Not to be cited without prior consent with the authors

SC/61/NPM9Stock structure scenario of common minke whales from theJapanese waters as revealed by genetic data
Mutsuo Goto, Naohisa Kanda, and Luis A. Pastene.

The Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, JapanABSTRACT
The Working Group on the in-depth assessment of western North Pacific common minke whales, with afocus on J stock, agreed to have four stock structure hypotheses for the J stock common minke whales inthe sub-areas (SA) 5 and 6. Although not listed as standing hypotheses, some members of the groupalso raised two additional stock structure hypotheses that extended covering geographic area to thePacific east of Japan. In this paper, although it is restricted to the Japanese waters, we evaluated how
consistent the results from our recent genetic studies of common minke whales from the area (Goto et al.,2009; Kanda et al., 2009a, 2009b) to these hypotheses.
KEYWORS: COMMON MINKE WHALES, J STOCK, STOCK STRUCTURE HYPOTHESES.

INTRODUCTION
In order to investigate stock structure of common minke whales in the Japanese and Korean waters,
Japanese and Korean scientists have worked together intersessionally to analyze the samples from Japanand Korea using genetic data (mtDNA and microsatellites). On the basis of the results from the theseanalyses, the Working Group on the in-depth assessment of western North Pacific common minke whales,with a focus on J stock, agreed the following four stock structure hypotheses (IWC, 2007).
(1) One stock in the SA5 and SA6.
(2) Two stocks in the SA5 and SA6. The one stock migrates along the Japanese coast, and the othermigrates along the Korean coast.
(3) Two stocks. One migrates up to the SA5 and the other migrates further north in the SA6 along theboth Japanese and Korean coast.
(4) Two stocks. Both migrate through the SA6 in different time of the year.
Last year at the SC meeting, some members of the group raised two additional stock structure scenarios(named (5) and (6) for this paper) that extended covering geographic area to the Pacific east of Japan, i.e.,
SA2 and SA7 although the working group disagreed to list as standing hypotheses (IWC, 2009).
(5) Two stocks in the SA5 and SA6. This one is same to the (3) except the stock in the SA6 extends its
distribution to the SA2 and co-occupies the SA7 with the O stock(6) Four stocks. One in the SA5, one migrating along the Korean coast, one migrating along the Sea ofJapan coast, and the other migrating along the Pacific coast.
Common feature for the (5) and (6) different from the first four is that the SA2 is not the area for the Ostock.
Plans were made to conduct analyses by the Japanese and Korean scientists to describe the stock structureof common minke whales around the Japanese and Korean waters and then to evaluate these hypotheses.
Unfortunately the Korean data set was incomplete for proceeding with the analyses, and was not finalized
by the 61SC. Therefore, the most recent genetic analysis we present to this SC meeting (Goto et al.,2009; Kanda et al., 2009a, 2009b) examined the stock structure of the common minke whales around theJapanese waters using only the Japanese samples.
Questions to be addressed under the above 6 hypotheses are (a) how many different stocks exists around
the Japanese waters, (b) if there are different stocks migrating along the Sea of Japan coast, and (c) if

