 5% level.
Constant
Date
Diatom Film
Sex
Year
Long（E+W)
BL (m)

Coefficient
B
SE
1.075
0.275
0.015
0.001
0.241
0.010
-0.3170.029-0.024
0.003
0.003
0.000
0.157
0.029

P-value
0.000
0.000
0.000
0.000
0.000
0.000
0.000

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95% confidence interval of B
Lower bound Upper bound
0.537
1.614
0.014
0.016
0.221
0.261
-0.374-0.259-0.029-0.019
0.003
0.004
0.099
0.215

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70°E

80°E

90°E 100°E 110°E 120°E 130°E 140°E 150°E 160°E 170°E 180° 170°W

60°S

Area IV

Area V

65°S

70°S

75°S

Ross Sea

Figure 1 Map showing sampling area.

Figure 2 Position of blubber thickness measurement.

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5
4.8

Mean blubber thickness (cm)4.6
y = -0.0436x + 4.3017

4.4
4.2
4
3.8
3.6
3.4
3.2
3

Mature male
87/88

89/90

91/92

93/94

95/96

97/98

99/00

01/02

03/04

Survey Year5y = -0.0337x + 4.5246

4.8

Mean blubber thickness (cm)

4.6
4.4
4.2
4
3.8
3.6

Pregnant female

3.4
3.2
3

87/88

89/90

91/92

93/94

95/96

97/98

99/00

01/02

03/04

Survey Year

Figure 3 Yearly trend of blubber thickness in Area IV and V.

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5

Mature Male in Area IV

Mean blubber thickness (cm)

4.8
4.6
4.4
4.2
4
3.8
3.6
3.4
3.2
3

87/88

89/90

91/92

93/94

95/96

97/98

99/00

01/02

03/04

5

Mature Male in Area V

Mean blubber thickness (cm)

4.8
4.6
4.4

y = -0.098x+ 4.6153
2
R = 0.5897

4.2
4
3.8
3.6
3.4
3.2
3

88/89

90/91

92/93

94/95

96/97

98/99

00/01

02/03

5

Pregnant female in Area IV

Mean blubber thickness (cm)

4.8
4.6

y = -0.1117x + 4.5923
2
R = 0.6016

4.4
4.2
4
3.8
3.6
3.4
3.2
3

87/88

89/90

91/92

93/94

95/96

97/98

99/00

01/02

03/04

5
4.8

Mean blubber thickness (cm)

4.6
4.4
4.2
4
3.8

Pregnant female in Area V

3.6
3.4
3.2
3

88/89

90/91

92/93

94/95

96/97

98/99

00/01

02/03

Survey Year

Figure 4 Yearly trend of blubber thickness in February in each maturity and area.

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8.0
Mature male

Blubber thickness (cm)

7.0
6.0
5.0
4.0
3.0
2.0

y = 0.0185x + 2.5149

1.0
0

20

40

60
Date (Dec. 1st = 1)

80

100

120

8.0
Pregnant female

Blubber thickness (cm)

7.0
6.0
5.0
4.0
3.0
2.0
y = 0.0151x + 3.0553
1.0
0

20

40

60

80

Date (Dec. 1st = 1)

Figure 5 Blubber thickness change with date in the Antarctic.

10

100

120

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8.0
Blubber thickness (cm)

7.0
6.0
5.0
4.0
3.0
2.0
y = 0.0063x + 2.8418

1.0
0.0

60

80

100

120

140

160

180

200

Longitude (in degrees East)

9.0
Blubber thickness (cm)

8.0
7.0
6.0
5.0
4.0
3.0
2.0

y = 0.005x + 3.4004

1.0
0.0

60

80

100

120

140

160

Longitude (in degrees East)

Figure 6 Blubber thickness change with longitude in Areas IV and V.

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180

200