SC56/O15

Preliminary analyses of prey consumption of three baleen
whales and their interaction with fisheries in the western
North Pacific
TSUTOMU TAMURA, KENJI KONISHI AND YOSHIHIRO FUJISEThe Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan,
Contact e-mail: tamura@cetacean.jp

ABSTRACTThe stomach contents of common minke whale Balaenoptera acutorostrata, Bryde’s whale B. edeni, sei whale B.
borealis sampled off Pacific region from May to September as part of the 1996-2003 JARPN II, were analyzed. The
main prey species of common minke whale consisted of one krill (Euphausia pacifica) and two fish (Japanese anchovy
Engraulis japonicus and Pacific saury Cololabis saira). The main prey species of Bryde’s whale consisted of one krill(Euphausia pacifica) and one fish (Japanese anchovy). There were seasonal, geographical and yearly changes of preyspecies in western North Pacific. The main prey species of sei whale consisted of two copepods (Neocalanus cristatus,
N. plumchrus), one krill (Euphausia pacifica) and two fish (Japanese anchovy and Pacific saury). The body length ofJapanese anchovy in the stomachs differed among whales. In western North Pacific, the total annual prey consumptionof Japanese anchovy and Pacific saury by three baleen whales were estimated as 2,720,000 tons and 230,000tons,
respectively. Based on these results, there is a possibility of direct competition between these whale and the fisheries forthese resources in western North Pacific. To evaluate this competition, more information of accurate abundance in preyspecies and these whales, and resident period of them off Pacific region are needed.
KEYWORDS: COMMON MINKE WHALE; BRYDE’S WHALE, SEI WHALE, NORTH PACIFIC; FISHERIESINTERACTION, SCIENTIFIC PERMITINTRODUCTION
Sustainable use of marine living resources is only way people have to choice. However, we have responsibility toevaluate and clear interactions between marine life and fisheries, because fishery industry can easily cause the depletion.
Baleen whales, which attained an ability to eat small zooplankton to fish, play an important role in the food chain
because of their large abundance.
The common minke whale Balaenoptera acutorostrata is widely distributed in the world. In the western North Pacifictwo stocks have been recognized: one in the Sea of Japan - Yellow Sea - East China Sea (J stock) and the another in the
Sea of Okhotsk – West Pacific (O stock) (IWC, 1983). The abundance of minke whales was estimated to be 19,209
animals with 95 % confidence interval (10,069 – 36,645) in the Sea of Okhotsk and 5,841 animals with 95 %confidence interval (2,835 – 12,032) in the Northwest Pacific during August and September in 1989 and 1990 (IWC,1992). In the western North Pacific, common minke whales are opportunistic feeders with a broad diet and with flexible
feeding habits. According to previous reports, they consume several prey species such as pelagic schooling fish andzooplankton (Kasamatsu and Hata, 1985; Kasamatsu and Tanaka, 1992).
The Bryde’s whale B. edeni is distributed in tropical and warm temperate waters throughout the year, in which thesurface temperature is above 16.4 oC (Ohsumi, 1977). The abundance of Bryde’s whales in the western North Pacific
was estimated to be 25,300 animals with 95 % confidence interval (17,700 – 36,200 (Okamura and Shimada, 1999).
Their feeding habits in the Northwest Pacific have been reported several authors. Bryde’s whales fed on krill, Japanese
anchovy (Engraulis japonicus) and Chub mackerel (Scomber japonicus) off the Sanriku Coast, krill and lantern fish in
the waters around Bonin Islands, and Japanese sardine (Sardinops melanostictus), Japanese anchovy and horsemackerel (Trachurus japonicus) off the West coast of Kyushu. In Japanese pelagic operations from 1952 to 1971,
Bryde’s whales fed on euphausiids (89 %) and fishes (11 %), indicating euphausiids and fish are main prey for Bryde’s
whale (Nemoto, 1959; Nemoto and Kawamura, 1977).
The sei whale B. borealis is widely distributed in the world’s ocean. In the western North Pacific one stock (west of 180
longitudinal degrees, excluding the Sea of Okhotsk, the Bering Sea and the Sea of Japan) have been recognized. The
abundance of sei whales was estimated to be 28,400 animals with 95 % confidence interval (13,100 – 61,600). In subareas 7, 8 and 9, the abundance was estimated to be 9,856 animals (Government of Japan, 2002a). The sei whale is1known to feed on a wide spectrum of marine animals such as krill, Japanese anchovy, chub mackerel, Pacific saury andJapanese common squid. According to Nemoto (1962), sei whales feed mainly copepods in the northern part of theNorth Pacific, but they likely feed on fishes and squids in the Sanriku and Hokkaido coastal waters of Japan. There isalso a report, which shows some sei whales ingested Pacific saury in the southern Aleutian waters. Twenty three speciesof zooplankton, 8 species of fishes and one species of squid were reported from stomachs of sei whales (Nemoto, 1959;
Kawamura, 1982).
The overall goal of the JARPN II was to contribute to the conservation and sustainable use of marine living resources
including whales in the western North Pacific, especially within Japan’s EEZ. In this JARPN II research special
attention was paid to the ecosystem surrounding cetaceans, and the data and materials related to cetaceans, prey species
and oceanographic conditions were collected. The main objectives of this research are the following:
1. Feeding ecology and ecosystem studies, involving studies of prey consumption by cetaceans, prey
preferences of cetaceans and ecosystem modelling.
2. Monitoring environmental pollutants in cetaceans and the marine ecosystem.
3. Stock structures of whales.
The research area includes sub-areas 7, 8 and 9 and the target species and sample sizes were set as follows: 150
common minke whales (100 in the offshore survey and 50 in the coastal survey); 50 Bryde’s whales (offshore survey);
50 sei whales (offshore survey) and 10 sperm whales (offshore survey) (Government of Japan, 2002a).
In this document, the abundances of common minke, Bryde’s, sei whales were estimated. And the consumption oftargeted fisheries species by these whales was calculated. Then, the interaction was evaluated by the comparison
between the prey consumption by these whales and fisheries catches.

