SC/64/NPM1

On the plausibility of stock structure hypotheses of western
North Pacific common minke whale
Luis A. Pastene1, Jung Youn Park2, An Yong Rock2 and Naohisa Kanda1
1

The Institute of Cetacean Research, Toyomi 4-5, Chuo-ku, Tokyo 104-0055, Japan
2Biotechnology Research Center, National Fisheries Research and Development Institute, Busan,
Korea
ABSTRACTTo assist the IWC SC in the assignment of plausibility to hypotheses on stock structure in thewestern North Pacific common minke whale we summarized and discussed here two kinds ofinformation a) the work conducted since 2003 to refine previous hypotheses based on O and J
stocks, which derived into current Hypotheses A and B, and b) our view and differences regarding
the main arguments presented in support of Hypothesis C (Wade and Baker, 2011). There are twocomponents that separate those hypotheses, first the question of J/O against the JW/JE andOW/OE sub-divisions, and second the occurrence or not of a different stock in the Yellow Sea (Ystock). The first component separates Hypotheses A and B from Hypothesis C, the second one
separate Hypothesis A from Hypotheses B and C. Regarding the first component current
Hypotheses A and B were refined based on new genetic analyses that followed a process andrecommendations from the IWC SC through the years, e.g. genetic separation of J/O stocks,
hypothesis testing on separated J and O stock samples, examination of the statistical power, etc. In
contrast, Hypothesis C that has to demonstrate division into JW/JE in the J stock and OW/OE in
the O stock, derived from genetic analyses conducted on total samples (pooled samples of the O
and J stocks), and no analytical effort was made to discriminate further the samples. Following
advice from the IWC SC several non-genetic data were examined by the proponents of Hypotheses
A and B and comparative analyses were conducted separately for J and O stock samples. Despitethat the proponents of Hypothesis C argued initially that several non-genetic data supported theirdivision into JW/JE and OW/OE, finally the only evidence cited by those proponents wasconception date, which is not a strong argument as explained in this document. Responding torecommendations from the IWC SC for specific analyses is important. One of the most relevant
recommendations from the IWC SC was to conduct analyses separately for J and O stock samples.
The proponents of Hypotheses A and B and other SC workers did. The proponents of Hypothesis
C did not. We believe that further analyses and scientific evidences are necessary for the actual useof Hypothesis C in the RMP Implementation. Regarding to the second component, we can notdeny the possibility of additional structure in the Yellow Sea (e.g. Y stock), however the current
scientific evidences (conception date and microsatellite) are weak and much more work is requiredon those data to confirm the possibility of additional structure. Therefore we believe that theplausibility of Hypothesis A should be higher than that of Hypothesis B.
KEYWORDS: COMMON MINKE WHALES, IMPLEMENTATION, WESTERN NORTH PACIFIC,STOCK STRUCTURE HYPOTHESES, PLAUSIBILITYINTRODUCTION
Three stock structure hypotheses (Hypotheses A, B and C) were proposed and specified on the new
sub-areas (Figure 1) for the current western North Pacific common minke whale Implementation.
Stock structure hypotheses A and B are refinement of the previous Hypotheses A and B of the 2003
Implementation, with some elements of these hypotheses (e.g. Y stock in Hypothesis B) mimicing
some of the aspects of the sensitivity tests considered during the 2003 Implementation (IWC, 2004). A
difference between current and previous Hypotheses A/B is the more extensive distribution of J stock
animals in the Pacific side of Japan and the spatial and temporal mixing of J and O stock animals in
sub-areas 7CS and 7CN is now more documented under Hypotheses A and B. Stock structure
Hypothesis C is a new one, derived basically from genetic analyses on pooled samples of the J and O
stocks, which proposes new stocks e.g. JW and JE and OW and OE.

