SC/62/NPM12rev

Progress in the development of stock structure hypotheses forwestern North Pacific common minke whales
Luis A. Pastene, Mutsuo Goto and Naohisa Kanda
Institute of Cetacean Research, Toyomi 4-5, Chuo-ku, Tokyo 104-0055, Japan
ABSTRACTThis paper examines recent progress in the development of stock structure hypotheses for western North
Pacific common minke whale (O and J stocks), and conduct a preliminary evaluation of these hypotheses
in the context of the available scientific information, mainly genetics, presented and discussed at the IWC
SC in recent years. The aim is to make a proposal of stock structure scenarios that is consistent with thedata. A total of four hypotheses are considered consistent with the available scientific data.
KEYWORDS: COMMON MINKE WHALES, PRE IMPLEMENTATION ASSESSMENT, WESTERNNORTH PACIFIC, STOCK STRUCTURE HYPOTHESES, PLAUSIBILITYINTRODUCTION
Based on the original works of Omura and Sakiura (1956) and Ohsumi (1977) the IWC SC recognized in
1993 two stocks of common minke whales, the Okhotsk Sea/Western Pacific (‘O’ stock) and the Yellow
Sea/East China Sea/Sea of Japan (‘J’ stock). Subsequently the IWC SC has been engaged in investigating
additional structure within these two stocks. Information on stock structure is important for the
Implementation Simulation Trials (ISTs), which are conducted to evaluate the conservation performance
before the calculation of a catch quota for commercial whaling under the RMP.
The RMP ISTs for western North Pacific common minke whales were completed in 2003. However these
ISTs were conducted in absence of the process for Implementation (e.g. pre-implementation assessment
followed by two-year Implementation), which was developed and accepted by the IWC SC in a later date.
For that reason instead of carrying out a typical Implementation Review after five years of completed theISTs, the IWC SC agreed to carry out a pre-implementation assessment and the two-year Implementation
process as specified in the process. This decision makes sense given the substantial amount of scientificinformation, notably genetics, accumulated after 2003.
Different stock structure scenarios have been listed by the IWC SC in recent meetings, which are basedon different levels of supporting evidences. The objective of this study is to summarize and conduct a
preliminary evaluation of these hypotheses in the context of the scientific information, mainly genetics,
presented and discussed at the IWC SC in recent years.
HYPOTHESES ON STOCK STRUCTURE
O stock
Several meetings were conducted by the IWC SC in the context of the RMP’s ISTs, which werecompleted in 2003 (IWC, 2004). Final hypotheses on stock structure were based mainly on genetic
analyses conducted using the past commercial samples and new samples collected in sub-areas 7, 8 and 9
by JARPN (1994-1999) and JARPN II between 2000 and 2001 (e.g. Goto et al., 2002; Kanda et al., 2002;
Taylor and Martien, 2002).
The following four hypotheses on stock structure were used in the RMP’ ISTs (Figure 1). Unfortunately
the IWC SC was unable to develop a scientific method to assign plausibility to these hypotheses. The
IWC SC arbitrarily assigned high plausibility to all these hypotheses (IWC, 2004).
Hypothesis A: three-stock scenario in the Pacific side of Japan (‘J’, ‘O’, ‘W’) with the ‘W’ stock foundonly in part of sub-area 9 and only sporadically (Figure 1-AB);1