MATERIALS AND METHODS
The abundance and average body weight of common minke, Bryde’s and sei whales in western North Pacific
The following abundance and average body weight of common minke, Bryde’s and sei whales in western North Pacificwere used (Table 1).
1.

Common minke whale: The abundance of common minke whales was estimated to be 19,209 animals with 95 %confidence interval (10,069 – 36,645) in the Sea of Okhotsk and 5,841 animals with 95 % confidence interval(2,835 – 12,032) in the Northwest Pacific during August and September in 1989 and 1990 (IWC, 1992). In subareas 7, 8 and 9, the abundance was estimated to be 11,521 animals. The average body weight is 4.3 tons (Average
body weight based on 1994 JARPN – 2003 JARPN II results).

2.

Bryde’s whale: The abundance of Bryde’s whales in the western North Pacific, was estimated to be 25,300 animals
with 95 % confidence interval (17,700 – 36,200). In sub-areas 7, 8 and 9, the abundance was estimated to be 2,439
animals (Okamura and Shimada, 1999). The average body weight is 13.5 tons (Average body weight based on
2000–2003 JARPN II results).

3.

Sei whale: The abundance of sei whales in the western North Pacific, was estimated to be 28,400 animals with
95 % confidence interval (13,100 – 61,600). In sub-areas 7, 8 and 9, the abundance was estimated to be 9,856
animals (Government of Japan, 2002a). The average body weight is 20.8 tons (Average body weight based on 2002
and 2003 JARPN II results).

The daily prey consumption by whales
We calculated daily prey consumption using the rate of prey intake per body weight of each cetacean species perday (feeding rate: % of body weight) and average body weight. This terminology of 'the feeding rate' was proposed
by Sergeant (1969). Some researchers have estimated the prey consumption by cetaceans on the basis of energy–
requirement (e.g. Hinga, 1979, Lockyer, 1981a, Armstrong and Siegfried, 1991; Tamura and Fujise, 2000a, 2002a).
In this study, we estimated the daily prey consumption and feeding rate by each cetacean species based on thefollowing three methods (Tamura, 2003).
Method-1 Estimation of daily prey consumption from average body weight
Innes et al.(1986) proposed the following method to estimate daily prey consumption of a cetacean from its average
body weight:
I = 0.42M0.67(1)

where I is daily prey consumption (kg per day) and M is average body weight (kg).

