SC/59/O9

Additional analysis of stomach contents weight of theAntarctic minke whale Balaenoptera bonaerensis in theSouthern Ocean
TSUTOMU TAMURA AND KENJI KONISHI
Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, JapanABSTRACT
The annual trend in stomach contents weight per capita in the Antarctic minke whale was examined using datafrom all 18 JARPA surveys. The results of stepwise multiple linear regression analyses showed that stomach
contents weight in the minke whales has decreased for almost two decades. The decrease in stomach contents isestimated at -0.694 kg/year in an animal, which was about 16 % for the whole JARPA period. Furthermore,“body length”, “time”, “latitude” and “date” were all identified as partially independent predictors of stomach
contents weight. “Longitude of sampling position”, “sex”, and “chasing time” were excluded. The directinterpretation of food availability on the decline in stomach contents in the minke whales with time (year) isdifficult, since no long term krill abundance series is available. However, competition among krill feeders and/orthe decrease of krill resources must be a candidate for the explanation of the decline.
KEYWORDS: ANTARCTIC, ANTARCTIC MINKE WHALE; FEEDING; TRENDS

INTRODUCTION
In the ecosystem in the Southern Ocean, although the Antarctic krill (Euphausia superba) is preyed upon by
many predators such as baleen whales, seals, sea birds, fish and squid, the prey consumption by whales can not
be disregarded in trying to understand the ecosystem because of their large biomass (Miller and Hampton, 1989).
The Antarctic minke whale (Balaenoptera bonaerensis) is the most abundant baleen whale species in theSouthern Ocean (south of 60°S) estimated to be as high as at 760,396 animals with 95 % confidence interval(510,000 – 1,140,000) in the austral summer of the 1980s (IWC 1991). New abundance estimation is being
discussed by the SC/IWC. Minke whales breed in austral winter at lower latitudes and migrate to the Antarctic
Ocean for feeding in austral summer (Kasamatsu et al. 1995). In Southern Ocean, Antarctic minke whales feedmostly on Antarctic krill in offshore areas (e.g. Kawamura 1980, Bushuev 1986, Ichii and Kato 1991), and on icekrill (E. crystallorophias) in coastal (shallow) areas of the continental shelf such as the Ross Sea and Prydz Bay(Bushuev 1986, Tamura 1998).
Previous estimates of the daily prey consumption of the whales were made on the basis of energy-requirement
calculations (Hinga 1979, Lockyer 1981a, 1981b, Armstrong and Siegfried 1991, Reilly et al. 2004) since there
was little quantitative data of stomach contents until now. Data such as stomach contents, sexual maturity, bodylength, and abundance estimates of Antarctic minke whale together with krill biomass estimates from theJapanese Whale Research Program under Special Permit in the Antarctic (JARPA) provided us to the means tounderstand the feeding ecology of Antarctic minke whales.
In this paper, we examined trends in stomach contents weight of the Antarctic minke whale during the JARPA
period (1987/88 – 2004/2005). A second objective was to examine other factors that could have affected stomach
contents of the minke whales in the Southern Ocean. Analyses showing a decline in the weight of stomach
contents in the minke whales were presented to the 2006 JARPA Review Meeting held in Tokyo. The meeting
recommended additional analyses to consider factors such as: i) determination of the duration of the feedingperiod; ii) examination at smaller spatial scales; iii) use multivariate analysis to examine trend, incorporating
covariates such as age, size and reproductive status of whales as well as the date and time of day. The additional
analysis presented here is in response to item iii). The following covariates were incorporated into the analyses:“undigested stomach contents weight per capita” (kg), “Body length” (in m), “Sighting time of captured whales”,

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“Latitude of sampling position”, “Longitude of sampling position”, “Date of sampling”, “Sex”, “Chasing time”
and “Year”.
MATERIALS AND METHODS
Sampling and measurements
Stomach contents weight 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 the Antarctic(JARPA) from 1987/88 to 2004/05 (Table 1).
The research area covers the International Whaling Commission (IWC) Management Areas III (East), IV, V andVI (West), south of 60°S in alternate years in austral summer seasons (Fig. 1). After capturing by threesighting/sampling vessels, the animals were placed aboard a research base vessel where they were examined.
After outer observations were completed, body length (from tip of snout to tail fluke notch in a straight line
along the axis) and other morphological measurements were taken. All Balaenopterid species have fourchambered stomach system (Hosokawa and Kamiya 1971, Olsen et al. 1994). As soon as the whale was on board,the four chambered stomach was removed and divided into four compartments: the forestomach, the funduschamber, the pyloric chamber and the duodenal ampulla. Each compartments mass were weighed to the nearest
0.1 kg. The freshness of forestomach contents was classified into one of four categories after examination by thenaked eye (F = fresh, fff = lightly digested, ff = moderately digested, f = heavily digested, See Appendix. 1). As
the prey consumption per capita, we applied the total of forestomach content and fundus chamber content weightof undigested status category data (F and fff) to avoid any bias resulting from digestion of stomach contents.
Because, the contents in both the forestomach and fundus chamber can move easily, the freshness offorestomach contents and fundus chamber contents was almost the same status.
Statistical analysis
Whales accumulate fat during the summer feeding period and then 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(Kasamatsu et al. 1995). Thus, the time spent in the feeding area and geographical and biological variables
which could be related to stomach contents weight should be considered. To take these factors into account andpossibly exclude some of them, we conducted stepwise multiple linear regression analyses. Stomach contents
weight per capita (in kg) was the dependent variable. In the run, we allowed the following independent variables:“Body length” (in m), “Sighting time of captured whales”, “Latitude of sampling position”, “Longitude ofsampling position”, “Date” (December 1st=day 1), “Sex” and “Chasing time”. “Year” was also included as anindependent variable to investigate a possible annual trend (87/88=1 88/89=2 89/91=3...). At each step of theregression analysis, the next variable was included in the equation if the corresponding p-value was below 5 %.

