SC/F16/JR41

Morphometric analysis on stock structure of the O stock
common minke whale in the western North Pacific
TAKEHARU BANDO AND TAKASHI HAKAMADA
Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
Contact E-mail: bando@cetacean.jp
ABSTRACT
Morphometric analyses were conducted to examine stock structure of western North Pacific common
minke whales (Balaenoptera acutorostrata) by using external measurement data collected during
1994 and 2014 JARPN and JARPNII surveys. Most of the analyses conducted followed
recommendations from the 2009 JARPNII review workshop. External measurements of mature males
were first compared between O and J stock animals assigned by the microsatellite DNA analysis. Then
only O stock animals were compared among sub-areas. The analytical procedures used were theAnalysis of Covariance (ANCOVA) and Discriminant Analysis (DA). Significant differences weredetected between O and J stock whales. J stock animals had longer head region compared to O stock
animals. No significant differences were detected in O stock animals among sub-areas. The results ofthe present morphometric analyses provided no evidences for sub-structuring of the O stock into Ow
and Oe as proposed in one of the hypotheses used in the RMP Implementation, as common minke
whales from coastal and offshore sub-areas did not differ in morphometric characters.
INTRODUCTION
In the western North Pacific, two stocks of common minke whales, the Okhotsk Sea – western North Pacificstock (O stock) and Sea of Japan – Yellow Sea – East China Sea stock (J stock) has been recognized basedon genetic (Goto and Pastene, 1997; Goto et al., 2009; Kanda et al., 2009) and non-genetic (Fujise et al.,
2000; Fujise and Kato, 1996; Goto et al., 2009; Kanda, et al., 2010; Kato, 1992; Mitani et al., 2000) studies.
Morphometric analysis has been conducted to clarify stock structure of common minke whales and theresults supported the view of only two stocks distribute in the western North Pacific (Fujise and Kato, 1995;
1996; Hakamada and Bando, 2009a; 2009b; Hakamada and Fujise, 2000; Kato, 1992).
During the Implementation Review for western North Pacific common minke whales conducted by the IWC
SC from 2010 to 2013, three stock structure hypothesis were proposed (IWC, 2014). Among the threehypothesis, Hypothesis C claimed three stocks (Je, Ow and Oe) distributed in coastal and offshore subareas of the Pacific side of Japan. In contrast, Hypotheses A and B proposed only O and J stocks distributedin the Pacific side of Japan, and mixing of both stocks in some sub-areas and periods.
Elucidation of stock structure in large whales is one of the objectives of JARPNII and several kinds ofsamples and data were collected for this purpose from the start of the program. In this study, the stock
structure of common minke whales in western North Pacific was investigated based on morphometricanalysis by using external measurement data collected during 1994 and 2014 by JARPN and JARPNII
surveys. The analyses followed most of the recommendations from the 2009 JARPNII review workshop(IWC, 2010).
Firstly external measurements between O and J stocks were compared. Second the external measurementsof only O stock animals were compared among sub-areas to examine the plausibility of sub-structuring of
the O stock into Ow and Oe as proposed under Hypothesis C. Animals were assigned to O and J stocks
based on microsatellite DNA analysis (Pastene et al., 2016a (SC/F16/JR38)).
MATEIALS AND METHODS
SamplesCommon minke whales sampled in sub-areas 7CN, 7CS, 7WR, 7E, 8 and 9 (Figure 1) during 1994 to 2014
JARPN and JARPNII offshore surveys were used in the analysis. Only mature males were used because