2Method-2 Estimation of daily prey consumption from the standard metabolism
Sigurjónsson and Víkingsson (1997) proposed a method for estimation of daily prey consumption from the standard
metabolism of each cetacean species. The daily prey consumption is given by:
D = 206.25M0.783 ; I = D/1,300 (2)where D is daily caloric value of prey intake (kcal per day), M is average body weight (kg) and I is daily prey
consumption (kg). We assumed that estimated caloric values of prey were 1,300 kcal/kg for baleen whales inNorthern Hemisphere in the world (Steimle and Terranova, 1985).
Method-3 Estimation of daily prey consumption from Klumov’s formula
Klumov (1963) proposed a method for estimating daily prey consumption from the average body weight of eachcetacean species. The daily prey consumption is given in the following formula by him:
I = 0.035M(3)

where I is daily prey consumption (kg per day), M is average body weight (kg)The each calculated daily prey consumption by whales is shown in Table 2. We used the average of three values.
Sampling of the stomach contents
Biological research such as measurement of body proportion were conducted for all whales sampled. The stomach
contents were all removed. After that the stomach contents were all removed. The contents from each stomach were
weighed to the nearest 0.1 kg, and sub-sample were removed from all individuals and frozen for later examination. The
contents were first classified to major prey groups, such as euphausiids, copepods, fish and others. The freshness of
stomach contents was categorized into four classes (1 = fresh, 2 = lightly digested, 3 = moderately digested, 4 = heavily
digested).
The sub-samples were identified to the lowest taxonomic level as much as possible. When undigested fishes were found,standard lengths and weight were measured to the nearest 1 mm and 1 g, respectively. When fishes occurred in the subsamples, the total number of each prey species was estimated by adding the number of undigested specimens and thenumber of intact skulls. The total weight of each prey species was estimated by using the average weight of fresh
specimens. The total number and weight of each prey species were estimated by using the results from the sub-samples.
Then, the relative prey importance by weight of each prey species (RW) was calculated as follows:
RW = (Wi / Wall) × 100
Wi = the weight of contents containing prey group i
Wall = the total weight of contents analyzed.
The prey species and composition in the stomach of whales from May to October
Based on JARPN and JARPNII results, the prey species of each whale was shown in Table 3. The diet composition(RW) of whales during feeding period in each sub areas showed in Table 4. It was assumed that the baleen whales spend
about 180 days in the feeding areas in the western North Pacific (Ohsumi, 1980, 1982). Lockyer (1981b) reported thatthe daily prey consumption by Antarctic minke whale in winter was equivalent to 10 % of that in the summer. The
seasonal prey consumption of whales during feeding period in each sub areas showed in Table 4.

RESULTS
Diversity of prey species
Common minke whale
A total of seventeen prey species, including 1 copepoda, 4 euphausiids, 1 squid and 11 fishes were identified in thestomachs of common minke whales based on JARPN and JARPN II surveys. Major prey species were Euphausia
pacifica, Japanese anchovy, Pacific saury (Cololabis saira), Japanese sand lance (Ammodytes personatus), walleye
pollock (Theragra chalcogramma) and Japanese common squid (Todarodes pacificus).
Bryde’s whale
A total of nine prey species, including 3 euphausiids and 9 fishes were identified in the stomachs of Bryde’s whales
based on JARPN II surveys. Major prey species were krill E. pacifica and Japanese anchovy.

3

Sei whale
A total of eight prey species, including 2 copepods, 2 Amphipods, 1 euphausiid, 2 fishes and 1 squid were identified in
the stomachs of sei whales based on JARPN II surveys. Major prey species were 2 copepods (Neocalanus cristatus, N.
plumchrus), 1 euphausiid (E. pacifica) and Japanese anchovy.
The daily prey consumption by common minke, Bryde’s and sei whales
The average of daily prey consumption weight of common minke, Bryde’s and sei whales based on three methods were
calculated to be 125 kg, 330 kg and 479 kg, respectively. The average of daily prey consumption rates of common
minke, Bryde’s and sei whales based on three methods were calculated to be 2.9 %, 2.4 % and 2.3 % of body weight,
respectively (Table 2).
The geographical and seasonal changes of prey species of common minke, Bryde’s and sei whales
Table 4 shows the prey composition of each whale during feeding period in each sub areas based on JARPN and
JARPNII.
The most important prey species of common minke whales were Japanese anchovy and Pacific saury in offshore. The
most important prey species in May and June was Japanese anchovy, and Pacific saury from July to September. In subarea 7, walleye pollock was also an important prey species during May and September.
The most important preys of Bryde’s whales were krill and Japanese anchovy in offshore. The most important preyspecies was krill in May and June, and Japanese anchovy from July to September.
The most important preys of sei whales were copepod, krill and Japanese anchovy in offshore. The most important preyspecies was copepod in July in sub-area 7 and in June and July in sub-area 9, while Japanese anchovy was the mostimportant one in sub area 8 and in August in sub-area 9.
The prey consumption of common minke, Bryde’s and sei whales in western North Pacific
Based on the average of three methods, the estimated total prey consumption by common minke, Bryde’s and sei
whales weights during feeding period in western North Pacific were 1,040,0000 tons, 2,760,000 tons and 4,500,000 tons,
respectively. In JARPN II research area (sub-areas 7, 8 and 9), the estimated total prey consumption of common minke,
Bryde’s and sei whales weights were 480,000 tons, 270,000 tons and 1,560,000 tons, respectively (Table 5).
The following assumptions were made for calculation.
1.

