SC/67a/SD

A synthesis of the work conducted on stock structure of western North
Pacific common minke whale in response to recommendations from the
IWC Scientific Committee
Mioko Taguchi, Mutsuo Goto and Luis A. Pastene
Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan
Contact e-mail: taguchi@cetacean.jp

ABSTRACT
A brief summary of the updated analyses on the stock structure of western North Pacific common minke whale conductedfollowing recommendations from the IWC SC is presented. The refined analyses provide strong support to stock structure
hypothesis A (proposing only J and O stocks), with a single O stock exhibiting a pattern of sexual and age segregation
during migration. Hypothesis C (proposing two J stocks and two O stocks) is contradicted by the data, and consequently
such hypothesis should now be rejected.
INTRODUCTION
Discussions on the stock structure of the western North Pacific common minke whale have taken place at the International
Whaling Commission Scientific Committee (IWC SC) since 1993, with the latest discussions occurring during the
JARPNII final review workshop (IWC, 2017a) and during the 2016 IWC SC Annual Meeting (IWC, 2017b).
Genetic analyses on stock structure by Japanese scientists have been conducted following specific recommendations fromthe JARPNII mid-term review (IWC, 2010). Results of the analyses were reviewed by the JARPNII final review workshop(IWC, 2017a) and by the subsequent IWC SC in 2016 (IWC, 2017b). The JARPNII final review workshop report states:‘agree that most of the recommendations and suggestions proposed in 2009 Panel have been addressed…’ (IWC, 2017a).
This document presents a synthesis of the refined analyses conducted in response to the recommendations from the IWC
SC, together with an evaluation of the plausibility of the stock structure hypotheses used in the latest RMP Implementationin 2013.

RESULTSA large number of analyses on stock structure of the western North Pacific common minke whale have been conductedfollowing recommendations from the JARPNII mid-term review (IWC, 2010) and JARPNII final review (IWC, 2017a)
workshops.
Assignment test
Pastene et al. (2016a) examined a total of 4,275 western North Pacific common minke whales with a set of 16 microsatellite
DNA loci and the program STRUCTURE (Pritchard et al., 2000) to assign whales to either J or O stocks. Regarding theunassigned whales in the STRUCTURE analyses, a simple simulation exercise showed that the number of unassigned
whales decreased with the increase in the number of microsatellite loci used, and that they were widely distributed
geographically (Figures 1a and 1b). The authors concluded that the unassigned whales are not related with the occurrenceof additional stock structure.

Tamura et al. (2017) reported some additional analyses on the unassigned whales. According to their report, a total of 217
common minke whales was selected among whales in sub-areas 6 and 7 in order to ensure a dataset with a nearly equal
proportion of J and O stock whales, which were genotyped with additional 10 microsatellite loci (GATA97408,
GATA91083, GATA43950, GATA38314, GATA6059012, GATA5946992, GATA6063862, GATA6064765,
GATA5943219 and GATA5890064). PCR amplifications for these loci were performed under the same condition as in
Pastene et al. (2016a). Thus, the total sample of 217 whales were genotyped at 26 microsatellite loci. The genotyped whales
were assigned to J/O stocks by applying the Bayesian clustering approach to each dataset with randomly selected 8, 11, 14,
17, 20, 23 and 26 microsatellite loci in the program STRUCTURE. These analyses were conducted with ten
independent runs for K = 2 without information on sampling locations. All runs were performed with 100,000
Markov chain Monte Carlo repetitions and 10,000 burn -in lengths using the admixture model which assumes
whales may have mixed ancestry, and with correlated allele frequencies which assumes frequencies in the different
stocks are likely to be similar due to migration or shared ancestry.