Hypothesis B: two stock scenario in the Pacific side of Japan (‘J’ and ‘O’) with no W stock as a limitingcase of Baseline A (Figure AB);
Hypothesis C: four-stock scenarios overall in the Pacific side of Japan, with ‘OW’, ‘OE’ and ‘W’ to theeast of Japan. Boundaries are fixed at 147°E and 157°E and there is no mixing between the stocks (Figure
1-C);
Hypothesis D: three-stock scenario in the Pacific side of Japan (‘J’, ‘O’, ‘W’), with ‘O’ and ‘W’ mixing
over 147°E and 162°E, O being dominant to the west and W to the east (Figure 1-D).
J-Stock
The IWC SC has been conducting an in-depth assessment of western North Pacific common minke whalewith emphasis on the J stock. The IWC SC listed a total of four primary hypotheses on stock structure of
the J stock (Figure 2).
Hypothesis 1: Single J stock in the Yellow Sea, Sea of Japan and Pacific coast of Japan.
Hypothesis 2: Two J stocks, one distributed around the Korean Peninsula and the other in the west andeast coasts of Japan.
Hypothesis 3: Two J stocks, one distributes in the Yellow Sea and the other in the Sea of Japan andPacific coast of Japan.
Hypothesis 4: Two J stocks migrate through the Sea of Japan at different times of the year.
These four hypotheses were agreed by the IWC SC in 2007. In subsequent meetings some members
proposed other hypotheses involving three J stocks but their proposal was not supported by any piece of
scientific evidence (Hypotheses 5, 6 and 7 in Figure 2).
EXAMINATION OF THE STOCK STRUCTURE HYPOTHESESNo quantitative method has been agreed in the IWC SC for evaluating plausibility of different hypotheses.
During the 2002 IWC SC meeting Japanese scientists presented the results of an AIC (Akaike’s
Information Criterion)-based evaluation of the plausibility of baseline stock scenarios for North Pacificcommon minke whales. These were based exclusively on mtDNA data. However there was no agreement
at the SC on the validity of using AIC for evaluating plausibility.
Recognizing that plausibility of stock structure hypotheses will be addressed by the IWC SC in a laterstage here a qualitative analysis is conducted by listing up the scientific evidences (genetics and nongenetics) pro and again different hypotheses.
O-Stock
Hypotheses A, B, C and D
Sub-structure within the O stock had been proposed and discussed by the IWC SC since 1993. In this year
The IWC SC proposed a complicated sub-stock scenario with several sub-stocks composing the O stockand hypothesized a western stock (‘W’ stock) in offshore areas in the Pacific side of Japan (IWC, 1994)(see Figure 3 for the definition of sub-areas for management by the IWC SC).
The issue of stock structure was discussed again by the IWC SC in 1996. During that meeting the IWC
SC discussed the new scientific information derived from JARPN and concluded that the sub-stock
scenario proposed in 1993 was not plausible (IWC, 1997).
The IWC SC reviewed the final results of JARPN during a Workshop conducted in 2000. The
information based on genetics, morphometric, ecological markers, biological parameters supported thesingle O stock scenario in sub-areas 7, 8 and 9 (Pastene et al., 2000). Based on the mtDNA information,
the Workshop did not discard the hypothesis of occurrence of W stock in offshore areas in the Pacific sideof Japan, at least in some years of the period of JARPN. The Workshop recommended that further
research was necessary to examine the hypothesis of the W stock (IWC, 2001).

