Past studies have indicated that there are two different stocks (J and O stocks) in the North Pacific common minke whale Balaenoptera acutorostrata population. These two stocks differ from each other in certain characteristics, such as body size, conception dates, and genetics. However, till date, few studies have investigated differences in the external body appearance between the two stocks. Therefore, in the present study, we focused on the unique white patch on the flipper of this whale species to elucidate inter-stock differences. We used the animals collected from JARPNII research during 2012 and 2013; stock information was determined by microsatellite DNA 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 stock. 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 “no GAL” 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 morphological differences in the flipper color pattern between J and O stocks, which enable to apply widely to other individuals.
The common minke whale Balaenoptera acutorostrata is the smallest species of the Balaenoptera whales. In the northern hemisphere, there are two distinguishable subspecies, one each in the North Pacific (B. a. scammoni) and North Atlantic (B. a. acutorostrata). In the southern hemisphere, there is another population, generally called as “dwarf” minke whales. They may provisionally be regarded as a racesubspecies of common minke whale, but their taxonomical status is not fixed yet due to the scanty data [
Historically, this species have been regarded as an important resource worldwide. For the proper management of wild resources, stock-based management is important as well as species-based management. For the North Pacific common minke whales, the attempts to clarify the stock structure began in the 1950s. In the 1980s, it was suggested that there were two minke whale stocks in seas around Japan [
Various aspects of the stock structure of the North Pacific minke whale population have been studied in-depth, including work on genetics [
Among North Pacific common minke whales, it has been reported that the area ratio of the white patch on the flipper differs between J and O stocks [
The samples were collected during offshore and coastal (Sanriku and Kushiro) research of the Japanese Whale Research Program under Special Permit in the Western North Pacific-Phase II (commonly known as JARPNII) (
Regulation of Whaling. 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 the methods of Kanda et al. (2009) [
In this 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 ance 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) (
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;
Type 3: GAL expands over the half-line of the flipper width.
In the process of the statistical analysis, Mann-Whitney U-test was adopted to compare the proportional size of the white patch on the flipper and Pearson’s chi-square test was adopted to compare the frequency of GAL types between two stocks. Each analysis were operated using the statistic software R 2.13.0.
The proportional length of Point A (distance from the tip of the flipper to the distal end of the white patch) 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) (
The proportional length of Point B (distance from the tip of the flipper to the mesial end of white patch) to the flipper length was 29.7 ± 4.5% (n = 49) and 30.3 ± 4.7% (n = 169) for J and O stocks, respectively. The values are quite similar between the two stocks, and therefore, no statistical differences were observed in this characteristic (Mann-Whitney U-test, p > 0.05) (
The 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 (
The present study reported that the pattern of the white patch correlate well with genetics and this character would be help to identify stocks from external appearance. 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 possibility of inter-stock differences [
No GAL | With GAL | Total | |||
---|---|---|---|---|---|
Type 1 | Type 2 | Type 3 | |||
J stock | 15 | 7 | 27 | 1 | 50 |
(93.8%) | (21.9%) | (18.9%) | (3.7%) | ||
O stock | 1 | 25 | 116 | 26 | 168 |
(6.3%) | (78.1%) | (81.1%) | (96.3%) | ||
Total | 16 | 32 | 143 | 27 | 218 |
The length of Point B was not significantly different between stocks, whereas the length of Point A was proportionally larger in O stock 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. Statistical analyses 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”. These findings may help in the future development of a useful index to identify different North Pacific common minke whale stocks (
At present, the stock of each animals are identified by the combination method of microsatellite and Bayesian clustering analyses following the methods of Kanda et al. (2009) [
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. For such animals, by combining other external characteristics, such as cookie cutter shark-induced scar marks and unsolved indicators that include body color pattern, the accuracy of whale stock identification in the ocean will be greatly improved and also would be able to clarify distribution areas of each stocks.
This study declared that white patch of the common minke whales was morphologically different between two stocks. The size of the white patch area relative to flipper area was not significantly different, however, the boundary area pattern of the white patch named as “Grayish Accessary Layer (GAL)” correlated highly with genetics. Using these characters, for the animal having typical character, it would be able to distinguish stocks only from its appearance.
We thank the former leaders of the JARPNII Survey, Genta Yasunaga and Takeharu Bando of the Institute of Cetacean Research, Toshiya Kishiro and Hideyoshi Yoshida of the National Research Institute of Far Seas Fisheries, Representative Director Yoshiichi Shimomichi, former Exploration Chief Shigeo Tabata, Takahiro Hara and the staff of the Association for Community-Based Whaling, the staff of Kyodo Senpaku Company who is the main proponent of offshore surveys. We also thank to the anonymous reviewers and the members of the Laboratory of Cetacean Biology, Tokyo University of Marine Science and Technology. The authors would like to thank Enago (www.enago.jp) for the English language review.
Gen Nakamura,Ichiro Kadowaki,Shouka Nagatsuka,Ryotaro Hayashi,Naohisa Kanda,Mutsuo Goto,Luis A. Pastene,Hidehiro Kato, (2016) White Patch on the Fore-Flipper of Common Minke Whale, as a Potential Morphological Index to Identify Stocks. Open Journal of Animal Sciences,06,116-122. doi: 10.4236/ojas.2016.62014