SC/62/NPM1 Rev

A hypothesis on the migration pattern of J-stock common minke
whales.
Mutsuo Goto¹, Tomio Miyashita², Naohisa Kanda¹, Luis A. Pastene¹ and Hiroshi Hatanaka¹¹The Institute of Cetacean Research, Toyomi-cho 4-5, Chuo-ku, Tokyo 104-0055, Japan²National Research Institute of Far Seas Fisheries, Fukuura 2-12-4, Kanazawa-ku, Yokohama, Kanagawa 236-8648,
Japan
ABSTRACT
A hypothesis on the migration pattern of J-stock common minke whale was proposed at the 61 IWC ScientificCommittee meeting (Hatanaka et al., 2010). Here this hypothesis was evaluated in the context of the available
information on pattern of mixing between O and J-stocks, sighting distribution,, sea ice condition and by-catch bycoastal fishing gear. The proposed hypothesis fix well with the above mentioned information, which was obtained in the
Sea of Japan and Okhotsk Sea.
INTRODUCTION
It is well known that there were two stocks of common minke whales, the Okhotsk Sea – West Pacific stock (O-stock)and the Sea of Japan – Yellow Sea – East China Sea stock (J-stock) (Omura and Sakiura, 1956). Kato (1992) reported
that the conception date of J-stock was autumn (October-November), while that of O-stock was winter (January-March).
Previous studies reported on the peak whaling seasons and possible migration route of the J-stock (Ohsumi, 1983).
Gong (2005) hypothesized a migration pattern for J-stock whales. In recent years genetic techniques have beendeveloped and used to allocate individual whales to O and J stocks and also information on by-catch has been
accumulated in a more systematic manner in recent years.
A new hypothesis on the migration of J-stock was presented to the 61 IWC Scientific Committee meeting in 2009
(Hatanaka et al., 2010). It was based on the information that the breeding season of the J-stock is approximately threemonth earlier than that of the O-stock. This suggested that the typical winter/breeding-summer/feeding migration
pattern of baleen whales could not be applied to J-stock.
In this study the hypothesis proposed by Hatanaka et al. (2010) is examined in the context of the available information
on pattern of mixing proportion of O and J-stocks, sighting distribution, sea ice condition and by-catch by coastal
fishing gear.
MATERIALS AND METHODSThe Hatanaka et al. (2010) hypothesis is characterized as follow:a.

Northward (feeding) migration begins in January – February.

1b.

Pregnant females migrate into the southern part of Okhotsk Sea in April following the retreat of sea ice.

c.

The main feeding season is April – June.

d.

Southward (breeding) migration starts in July.

e.

Segregation by sex and maturity occurs.
e-1. Pregnant females migrate to northernmost distribution area.
e-2. In general, adult animals migrate and distribute in offshore waters in the Sea of Japan.
e-3.The migration of juveniles is different from adult animals. They stay close to the coasts of Japan and Korea
almost all year around.

Various kinds of data have been considered to examine this hypothesis:1) Mixing proportion of J-stock in sub-area 11 based on mtDNA (Table 1). The mixing proportion was
estimated using a Bayesian approach (Punt, 2003), which was previously employed during the

Implementation Simulation Trials (IST) for North Pacific common minke whales and included
estimation of the standard deviations. In these estimations, the haplotype composition of samples fromJapanese by-catches in sub-area 6 (Sea of Japan) (n=362) during 2001 to 2007 and that of samples fromsub-areas 8 and 9 taken in JARPN and JARPNII surveys (n=690) between 1994 and 2007 were used as
representative samples of J and O stocks, respectively.
2) Mixing proportion of J-stock in sub-area 12 based on cookie-cutter shark scars (Miyashita et al., 2010).
3) Sighting survey data
- Sighting positions in April – June (Figure 1) (Miyashita et al., 2009).
- Sighting positions in July - September (Figure 2).
- Estimated length composition of sighted animals (estimated by eye from the vessel) (Figure 3).
4) Sea ice coverage in the Okhotsk Sea (Figure 4).
5) By-catch information- Geographical location of by-caught J-stock animals in the coast of Japan, by month (Figure 5).
- Monthly change in the number of by-caught J-stock animals in the coast of Japan (Figure 6).
- Length composition of by-caught animals (Figure 3).
The consistency between the hypothesis characteristics (items a. to e. above) and the available data (items 1) to 5)
above) was examined.
RESULTS1.

Mixing proportion of J-stock animals in sub-area 11
The mixing proportion of J stock in April is high for both sexes and the proportion of females is extremely higher
than males (commercial samples in Table 1). This is consistent with items b and e-1.
The mixing proportion decreases in May (Table 1). This might suggest that J-stock animals disperse widely to thesouthwest part of the Okhotsk Sea as sea ice melts in May. The alternative explanation is that the lower proportionof J stock reflects an increase of O stock animals in this sub-area.
The mixing proportion of J stock becomes slightly high again in July and August ( JARPN samples in Table 1). Thismight suggest that J stock animals return to sub-area 11 from the southwest part of Okhotsk Sea on the course of
breeding migration to lower latitude areas. This is consistent with item d.