1Descriptions of the three hypotheses were given in Appendix F of IWC (2011a). Our summary of thescientific evidences pro- and against those hypotheses and our plausibility ranking were alreadypresented during the 2011 IWC SC meeting (Pastene and Hatanaka, 2011; Pastene et al., 2011). We
concluded that Hypothesis A should be assigned a ‘High’ plausibility ranking and Hypotheses B and Ca ‘Low’ ranking (Pastene et al., 2011).
To assist the IWC SC in the assigning of plausibility to stock structure hypotheses at the 2012 IWC SC
meeting we summarized and discussed here two kind of information a) the analytical work conductedsince 2003 to refine previous hypotheses based on O and J stocks, which derived into the current
Hypotheses A and B, and b) our view and differences regarding the main arguments presented insupport of Hypothesis C (based on the latest paper by Wade and Baker, 2011).
There are two main components that separate those hypotheses, first the question of J/O against the
JW/JE and OW/OE sub-divisions, and second the occurrence or not of a different stock in the Yellow
Sea (Y stock). The first component separates Hypotheses A and B from Hypothesis C, the second one
separates Hypothesis A from Hypotheses B and C. The first component is examined in the next two
sections below while the second component is examined in the third section below.
Regarding the first component in 2010 the IWC SC noted that much of the remaining disagreement
about competing stock structure hypotheses centres on the question of whether minke whales in subarea 7 and 2 represent a mixture of ‘O’ and ‘J’ stock animals or a single stock with intermediatecharacteristics. It agreed that trying to resolve this issue should be a top priority using both genetic andnon-genetic data (IWC, 2011b).
REFINEMENT OF PREVIOUS HYPOTHESIS BASED ON O AND J STOCKS
Table 1 shows examples of genetic analyses conducted by Japanese scientists to examine and refine
previous hypotheses based on O and J stocks, which followed advices from the IWC SC. Results of
those analyses were used to propose the current Hypotheses A and B in the current Implementation.
Detailed results of these analyses were already presented in Appendix F of IWC (2011a) and Pastene etal. (2011). The next paragraphs summarize only the key elements.
Genetic analyses conducted separately for J and O stocks
During the previous Implementation of western North Pacific common minke whale, the IWC SC had
recommended genetic analyses separately for J and O stocks, and specifically recommended the use ofalternative methods for exclusion of ‘J’ stock animals (IWC, 2003). This would facilitate theinterpretation of results. Then the first task by Japanese scientists was to identify a genetic method able
to discriminate individual J and O stocks.
Kanda et al. (2009a) used 16 microsatellite loci and the program STRUCTURE to discriminate
individual J and O stocks in a large sample from JARPN and JARPN II and from by-catches (n=2,542).
The animals with the membership probability of over 90% for either two stocks were assigned as‘pure’ individuals. A total of 2,302 animals (91%) were assigned as the pure individual to the either
stock (770 to the J stock and 1,532 to the O stock). There were a 9% of unassigned animals.
During the 2009 annual meeting the IWC SC welcomed these results and recommended several
additional analyses based on STRUCTURE, particularly to elucidate the problem of ‘unassigned’
animals (IWC, 2010a). Some of the recommended analyses were made and presented to the 2010 IWC
SC by Kanda et al. (2010a). Basically the results of the additional analyses failed to detect evidence ofadditional stock structure and they suggested that unassigned individuals were probably either J or O
stocks.
After examining the extensive discussions by the IWC SC on this particular topic we believe that there
is agreement in the Committee that the approach used by Kanda et al. (2009a) is useful to assign
individuals to stock differentiated at the level of O and J stocks (e.g. Fst= 0.04-0.05) and that there isremaining questions on whether the approach can assign individuals to less differentiated stocks.
Therefore we consider that any genetic study focused to investigate additional genetic structure within
the J and O stocks should start separating the O and J stocks animals as in Kanda et al. (2009a). Other
more sensitive approaches could be tried then on the discriminated samples of J and O stock animals.