1SC/F16/JR41

body proportion could differ between mature and immature animals and the very limited number of mature
females in the research area. Males of minke whales were defined as sexually mature based on testis weight(heavier side) of more than 290g (Bando et al., unpublished data). All mature males were used to compare
O and J animals (Table 1). Only morphometric data obtained by the same researchers who measured whales
from all sub-areas (three persons) were used to compare O stock animals among sub-areas (Table 2). Thiswas done in order to reduce possible bias derived from measurement differences among researchers.
External measurements
External measurements were conducted on the research vessel. The measurements that were less susceptible
to differences among researchers were selected for the analysis. Selection of these measurements took into
consideration the opinion of experienced researchers. We also excluded girth because they are likely tochange with progress of the feeding season. The ten external measurements used are shown in Figure 2.
Logarithms of the measurements were used for the analyses.
Assignment of O and J stocks
Each whale was assigned to the O and J stock based on the microsatellite DNA analysis presented in
Pastene et al. (2016a: SC/F16/JR38). Only animals with membership probability of over 90% were includedin the analysis (see discussion of Pastene et al. (2016a: SC/F16/JR38) on the validity of this approach).
Analytical procedures
Following recommendations from the 2009 JARPNII review workshop, the following procedures were
used:
ANCOVA
Analysis of covariance (ANCOVA) using body length as a covariate was used to test if there are significant
differences in external measurements among groups. In these analyses, it was assumed that lengths of themeasurements depend on body length and that their relation can be described by the formula,
log
log
(1)
where x1 is body length, xn is length of the measurement Vn, i is an index representing group to be compared,
αi is the slope and βi is intercept of the formula (1) for group i. Using formula (1), ANCOVA can be
explained as follows. ANCOVA consists of two steps. The first one is to test null hypothesis that⋯(2)
where m is the number of the groups to be compared by ANCOVA. If the null hypothesis was not rejected,
the second test would be conducted. Assuming that equation (2) is true, formula (1) would be
α log
(1)’.
log
The second one is to test null hypothesis that⋯(3)
in formula (1)’ when the null hypothesis (2) was not rejected at the 5% significant level. When m is morethan 2, multiple comparison tests were conducted if the null hypothesis (3) was rejected.
Discriminant analysis
Discriminant analysis using principle components were conducted in two steps. First, PCA was used to
extract principal components (PC) from the 11 measurements. A total of 11 PCs was obtained from 11
measurement. If a sample has missing value in some measurements, it was excluded from the dataset.
Second, 11 PCs were used to construct discriminant function to predict groups. To determine thediscriminant function, stepwise variable selection was conducted. If F-statistics is more than 3.84, the PC
was included in the discriminant function. If F-statistics was less than 2.71, the PC was removed from thefunction. This procedure is a combination of forward selection and backward selection (Landau and Everitt,2001).
RESULTS
Difference between O and J stock animals
ANCOVA
There was no significant interaction between logarithm of body length and stocks (i.e. the null hypothesis(2) was not rejected) for all measurements and therefore ANCOVA can be applied. Table 3 shows the resultsof the ANCOVA test for the null hypothesis (3) for each measurement. There were significant differences
between the two stocks for measurements V2, V3, V4, V5 and V10. This result suggested there are2

SC/F16/JR41

significant difference in measurements between the two stocks.
Discriminant analysis
Table 4 shows coefficient of PCs to construct discriminant functions to discriminate between O and J stocks.
PC1 was positively correlated to all measurements, which can be regarded as an index of growth. PC2 waspositively correlated to V2-V5 and V10-V11, which can be regarded as an index of length of head region.
Table 5 shows prediction of the stock from the discriminant functions which have the PCs as covariates.
PC1, PC2, PC3, PC8 and PC9 were selected as variables of discriminant function. The table shows theprediction is correct for about 80% of the samples. Figure 3 shows a histogram of score of discriminantfunctions for O and J stocks. When the score is more than 0, the sample was predicted as J stock. This figure
shows that discriminant function predicts the stock almost correctly for each stock. These results suggestthat two stocks are different in measurements.
Difference of O stock animals among sub-areas
ANCOVA
Table 6 shows test for the null hypothesis (3) for each measurement to examine if there are significant
difference in ‘O’ stock animal’s measurement among sub-areas 7CS, 7CN, 7WR, 7E, 8 and 9. There wasno significant difference for all the measurements examined.
Discriminant analysis
Table 7 shows the coefficient of PCs to construct discriminant functions to discriminate O stock animals
among sub-areas 7CS, 7CN, 7WR, 7E, 8 and 9. PC1 is positively correlated to all measurements, which
can be regarded as an index of growth. PC2 is negatively correlated to V2-V5 and V10-V11, which can be
regarded as an index of length of head region. Effort was made to construct discriminant function by the
stepwise variable selection, however no variable was selected.
DISCUSSIONS
Significant difference was detected in morphometry between O and J stock common minke whales,confirming the findings of previous studies. J stock animals have longer head region compared to O stock
animals, which confirmed the results of previous studies (Hakamada and Bando, 2009a; Hakamada andFujise, 2000). More than 80% of morphometrically discriminated animals matched with DNA-identified
stock, which supported the separation of two stocks. Significant difference was also reported in colorationsof flipper between O and J animals (Nakamura et al., 2016 (SC/F16/JR39)). These results revealed thatmorphology and morphometry can be used to identify O and J stocks of North Pacific common minke
whales.
On the other hand, no significant differences were detected in O stock animals among sub-areas. This result
was consistent with those of previous studies (Hakamada and Bando, 2009a; 2009b; Hakamada and Fujise,
2000). Putative Ow and Oe stocks were proposed for coastal sub-areas (7CS and 7CN) under Hypothesis
C. However the results of the present morphometric analyses provided no evidences for such substructuring as common minke whales from coastal and offshore sub-areas did not differ in morphometriccharacters. These morphometric results are consistent with those from microsatellite and mtDNA analyses
presented to this workshop (Pastene et al., 2016a (SC/F16/JR38); 2016b (SC/F16/JR40)).
Only mature males were analysed in this study because sample size of mature female was not large enough,due to sexual segregation of common minke whales in the research area. Migration strategy of O stock
common minke whale differs by sex and maturity status, and majority of mature females are thought tomigrate to Okhotsk Sea (Hatanaka and Miyashita, 1997). Further analysis by using mature female samples
will contribute to elucidate further the stock structure and migration strategy of common minke whales in
the western North Pacific.
ACKNOWLEDGEMENT
We would like to thank all researchers and crews involved in the JARPN and JARPNII surveys. We express
our gratitude to Luis A. Pastene (Institute of Cetacean Research) for valuable comments and suggestions.
REFERENCES