2

2.

Scars of cookie-cutter shark
The analysis of shark scars (Miyashita et al., 2010) showed that the proportion of J stock animals is zero in July –August in Sub-area 12. This coincides with item d.

3.

Sighting distribution
Common minke whales are distributed widely in the whole Sea of Japan in April to June (Figure 1). This supports
item c.
Common minke whales in the Sea of Japan are less frequent in July-September than in April – June (Figure 2).
Density (number of sighting per 100 nautical miles) is one third of that in April – June (0.49 versus 1.49). This
supports items c and d.
Although estimate of body length by eye from the vessel is not precise, Figure 3A suggests that most of the sighted
animals are adult (larger than 6.5 m). Sightings are distributed in offshore waters (Figures 1 and 2). This supports
item e-2.

4.

Sea ice coverage in the Okhotsk Sea
Sea ice covers the southwest part of the Okhotsk Sea and the La Perous Strait was closed in March (Figure 4).
Therefore J stock animals can not get into the Okhotsk Sea in March. Then the Strait opens in April and sea ice
apart slightly from the north coast of Hokkaido and southeast coast of Sakhalin. Therefore the distribution of J
stock animals should be restricted to these coastal areas in April. Then they disperse into the southwestern part ofOkhotsk Sea following the retreat of the sea ice, but probably they can not reach the north part of the Sea. It is said
that large whales can not pass the Tatarskij (Mamiya) Strait between the continent and Sakhalin Island. This
supports item b.

5.

By-catch information
J stock animals are caught in coastal areas of Japan (Figure 5) though catch are fewer in some months especially in
September. This supports item e-3.
Monthly number of by-catch shows that animals occur through all year (Figure 6). There is a peak in January, whichcoincides with item a.
unknown.

The reason of a decrease in the number of by-caught animals in September and October is

However these months correspond to the breeding season of this stock.

Most of by-caught animals are juvenile of 3-5.5 m (Figure 3). This supports item e-3.
DISCUSSION
The hypothesis proposed by Hatanaka et al. (2010) is supported by the various kind of available data from the Sea ofJapan and Okhotsk Sea. There is a possibility that more than one sub-stock exist within the J-stock (Kanda et al., 2010).
The hypothesis treated here is regarding large-scale migration and may not applicable to resident group in the Yellow
Sea. The feeding migration route is schematically shown in Figure 7 for adults and Figure 8 for juveniles. Breeding
migration is assumed to be the reverse in the case of adults. There are two possibilities of juveniles, one is that theymigrate from coastal area to adults’ feeding area when they grow and the other is that they return to breeding area once
and then migrate to adults’ feeding area.
Although there is no direct information on the breeding area, it is supposed to be in the East China Sea as juveniles are
distributed along the Pacific coast of southwest Japan, because the movement through the Kanmon Strait and Seto

3

Inland Sea from the Sea of Japan is not plausible as by-catch is seldom reported within the Seto Inland Sea.
In general the typical migration pattern of baleen whales is that the feeding migration starts in spring and return tobreeding area in autumn (breeding in winter). However in the case of J stock a different migration pattern is assumed asthey appear to be autumn breeders. This unusual pattern might be caused from the closing and openings of Sea of Japanin the past ice age. It was suggested that the divergence time of the J and O stocks was 0.14 Ma (Pastene et al., 2007)
which was much older than the opening of Sea of Japan after the last glaciation some 10 thousands years ago. Therefore,
the two stocks might have used different refugia around the area while closing, that prevented them from interbreeding,and only the J stock subsequently entered Sea of Japan. Although its peak is different from each other, the conception
dates are actually variable within the stocks (Kato, 1992). That variation could have also existed at the past. The
breeding season of the J stock could have shifted forward since then, due to the whales’ adaptation to the environments
in Sea of Japan.
REFERENCES
Gong, Y. 2005. Distribution and abundance of the East/Japan Sea – Yellow Sea – East China Sea stock of minke whales.
SC/57/NPM13.
Hatanaka, H., Miyashita, T. and Goto, M. 2010. A hypothesis on the migration of J-stock minke whales. Report of the
Scientific Committee, Annex G1, Appendix 6.
Kanda, N., Park, J-Y., Goto, M., An, Y-R., Choi, S-G., Moon, D-Y., Kishiro, T., Yoshida, H.,Kato, H. and Pastene, L.A.
2010. Genetic analysis of western North Pacific minke whales from Korea and Japan based on microsatellite
DNA. SC/62/NPM11.
Kato, H. 1992. Body length, reproduction and stock separation of minke whales off northern Japan. Rep. Int. Whal.
Commn. 42:443-453.
Miyashita, T., Goto, M., Yoshida H. and Kanaji Y. 2010. Estimation of mixing proportion of O and J common minke
whales in sub-area 12 using cookie-cutter shark scar as ecological marker. SC/62/NPM10..
Miyashita, T., Okamura, H. and Kitakado, T. 2009. Abundance of J-stock common minke whales in the Sea of Japan
using the Japanese sighting data with g(0)=1. SC/61/NPM7.
Ohsumi, S. 1983. Minke whales in the coastal waters of Japan in 1981, with special reference to their stock boundary.
Rep. Int. Whal. Commn. 33: 365-371.
Omura, H. and Sakiura, H. 1956. Studies on the little piked whale from the coast of Japan. Sci. Rep. Whales Res. Inst.,
Tokyo 11: 1-37.
Pastene, L.A., Goto, M., Kanda, N., Zerbini, A.N., Kerem, D., Watanabe, K., Bessho, Y., Hasegawa, M., Nielsen, R.,
Larsen, F., Palsboll, P.J. 2007.