1

Whale assignment was conducted using respective membership probabilities of more than 90%, 80% and 70%, and thenumber of unassigned whales for each number of microsatellite loci was examined. As expected, the number of unassigned
whales decreased with the increase in the number of microsatellite loci used (Figure 2), which confirms that the number ofloci used for assignment to the J/O stock is one of factors leading to the unassigned individuals in the STRUCTURE
analysis of common minke whale. This was also supported by the previous study suggesting that the accuracy of theassignments depends in practice on the number of loci (Prichard et al., 2000). For the three membership probabilities (70%,
80% and 90%), the trend was the same, with the trend for the 70% probability approaching zero when the number of lociwas 26. It is reasonable to assume that the same result would be obtained for 80% and 90% probabilities if the number ofloci is increased further.
Based on these results, the authors considered that only the whales assigned to the O stock with assignment probability
greater than 90% could be used to investigate additional structure with the O stock using alternative analytical approaches.
Hypothesis testing
Pastene et al. (2016b) examined the genetic population structure of ‘O’ stock common minke whale in the western North
Pacific based on mitochondrial DNA (mtDNA) control region sequencing (487bp) and microsatellite DNA (16 loci).
Whales used in the tests of homogeneity were obtained during the surveys of the JARPN and JARPNII in sub-areas of the
Pacific side of Japan between 1994 and 2014 (n= 2,071 for microsatellite; n=2,070 for mtDNA). Whales were assigned tothe ‘O’ stock by the analysis of STRUCTURE presented in Pastene et al. (2016a). Tests based on both genetic markers anddifferent grouping of the samples showed no evidence of sub-structuring in the ‘O’ stock common minke whale on the
Pacific side of Japan. A simulation exercise showed that the statistical power of the homogeneity test was high.
DAPC analysis
A Discriminant Analysis of Principal Components (DAPC) based on the total whale samples used in Pastene et al. (2016a)
showed clear differentiation between J and O stock whales but no evidence of sub-structuring within the O stock whales(Pastene et al., 2016b).
Morphometric analysis
Bando and Hakamada (2016) conducted a morphometric analysis to examine stock structure of western North Pacificcommon minke whales by using external measurement data collected during the 1994-2014 JARPN and JARPNII surveys.
External measurements of mature males were first compared between O and J stock whales using the Analysis of
Covariance (ANCOVA) and Discriminant Analysis (DA). Then only O stock whales were compared among sub-areas.
These analyses showed that J stock whales had a longer head region compared to O stock ones but no significant differenceswere found within O stock whales.
Catch-at-age analysis
Kitakado and Maeda (2016) used catch-at-age data for common minke whales in the western North Pacific provided by
the JARPN/JARPNII program to refine existing RMP Implementation Simulation Trials (ISTs) in a simple way so as toinvestigate the relative plausibility of the single and two (Ow and Oe) stock hypotheses for the O stock whales in the Pacificside of Japan. While the single stock scenario seems consistent with these age data, it is difficult to reconcile the two stock
hypothesis with these data because of the relative absence of particularly younger whales in a supposedly separate discrete
Oe stock.
Kinship analysis
Tiedemann et al. (2017) used a dataset of complete genotypes at 16 microsatellite loci of 4,554 North Pacific common
minke whales, accompanied by mtDNA and biological information to infer Parent-Offspring relationships using a
Maximum-Likelihood approach. A total of 75 pairs were confirmed, among them 26 pairs were mother-fetus. Most of thematches between sub-areas for O stock whales were between adult offshore fathers and coastal offspring, providing support
to the previous view of sexual and age segregation during migration of this stock.

CONCLUSIONSA substantial amount of new information on stock structure of common minke whale has been accumulated since the last
Implementation Review, which was based on data collected until 2007. In particular this includes a larger number of new
samples (around 1,700), new analytical procedures (DAPC, kinship, statistical power of the heterogeneity test, etc.), andthe availability of age data.
Most of the analyses in the documents summarized above responded to recommendations made during the 2009 mid-term
and the 2016 final reviews of JARPNII. A parsimonious and consistent interpretation of all different refined analyses and2results points to a single O stock distributed from the Japanese coast to approximately 170°E. Also the available information
provides evidence for a single J stock around Japan.
Stock structure hypothesis C, which proposes a coastal (Ow) and an offshore (Oe) stocks, is very difficult to reconcile withthe current genetic and non-genetic evidence summarized above. In particular, this hypothesis cannot be reconciled withthe finding of Tiedemann et al. (2017) showing a substantial number of Parent-Offspring matches between offshore (Oe)and coastal (Ow) areas.
Tables 1A and 1B show the number of Parent-Offspring pairs observed by sub-area pairings for O and J stock whales,
respectively. The different colors show which sub-areas pairings are and are not consistent in terms of the Hypothesis C
mixing matrices for the assumed Ow/Oe and Jw/Je stocks. Table 1A indicates 20 examples of Parent-Offspring pairs thatare inconsistent with the Hypothesis C split of O whales into Ow and Oe stocks. Table 1B indicates two examples of
Parent-Offspring pairs that are inconsistent with the Hypothesis C split of J whales into Jw and Je stocks.
Hypothesis C is also hardly consistent with the available age information (Kitakado and Maeda, 2016). The missing
younger whales from the ‘Oe stock’ raise further serious questions about the plausibility of the hypothesis C.
In summary, the latest scientific information is consistent with a single J stock and a single O stock which is sexuallysegregated during the migration, with younger animals migrating mainly along the coast and mature animals mainly
through offshore areas; this new evidence necessitates the rejection of Hypothesis C.