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Fujise, Y. and Kato, H. 1995. Preliminary report of morphological differences of mine whales between
coastal Japan (sub-area 7) and offshore area (sub-area 9) of western North Pacific. Paper SC/47/NP6
presented to IWC Scientific Committee, May 1995 (unpublished). 9pp.
Fujise, Y. and Kato, H. 1996. Some morphological aspects of the western North Pacific minke whales;
Preliminary analyses of materials from JARPN surveys in 1994-5. Paper SC/48/NP11 presented to
IWC Scientific Committee, May 1996 (unpublished). 10pp.
Fujise, Y., Hakamada, T., Aoki M., Niimi, S., Nakata, H., Honda, K. and Tanabe, S. 2000. An attempt toidentify stocks in the western North Pacific minke whale (Balaenoptera acutorostrata) using
accumulation levels of heavy metals and organochlorines as ecological tracers. Paper SC/F2K/J18
presented to the IWC Scientific Committee Workshop to Review the Japanese Whale Research
Programme under Special Permit for North Pacific Minke Whales (JARPN), Tokyo, 7-10 February
2000 (unpublished). 18pp.
Goto, M. and Pastene, L.A. 1997. Population structure of the western North Pacific minke whale based on
an RFLP analysis of the mtDNA control region. Rep. int. Whal. Commn 47:531-537.
Goto, M., Kanda, N., Kishiro, T., Yoshida, H., Kato, H. and Pastene, L.A. 2009. Mitochondrial DNA
analysis on stock structure in the western North Pacific common minke whales. Paper SC/J09/JR29
presented to the JARPN II Review Workshop, Tokyo, January 2009 (unpublished). 10 pp.
Hakamada, T. and Bando, T. 2009a. Morphometric analysis on stock structure in the western North Pacificcommon minke whales (Balaenoptera acutorostrata). Paper SC/J09/JR27 presented to the JARPNII Review Workshop, Tokyo, January 2009 (unpublished). 13 pp.
Hakamada, T. and Bando, T. 2009b. Further morphometric analysis on stock structure in the western North
Pacific common minke whales (Balaenoptera acutorostrata). Paper SC/61/JR6 presented to the
IWC Scientific Committee, May 2009 (unpublished). 4 pp.
Hakamada, T. and Fujise, Y. 2000. Preliminary Examination of the heterogeneity of external measurementsof minke whales in the western North Pacific, using data collected during 1994-1999 JARPN
surveys. Paper SC/F2K/J15 presented to the IWC Scientific Committee Workshop to Review theJapanese Whale Research Program under Special Permit for North Pacific Minke Whales (JARPN),Tokyo, 7-10 February 2000 (unpublished). 12pp.
Hatanaka, H. and Miyashita, T. 1997. On the feeding migration of the Okhotsk Sea – West Pacific stock ofminke whales, estimates based on length composition data. Rep. Int. Whal. Commn 47: 557-564.
International Whaling Commission. 2010. The report of the Expert Workshop to review the ongoing
JARPN II Programme. J. Cetacean Res. Manage. 11 (Supple 2): 405-449.
International Whaling Commission. 2014. Report of the working group on the Implementation Review for
western North Pacific common minke whales. J. Cetacean Res. Manage. 15 (Suppl.): 112-188.
Kanda, N., Goto, M., Kishiro, T., Yoshida, H., Kato, H. and Pastene, L.A. 2009. Microsatellite analysis ofminke whales in the western North Pacific. Paper SC/J09/JR30 presented to the JARPN II Review
Workshop, Tokyo, January 2009 (unpublished). 14 pp.
Kanda, N., Goto, M., Nagatsuka, S., Kato, H., Pastene, L.A. and Hatanaka, H. (2010). Analysis of genetic
and non-genetic data do not support the hypothesis of an intermediate stock in sub-area 7. Paper
SC/S10/NPM9 presented to the First Intersessional Workshop for western North Pacific common
minke whales, Tokyo, September 2010 (unpublished).
Kato, H., Kishiro, T., Fujise, Y. and Wada, S. 1992. Morphology of minke whales in the Okhotsk Sea, Sea
of Japan and off the East Coast of Japan, with respect to stock identification. Rep. Int. Whal. Commn
42: 437-442.
Landau, S. and Everritt, B. S. 2001 A handbook of statistical analyses using SPSS. Chapman & Hall/CRC.
354pp.
Mitani, Y., Bando, T., Takai, T. and Sakamoto, W. 2000. Diet record and stock structure of minke whales
Balaenoptera acutorostrata around Japan examined by δ13C and δ15N analyses. Paper SC/F2K/J20
presented to the IWC Scientific Committee Workshop to Review the Japanese Whale Research
Programme under Special Permit for North Pacific Minke Whales (JARPN), Tokyo, 7-10 February
2000 (unpublished). 12pp.
Nakamura, G. et al. 2016. Investigation of the spatial and temporal mixing of ‘O’ and ‘J’ stocks based onmorphology and coloration of the flippers. Paper SC/F16/JR39 presented to the JARPNII special
permit expert panel review workshop, Tokyo, February 2016 (unpublished).
Pastene, L.A., Goto, M., Taguchi, M. and Kitakado, T. 2016a. Temporal and spatial monitoring of the ‘J’and ‘O’ stocks of common minke whale in waters around Japan based on microsatellite DNA. Paper
SC/F16/JR38 presented to the JARPNII special permit expert panel review workshop, Tokyo,

