SC/F16/JR39

Morphological differences in the white patch on the flipper between J and O stocksof the North Pacific common minke whale
GEN NAKAMURA1*, ICHIRO KADOWAKI1, SHOUKA NAGATSUKA1, RYOTARO HAYASHI1, NAOHISAKANDA3, MUTSUO GOTO2, LUIS A. PASTENE2 AND HIDEHIRO KATO1
1 Laboratory of Cetacean Biology, Tokyo University of Marine Science and Technology, 4-5-7Konan, Minato-ku, Tokyo 108-8477, Japan
Institute of Cetacean Research, Toyomi Promotion Building 5F, 4-5 Toyomi-cho, Chuo-ku, Tokyo104-0055, Japan
3Japan NUS Co. Ltd., Nishi-Shinjuku Kimuraya Building 5F, 7-5-25 Nishi-Shinjuku, Shinjuku-Ku,Tokyo 160-0023, Japan2ABSTRACT
Past studies have indicated that there are two different stocks (J and O stocks) in the North Pacificcommon minke whale Balaenoptera acutorostrata population. These two stocks differ from each otherin certain characteristics, such as body size, conception dates, and mtDNA haplotypes. However, till
date, few studies have investigated differences in the external body appearance between the twostocks. Therefore, in the present study, we focused on the unique white patch on the flipper of thiswhale species to elucidate inter-stock differences. We used animals collected from JARPNII researchduring 2012 and 2013; stock information was determined by microsatellite analysis (n = 220). We
focused on the morphological differences in the size and pattern of the white patch on the flipper of
each whale. The length of the white patch along the anterior (ventral) margin of the flipper tends to be
proportionally larger in O stocks. The pattern of the boundary area of the white patch named as the“Grayish Accessary Layer (GAL)” was remarkably different between stocks. Within animals with “noGAL” type, 94% were J stock. Conversely, of animals with GAL expanding over the half the flipper
width, 96% were O stock. We concluded from our study that there were clear morphologicaldifferences in the flipper color pattern between J and O stocks.
INTRODUCTION
The common minke whale Balaenoptera acutorostrata has been regarded as an important resource
worldwide. However, commercial whaling has been under a moratorium since 1988. The International
Whaling Commission (IWC) recommends stock-based as well as species-based management(Government of Japan, 1994). Attempts to clarify the stock structure of the North Pacific minke whale
population began in the 1950s. In the 1980s, it was suggested that there are two minke whale stocks
in seas around Japan (Ohsumi, 1983) comprising J stocks, which are mainly distributed from the Sea
of Japan to the East China Sea, and O stocks, which are distributed from the Okhotsk Sea to the western
North Pacific. The two whale stocks share the same feeding ground and they are mixed in the Okhotskand Japanese coasts (Ohsumi, 1977; Hatanaka and Miyashita, 1997).
When preparing plans for the implementation of the Revised Management Procedure, the issue ofstock structure was considered as one of the most important issue in the IWC. From 1994, the JapaneseWhaling Research Programs under Special Permit in the Western North Pacific (JARPN) wereconducted by the Government of Japan from 1994 to 1999. The main objective of that research wasto clarify the stock structure of common minke whales in the western North Pacific. After 2000,
additional research (JARPNII) has been conducted by the Government (Government of Japan, 1994;
2000).
Various aspects of the stock structure of the North Pacific minke whale population have been studied
in-depth, including work on genetics (Wada, 1991; Kanda et al., 2009; Goto et al., 2009), conception
1dates (Kato, 1992), and cookie cutter shark-induced scar marks (Bando et al., 2010). Morphologicalstudies comparing body length; body proportions; and color patterns of the baleen plates, flippers,
and tail flukes have also been conducted (Ohsumi, 1983; Kato, 1992; Hakamada and Bando, 2009;
Kanda et al., 2010).
The white patch on the flipper is a unique characteristic of the common minke whale. Moreover, theposition, size, and shape of the white patch on the flipper are clearly different between the North
Pacific, North Atlantic, and southern hemisphere populations. Therefore, the white patch on theflipper may serve as a useful distinguishing characteristic for taxonomy classification purposes(Nakamura et al., 2014). Among North Pacific common minke whales, it has been reported that thearea ratio of the white patch on the flipper differs between J and O stocks (Kanda et al., 2010).
Kanda et al. (2010) suggested that the white patch length in proportion to the overall flipper length
may differ; however, there was still no clear indicator to identify the two stocks. Therefore, wefocused on the white patch of the flipper of the North Pacific minke whale in more detail. The aim of
this study was to clarify the morphological differences between stocks so that a morphological index
could be developed that will help to correctly identify whale stocks from their appearance.
MATERIALS AND METHODS
The samples were collected during offshore and coastal (Sanriku and Kushiro) research of theJapanese Whale Research Program under Special Permit in the Western North Pacific-Phase II(commonly known as JARPNII), during 2012 and 2013. The survey was conducted in accordancewith the Article VIII of the International Convention for the Regulation of Whaling and in accordancewith the Japanese law. The animals were identified and classified into either J or O stock using genetic
analyses along with a combination of microsatellite and Bayesian clustering analyses following themethods of Pastene et al. (2016: SC/F16/JR38). We conducted research on 220 animals (138 males,body length: 3.82–8.16 m; 82 females, body length: 4.06–8.68 m), which were assigned into theappropriate stock based on their high membership probability (>90%).
In this inter-stock comparison study, we focused on the proportional size and boundary area pattern of