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These analyses and agreements by the IWC SC were the basis for the formulation of Hypotheses A and B.
At the later stage of the ISTs new analyses were presented based on genetics (mtDNA and microsatellites),
body proportion and biological parameters, which were consistent with the single O stock scenario(Hypothesis B). As noted above the mtDNA analysis suggested the possibility of sporadic occurrence of adifferent stock (W stock) in part of sub-area 9, which was the basis to formulate Hypothesis A.
The only evidence suggesting further division of the O stock (apart from the W stock) comes from theBoundary Rank analysis on mtDNA data, which proposed division of the O stock into Ow and Oe(Hypothesis C). Some authors interpreted these results with the occurrence of two stocks, one coastal andother offshore that mix in a transition area (Hypothesis D). However the result of the Boundary Rank
analysis was not supported by other lines of evidence. Furthermore the proposed existence of a small
coastal O stock (Ow) was inconsistent with the pattern of CPUE series of the former commercial minke
whaling: given the past level of catches, a small coastal stock would have been depleted, which could not
be revealed by the CPUE series.
Recent genetic analyses based on a larger set of samples (1994-2007) have been valuable to examine
these past hypotheses. Papers were presented to the JARPN II review meeting (IWC, 2009a) andsubsequently revised versions of those papers that responded to some of the suggestions of the JARPN II
review meeting were presented to the IWC SC meeting in 2009 (IWC, 2009b). One of the most valuablepieces of information was the work on microsatellites to assign individuals to J and O stocks (Kanda et al.,2009a). This information allowed subsequent hypothesis testing analyses to be conducted separately forthese two stocks within sub-area 7W. Hypothesis testing analyses were conducted for the samples
collected in 1994-2007, grouping the samples as suggested by Hypotheses A, C and D for both mtDNA(Goto et al., 2009a) and microsatellites (Kanda et al., 2009b). Results of these analyses were inconsistent
with stock scenarios C and D.
The microsatellite analysis however found significant differences between western and eastern sectors of
sub-area 9 (Kanda et al., 2009b) while the mtDNA analysis found significant differences between those
sectors using Fst for a particular year (1995) (Goto et al., 2009a). These results were consistent with thesporadic intrusion of an offshore stock into sub-area 9 (Hypothesis A).
Hypotheses A and B, which are supported by the best available genetic evidence, is consistent with othernon-genetic analyses presented to the JARPN review meeting (Pastene et al., 2000; IWC, 2001) and also
summarized when the ISTs were completed in 2003 (IWC, 2004).
Results of the most recent genetic analyses (Kanda et al., 2010; Park et al., 2010) supported the single O
stock scenario. In addition an update of the Boundary Rank analysis based on a large number of samples(Gaggiotti and Durand, 2010) provided no support to stock scenario C.
J-Stock
Different genetic analyses have been conducted in the Sea of Japan by combining Japanese and Korean
genetic data. Some preliminary results were presented at the 2008 IWC SC meeting. Due to time
constraint, data set could not be completed in time for the IWC SC 2009 meeting so the analyses on the J
stock structure in this year were based only on Japanese samples (Goto et al., 2009b; Kanda et al., 2009c).
More recently Kanda et al. (2010) completed a microsatellite analyses on O and J stock structure in
response to some suggestions by the IWC SC in 2009. Based on the information from these studies apreliminary evaluation of the stock structure of J stock listed by the IWC SC in 2009 is made below.
Hypothesis 1
Results of recent genetic analyses are partially consistent with this hypothesis. J stocks animals from bothsides of Japan showed no significant differences in both mtDNA and microsatellites (Goto et al., 2009b;
Kanda et al., 2009c; 2010; Park et al., 2010). Weak differences were found by the microsatellite analysis
between Japan and Korean J stock animals, which was attributed to a different stock in the Yellow Sea(Kanda et al., 2010). Significant differences were found between Korean and Japan by the mtDNA
analysis, which became no significant when samples from the Yellow Sea were excluded from theanalysis (Park et al., 2010). No other evidence is available to support strong differentiation among J stock
animals from Japan (Sea of Japan and Pacific), Korea sub-area 6 and Korea sub-area 5.
Hypothesis 2