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February 2016 (unpublished).
Pastene, L.A., Goto, M., Taguchi, M. and Kitakado, T. 2016b. Updated genetic analyses based on
mitochondrial and microsatellite DNA indicated no sub-structure of the ‘O’ stock common minke
whale in the western North Pacific. Paper SC/F16/JR40 presented to the JARPNII special permit
expert panel review workshop, Tokyo, February 2016 (unpublished).

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Table 1. Sample size of mature male common minke whales used for comparison between O and J stock
animals.
Year
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
Total

Stock
Total
J
O
15
15
63
63
3
30
33
64
64
60
60
4
38
42
2
16
18
56
56
3
54
57
64
64
3
54
57
1
42
43
1
52
53
2
49
51
35
35
20
20
7
7
21
21
5
17
22
3
3
24 760 784

Table 2. Sample size of mature males common minke whales used for comparison of O stock animals
among sub-areas (only morphometric data obtained by three researchers who measured whales from all
sub-areas, were used)
Year
1998
2000
2001
2002
2003
2006
2009
Total

Sub-area
7CS 7CN 7WR 7E 8
0
0
0
6 30
1
4
0
0
0
3
7
10
5 12
0
28
1
0
5
2
0
3
5 26
7
2
6
0 23
0
2
6
1
7
13 43
26 17 103

9
0
11
19
20
28
14
4
96

Total
36
15
53
54
62
45
20
285

Table 3. Results of ANCOVA to test morphometric differences between O and J stock animals.
Measurement p -value
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11

p <0.05
p <0.05
p <0.05
p <0.05
n.s.
n.s.
n.s.
n.s.
p <0.05n.s.

J stock
n =24
97.0
117.3
155.6
314.8
405.9
350.7
241.3
193.4
157.3
83.6

O stock
n =760
93.1
111.8
150.4
307.7
409.5
353.5
242.7
194.8
151.7
84.1

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Table 4. Component matrix extracted by PCA to test morphometric differences between O and J stock
animals. Bold letter indicates the PCs selected for discriminant function. PCx indicates the xth PC.
PC3