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For example several genetic analyses based on hypothesis testing and mtDNA and microsatellite wereconducted separately for O and J stocks (Table 1; see also Appendix F of IWC, 2011a and Pastene etal., 2011). None of these analyses showed significant heterogeneity within the J and O stock samples.
Another example is the Principal Component Analysis (PCA) used by Gaggiotti and Gascuel (2011),
which was applied on samples of J and O stocks discriminated by the approach of Kanda et al. (2009a)(see discussion on the PCA results below).
However despite these methodological advances in separating O and J stocks animals, which wererecognized by the IWC SC, the proponents of Hypothesis C conducted genetic analyses based onpooled samples of O and J stocks. Interpretations of results derived from such analyses are very
difficult (see section below). No analytical effort was made by those proponents to demonstrate subdivision in the J and O stocks using the genetic data.
Examination of the statistical power
The IWC SC had recommended analysis of the statistical power of the hypothesis testing (IWC, 2010b).
Japanese scientists presented results of the power analyses at the 2009 IWC SC annual meeting (Kanda
et al., 2009b). The study suggested high statistical power providing confidence to the conclusion of asingle O stock. The IWC SC noted that the work was a direct response to a suggestion by the JARPN II
Panel (IWC, 2010b), and thanked the authors for the substantial work involved in conducting the power
analyses and agreed that the results represented a very valuable contribution (IWC 2010a).
Examination of non-genetic information for O and J stock
Other of the recommendations of the IWC SC was the analyses of non-genetic data in addition to
genetic data. The IWC SC had made this recommendation several times, the last being in 2010 (IWC,
2011b). Several analyses of non-genetic data (morphometric, flipper color pattern, fluke color pattern,conception date) were conducted separately for J and O stock samples (Table 1). None of theseanalyses showed significant heterogeneity within the J and O stocks although in some cases the sample
size was small (see results in Appendix F of IWC, 2011a and Pastene et al., 2011).
EXAMINING THE SCIENTIFIC BASIS FOR THE SUB-DIVISION OF JW/JE AND OW/OE
STOCKS
Spatial genetic heterogeneity of pooled J and O stock samples
The analytical approach used by the proponents of Hypothesis C to examine genetic data was different
from that followed by the proponents of Hypotheses A and B and other IWC SC workers. The proposal
for the division of the J stock into JW and JE and of the O stock into OW and OE was based on genetic
analyses around the Japanese coast that considered all samples of O and J stocks pooled.
Our fundamental concern here is that the proponents of Hypothesis C did not take advantage of theapproach previously used to separate J and O stock animals. As noted earlier there is agreement that theapproach used (STRUCTURE) can assign individuals to stocks differentiated at the level of J and O
stock. New genetic analyses could start from this point as Gaggiotti and Gascuel (2011) did. As aconsequence of using pooled samples for the genetic analyses the proposal of JW/JE and OW/OE is notbased on hard genetic information, and then the proposal is highly speculative.
As noted above the mtDNA analysis by Baker et al. (2010) and the microsatellite analysis by Slikas
and Baker (2010) in the Pacific side of Japan were conducted for all J and O stock animals combined.
As expected several ‘significant’ differences among sub-areas were found. The proponents ofHypothesis C provided two explanations for their results, one was that the significant differences foundderive from differing proportion of just two stocks (J and O stocks) in each of the sub-areas examined,
the other was that there are differentiated stocks in these sub-areas (Wade and Baker, 2011). They
finally favored the latter explanation but conducted no further analyses or showed no evidence tojustify such preference.
In favoring the second explanation of additional stock structure, the proponents of Hypothesis C
alleged that allozyme and microsatellite allele frequencies only showed strong evidence for mixing ofstocks (evidenced by HW disequilibrium) along the Korean coast of the Sea of Japan, and north of
Hokkaido and that the areas in the Pacific side of Japan do not show strong evidence for mixing ofstocks (e.g. J and O stocks) (Wade and Baker, 2011).