The prey composition of common minke whale in the Sea of Okhotsk was assumed that they fed mainly on krill,
herring and walleye pollock (33.3% of wet weight) (Kasamatsu and Hata, 1985).

2.

The prey composition of sei whales distributing excluding sub-areas 7, 8 and 9 was assumed that they fed mainly
on copepods (75%), krill (15%) and other fishes (10% of total wet weight) (Nemoto, 1962; Kawamura, 1982).

In western North Pacific, the prey consumption of economically important Pacific saury, walleye pollock and herringby common minke whales were calculated as 198,000 tons, 201,000 tons and 192,000 tons, respectively. The prey
consumption of economically important chub mackerel by Bryde’s whales was calculated as 25,000 tons. The prey
consumption of economically important Pacific saury by sei whales was calculated as 29,000 tons (Table 6).
The length composition of Japanese anchovy in the stomach of each whale
Even though, three whale species feed on Japanese anchovy, the sizes of the fish differ. The average body length ofJapanese anchovy in the stomachs of common minke whale, Bryde’s whale, sei whale, were 123 mm, 97and 119 mm,
and from 29 to 70 mm, respectively (Fig. 2).

DISCUSSION
Our estimates of the prey consumption using the three methods, was similar to by Nordøy et al. (1995), whichinvestigated the eastern North Atlantic minke whales and these obtained by Lockyer (1981a), which investigated thelarge baleen whales.
The results of JARPN and JARPN II showed that common minke whales have wide range of feeding habits and it
varied both geographically and temporally. This euryphagous is similar to that in Northeast Atlantic.
In western North Pacific, the prey consumption of Pacific saury by common minke whales was calculated as 198,000
tons. In 2002, the total catch of Pacific saury by Japanese fisheries was about 200,000 tons. The biomass of Pacificsaury was estimated 8,000,000 tons, but the total consumption of Pacific saury by minke whales was equivalent to thefisheries catch. The fisheries season of Pacific saury is from August to September, and common minke whales also feedon Pacific saury in a same season. The catch quota and season of Pacific saury is regulated by the fisheries conference4

