Technical Reports of the Ins�tute of Cetacean Research (2021)

pp. 49–57

Technical Report (not peer reviewed)

Review of biological informa�on of the J stock common minke whalebased on gene�cally iden�ﬁed individuals
Mutsuo Goto*, Tsutomu Tamura, Takeharu Bando and Genta Yasunaga
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan*Contact e-mail: goto@cetacean.jp

ABSTRACT

The ﬁndings from a review of the biological and ecological informa�on of the J stock common minke whale
held at the Ins�tute of Cetacean Research are summarized in this paper. Informa�on reviewed included
spa�al/temporal distribu�on, morphology, morphometrics, reproduc�ve characteris�cs, feeding ecology,
environmental pollutants, and ecological markers, based on the individuals of this stock iden�ﬁed by gene�cmarkers. Results of this review conﬁrm biological and ecological diﬀeren�a�on of the J stock within the North
Paciﬁc common minke whale, and corroborate the view that this stock should be managed independently.

INTRODUCTION
In the western North Paciﬁc, at least two biological stocksof common minke whales Balaenoptera acutorostrata
are known to exist: the Okhotsk Sea-West Paciﬁc (Ostock) and the Sea of Japan-Yellow Sea-East China Sea (Jstock) (Omura and Sakiura, 1956; Ohsumi, 1977; 1983).
The two stocks are diﬀeren�ated in morphological andreproduc�ve characteris�cs (Omura and Sakiura, 1956;
Ohsumi, 1977; Kato, 1992), as well in gene�cs (Wada andNumachi, 1991 for allozymes; Goto and Pastene, 1997 for
mtDNA; and Kanda et al., 2009a; b for microsatellites),sugges�ng their reproduc�ve isola�on. The Interna�onal
Whaling Commission (IWC) had proposed some boundaries for these stocks (Donovan, 1991).
Previous gene�c studies showed that both stocks mix
with each other spa�ally and temporally in the southernpart of the Okhotsk Sea (northern Hokkaido) (Wada, 1991;
Pastene et al., 1998). Since then, a substan�al number ofgene�c samples of western North Paciﬁc common minke
whales became available, and modern and more powerful gene�c markers have been applied to those samples inrecent years. The applica�on of such markers to the new
samples enabled ﬁner studies on stock structure of thisspecies in this ocean basin (Pastene et al., 2016a; b).
In recent years microsatellite DNA data from a substan�alnumber of gene�c samples of common minke whales in thewestern North Paciﬁc have been used in gene�c analyses forindividual assignment, for example, by using the program
STRUCTURE (Pritchard et al., 2000). Gene�c individual iden�ﬁca�on has allowed the study of several biological and eco-logical aspects separately for both J and O stocks common
minke whales. The objec�ve of this document was to review
the biological and ecological informa�on accumulated on
the J stock based on these approaches.

INDIVIDUAL IDENTIFICATION BASED ON MICROSATELLITE DNA ANALYSES
Microsatellite DNA data and the program STRUCTURE
version 2.0 (Pritchard et al., 2000) were used to determine the most likely number of gene�cally dis�nct stocks
present in the samples of common minke whales in thewestern North Paciﬁc. The program is a model-based
clustering method for inferring stock structure (K, thenumber of stocks in the model) using mul�locus genotype data with and without informa�on on sampling loca�ons. Individual assignment was conducted for the mostplausible K using the es�mated individual propor�on ofmembership probability. The ancestry model used for thesimula�on was the admixture model, which assumes individuals may have mixed ancestry. The allele frequency
model used was the correlated allele frequencies model,
which assumes frequencies in the diﬀerent stocks arelikely to be similar due to migra�on or shared ancestry.
Further details of the gene�c analyses on individual
iden�ﬁca�on of common minke whales can be found in
Goto et al. (2017).

BIOLOGICAL AND ECOLOGICAL CHARACTERIZATION OF THE J STOCK
Several biological and ecological studies have been conducted based on gene�cally-iden�ﬁed J stock individuals.
49Technical Reports of the Ins�tute of Cetacean Research (2021)
Table 1
Summary of diﬀerent biological and ecological studies of the J stock common minke whales including focal points, samples used andkey references. JARPN: Japanese Whale Research Program under Special Permit in the western North Paciﬁc; JARPNII: JARPN Phase II;
NEWREP-NP: New Scien�ﬁc Whale Research Program in the western North Paciﬁc.
Focal points
Distribu�on and movementSpa�al distribu�on
Temporal distribu�on
Distance from the coastal line
Morphology and morphometry
Flipper color pa�ern
Fluke color pa�ern
Morphometry
Reproduc�onConcep�on dateFeeding ecology
Stomach contents
Ecological markers
Total Hg levels
Cookie cu�er shark scar

Figure 1.