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SC/61/NPM9
there are different stocks in the SA6 and SA2 in addition to the O stock.
RESULTS AND DISCUSSION
Kanda et al. (2009a, i.e., SC/61/JR5) attempted to distinguish sampled minke whales into geneticallydistinct stocks using a combination of microsatellite analysis and a Bayesian clustering approach
implemented in the computer program STRUCTURE (Pritchard et al., 2000). Samples of 2542 minke
whales were collected during the offshore component of JARPN and JARPNII from 1994 to 2007, during
the coastal component of JARPNII from 2002 to 2007, and from bycatches in the set net fishery along theJapanese coast from 2001 to 2007, and were analyzed using 16 microsatellite loci. Result of theclustering analysis indicated that the samples came from two genetically different groups of minke whales.
Assignment of the individuals in the samples was conducted based on the membership probabilities
obtained from the program, and more than 90% of the individuals were assigned into the either stocks
with high membership probabilities. After the assignment, these individuals were then grouped basedon their sampling origins. Spatial distribution of these individuals clearly indicated that these two stocks
were the J and O stocks because individuals from the Sea of Japan and offshore North Pacific tended tobe assigned in the different group. The results also showed that the SA2 was mainly occupied by the Jstock with some O stock individuals entered, and that both the J stock and O stock co-occupy the area in
the SA7 very close to the coastal line. In addition, the number of the J and O stock individuals and thatof the immature mature individuals differed in between the bycatch and JARPN/JARPNII samples in theSA7 as well as SA11.
Kanda et al. (2009b) and Goto et al. (2009) then looked for any evidence of genetic differences in
microsatellite allele and mitochondrial DNA (mtDNA) haplotype frequencies among these samples
around Japan in order to look for evidence of sub-structuring of the J stock. They used the JARPN andJARPNII samples from 1994 to 2007 at the SA7W (140-147°E) and SA11 and the bycatch samples fromset net fishery along the Japanese coast from 2001 to 2007. The individuals in the samples were labeled
as either the J stock, O stock, or unknown origin based on the assignment in the Kanda et al. (2009a) forthe heterogeneity tests. The heterogeneity tests were conducted in the two ways: one used all theindividuals in the samples and the other used only the J stock individuals in the samples. The samples of
all individuals contained not only the J stock individuals but also the O stock individuals and theindividuals of unknown origins. The proportions of the J and O stock individuals were quite different
among the subareas while the proportion of the individuals of unknown origins was similar to each of thesubareas. Both analyses showed the same results. Among all of the heterogeneity tests conducted,
significant differences were detected only from the tests that included all individuals. The genetic
heterogeneity was found among the samples from the different sub-areas. In addition, the genetic
heterogeneity was observed in the SA7 between the samples collected from the different seasons of theyear and between the samples from the JARPN/JARPNII and bycatch samples. The authors of thesepapers then concluded that the genetic heterogeneities observed were most likely due to the spatial andtemporal differences in the proportion of the J and O stocks, but not the existence of different stocks,
indicating that only the single J stock with no sub-structuring existed around the Japanese waters.
With respect to the evaluation of how consistent the results of our genetic studies to the hypothesis, webelieve useful new information has been obtained. First of all, only two stocks distribute around theJapanese waters, the J and O stocks. If the assignment of the whales to the J and O stocks conducted inKanda et al. (2009a) had been unreliable, we could have detected statistically significant differences
between some of the samples even in the only J stock individuals group.
Secondly, our results do not support the hypotheses proposing that minke whales from two different
stocks migrate along the Japanese coast in the Sea of Japan at the different time of the year because wefound no seasonal genetic difference in the SA6 samples. We believe that the sample sizes for this test
(Early = 188, Late = 223) was reasonably large to say that (Goto et al., 2009; Kanda et al., 2009b).
Thirdly, the SA2 is not the area for the O stock. With respect to this, hypotheses (1) to (4) are equallyunlikely.
Finally, no sub-structuring of the J stock was found around Japan, that is, the individuals from the same
single J stock occupy the Japanese waters. Simulation study we conducted indicated that from genetics
standpoint the statistical power for testing the J sub-stocks with our data set was quite high (Kanda et al.,Not to be cited without prior consent with the authors

SC/61/NPM9
2009b). It might not be surprising that the microsatellite study (Kanda et al., 2009b) failed to find thegenetic differences among the samples in the case of the only J stock individuals because the samples
were grouped a priori based on the genetic variation at the microsatellite loci (i.e., Kanda et al., 2009a).
However, that means one agrees our assignment analysis was reliable. We also stress that the analysis
using independent mtDNA marker showed the same results, increasing the confidence to our results(Goto et al., 2009). Our results indicated the hypothesis (6) is unlikely.
Stock structure scenario we propose from our results is that only one J stock exists around Japan whose
main distribution area is the Sea of Japan and SA2 of the North Pacific and share coastal zone of the SA7
with the O stock. In the SA11, the J stock individuals appear to be dominant in the coastal area whileboth of the O and J stock ones exist in the offshore area. In the SA12 of the Okhotsk Ocean, althoughsmall proportion of the J stock individuals appear to exist only south part and only the O stock ones enter
the north part (e.g., Hakamada et al., 2009), no genetic data has been available so far.
It is important to note at this last moment is that sampling locations of the bycatches are restricted within
3 n.m. and most of them are immature whales (see Kanda et al., 2009a), suggesting that the number of theO stock individuals migrating into the offshore area of the SA2 may be higher than that expected fromour studies as seen in the SA7 and SA11.
ACKNOWLEDGEMENTS
We thank H. Hatanaka for the useful comments to the manuscript.
LITERATURE CITED
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