SC/D06/J19
THIS PAPER CAN NOT BE CITED WITHOUT WRITTEN PERMISSION FROM THE AUTHORS

Decline in energy storage in the Antarctic minke whale
Balaenoptera bonaerensis in the Southern Ocean
KENJI KONISHI1), TSUTOMU TAMURA1) AND LARS WALLØE2)
1)Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan2)Department of Physiology, University of Oslo, P.O. Box 1103, Blindern N-0317 Oslo,Norway
KEYWORDS: ANTARCTIC MINKE WHALE; BODY CONDITION; BALAENOPTERA;BLUBBER THICKNESS; SCIENTIFIC PERMITSABSTRACT
The annual trend in energy storage in the Antarctic minke whale was examined using catch data from all 18JARPA survey years. The results of stepwise multiple linear regression analyses show that blubber
thickness in minke whales has been decreasing for nearly two decades. The decrease in blubber thickness isestimated at -0.0190 cm/year, or close to 10 % for the whole JARPA period. Furthermore, “date”, “extentof diatom adhesion”, “sex”, “body length”, “fetus length” and “longitude” were all identified as partiallyindependent predictors of blubber thickness. The direct interpretation of the decline in energy storage in theminke whales with time in terms of food availability is difficult, since no long term krill abundance series isavailable. However, competition among krill feeders must be considered as the most likely explanation of
the decline, given the recent recovery of the stocks of large baleen whales.

INTRODUCTION
The Southern Ocean supports a simple food web with high productivity, and is an important feeding groundfor many consumers. The Antarctic krill Euphausia superba is the most abundant and main prey for large
baleen whales, seals and seabirds. The Antarctic minke whale B. bonaerensis, which is a small baleen
whale, also depends largely on E. superba (Ohsumi 1979, Armstrong and Siegfried 1991, Ichii and Kato1991, Ichii 1997).
Krill availability in the Antarctic Ocean is generally considered to be the most important limiting factor for
population sizes of many krill feeders. Krill biomass is very high and many animals depend on this species,
so that a sudden decrease in certain krill consumers could cause drastic changes in the population sizes ofother consumers in Antarctic ecosystem. It has been hypothesised that after more than 50 years ofcommercial whaling and overexploitation of the large baleen whales, the relative abundance of the whale
species was totally changed from the situation in the pre-whaling period (Laws 1977, Brown and Lockyer
1984). In parallel with the decline in abundance of the large baleen whales, the minke whale population
presumably started to grow. According to the “whale reduction” or “krill surplus” hypothesis (Laws 1977),
this growth is considered to be a result of abundant food supplies available to the minke whale after thedecline of the large baleen whale populations.
The main objective of the present study was to examine trends in body condition of the Antarctic minke
whale during the JARPA period. A second objective was to examine other factors that could have affectedbody condition and to review the energetic status of minke whales in the Southern Ocean.

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MATERIALS AND METHODS
Sampling and measurements
Blubber thickness and all other variables used in the present investigation were measured in nearly all
Antarctic minke whales taken by the Japanese Whaling Research Program under Special Permit in theAntarctic (JARPA) from 1987/88 to 2004/05. To avoid any bias resulting from growth or lactation, we used
only mature males and pregnant females, but not lactating females or immature animals, for the presentanalyses (Table 2).
The research area covers the International Whaling Commission (IWC) Management Area III (East), AreaIV, Area V and Area VI (West), south of 60°S in alternate years in austral summer seasons (Fig. 1). After
capturing by three sighting/sampling vessels, the animals were placed aboard a research base vessel where
they were examined. After outer observations were completed, body length (from snout tip to tail fluke
notch in a straight line along the deck) and other morphological measurements were taken. Blubber
thickness was measured to the nearest mm, by dissecting perpendicularly from skin to muscle without
including connective tissue or black surface skin. The blubber thickness at mid-lateral region was usedbecause this is a good indicator of body condition (Konishi 2006) (Fig. 2). Sex and maturity were recorded
for each whale on the basis of routine observations of reproductive organs during dissection and tissue
observations in the laboratory. The relevant stocks of Antarctic minke whale were defined as follows:
Eastern Indian Ocean Stock (I-stock) Areas IIIE, IV and VW, Western South Pacific Stock (P-stock) Areas
VE and VIW (see paper in this meeting). A diatom film is sometimes observed on the surface of the whales
and may cover the entire body. The extent of diatom adhesion on the skin is a rough indicator of how longa whale has spent in cold waters (Table 1).
Statistical analysis
Whales accumulate fat during the summer feeding period and migrate to low latitude areas for reproduction.
Like other baleen whales, the minke whale migrates to the Antarctic where it spends the austral summer
feeding (Kato and Miyashita 1991, Kasamatsu et al. 1995). Thus, the time spent in the feeding area andgeographical and biological variables which could be related to blubber thickness should be considered inbody condition analysis. To take these factors into account and possibly exclude some of them, weconducted stepwise multiple linear regression analyses. Blubber thickness (in cm) was the independent
variable. In the first run, we allowed the following independent variables: “date” (December 1st=day 1),“diatom adhesion” (0 to 4, Table 1), “sex” (1: male, 2: female), “stock” (1: I-stock, 2: P-stock), “bodylength” (in m) and “fetus length” (in cm). “Year” was also included as an independent variable toinvestigate a possible annual trend (87/88=1 88/89=2 89/91=3...). In a second run, “longitude” (in degreeseast) was substituted for “stock”. At each step of the regression analysis, the next variable was included inthe equation if the corresponding p-value was below 5 %.