Samples/Years/Sources
n=4,275 (1994 to 2014), JARPN/JARPNII; bycatch
n=2,522 (2001 to 2014), JARPNII; bycatch
n=986 (1994 to 2007), JARPN/JARPNII; bycatch

Goto et al. (2017)
Goto et al. (2017)
Kanda et al. (2010)

n=220 (2012 and 2013), JARPNII
n=164 (2007), JARPNII
n=500 (2000 to 2007), JARPN/JARPNII

Nakamura et al. (2016)
Nagatsuka (2008; 2010)
Hakamada and Bando (2009)

n=107 (1994 to 2007), JARPN/JARPNII

Bando et al. (2010a)

n=742 (1996 to 2018), JARPN/JARPNII; NEWREP-NP

Present study

n=59 (2012 and 2013), JARPNII
n=1,037 (2002 to 2007), JARPN/JARPNII

Yasunaga and Fujise (2016)
Bando et al. (2010b)

Spa�al occurrence of O and J stocks common minke whales in waters around Japan based on gene�c individual iden�ﬁca�on (modiﬁed a�er Goto et al., 2017).

Descrip�on of the results of the main focal points arepresented in this sec�on (Table 1).
Distribu�on and movement
Spatial distribution along the Japanese coast
As shown in Figure 1, almost all of the individuals collected from the Sea of Japan side belong to the J stock,
50

Referenceswhereas almost all of the individuals from the oﬀshore
North Paciﬁc belong to the O stock. The southern partof the Paciﬁc side of Japan was mainly occupied by the
J stock. Northern Hokkaido and the northern part of the
Paciﬁc side of Japan represent areas where both stocks
overlap geographically.

Review of biological information of the J stock common minke whale based on genetically identified individuals

Figure 2.

Propor�on of the minke whales assigned to the J
and O stocks collected from sub-area 7W by the
distance from the Japanese coastal line (a�er Kanda
et al., 2010).

Temporal distribution along the Paciﬁc coast of Japan
In the southern part of the Paciﬁc side of Japan, the J
stock was predominant throughout the year (around 80%
in propor�on). In the Northern part of the Paciﬁc side ofJapan, the propor�on of the J stock increased in autumn/
winter and decreased in spring/summer. Conversely, thepropor�on of O stock decreased in autumn/winter andincreased in spring/summer. The fact that the J stock
is distributed in the southern part of the Paciﬁc side ofJapan throughout the year suggests that the Kuroshio
Current—one of the strongest west-boundary currentsof the subtropical gyre—is serving as the stock boundary
between O and J stocks (Goto et al., 2017).
Distance from the coastal line
In terms of the distance from the Japanese coastal linein the North Paciﬁc side e.g., sub-area 7W (sub-areas
Ba and Bb in Figure 1), the propor�on of the common
minke whales was diﬀerent between the J and O stocks(Figure 2). The propor�on of the J stock whales decreased
from coastal areas towards oﬀshore areas. Such a clinal
distribu�on supports the mixing of the two stocks in subarea 7W.
Migratory routes
The migratory routes to feeding areas of adult and juvenile J stock animals are schema�cally shown in Figure 3.
Migratory routes to breeding areas are assumed to be
the reverse in the case of adults. Adult animals were assumed to migrate northward and southward for feeding
and breeding, respec�vely, through the central corridorof the Sea of Japan. The northward migra�on limit was
not clear at this stage because there were no gene�csamples available from the central and northern parts of
the Okhotsk Sea. In the case of juveniles, it was assumed
that they were making short northward and southward

Figure 3.

Assumed feeding migra�on route of J stock animals(modiﬁed a�er Hatanaka and Miyashita, 1997 andGoto et al., 2010). Solid line: mature, do�ed line:
immature.

migra�ons along the coastal area for feeding because
bycatch juvenile animals were observed throughout theyear on the Japanese coast.
Morphology and morphometry
Morphology
Nakamura et al. (2016) studied the unique white patchon the ﬂipper of whales sampled by JARPNII during 2012
and 2013 to elucidate stock diﬀerences. They focused on
the morphological diﬀerences in the size and pa�ern of
the white patch on the ﬂipper of each whale. The lengthof the white patch along the anterior (ventral) marginof the ﬂipper tends to be propor�onally smaller in the
J stock. The pa�ern of the boundary area of the whitepatch named as the ʻGrayish Accessary Layer (GAL)ʼ wasremarkably diﬀerent between the two stocks (Figure 4).
Among animals with ʻno GALʼ type, 94% were J stock.
Conversely, 96% of the animals with GAL expanding overthe half the ﬂipper width were O stock.
Fluke color pattern
Nagatsuka (2008) found that common minke whales had
diﬀerent black and white pa�erns on the underside of
their ﬂukes. She then separated the sampled whales into
three diﬀerent ﬂuke color types (Figure 5). Diﬀerences
51

Technical Reports of the Ins�tute of Cetacean Research (2021)

Figure 4.