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Results of recent genetic analyses are not consistent with this hypothesis. The weak microsatellite and
mtDNA differentiation found between Japan and Korean J stock animals were attributed to whales in subarea 5 (Yellow Sea) (Kanda et al., 2010; Park et al., 2010).
Hypothesis 3
Results of recent genetic analyses (microsatellites) are consistent with this hypothesis. Some levels of
seasonal genetic differentiation were found in the Korean samples but not in the Japanese sample. Yearly
heterogeneity and the very weak heterogeneity between the Japanese and Korean samples from sub-area 6
could be due to a different stock in the Yellow Sea (Y stock) moving north at some extent along the
Korean coast (Kanda et al., 2010). MtDNA differentiation between Japan and Korea was attributed towhales from the Yellow Sea (Park et al., 2010).
Hypothesis 4
Results of recent genetic analyses are not consistent with this hypothesis. J stocks animals from both sideof Japan showed no significant differences in both mtDNA and microsatellites. Furthermore no seasonal
genetic differences were found in the Sea of Japan coast of Japan (Goto et al., 2009b; Kanda et al., 2009c;
2010; Park et al., 2010).
Hypotheses 5
Results of recent genetic analyses are not consistent with this hypothesis. Weak microsatellite and
mtDNA differentiation between Korean and Japan J stock was attributed to animals in sub-area 5 (Kanda
et al., 2010; Park et al., 2010).
Hypotheses 6 and 7
Results of recent genetic analyses are not consistent with this hypothesis. J stocks animals from both sides
of Japan showed no significant differences in both mtDNA and microsatellites (Goto et al., 2009b; Kanda
et al., 2009c; 2010; Park et al., 2010).
SUMMARY OF PLAUSIBLE HYPOTHESESThe best available scientific evidence is consistent with the following hypothesis, which is considered themost plausible:
1) Single J stock distributed in the Yellow Sea, Sea of Japan and Pacific side of Japan (pattern ofinteraction between J and O stocks along the Japanese coast as proposed by Kanda et al., 2009a).
Migration pattern of adult and juvenile J stock is as suggested by Goto et al. (2010). Single O
stock in sub-areas 7, 8 and 9. Migration of O stock is as suggested by Hatanaka and Miyashita(1997).
Three less plausible hypotheses are also postulated:
2) Same as in 1) but W stock sporadically intrudes into sub-area 9.
3) Same as 1) but a different stock (Y stock) resides in the Yellow Sea and interacts with the J stockin the south part of sub-area 6.
4) Same as in 1) but with W stock sporadically intrudes into sub-area 9 and a different stock (Ystock) residing in the Yellow Sea, which interacts with the J stock in the south part of sub-area 6.
Most of the recent genetic analyses have considered useful suggestions made by the IWC SC and JARPN
review workshop. In particular the IWC SC welcomed the power analysis conducted for the hypothesis
testing analysis of microsatellite data (IWC, 2009b). That study suggested that the statistical power for
testing stock structure using the Japanese samples was quite high.
The most recent hypothesis testing analyses based on mtDNA and microsatellites were conducted for‘total’ samples as well O stock and J stock samples separately. This was possible by the microsatellite
assignment study by Kanda et al. (2009a). There are some discussions on the effect of unassigned
individuals and on how these samples can affect the conclusion of the stock structure. The unassigned
individuals could be whales from some other additional weakly differentiated stocks or simply could be
whales that could not be assigned to J and O stocks simply due to the low statistical power of the
STRUCTURE analysis. The IWC SC provided some useful suggestions to elucidate this problem (IWC,
2009b). Some of those suggestions were responded by Kanda et al. (2010) in their microsatellite analysis,

4and they concluded that the possibility of a third stock (intermediate stock between O and J stocks)
represented by the unassigned samples is low.
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including JARPN and JARPN II samples from 1994 to 2001. Paper SC/J02/NP10 presented to the IWC
SC North Pacific minke whale RMP/ISTs Workshop, January 2002 (unpublished). 9pp.
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pattern of J-stock common minke whales. Paper SC/62/NPM1 presented to this meeting.
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whales including JARPN and JARPN II samples from 1994 to 2001. Paper SC/J02/NP11 presented to the
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Kanda, N., Goto, M. and Pastene, L.A. 2009c. Update of the microsatellite analysis on sub-stock structureof the J stock common minke whales from the Japanese waters. Paper SC/61/NPM8 presented to the IWC
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January 2002 (unpublished). 9pp.

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Figure 1: Hypotheses on stock structure of North Pacific common minke whale used in the
Implementation Simulation Trials of the RMP (IWC, 2004). See text for details.

7Figure 2: Hypotheses on stock structure of J stock listed during the in-depth assessment of western North
Pacific common minke whales with emphasis on the J stock (IWC, 2009b). See text for details.

8Figure 1: Sub-areas used by the IW SC for management purposes of the common minke whale.

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