the white patch on the flipper of each whale. We measured the following three flipper characteristics
after the flipper was dissected from the whale’s body: flipper length, distance from the tip of the flipper
to the distal end of the white patch (Point A), and the tip to mesial end of the white patch (Point B)(Fig. 1). These points were measured to millimeter scale accuracy using measuring tape or a stainlesssteel caliper. The boundary area pattern of the white patch named as “Grayish Accessary Layer (GAL)”
were classified into groups, based on the criteria described below (Fig. 2):
no GAL type: Animals with no grayish band near the boundary,Type 1: With small GAL not fused to the other side,
Type 2: GAL is connected to the other side, andType 3: GAL expands over the half-line of the flipper width.
RESULTS
Proportional size of the white patch on the flipper
The proportional length of Point A (distance from the tip of the flipper to the distal end of the whitepatch) to the flipper length was 61.3±4.7% (n = 47) and 63.8±4.5% (n=168) for J and O stocks,respectively. Although the values overlapped, the distal end of the white patch tends to be relatively
far from the tip of flipper in O stocks compared with that in J stock whales (Mann–Whitney U-test, p< 0.01) (Fig. 3 a).
The proportional length of Point B (distance from the tip of the flipper to the mesial end of whitepatch) to the flipper length was 29.7±4.5% (n = 49) and 30.3±4.7% (n = 169) for J and O stocks,2respectively. The values are quite similar between the two stocks, and therefore, no statisticaldifferences were observed in this characteristic (Mann–Whitney U-test, p > 0.05) (Fig. 3 b).
Classification by GAL typeThe relationship between the type of stock and pattern of the boundary in the white patch (GAL) of
individual whales was studied. More than 80 % whales were categorized as Types 1 (n = 32) and 2 (n= 143) comprised both stocks. However, 94% of the “no GAL” type (n = 16) whales comprised J stock,and 96% of Type 3 (n = 27) whales comprised O stock (Fig. 4). The frequency of the J and O stocks
were distinct among GAL types (Pearson’s chi-square test, p < 0.01).
DISCUSSION
The present study reported the morphological differences in the white patch pattern on the flippers ofindividual whales with the aim of improving the identification of North Pacific common minke whales
by stock. Previous studies mentioned the possibility that the pattern of the white patch on the flipper
differs in these whales within as well as between particular oceans/seas, thus suggesting the possibilityof inter-stock differences (Kato, 1992; Nagatsuka, 2010; Nakamura et al., 2014). As indicated by thesestudies, definite morphological differences in the white patch of the flipper were observed between
stocks.
The length of Point B was not significantly different between stocks, whereas the length of Point A
was proportionally larger in O stocks than in J stocks. This meant that the length of white patch along
the anterior (ventral) margin of the flipper tends to be proportionally larger in O stocks. Statisticalanalyses also support these results. However, the range of the proportion are overlapped between
stocks, and therefore, it would be difficult to classify whales into either stock with 100% accuracy
based only on this characteristic.
The GAL pattern was the clearest characteristic for stock identification. We were able to define
separate stocks with high probability (>90%) when the target animal was “no GAL” type or “Type 3”.
On the other hand, more than 80% of observed animals were classified into Type 1 (15%, n = 32) or 2
(66%, n = 143), and these were from both J and O stocks.
We conclude that distinct morphological differences in the flipper’s white patch pattern were observed
between J and O stocks whales, particularly in the GAL pattern (Fig. 5). These findings may help in
the future development of a useful index to identify different North Pacific common minke whale
stocks. Combined with the observations of other external characteristics, such as cookie cutter sharkinduced scar marks and unsolved indicators that include body color pattern, the accuracy of whale
stock identification in the ocean will be greatly improved.
ACKNOWLEDGMENTS
We thank the former leaders of the JARPNII Survey; Genta Yasunaga and Takeharu Bando of theInstitute of Cetacean Research; Former Chief of the JARPNII Survey; Shigeo Tabata; Takahiro Haraof the National Research Institute of Far Seas Fisheries; the staff of Kyodo Senpaku Co., which is themain proponent of offshore surveys; and Representative Director Yoshiichi Shimomichi and the staffof the Association for Community-Based Whaling. We also thank the members of the Laboratory ofCetacean Biology, Tokyo University of Marine Science and Technology. The authors would like tothank Enago (www.enago.jp) for the English language review.

3REFERENCES
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Fig. 1 Measurement points on the flipper.

Fig. 2 Basis for the classification based on the GAL types (GAL: surrounded by dotted line).

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Fig. 3 Proportional size of white patch in the flipper. a) The proportional length of Point A (The distancefrom the tip of the flipper to the distal end of white patch) to the flipper length. b) The proportional lengthof Point B (The distance from the tip of the flipper to the mesial end of white patch) to the flipper length.
In Point A, proportional length was larger wain O stocks compared to J stock whales (Mann–Whitney Utest, p < 0.01)

Fig. 3 The relationships between stock and the pattern of the boundary in white patch (GAL).

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Fig. 4 Typical diagram of the white patch of the flipper in J and O stocks.

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