Characteris�cs of the white patch along the ventral part of the ﬂipper in common minke whales from thewestern North Paciﬁc. The pictures show the basis for the classiﬁca�on based on the GAL types (GAL: surrounded by do�ed line) (a�er Nakamura et al., 2016).

Figure 5.

Propor�on of the minke whales assigned to J and O stock collected from sub-area 7W by the ﬂuke colorpa�ern, and diagram of the ﬂuke color pa�ern (a�er Kanda et al., 2010).

Figure 6.

External measurements of western North Paciﬁc common minke whales examined for compara�ve stock
analyses (a�er Hakamada and Bando, 2009). Measurements V3, V4, V7, V10 and V11 showed signiﬁcantdiﬀerences in morphometrics between J and O stocks.

in the frequencies of these three types were observed
between the individuals assigned gene�cally as J and O
stocks sampled during the 2007 JARPNII coastal and oﬀshore surveys (Nagatsuka, 2010). The frequencies of the52

three types were recalculated for the samples collected
only from sub-area 7W (Figure 5). The frequencies were
diﬀerent between the individuals assigned to J and O
stocks (p<0.001).

Review of biological information of the J stock common minke whale based on genetically identified individuals

Morphometry
Hakamada and Fujise (2000) examined the diﬀerence
between the J and O stocks, using the external measurement data obtained during the 1994–1999 JARPN
surveys. Signiﬁcant diﬀerences were observed between
the two stocks, especially that J animals exhibited shorter
lower body than O stock animals. Hakamada and Bando(2009) examined morphometric data of common minke
whales sampled by JARPNII to inves�gate the diﬀerences
between J and O stocks (Figure 6). Analysis of covariance(ANCOVA) using body length as a covariate was used totest if there were signiﬁcant diﬀerences in morphometricmeasurements among the groups compared. Measure-

Figure 7.

ments V3, V4, V7, V10 and V11 showed signiﬁcant diﬀerences in morphometrics between J and O stocks.
Reproduc�onConception date
Bando et al. (2010a) examined the distribu�on of concep�on dates of J and O stock animals collected from thecoastal area of western North Paciﬁc (sub-area 7W) andthe Okhotsk Sea (sub-area 11) during 1994–2007 JARPN/
JARPNII surveys (Figure 7). In sub-area 7W the concep�on date of the J stock was in August (n=1) and January(n=2); in sub-area 11 they were distributed between October and March (n=8). These results suggested that the

Seasonal distribu�on of concep�on dates of the J and O stock common minke whales in ten-day periods(a�er Bando et al., 2010a).

Figure 8.

Yearly and geographical change of main prey species of J stock common minke whales.
53Technical Reports of the Ins�tute of Cetacean Research (2021)

concep�on period of the J stock extends from autumn towinter. However, the sample sizes are too small to reach adeﬁni�ve conclusion.
Feeding ecology
The feeding ecology of common minke whales around
Japan has been examined in previous published papers(Kasamatsu and Tanaka, 1992; Tamura and Fujise, 2002;
Konishi et al., 2009). However, there have been few published papers speciﬁcally on the feeding habits of J stock
common minke whales. Results of the analyses of gene�cally iden�ﬁed J stock individuals showed that thesewhales fed on various prey species such as Paciﬁc krill(Euphausia paciﬁca), Japanese sandlance (Ammodytes
personatus), Japanese anchovy (Engraulis japonicus),
Paciﬁc saury (Cololabis saira), walleye pollock (Gadus
chalcogrammus), and Japanese common squid (Todarodes paciﬁcus), and that the main prey species changed
both yearly and geographically (Figure 8). These results
suggested that J stock common minke whales appear to54

be an opportunis�c feeder, changing their prey species in
response to prey availability.
Environmental pollutant (total Hg levels)
Yasunaga and Fujise (2016) compared the accumula�on patterns of total Hg concentra�ons in muscle and liver between
J and O stocks of common minke whales. The analyses werebased on 35 O stock and 24 J stock immature animals takenfrom sub-area 7W in the 2012 and 2013 JARPNII surveys(Figure 9). Mul�ple linear regression analyses in total Hgconcentra�ons of the whales adjusted for confounders (ageindex, sex, blubber thickness and year) showed no discernible eﬀect between the two stocks. Results suggested that
there is no stock-dependent diﬀerence of total Hg and that
there is no exposure risk among the common minke whales
from the coastal waters of Japan.
Ecological markers
Cookie cutter shark scar
Bando et al. (2010b) inves�gated cookie cu�er shark-

Figure 9.