in order to protect the resources of Pacific saury and price control; however the consumption of Pacific saury bycommon minke whales is not considered in catch quota calculation. Our results shows, therefore, consumption bycommon minke whales should be taken into account for fishery management of Pacific saury in the future.
It was occurred to replacement from Japanese sardine to Japanese anchovy from 1980’s in Pacific region of Japan.
Kasamatsu and Tanaka (1992) examined annual changes of prey species based on the catch records of small typewhaling in the seven whaling grounds off Japan from 1948 to 1987. In Pacific coast of Hokkaido (a part of sub-area7W) from April to October, prey species recorded were krill, squid, Japanese sardine, Japanese anchovy, chub mackerel,
walleye pollock, cod, sand lance, Pacific saury and so on. They noted that the change of prey of common minke whales
from Chub mackerel to Japanese sardine in 1977 corresponded with a change of the dominant species taken bycommercial fisheries in the same area in 1976. They also reported krill was dominant prey species from 1964 to 1987 in
the Okhotsk Sea. In addition, the change of prey species of common minke whales from chub mackerel to Japanesesardine in 1977, from Japanese sardine to Pacific saury in 1996 corresponded with a change of the dominant species
taken by commercial fisheries in the same area in 1976, 1996, respectively. Since it is reasonable to assume that
common minke whales do not have a strong preference for a particular prey species (Kasamatsu and Tanaka, 1992).
The estimated total prey consumption of common minke whales weights during feeding period in western North Pacificwere 1,040,000 tons, the composition of prey species probably reflect changes in the abundance of available preyspecies in the area.
Between 2000 and 2003 JARPN II, the prey species of Bryde’s whales in the western North Pacific during May and
September, were occupied various pelagic prey species of zooplankton and fishes. Prey species of Bryde’s whales
varied both geographically and temporally. The dominant prey species of Bryde’s whales was krill during May andJune, Japanese anchovy between July and September.
In western North Pacific, the biomass of Japanese anchovy estimated to 1,300,000 tons, but the total consumption ofJapanese anchovy by Bryde’s was more than the biomass of Japanese anchovy. The prey consumption of Japanese
anchovy by Bryde’s whales was calculated as 1,450,000 tons. Therefore, consumption by Bryde’s whales should betaken into account for fishery management of Japanese anchovy in the future.
Between 2002 and 2003 JARPN II, the prey species of sei whales in the western North Pacific during May and
September, were occupied various pelagic prey species of zooplankton, fishes and squid. Prey species of sei whales
varied both geographically and temporally. The most important prey species of sei whales was copepod in July in subarea 7, in June and July in sub-area 9, while Japanese anchovy was the most important one in sub area 8 and in August
in sub-area 9. In western North Pacific, the prey consumption of Japanese anchovy by sei whales was calculated as
1,050,000 tons. Therefore, consumption by Bryde’s whales should be taken into account for fishery management ofJapanese anchovy in the future.
The body length of Japanese anchovy ingested by each whale was different. The common minke whales fed mainly
on matured Japanese anchovy, on the other hand, Bryde’s whales fed mainly on pre-matured Japanese anchovy. Thisdifferent probably reflect the distribution of Japanese anchovy in the area (Murase et al, 2002). It seems that there issize selectivity of prey among whales.
In addition, more data are needed on seasonal, geographical and annual variations in the prey of common minke
whales, Bryde’s whales and sei whales before conclusions can be drawn with regard to their food consumption inwestern North Pacific from spring to autumn.
To evaluate the competition between each whale and some fisheries, more information of accurate abundance in preyspecies and each whale, and the accurate resident period of each whale are needed. Furthermore, there is a need tounderstand the potential for each whale to have an impact on commercial some fisheries, either directly or indirectly
using simulation models for specific geographical regions in future.

ACKNOWLEDGEMENTS
We would like to thank all captains, crews and researchers, who were involved in offshore compornent of JARPN and
JARPN II surveys from 1994 to 2003. Our sincere thank to Dr. H. Hatanaka and Dr. Luis A. Pastene of the Institute of
Cetacean Research (ICR) for their valuable suggestions and useful comments on this paper.
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Table 1. The abundance and average body weight of common minke, Bryde’s and sei whales in western North Pacific

Species

Westen North PacificSub-areas 7, 8, 9

Body weight

Abundance (95％ confidence level )Abundance(tons)

Common minke whale

25,000( 12,900 - 49,000 )

11,522

4.3

Bryde's whale

25,300( 17,700 - 36,200 )

2,439

13.5

Sei whale

28,400( 13,400 - 61,600 )

9,856

20.8

Table 2. The daily prey consumption of common minke, Bryde’s and sei whales in western North Pacific

Species

Daily prey consumption (kg)

Average

Feeding

Method-1

Method-2

Method-3(kg)

rate (%)

Common minke whale

114

111

151

125

2.9

Bryde's whale

246

272

473

330

2.4

Sei whale

328

381

728

479

2.3

8

Table 3. The prey species of common minke, Bryde’s and sei whales in western North PacificA. Common minke whale
Prey species
Main prey

Copepods

Neocalanus cristatus

Krill

Euphausia pacifica
Thysanoessa longipes
T. inermis

Japanese sand lance

Ammodytes personatus

Japanese anchovy

Engraulis japonicus

Pacific saury

Cololabis saira

Chub mackerel

Scomber japonicus

Japanese pomfretBrama japonica

Walleye pollock

Theragra chalcogramma

Pink salmon

Oncorhynchus gorbuscha

Japanese common squid Todarodes pacificus
Minor prey Krill

T.inspinata

Japanese sardine

Sardinops melanostictus

Coho salmon

O. kisutch

Duckbill barracudina

Paralepis atlantica atlantica

Daggertooth

Anotppterus pharao

B. Bryde's whale
Prey species
Main prey

Krill

Euphausia pacifica

Japanese anchovy

Engraulis japonicus

Chub mackerel

Scomber japonicus

Minor prey Krill

E. similis
Thysanoessa inspinata
Decapterus tabl

Lantern fish

Diaphus theta
Tarletonbeania taylori

Gonostomatid fish

Vinciguerria nimbaria

Starry toado

Arothron firmamentum

Snipe eel

Nemichthys scolopaceus
Lestidiops jayakari

C. Sei whale
Prey species
Main prey

Copepods

Neocalanus cristatus
N. plumchrus

Krill

Euphausia pacifica

Japanese anchovy

Engraulis japonicus

Pacific saury

Cololabis saira

Japanese common squid Todarodes pacificus
Minor prey Amphipods

Themisto japonica
T. pacifica

9

Table 4. The prey composition (% of wet weight) of each whale during feeding period in each sub areas in western
North PacificA. Common minke whale
Sub area