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Notwithstanding our fundamental differences on the approach used in the genetic analysis by the
proponents of Hypothesis C, we discussed below the interpretation of the proponents of Hypothesis C
on the results of HW tests.
Results of the HW disequilibrium test
The argument given by the proponents of Hypothesis C is based on the allozyme results obtained by
Wada (1991) who found no Hardy-Weinberg deviations in samples from small-type coastal whaling
sub-area 7W (current 7CS, 7CN, 7WR) based on a single locus. According to the proponents ofHypothesis C, this result provides no support for the mixing of two stocks (J and O stocks) in that subarea.
However Kanda et al. (2011a) examined a large number of samples from recent by-catch and
JARPN/JARPN II samples in the same sub-area 7W (n= 1,106) with 16 microsatellite loci. Four of the
16 loci showed significant deviation from the Hardy-Weinberg expected genotypic proportion. The
same result was found for all loci combined. The authors noted that these significant differences were
due to the homozygote excess, which support the mixture of two stocks in that sub-area (including 7CS,
7CN and 7WR).
We believe that, in examining the plausibility of hypotheses, the IWC SC should discuss and conclude
on these two different results. After all the results of Wada (1991) of no HW deviation in sub-areas 7W
is being used by the proponents of Hypothesis C in support of their preferred explanation for their
genetic results (e.g. that additional stocks occur in that sub-area).
One of the elements that the IWC SC could consider is the distribution of the samples used in bothanalyses. We know for sure about the distribution of the samples used in the analysis by Kanda et al.
(2011a), which occur from very near the coast to more offshore waters. On the other hand there isagreement in the Committee that the distribution of J stock animals is within the 10n. miles from thecoast, but concentrated mainly within 3n. miles (Kanda et al., 2011b). The results of Wada (1991)
could be explained by either the few commercial catches taken in very coastal waters, where thefraction of J stock animals is high (see maps in Kanda et al., 2011b), or simply by the low power of theallozyme analysis.
Other genetic analyses that apparently support Hypothesis C
Gaggiotti and Gascuel (2011) used PCA to study the heterogeneity within J and O stock animals,
respectively, as defined by STRUCTURE analyses of Kanda et al. (2009a). Two clusters separated
clearly J and O stock animals. The authors found some degree of heterogeneity within the O stock
samples, but none between J stock samples from the east and west coasts of Japan.
Whether or not such heterogeneity within the O stock could be associated to OW and OE stocks ofHypothesis C is unclear as the precise geographical specification is not possible based on PCA analysis(IWC, 2011c). Furthermore heterogeneity detected is not necessarily related to additional stock
structure. It could be explained for example for pairs of closely related samples (kinship).
Gaggiotti and Gascuel (2011) noted that the structuring within the O stock was not related with bodylength and geographical position, and that such results do not have clear biological interpretation.
We believe that the results by Gaggiotti and Gascuel (2011) on the O stock, at least at this stage, can
not be used in support of the idea of additional structure within this stock as proposed in Hypothesis C.
‘Pure stock’ and mixing proportion
Proponents of different hypotheses have defined ‘pure’ stock samples with the aim of estimating
mixing proportion in sub-areas. In the case of Hypothesis C the ‘pure’ samples for the OW and JE
stocks is as follow:
OW: whales in sub-area 7CN in June (>8.8n. miles)
OE: whales in sub-area 8 and 9 (all months) (excluding 9W in 1995)
JW: whales in sub-area 6E (all months)
JE: whales in sub-area 2C in July-December