PC4

PC5

-0.181

0.016-0.035-0.1400.505-0.114

0.387

0.037

0.808

0.517

-0.152-0.030

0.068

0.098

logV4

0.859

0.430

-0.111-0.125

0.021

logV5

0.897

0.225

-0.029-0.252-0.104

logV6

0.805

-0.482

0.010

0.112

-0.268

0.010

logV7

0.860

-0.454-0.0530.075-0.042

0.011

logV8

0.825

-0.501-0.096-0.0200.176-0.038-0.010

PC1

PC2

logV1

0.951

logV2

0.748

logV3

PC6

PC7

PC8

PC9

PC10

PC11

0.022

-0.034-0.129-0.136-0.036-0.0590.047-0.1350.016-0.022

0.011

0.015

0.156

0.030

0.058

-0.043-0.102

0.104

0.143

-0.040-0.047

0.050

0.106

0.070

-0.238

0.069

0.049

0.005

0.003

0.116

0.147

0.001

-0.004

0.016

0.002

-0.158

0.140

0.002

0.022

0.038

-0.0230.146-0.047
0.041

logV9

0.818

-0.480-0.141-0.0310.238-0.035-0.0250.052-0.026-0.125logV10

0.816

0.447

0.070

-0.030-0.032-0.3570.027-0.0040.012-0.001

0.004

logV11

0.670

0.037

0.732

0.024

0.095

0.069

0.014

0.010

0.008

-0.003

0.002

Table 5. Comparison between DNA and morphometrically discriminated stocks of common minke
whales.
M orphologically discriminated stock

DNA based
stock

Frequency%

J stockO stockJ stockO stockJ stock
20
146
83.3
19.2

O stock
4
614
16.7
80.8

Total
24
760
100.0
100.0

Table 6. Results of ANCOVA to test morphometric differences of O stock animals among sub-areas.
7CS
7CN
7WR
7E
8
9
Measurement p -value
n =13
n =43
n =26
n =17
n =103
n =96
V2
n.s.
93.5
93.5
92.9
93.4
93.8
94.6
V3
n.s.
113.3
111.9
112.2
112.8
112.2
113.1
V4
n.s.
152.0
151.1
150.8
152.2
151.1
152.2
V5
n.s.
312.6
309.2
307.0
310.8
309.8
310.4
V6
n.s.
410.8
414.1
412.8
414.9
413.6
412.4
V7
n.s.
358.5
359.6
357.5
358.2
356.7
356.0
V8
n.s.
246.4
246.7
246.2
244.9
244.9
243.7
V9
n.s.
198.1
197.9
198.1
196.8
197.0
195.8
V10
n.s.
150.7
151.7
150.2
152.5
151.9
153.1
V11
n.s.
82.8
84.4
84.2
85.6
84.4
85.0

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Table 7. Component matrix extracted by PCA to test morphometric differences of O stock animals among
sub-areas.
PC1

PC2

PC3

PC4

PC5

PC6

PC7

PC8

PC9

PC10

PC11
-0.089

logV1

0.954

0.199

0.026

0.018

-0.128-0.079-0.016-0.043-0.029-0.127

logV2

0.771

-0.493-0.130

0.351

0.094

-0.097

0.051

0.022

-0.022

0.004

0.017

logV3

0.817

-0.504-0.175-0.073-0.007

0.106

0.107

-0.001

0.111

0.043

-0.075

logV4

0.867

-0.413-0.116-0.152-0.036

0.093

0.110

-0.032-0.104-0.051

0.072

logV5

0.916

-0.221-0.022-0.167-0.132-0.233-0.051

0.065

0.020

0.058

0.028

logV6

0.798

0.490

0.074

0.139

-0.259

0.135

0.013

0.108

0.007

0.030

0.025

logV7

0.850

0.470

-0.0420.045-0.020-0.017-0.006-0.213

0.049

0.046

0.041

logV8

0.809

0.530

-0.094-0.051

0.165

0.008

0.010

0.030

-0.1170.088-0.058

logV9

0.797

0.516

-0.164-0.055

0.211

0.001

-0.014

0.089

0.088

-0.0790.045

logV100.833-0.466

0.055

0.007

0.051

0.119

-0.263-0.012-0.0100.002-0.001logV110.754-0.1050.633-0.0190.115-0.004

0.078

0.004

0.017

0.002

0.003

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Figure 1. The sub-areas used for the Implementation Review for western North Pacific common minke
whales (IWC, 2014).

V5
V4
V3
V2

V9
V8
V7
V6
V1

V1: Body length
V2: From the tip of snout to blow hole
V3: From the tip of snout to center of eye
V4: From the tip of snout to ear
V5: From the tip of snout to tip of flipper
V6: From notch of flukes to end of ventral gloves
V7: From notch of flukes to center of umbilicus
V8: From notch of flukes to sexual apparatus
V9: From notch of flukes to anus
V10: Length of skull
V11: Maximum width of skull
Figure 2. External measurement points of common minke whale used in this study.

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200

O stock
150
100
50
0
-7

-5

-3

-1

1

3

5

7

-3

-1

1

3

5

7

6

J stock

5
4
3
2
1
0
-7

-5

Figure 3. Results of discriminant analysis between O and J stock animals.

10