RESULTS AND DISCUSSION
Factors affecting stomach contents weight and its yearly trend
The stepwise multiple linear regression analyses included all possible dependent variables as predictors ofstomach contents weight at the 5 % level (Table 2). “Body length” was the best predictor of stomach contents
weight followed by “Sighting time”, “Latitude of sampling position”, “Date” and “Year” in that order.
“Longitude of sampling position”, “Sex”, and “Chasing time” were excluded. The coefficients of “year” were-0.693 kg/year (±0.199 SE), indicating that stomach contents weight decreased over the years (Table 2).
Annual trend in stomach contents weight
The results from the statistical analyses clearly show that stomach contents weight per capita in Antarctic minke
whales has decreased during the last 18 years. This phenomenon indicates that the krill abundance has decreased
year by year in the JARPA research area.
We calculated the stomach contents weight (M) during the austral summer of Antarctic minke whale according
to following equations:
M = α + β1BL + β2Time + β3Latitude+ β4Date + β5Year
M: The both contents in forestomach and fundus chamber weight (kg), α: Constant (68.217), BL: Bodylength (m), Time: Sighting time of captured whales, Latitude: Latitude of sampling position,
Date: Sampling date (December 1st=day 1), Year: Research season (e.g. 1987/1988 season=1).
We assumed the following scenarios for detection of the yearly trend.

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Scenario1: Body length is 6 m. Time is 10 o’clock. Latitude is south of 65 degree.
Date is 90 days (February 1st). Year are 1, 2, 3 …18.
Scenario2: Body length is 7 m. Time is 10 o’clock. Latitude is south of 65 degree.
Date is 90 days (February 1st). Year are 1, 2, 3 …18.
Scenario3: Body length is 8 m. Time is 10 o’clock. Latitude is south of 65 degree.
Date is 90 days (February 1st). Year are 1, 2, 3 …18.
Scenario4: Body length is 9 m. Time is 10 o’clock. Latitude is south of 65 degree.
Date is 90 days (February 1st). Year are 1, 2, 3 …18.
These results are shown in Appendix 2. It suggests the decline of food availability per capita of Antarctic minke
whale in the JARPA survey area from the 1987/88 to the 2004/05 season. Possible explanations for this include:
1) inter-specific competition, 2) intra-specific competition among baleen whales and other organisms, 3)
decreasing of krill resources due to environment change. Long-term data collection of baleen whales and krill
should be continued to monitor changes in the Antarctic marine ecosystem. Construction of ecosystem models isneeded to consider the existence of inter-specific competition and/or intra-specific competition among baleen
whales and other organisms and to improve management of whale resources in the JARPA research area.

ACKNOWLEDGEMENTS
We thank Dr. Hidehiro Kato, Dr. Yoshihiro Fujise, Mr. Kazuo Yamamura, Mr. Shigetoshi Nishiwaki, Mr. Hajime
Ishikawa, and the late Dr. Fujio Kasamatsu research leaders of these cruises for giving us the opportunity tocarry out this study. The stomach contents examined in this study were collected by many researchers and crews.
We also thank the captains, crews and researchers who were involved in the JARPA research cruises for theirefforts. Our sincere thank to Dr. Hiroshi Hatanaka, Dr. Luis A. Pastene of the Institute of Cetacean Research(ICR) for their valuable suggestions and useful comments on this paper.