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First there is no scientific evidence supporting these definitions as ‘pure’ stocks, even thought as aproxy. If these definitions represent the best proxy for the ‘pure’ stocks, then at least significant
differences should be found in the comparison between OW/OE and JW/JE. However this is not thecase.
Results of a heterogeneity test for these ‘pure’ samples based on 16 microsatellites were the following:
OW against OE (P=0.554); JW against JE (P=0.393). The implication of this result is that the estimatesof mixing proportion in sub-areas e.g. 7WR (OW and OE mixing in April-September) or in sub-area 11(mixing of JW, OW and OE in April-September), are of limited value as there is not significant
differences among the baseline samples.
In summary we believe that there are no strong or conclusive genetic information supporting the JW/JE
and OW/OE divisions.
Non-genetic data
Conception date
Initially several non-genetic data were presented in support of the stock divisions proposed under
Hypothesis C (Table 1 in Wade and Baker, 2010). However recently, the conception date is the only
non-genetic data mentioned by the proponents of Hypothesis C to support their hypothesis (Wade andBaker, 2011).
Regarding conception date, initially Wade and Baker (2010) claimed that different J stocks distributeon each side of Japan because whales in the Sea of Japan had bimodal (autumn and winter) distributionof conception date and single winter peak in Sanriku. Bando et al. (2010) demonstrated that J stock
whales in sub-area 7W have also bimodal distribution of conception dates. Furthermore this author
showed that the conception date of O stock animals in sub-area 7W and sub-areas 7E, 8 and 9 have
similar conception date (winter).
Recently the proponents of Hypothesis C noted that under their hypothesis the rationale for JE vs JW
does not depend heavily on differences in conception dates (IWC, 2011c).
Other non-genetic data
Another of the questions that the proponents of Hypothesis C have failed to respond appropriately is on
the completeness of their proposed stocks. Analyses of length composition and sex ratio in the Pacificside of Japan and Sea of Japan were conducted separately for J and O stocks (Kanda et al., 2010b). At
the Pacific side of Japan O stock immature animals distributed mainly in coastal areas whereas O stock
mature animals distribute mainly in offshore areas, which is consistent with a single O stock withspatial segregation by sex and maturity classes. J stock animals from the Sea of Japan and Pacific coast
showed quite similar characteristics in the data. It should be noted that these analyses were based onlarge sample sizes.
The proponents of Hypothesis C suggested that stock OW and JE distributed along the Japanese coast
year round. The question here is how is it possible that a stock will be composed by a single
reproductive class, only immature animals in this case? In the case of their OE stock, how is it possiblethat a stock will be composed only by mature animals?
In summary contrary to initial allegations by the proponents of Hypothesis C that there are several nongenetic data supporting their hypothesis, our conclusion is that there is no non-genetic data supporting
the division of JW/JE and OW/OE.
EXAMINING THE SCIENTIFIC BASIS FOR THE Y STOCK
Data that apparently support of additional stock structure in the Yellow Sea (Y stock) come fromgenetics and conception date.
There is a evidence of microsatellite differences between the Yellow Sea and Sea of Japan whales in
winter (Kanda et al., 2010a) and for HW disequilibrium along the Korean coast of the Sea of Japan insummer (Slikas and Baker, 2011). However in the latter case the deviations was very strong comparedto slight or not differences (based on Fst) found between sub-areas 5 (Yellow Sea) and 6 (Sea of Japan)
for both mtDNA and microsatellites. We believe that the genetic evidences are very preliminary in5nature, and obviously additional work is required to get final conclusions on differences between the
Yellow Sea and Sea of Japan. For example the differences in the results between the mtDNA andmicrosatellite should be appropriately interpreted. Also the strong deviation from HV equilibrium inrelation to the very low Fst should be appropriately explained.
It has been suggested that the Yellow Sea have only fall conception dates whereas whales in the Sea ofJapan have a mixture of both autumn and winter conception date (Wang, 1985). However the quality ofthe data used by this author has been questioned in that a number of small foetuses could have been
missed in the commercial whaling samples (e.g. whales with conception date in winter could have been
missed) (Bando, 2011). Without further information on the collection method of the samples used in
Wang (1985) it will be very difficult to get a conclusion on the conception date for the whales in the
Yellow Sea. The possibility of collecting new conception date data from stranded or incidentallycaught common minke whales in the Yellow Sea should be considered and evaluated.
The IWC SC has put particular emphasis in the examination of the quality of the genetic data used for
management advice. We believe that the quality of non-genetic data used for management advices
should be also considered by the IWC SC.
CONCLUDING REMARKS
•

•

•

•

•

Genetic analyses conducted by the proponents of Hypotheses A and B to refine their
hypothesis of single J and O stocks followed a process and recommendations from the IWC
SC through the years (see Table 1). The genetic analyses conducted by the proponents ofHypothesis C in support of their JW/JE and OW/OE sub-divisions were based on pooled
samples of the J and O stocks, which are of limited utility.
Because the nature of the genetic analyses by the proponents of Hypothesis C, several
interpretations on stock structure of O and J stocks emerged. The proponents selected one of
the interpretations (additional stock structure) but they did not provide a strong rational forsuch preference. The absence of HW disequilibrium in sub-area 7W based on allozyme data is
not a strong argument for the reasons given in this document.
Several non-genetic data were examined by the proponents of Hypotheses A and B, andcomparative analyses were conducted within the J and O stock samples. Despite the fact thatthe proponents of Hypothesis C argued initially that several non-genetic data supported theirJW/JE and OW/OE sub-divisions, finally the only evidence presented was conception date,
which is not a strong argument for the reasons given in this document.
While the possibility of the occurrence of stock Y in the Yellow Sea can not be denied at thisstage, the genetic results are very preliminary and contradictory, and further analyses arenecessary to confirm this possibility. Further the quality of conception date data available for
the Yellow Sea should be further examined.
Responding to recommendations from the IWC SC for specific analyses is important. One of
the most relevant recommendations from the IWC SC was to conduct analyses separately for J
and O stock samples. The proponents of Hypotheses A and B and other SC workers did. The
proponents of Hypothesis C did not. We believe that further analyses and scientific evidences
are necessary for the actual use of Hypothesis C in the RMP Implementation. Given the
preliminary and contradictory nature of the available evidences on the Y stock, we believe thatthe plausibility of Hypothesis A should be higher than that of Hypothesis B.