Month

N

Krill

Anchovy

Saury

Pollock

Squid

Other

7

May-June

156

8.3

84.0

0.0

7.1

0.0

0.6

July-September 106

22.6

28.3

30.2

12.3

6.6

0.0

8

9

May-June

67

4.5

89.6

6.0

0.0

0.0

0.0

July-September

67

3.0

7.5

86.6

0.0

1.5

0.0

May-June

81

2.5

72.5

22.5

0.0

0.0

2.5

July-September 188

8.5

12.8

75.0

0.0

0.0

3.7

Squid

Other

B. Bryde's whale
Sub area

Month

N

Krill

7

May

5

80.0

20.0

June

41

93.2

4.4

2.4

July

14

69.6

24.1

6.4

August

21

25.3

74.5

September

8

14.9

85.1

June

9

60.0

40.0

0.03

July

7

99.9

0.07

August

26

99.9

0.05

0.03

July

14

28.6

71.4

<0.01

<0.01

Sub area

MonthN

Copepods

Krill

Anchovy

Saury

7

July

3

85.4

8

May

3

June

9

July

3

73.9

September

2

100.0

June

10

77.8

11.1

0.4

10.7

July

40

46.4

39.3

11.2

2.5

August3

8

9Anchovy Mackerel

Other

0.17

C. Sei whale

9

14.6
100.0

10.0

38.1

51.9

94.9

10

0.01
9.2

16.2

0.7

0.5

0.02
5.1

Table 5. The seasonal prey consumption of each whale during feeding period in western North Pacific
Species

Abundance (inds.)

Body

Daily

Seasonal prey consumption (ten thousand tons)

Sub-areas

westernweightconsumption

Sub-areas

western

7, 8 and 9

North Pacific(tons)(kg)

7, 8 and 9

North PacificCommon minke whale

11,52225,000

4.3

125

47.7

103.5

Bryde's whale

2,439

25,300

13.5

330

26.6

276.4

Sei whale

9,856

28,400

20.8

479

156.3

450.4

Table 6. The each prey consumption of each whale during feeding period in western North PacificA. Common minke whale
Area

Annual consumption (ten thousand tons)
Krill

Anchovy

Saury

Pollock

Squid

Herring

Other

7+8+9

3.2

22.4

19.8

0.9

0.5

0.0

1.0

WNP*

22.4

22.4

19.8

20.1

0.5

19.2

1.0

B. Bryde's whale
Area

Annual consumption (ten thousand tons)
Krill

Anchovy Mackerel

Other

7+8+9

12.4

14.0

0.2

0.0004

WNP

128.7

145.2

2.5

0.005

C. Sei whale
Area

Annual consumption (ten thousand tons)
Copepoda

Krill

Anchovy

Saury

Other

7+8+9

28.6

18.2

104.5

29.0

0.0

WNP**

220.6

44.1

104.5

2.9

29.4

*: The prey composition of common minke whale in the Sea of Okhotsk was assumed that they fed mainly on krill,
herring and walleye pollock (33.3% of wet weight).
**: The prey composition of sei whales distributing excluding sub-areas 7, 8 and 9 was assumed that they fed mainly on
copepods (75%), krill (15%) and other fishes (10% of total wet weight).

11

50°N

Sub area 9

40°N

Sub area 8
Sub area 7

140°E

150°E

160°E

Fig.1. Research area

12

170°E

a. Common minke whale75
Sub area 8
N =181

%

50

25

0
0

50

100

150

mm

b. Sei whale
5050

Sub area 9
N =20025%%

Sub area 8
N =200

0

25

0

0

50

100

150

0

50

mm

100

150

100

150

mm

c. Bryde’s whale
5075

Sub area 9
N = 120Sub area 8
N = 50

%

%

50
25

25

0
0

50

100

0

150

0

mm

50
mm

75
Sub area 8
N = 50

%

50

25

0
0

50

100

150

mm

Fig.2. Length composition (S.L.) of Japanese anchovy in the stomachs of each whale13