RESULTS
Factors affecting body condition and its yearly trend
Blubber thickness ranged from 1.1 to 7.2 cm in mature males, and from 1.5 to 7.7 cm in pregnant females
in both areas over the whole research period (Table 2).
In preliminary analyses, the yearly trend in blubber thickness near the end of the feeding season (onlywhales caught in February) for each sex and stocks (I and P) was examined. The results from the I-stock
are displayed in Figure 3. Results from the P-stock were similar. In this analysis the variability the othervariables (“date”, “diatom adhesion”, “body length”, “fetus length”) result in the large SDs displayed in thefigure.
The stepwise multiple linear regression analyses included all possible dependent variables as predictors of
blubber thickness at the 5 % level in both runs (Table 3 and 4). Thus, no variable was excluded. “Date” wasthe best predictor of blubber thickness followed by “diatom”, “sex”, “fetus length”, “year”, “stock” and“body length” in that order. The second run gave similar results. “Date” was the best predictor of theblubber thickness followed by “diatom”, “sex” and “fetus length”, “year”, “longitude” and “body length”.
The coefficients of “year” were -0.0191cm/year (±0.0022 SE) and -0.0189 cm/year (±0.0022 SE),
respectively in the two runs, showing that blubber thickness decreased significantly over the years (Table 3and 4). The results also indicated that blubber thickness increased with time spent feeding and from west toeast. The blubber layer was found to be 0.366 cm thinner in males than in females.