ACKNOWLEDGEMENTS
We thank H. Hatanaka, Institute of Cetacean Research, for useful comments.
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Bando, T., Miyashita, T., Kishiro, T., Yoshida, H. and Hatanaka, H. 2010. An analysis of conception
dates of common minke whales sampled by JARPN and JARPN II in the context of stock
structure hypotheses. Paper SC/S10/NPM10. 5pp

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Wang (1985). Small fetuses were overlooked? Appendix 5 of Annex D1 (in press).
Gaggiotti, O.E. and Gascuel, F. 2011. Stock structure of North Pacific minke whales as revealed by
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Goto, M., Kanda, N., Kishiro, T., Yoshida, H., Kato, H. and Pastene, L.A. 2009. Mitochondrial DNA
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JARPN II Programme, Yokohama, Japan, January 26-30 2009 (unpublished). 10pp.
Hakamada, T. and T. Bando. 2009 Morphometric analysis on stock structure in the western North
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Manage. 11 (Suppl. 2): 198-217.
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Kanda, N., Goto, M., Kishiro, T., Yoshida, H., Kato, H., and Pastene, L.A. 2009a. Update of theanalyses on individual identification and mixing of the J and O stocks of common minke whales
around Japanese waters examined by microsatellite analysis. Paper SC/61/JR5 presented to the
IWC Scientific Committee, May 2009, Madeira, Portugal (unpublished), 14pp.
Kanda, N., Goto, M., Kishiro, T., Yoshida, H., Kato, H., and Pastene, L.A. 2009b. Further
microsatellite analysis of minke whales in the western North Pacific. Paper SC/61/JR8 presented
to the IWC Scientific Committee, May 2009, Madeira, Portugal (unpublished), 14pp.
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. 2010a. Genetic analysis of western North Pacific minke whales from Korea
and Japan based on microsatellite DNA. Paper SC/62/NPM11. 13pp.
Kanda, N., Pastene, L.A. and Hatanaka, H. 2010b. Length composition and sex ratio of western North
Pacific minke whales and their consistencies with stock structure hypotheses. Paper
SC/D10/NPM7. 12pp.
Kanda, N., Goto, M., Nagatsuka, S., Kato, H., Pastene, L.A. and Hatanaka, H. 2010c. Analyses ofgenetic and non-genetic data do not support the hypothesis of an intermediate stock in sub-area 7.
Paper SC/S10/NPM9. 7pp

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Table 1. Examples of analyses conducted following suggestions and recommendations from the IWC
SC to refine previous stock structure hypothesis based only on O and J stocks
Suggestions/recommendations fromthe IWC SC
Conduct genetic analyses separately for
J and O stock animals
Conduct genetic analyses separately for
J and O stock animals

Reference

Response

Reference

IWC (2003)

Analyses with 17 loci andprogram STRUCTURE
Hypothesis testing on O
stock samples(microsatellite; mtDNA)
Hypothesis testing on J stock
samples (microsatellite;
mtDNA)
Additional STRUCTURE
runs and PCA

Kanda et al. (2009a)

IWC (2003)

Conduct genetic analyses separately for
J and O stock animals

IWC (2003)

Conduct additional STRUCTURE runs
and Principal Coordinate Analyses(PCA)
Conduct power analysis

IWC (2010a)
IWC (2010b)

Conduct analyses on non-genetic data

IWC (2011b)

Statistical power for the
hypothesis testing analysis
on O stock based on
microsatellites
Morphometry, flipper color
pattern, fluke color pattern,conception dateKanda et al. (2009b); Goto
et al. (2009)
Kanda et al. (2010a); Parket al. (2010)
Kanda et al. (2010a)
Kanda et al. (2009b)

Hakamada and Bando(2009); Kanda et al.
(2010c); Bando et al.
(2010)

Figure 1. New sub-areas defined in the current RMP Implementation of western North Pacific common
minke whales

9