Rela�onship between Hg concentra�ons (ppm wet wt.) in muscle in rela�on to age (le�) and Hg concentra�on in liver in rela�onship to age (right) in common minke whales from sub-area 7W (a�er Yasunaga andFujise, 2016).

Figure 10.

Three types of cookie cu�er shark-induced scars in common minke whales. J stock animals had fewer
scars than O stock animals.

Review of biological information of the J stock common minke whale based on genetically identified individuals
Table 2
Types of cookie cu�er shark-induced scars in common minke
whales in the coastal areas of western North Paciﬁc oﬀ sub-area
7W (North: oﬀ Hokkaido, South: oﬀ Sanriku) and Paciﬁc oﬀshore
area (sub-areas 8 and 9) (modiﬁed from Bando et al., 2010b).
Subarea

7W
North

Type of scar
Type 1 None
Type 2 1–20 scars
Type 3 more than 20 scars
Total

7W
South

Type 1 None
Type 2 1–20 scars
Type 3 more than 20 scars
Total

8

Type 1 None
Type 2 1–20 scars
Type 3 more than 20 scars
Total

9

Type 1 None
Type 2 1–20 scars
Type 3 more than 20 scars
Total

Microsatellite DNA
O

?

J

0
14
291

2
5
31

17
31
5

305

38

53

0
10
213

3
5
25

19
22
2

223

33

43

0
3
95

0
1
11

0
0
0

98

12

0

0
4
203

0
1
23

0
0
1

207

24

1

induced scars as an ecological marker to determine stock
structure in western North Paciﬁc common minke whales
collected by JARPNII surveys during 2002–2007. Threetypes of common minke whales were iden�ﬁed from thedensity of scars (Figure 10). Prevalence of scars diﬀered
clearly between J and O stock animals. However, thisecological marker cannot be considered as an absolutemarker to diﬀeren�ate the two stocks as evidenced inTable 2. J stock animals had fewer scars than O stock
animals. In both stocks, prevalence increased with bodylength, and almost all animals of more than 7 m in bodylength had scars. Nevertheless, these results were consistent with the occurrence of the two stocks J and O, theformer with fewer scars distribu�ng in coastal areas while
the la�er with more scars distribu�ng in both coastal andoﬀshore areas.

SUMMARY
Table 3 shows a summary of the biological and ecologicalcharacteris�cs of J stock individuals. The individual iden�ﬁca�on by the gene�c markers has been very useful inlooking for stock characteriza�on and diﬀerences in several traits, such as distribu�on and movement, morphology and morphometry, reproduc�on, feeding ecology
and environmental pollutant, as presented in this paper.
Except for feeding ecology and environmental pollutant(total Hg levels), these traits indicate that J stock individuals are biologically and ecologically dis�nguished from O

Table 3
Summary of the studies on biological and ecological characteris�cs of the gene�cally iden�ﬁed individuals of J stock common minke
whales reviewed in this study.
Focal point

Characteris�c

Spa�al distribu�on

Whales occupy the Sea of Japan side and the southern part of the Paciﬁc side of Japan; they overlapped geographically with O stock in northern Hokkaido and the northern part of the Paciﬁc sideof Japan.
Temporal distribu�on
Whales are predominant throughout the year in the southern part of the Paciﬁc side of Japan.
Their propor�on increases in autumn/winter and decreases in spring/summer in the Northernpart of the Paciﬁc side of Japan.
Distance from the coastal line Their propor�on decreases from coastal areas towards oﬀshore.
Flipper color pa�ern
Characterized for almost no GLA in their ﬂippers in comparison with the O stock (Figure 4).
Fluke color pa�ern
Characterized by a higher propor�on of white color in ﬂukes in comparison with the O stock (Figure 5).
Morphometry
Characterized by short lower body in comparison with the O stock.
Concep�on date
Most likely extends from autumn to winter.
Feeding ecology
Feed on various prey species such as krill, schooling pelagic ﬁshes, and Japanese common squid.
They appear to be an opportunis�c feeder changing their prey species in response to prey availability in their feeding ground.
Environmental pollutant (total Levels are similar to those of the O stock animals in the coastal waters of Japan. Levels suggest
Hg levels)that the health risk is low.
Cookie cu�er shark scar
Fewer scars in comparison with the O stock (Figure 10).
55

Technical Reports of the Ins�tute of Cetacean Research (2021)

stock individuals. The J stock can be deﬁned as a groupof individuals sharing a common gene pool maintainedby random ma�ng and should therefore be managed
independently.

ACKNOWLEDGEMENTS
We thank Luis A. Pastene (ICR) for his assistance in preparing this document.

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57