Radiation and speciation of pelagic organisms during periods of global warming:

the case of the common minke whale, Balaenoptera acutorostrata. Mol. Ecol. 16: 1481-1500.
Punt, A.E. 2003. A Bayesian Approach to Estimating ‘J’-‘O’ Mixing Proportions. Annex F. Report of theWorkshop on North Pacific Common Minke Whale (Balaenoptera acutorostrata) Implementation

Simulation Trials (SC/54/Rep1). J. Cetacean Res. Manage. (Suppl.) 5:482.

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Figure 1. Sighting positions (black circle) of common minke whales in the Sea of Japan with track line on effort
indicated. Data from six Japanese cruises in April to June between 2003 and 2007

5

Figure 2. Track line on effort in July – September and sighting positions of common minke whales. Data from
Toshimaru No.25 in 1988 and Shunyo-maru in 1994

6

60

Number

50
40
30
20
10
0
≤3.0 ≤3.5 ≤4.0 ≤4.5 ≤5.0 ≤5.5 ≤6.0 ≤6.5 ≤7.0 ≤7.5 ≤8.0 ≤8.5
Estimated body length (m)Ａ：Animals
sighted during the sighting surveys (n=201).
Ａ：

140

120

100

Number

80

60

40

20

0
≤2.5

≤3.0

≤3.5

≤4.0

≤4.5

≤5.0

≤5.5

≤6.0

≤6.5

≤7.0

≤7.5

≤8.0

≤8.5

≤9.0

Body length (m)B:

By caught animals (n=419)

Figure 3. Body length composition of animals sighted during sighting surveys (A) and by-caught animals (B) in the Sea
of Japan7

5 March

5 April

La Pérous Strait

20 April

30 AprilFig. 4. Average sea ice coverage in the Sea of Okhotsk during 1971 to 2000.
From the website of the Japan Meteorological Agency(http://www.data.kishou.go.jp/kaiyou/db/seaice/okhotsk/okhotsk_normal.html).

8

45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

January

30N
138E

February

45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

March

30N
138E

April

45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

May

30N
138E

June

9

(Fig. 6. continued)
45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

30N
138E

July

August

45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

September

30N

October

138E

45N

45N

45N

45N

40N

40N

40N

40N

35N

35N

35N

35N

30N

30N
128E

30N
128E

138E

November

30N

December

138E

Figure 5. Geographical distribution of by-caught animals, by month. Stocks were indentified using microsatellite DNA.
Pink triangle: Female less than 6m, red triangle: Female more than 6m, light blue rhomb: male less than 6m and blue
square: male more than 6m.

10

70
60

Number

50
40
30
20
10
0

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Month
Figure 6．Number of by-caught of J-stock animals by month in sub-area 6

11

Nov

Dec

Figure 7. Assumed feeding migration route of adult J-stock animals

12Figure 8. Assumed feeding migration route of juvenile J-stock animals

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Table 1. The mixing proportion of the J stock in SA11 based on mtDNA data obtained from samples of pastcommercial whaling, JARPN, and bycatches.

Sample source

PeriodN

Mix. Prop.

S.D.

Commercial whaling
Female
April
May
June
July
AugustSeptember

1983-1987
53
51
24
2
5
2

0.645
0.055
0.263
0.253
0.143
0.257

0.066
0.036
0.086
0.196
0.124
0.199

2
14
9
2
3
4

0.747
0.125
0.092
0.250
0.204
0.167

0.196
0.080
0.084
0.194
0.167
0.141

1999
1996

22
11

0.453
0.137

0.103
0.097

1999
1996

27
18

0.219
0.441

0.079
0.112

11

0.921

0.073

Male
April
May
June
July
AugustSeptember
JARPN
FemaleJuly
AugustMale
July
AugustBy-caches
Female and male
2001-2007
N = sample size, S.D. = standard deviation

14