REFERENCES
Armstrong, A.J., Siegfried, W.R. 1991. Consumption of Antarctic krill by minke whales. Antarctic Science,3(1):13-18.
Bushuev, S.G. 1986. Feeding of minke whales, Balaenoptera acutorostrata, in the Antarctic. Rep. int. Whal.
Commn 36:241-245.
Hinga, K.H. 1979. The food requirements of whales in the southern hemisphere. Deep-Sea Research.
26A:569-577.
Hosokawa, H. and Kamiya, T. 1971. Some observations on the cetacean stomachs, with special considerations
on the feeding habits of whales. Sci. Rep. Whales Res. Inst., 23:91-101.
Ichii, T. and Kato, H. 1991. Food and daily food consumption of southern minke whales in the Antarctic. PolarBiol., 11:479-487.
International Whaling Commission. 1991. Report of the Scientific Committee. Rep. int. Whal.Commn 41:51-219.
Kasamatsu, F., Nishiwaki, S. and Ishikawa, H. 1995. Breeding areas and southbound migrations of southernminke whales Balaenoptera acutorostrata. Mar. Ecol. Prog. Ser. 119: 1-10.
Kasamatsu, F., Joyce, G.G. Ensor, P. and Mermoz, J. 1996. Current occurrence of baleen whales in Antarcticwaters. Rep. int. Whal. Commn 46:293-304.
Kawamura A. 1980. A review of food of Balaenopterid whales. Sci Rep Whales Res Inst 32:155-197
Lockyer, C. 1981a. Estimation of the energy costs of growth, maintenance and reproduction in the female minke
whale, (Balaenoptera acutorostrata), from the southern hemisphere. Rep. int. Whal. Commn 31:337-343.
Lockyer, C. 1981b. Growth and energy budgets of large baleen whales from the Southern Hemisphere. FAO Fish.
Ser. (5) [Mammals in the Sea] 3:379-487.
Miller, D.G.M. and Hampton, I, 1989. Biology and ecology of the Antarctic krill (Euphausia superba DANA): A
review. Biological investigations of marine Antarctic systems and stocks (Biomass) 9: 1-166.
Mori, M. and Butterworth, D.S. 2004. Consideration of multispecies interactions in the Antarctic: A preliminarymodel of the minke whale-Blue whale- Krill interaction. Ecosytem Approches to Fisheries in the Southern
Benguela Afr. J. mar. Sci. 26:245-259.
Olsen, M.A., Nordøy, E.S., Blix, A.S. and Mathiesen, S.D. 1994. Functional anatomy of the gastrointestinal
system of Northeastern Atlantic minke whales (Balaenoptera acutorostrata). J. Zool., Lond., 234:55-74.
Reilly S, Hedley S, Borberg J, Hewitt R, Thiele D, Watkins J, Naganobu, M. 2004. Biomass and energy transferto baleen whales in the South Atlantic sector of the Southern Ocean. Deep-Sea Research II 51A:1397-1409

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Tamura, T. 1998. The study of feeding ecology of minke whales in the Northwest Pacific and the Antarctic. D. C.
Thesis. Hokkaido University. 125pp. [In Japanese].

4Table 1 Data used in the analyses

Year
1987/88
1988/89
1989/90
1990/91
1991/92
1992/93
1993/94
1994/95
1995/96
1996/97
1997/98
1998/99
1999/2000
2000/01
2001/02
2002/03
2003/04
2004/05
Total

Male
37
14
56
25
31
45
66
67
92
79
139
111
108
120
119
112
100
91
1,412

Female
23
30
30
27
16
42
36
62
30
85
58
40
106
73
127
105
103
114
1,107

Total
60
44
86
52
47
87
102
129
122
164
197
151
214
193
246
217
203
205
2,519

Table 2 Results of the multiple stepwise regression analyses

Constant
Body length (m)TimeLatitude
DateYear (87/88=1)

CoefficientBSE
68.217 16.122
19.2510.936-2.9090.260-1.980
0.245
0.201
0.035
-0.6930.199

P value
0.000
0.000
0.000
0.000
0.000
0.001

95% confidence interval of B
Lower boundUpper bound36.60499.83017.41521.087-3.419-2.399-2.460-1.5000.133
0.270-1.083-0.303F value R square394.424
278.347205.008
166.166
135.943

0.135
0.181
0.196
0.209
0.213

Results of step-wise multiple regression at 5% level for JARPA all year (87/88-04/05) results
Area 3-6
Dependent variable= SC (kg)Explanatory variables=Body length, Time (sighting), Latitude, Date, Year
Date:Dec.1st=1,sex:male(1) female(2), Year(87/88=1)

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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).

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Appendix. 1. Classification of digestion status of stomach contents.
CodeFClass
FreashDescription
Prey not affected by digestion

fff

Lightly digested

Prey slightly affected by digestion

ff

Moderately digested

Prey moderattory to highly fragmented

fHeavily digestedUnidentifiable remains or indigestible parts onlyAppendix. 2. The yearly trend of stomach contents of Antarctic minke whales in JARPA
research area from the 1987/88 season to the 2004/2005 season.
120.0

kg

90.0
60.0
30.0
0.0
1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005
Year
BL: 6mBL: 7m7

BL: 8mBL: 9m