REFERENCES
Bando, T. and Hakamada, T. 2016. Morphometric analysis on stock structure of the O stock common minke whale in thewestern North Pacific. Paper SC/F16/JR41 presented to the JARPNII special permit expert panel final review
workshop, February 2016 (unpublished). 10pp.
International Whaling Commission. 2010. Report of the Expert Workshop to Review the Ongoing JARPN II Programme.
J. Cetacean Res. Manage. 11 (suppl. 2): 405-449.
International Whaling Commission. 2017a. Report of the Expert Panel of the Final Review on the Western North PacificJapanese Special Permit Programme (JARPN II). J. Cetacean Res. Manage. 18 (suppl.): 527-592.
International Whaling Commission. 2017b. Report of the Scientific Committee. Annex I. Report of the Working Group on
Stock Definition. J. Cetacean Res. Manage. 18 (suppl.): 264-276.
Kitakado, T. and Maeda, H. 2016. Fitting to catch-at-age data for North Pacific common minke whales in the Pacific sideof Japan. Paper SC/F16/JR43 presented to the JARPNII special permit expert panel final review workshop, February
2016 (unpublished). 15pp.
Pritchard, J.K., Stephens, M. and Donnelly, P. 2000. Inference of population structure using multilocus genotype data.
Genetics 155: 945-959.
Pastene, L.A., Goto, M., Taguchi, M. and Kitakado, T. 2016a. Temporal and spatial distribution of the ‘J’ and ‘O’ stocksof common minke whale in waters around Japan based on microsatellite DNA. Paper SC/F16/JR38 presented to the
JARPNII special permit expert panel final review workshop, February 2016 (unpublished). 14pp.
Pastene, L.A., Goto, M., Taguchi, M. and Kitakado, T. 2016b. Updated genetic analyses based on mitochondrial andmicrosatellite 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 final review workshop, February 2016
(unpublished). 19pp.
Tamura, T., Kitakado, T. and Pastene, L. 2017. Progress report of the work responding recommendations from the JARPN
II final review workshop. Paper SC/J17/JR02 Rev1 presented to the NEWREP-NP special permit proposal expert
panel review workshop, January 2017 (unpublished). 50pp.
Tiedemann, R., Tiedemann, M.R., Goto, M., Taguchi, M. and Pastene, L.A. 2017. Finding parent-offspring pairs among
western North Pacific common minke whale. Paper SC/67a/SD/xx presented to the IWC Scientific Committee, May
2017 (unpublished). 17pp.

3

Table 1A. The number of Parent-Offspring pairs of O stock whales within and between sub-areas(from Table 2 of Tiedemann et al., 2017). The blue color indicates those sub-area pairings that are notconsistent with the Hypothesis C mixing matrices for the assumed Ow/Oe stocks; the orange color
indicates those sub-areas that are not inconsistent with this hypothesis.1
O stock
1E

2C

6E

7CN

1E

1

2C

1

7CS

7WR

7
5

7E

8

9

11

1

3

7

1

1

1

6

6E
1

7CN
7CS
7WR
7E

2

8

2

9

1

11
Table 1B. The number of Parent-Offspring pairs of J stock whales within and between sub-areas (fromTable 2 of Tiedemann et al., 2017). The blue color indicates those sub-area pairings that are notconsistent with the Hypothesis C mixing matrices for the assumed Jw/Je stocks; the orange color
indicates those sub-areas that are not inconsistent with this hypothesis.
J stock
2C

6E

7CN

7CS

2C
6E
7CN
7CS

4

2
1
2

1Rev1 differs from the original version of the paper only in that the color coding of the 2C-7CN

cell and the 7CN-11 cells in Table 1a were changed from blue to orange.

(b)

(a)

Figure 1. Locations of the common minke whales used in the different analyses on stock structure in the western North
Pacific. Stock assignment is according to the STRUCTURE analysis: O-stock (green), J-stock (blue) and unassigned (red);(a) whole sampling area; (b) enlarged map of the coastal area

Figure 2. Change in the proportion of unassigned individuals in relation to the number of microsatellite loci used in eachassignment criteria.

5