2DISCUSSIONAnnual trend in body condition
The results from the statistical analyses clearly show that blubber thickness in Antarctic minke whales hasdecreased during the last 18 years. This is the first time a long-term decline in energy storage in minke
whales has been demonstrated, although Ohsumi et al. (1997) found some non-significant indications of adecline starting in the early 1980s. Blubber thickness varies considerably between years. Recent studies,such as Ichii et al. (1998), have shown that this variation in body condition is correlated with the extent of
ice coverage in the JARPA survey area. The large inter annual variation in blubber thickness may be themain reason why the decline has not been recognised earlier.
Elucidation of the role of cetaceans in the Antarctic ecosystem
At the mid-term JARPA review meeting in 1997, there was discussion about which of the two hypotheses“whale reduction/krill surplus” or “environmental change” was more likely to explain a putative decline inbody condition in the Antarctic minke whale over the previous decade. However, neither of the hypotheses
could be excluded because there was little available information either on trends in minke whale conditions
or on krill abundance (see Ichii 1997). Environmental change could be an important factor since it is known
to affect krill abundance. The abundance of krill around the Antarctic Peninsula is known to have declined
since the 1970, because the extent of the sea ice has decreased as a result of high temperatures (Loeb et al.
1997, Atkinson et al. 2004). However, no such environmental trend has been observed in the JARPA
survey area (Watanabe et al. 2005).
The results of this study and of Ohsumi et al (1997) indicate that food availability for the minke whale hasbeen declining since about 1980, and that the decline in blubber thickness over this period has been about
10 %. It has been estimated that the recruitment of minke whales reached a peak around 1970 (Mitsuyo this meeting), and multi-species modelling work by Mori and Butterworth (2006) indicates that a decline in
krill biomass started somewhat earlier. If the physical environment has been stable, it is conceivable thatthe total biomass of consumers was already near carrying capacity about thirty years ago. Sighting surveys
carried out by the Southern Ocean Whale and Ecosystem Research (SOWER) programme and JARPA havedemonstrated the recent recovery of humpback (Megaptera novaeangliae) and fin whales (B. physalus)(Matsuoka et al. 2005). Branch et al. (2004) also reported that the Antarctic blue whale (B. musculus)
population has increased, although it is still small. These large whales also feed on the Antarctic krill andshare same food niche as the minke whale (Nemoto 1962, Kawamura 1980), so their ongoing recovery
could cause a further decline in food availability for the minke whale.
Our results also show that blubber thickness increases from west to east, suggesting that there has been ageographical difference in food availability for minke whales.
Understanding how krill demography is affected by changes in physical environmental factors and by
predator consumption, and how, in turn, this influences predator performance and survival, is an essential
basis for predicting future change in the Antarctic marine ecosystem (Reid and Croxall 2001). However,investigating and understanding the dynamics of the widely distributed krill population is difficult, so thatmonitoring energy storage by a krill consumer, such as the minke whale, can be very useful. Understanding
the status of the Antarctic minke whale is one of the keys to predicting changes in the populations of large
krill consumers, and also of crucial importance in detecting food web interactions and changes in the krill
population itself.
Conclusions and further studies
The results of our study lead to the conclusion that the “krill surplus” period ended more than twenty years
ago. The JARPA data have not as yet provided evidence of environmental change (Watanabe et al. 2005).
However, there is substantial overlap between the habitats of different whale species, suggesting possible
interference between minke and blue whales in Antarctic waters (Kasamatsu et al. 2000). If other krill
consumers also increase in abundance, the body condition and abundance of the Antarctic minke whale
may continue to decline, because the ongoing recovery of large baleen whales could further reduce foodavailability for the minke whales.
Therefore, we need to focus on interactions between krill consumers and study intra- and inter-specificpopulation dynamics in whale species. The study of body condition is a necessary part of ecosystemmonitoring in the Southern Ocean. Simultaneous monitoring of the physical and prey environment is also
important (Ichii 1997). The comprehensive work will provide important information on what is likely tohappen in Southern Ocean ecosystem in the near future, which will be valuable for sustainable managementof whales.

3ACKNOWLEDGEMENTS
We would like to thank all the captains and crews of the ships that took part and the scientists who wereinvolved in JARPA surveys. Thanks are also due to Mrs. R. Zenitani, Mr. T. Bando and Dr. Y. Fujise forproviding us with maturity and fetus length data for minke whales, and to Mrs. Y. Hosone for helping us toorganise the data set. We would also like to thank all those people who have made useful comments on thispaper. Finally, Thanks all participants read our paper at JARPA review meeting 2006 (from first author on
the board of Nisshin-maru).

REFERENCES
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Atkinson, A., Siegel, V., Pakhomov, E. and Rothery, P. 2004. Long-term decline I krill stock and increasein salps within the Southern Ocean. Nature. 432: 100-103.
Branch et al. 2004. Evidence for increases in Antarctic blue whales based on Bayesian modelling. Marine
Mammal Science 20:726-754.
Brown S G,Lockyer C H Whales. In: R. M. Laws (eds). Antarctic Ecology Volume Two. Academic Press,Cambridge. 1984; 717-781.
Butterworth, D.S., Punt, A.E., Geromont, H.F., Kato, H. and Fujise,Y. 1999. Inferences on the dynamics ofSouthern Hemisphere minke whales from ADAPT analyses of catch-at-age information. J. Cetacean Res.
Manage. 1(1): 11-32.
Ichii T., Kato H. Food and daily food consumption of southern minke whales in the Antarctic. PolarBiology. 1991; 11: 479-487.
Ichii, T. 1997. A review of the Marine Ecosystem Studies under the JARPA. Paper SC/M97/4 presented tothe JARPA review meeting, May 1997 (unpublished). 9p
Ichii, T., Shinohara, N., Fujise, Y., Nishiwaki, S. and Matsuoka, K. 1998. Interannual changes in body fatcondition index of minke whales in the Antarctic. Mar. Biol. Prog. Ser. 175: 1-12.
Kasamatsu F., Nishiwaki S. and Ishikawa, H. 1995. Breeding areas and southbound migrations of southernminke whales Balaenoptera acutorostrata. Marine Ecology Prograss Series. 119: 1-10.
Kasamatsu F., Matsuoka K. and Hakamada T. 2000. Interspecific relationships in density among the whale
community in the Antarctic. Polar Biology. 23: 466-473.
Kato H, Miyashita T Migration Strategy of Southern Minke Whales in relation to Reproductive Cycle
Estimated from Foetal Length. Rep. Int. Whal. Commn. 1991; 41: 363-369.
Kawamura A. 1980. A review of food of Balaenopterid whales. Sci. Rep. Whales Res. Inst. 32: 155-197.
Konishi, K. 2006. Characteristics of the blubber and body condition indicator for the Antarctic minke
whales (Balaenoptera bonaerensis) Mammal Study 31:14-22.
Laws, R. M. 1977. Seals and whales of the southern ocean. Philos. Trans. R. Soc. Lond. B. 279: 81-96.
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.
Matsuoka, K., Hakamada, T. and Nishiwaki, S. 2005. Distribution and abundance of humpback, fin andblue whales occurring in the Antarctic Areas IIIE, IV, V and VIW (35°E-145°W). Paper JA/J05/PJR10
presented to the Pre-JARPA Review meeting, January 2005.
Mori, M., Butterworth, D.S., and Kitakado, 2006. T. Application of adaptive-VPA to Antarctic minkewhales in the JAPRA research area (Document prepared for the JARPA review meeting: this meting).
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4Table 1: Extent of diatom film
Scale01234

Levelnone
LimitedModerate
Most of body
Entire body

Table 2 Data used in the analyses:
Mature males
Pregnant females

Body length (m)Blubber thickness (cm)Body length (m)Blubber thickness (cm)

n2890
2872
1814
1806

mean
8.4
3.6
8.9
4.0

SD
0.4
0.9
0.4
1.0min
6.3
1.1
7.6
1.5

max
9.6
7.2
10.2
7.7

Table 3 Results of one of the multiple stepwise regression analyses
ConstantDate (Dec. 1st=1)
DiatomSex (male=1, female=2)
Fetus length（cm）
Year (87/88=1)
Stock (P=1,I=2 )Body length (m)CoefficientBSE
0.96930.22630.01390.00040.18180.00990.36590.02750.00570.0004-0.01900.00220.16520.02410.08230.0276

P value
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.003

95% confidence interval of B
Lower boundUpper bound0.52571.41300.01310.01470.16240.20120.31190.41980.00480.0065-0.0234-0.01470.11800.21240.02810.1365

Table 4 Results of another regression analysis. In this analysis, the variable “Longitude”(in degrees east) was substituted for “Stock”
Constant
Date (Dec. 1st=1)
DiatomSex (male=1, female=2)
Fetus length（cm）
Year (87/88=1)
Longitude (deg E)
Body length (m)CoefficientBSE
0.91190.22400.01350.00040.18090.00990.36660.02730.00560.0004-0.0189
0.00220.00220.00030.08390.0274

P value
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.002

5

95% confidence interval of B
Lower boundUpper bound0.47281.35100.01270.01430.16150.20020.31320.42010.00480.0065-0.0232-0.01460.00170.00270.03020.1376

Area III(east)

Area IV

Area V

Area VI
west

Ross Sea(Area V)

Figure 1 Map of JARPA survey area (Areas III-East, IV, V and VI-West).

Figure 2 Position of blubber thickness measurement

Mean blubber thickness (cm)

65.5
5
4.5
4
3.5
3
2.5
21986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006
YearFigure 3 Trend in blubber thickness in the Antarctic minke whale over the JARPA
survey period. P-stock in February. (Error bars represent SD) closed circle:
mature male, open circle: pregnant female.

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