Technical Reports of the Ins�tute of Cetacean Research (2020)

pp. 48–53

Technical Report (not peer reviewed)

Progress in the research on earplug-based age determina�on andbiological parameters of North Paciﬁc sei whales at the Ins�tute ofCetacean Research
Takeharu Bando1, * and Hikari Maeda2
1

Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan
Japan Fisheries Research and Education Agency, 2–12–4 Fukuura, Kanazawa-ku, Yokohama-shi, Kanagawa
236–8648, Japan

2*Contact e-mail: bando@cetacean.jp

ABSTRACTThis study summarizes the progress in the research on earplug-based age determina�on and es�ma�on ofbiological parameters of the western North Paciﬁc sei whales sampled by the JARPNII/NEWREP-NP surveys
between 2002 and 2018. A total of 1,622 whales (744 males and 878 females) were sampled in the western
North Paciﬁc north of 35°N and between the Paciﬁc coast of Japan and 170°E. Earplugs were read for whales
collected up to 2016 (n=1,354). Age informa�on was obtained for 887 whales (65.5%). Earplug readability washigher for mature (69.7%) than immature (52.1%) whales, and no diﬀerence between sexes was observed.
The growth curves for males and females were Lt =14.14 (1−e−0.174(t+6.650)) and Lt =15.17 (1−e−0.150(t+7.407)),
respec�vely. The age at sexual maturity for males and females were 6.7 (SE=0.29) and 6.9 (SE=0.27) years,
respec�vely. The body length at sexual maturity was es�mated as 12.72 m (SE=0.12) for males and 13.31 m(SE=0.12) for females. The annual ovula�on rate was es�mated as 0.744/year. Substan�al biological informa�on was obtained for the North Paciﬁc sei whale during the 17 year of JARPNII and NEWREP-NP. The analysesof biological data will contribute to the management of this whale species in the North Paciﬁc.

INTRODUCTION
Sei whale (Balaenoptera borealis) is widely distributed
throughout the world. As with other baleen whale species, they undertake seasonal migra�on spending thesummer feeding season in subpolar waters and returning
to low la�tude waters to calve in winter (Horwood, 2017).
In the North Paciﬁc, sei whales (Figure 1) distributewidely in the oﬀshore area associated with the produc�ve polar and subarc�c fronts during the summer feeding
season (Murase et al., 2014).
The second phase of the Japanese Whale Research
Program under Special Permit in the western North
Paciﬁc (JARPNII) started in 2000 (GOJ, 2000; 2002), andwas completed in 2016. The sei whale was added as
one of the target species for sampling in 2002. The New
Scien�ﬁc Whale Research Program in the western North
Paciﬁc (NEWREP-NP) started in 2017 (GOJ, 2017) and also
had the sei whale as one of the target species. During theannual surveys, biological data and samples such as bodylength measurements and reproduc�ve organs, wereobtained systema�cally. A total of 1,622 sei whales were48

sampled during a 17 year survey period. The NEWREP-NP
was completed in 2018.
Age data is one of the most important informa�on forstock assessment and management of large whales. Agees�ma�on based on coun�ng of growth layers accumulated in the earplugs is considered the most reliable tool
for age determina�on in baleen whales (Lockyer, 1984a;

Figure 1.

A sei whale feeding on plankton near the sea surface
in the western North Paciﬁc.

Age determina�on and biological parameters of North Paciﬁc sei whales at the Ins�tute of Cetacean Research

Maeda et al., 2016). During JARPNII and NEWREP-NP, earplugs were collected from each whale sampled by experienced biologists in order to increase the readability. Someage-related biological parameters are known to change in
response to changes in abundance, food availability orcompe��on among whale species (Gambell, 1973; Katoand Sakuramoto, 1991; Lockyer, 1984b). Therefore, monitoring of biological parameters is indispensable for theassessment and management of whale stocks.
This study summarized the progress in the researchon earplug-based age determina�on and es�ma�on of
the biological parameters of the western North Paciﬁc sei
whales sampled by the JARPNII and NEWREP-NP surveys
between 2002 and 2018.

est 1 cm, from the �p of the snout to the notch of theﬂukes using a stainless measure. Sexual maturity in males
was determined by examining the histological samples
from the tes�s. Males with seminiferous tubules over
100 µm diameter, sperma�d or open lumen in the tubules
were determined to be sexually mature (Masaki, 1976;
Lockyer, 1984b). Maturity of females was preliminarily
determined in the ﬁeld by the presence or absence of at

MATERIALS AND METHODS
Whale sampling
A total of 1,622 (744 males and 878 females) sei whales
were sampled by JARPNII and NEWREP-NP during
2002–2018. The geographical distribu�on of the sampledwhales is shown in Figure 2. Table 1 shows the number ofsampled whales by year and sexual maturity status.
Figure 2.

Biological data
Body length was measured in a straight line to the near-

Research area of JARPNII and NEWREP-NP andsigh�ng posi�ons of sei whales sampled during the2002–2018 surveys.

Table 1
Number of sei whales sampled by JARPNII and NEWREP-NP in the western North Paciﬁc in the period 2002–2018, by year, sex andsexual maturity status.
YearMale

Female

Total

Immature

MatureUnknown

Total

Immature

Mature

Total

2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018

3
4
11
6
18
21
6
10
12
8
12
15
7
9
10
13
12

12
19
36
45
30
33
38
36
31
46
32
29
31
20
28
50
50

00000000000000001

15
23
47
51
48
54
44
46
4354
44
44
38
293863634414141416
179101311611151615
1720
23
39
35
38303945
47
2845
50
41
46365654242753
4952
4656
5457
4156
5652
6152
71
71

39
50
100
100
100
100
100
100
100
95
100
100
90
90
90
134
134

Total

177

566

1744

206

672

878

1622
49

Technical Reports of the Ins�tute of Cetacean Research (2020)

Tm = g − 0.5 +var(tm ) =hBisected earplug surface of sei whale with light anddark laminae of growth layer groups (GLGs). Thescale shows a 1 mm interval.

least one corpus luteum or corpus albicans in both sidesof the ovaries, and conﬁrmed later in the laboratory bycoun�ng number of corpora.
Earplug sampling and age determina�on
Earplugs were collected from all sampled animals, following the method developed for baleen whale (Omura,1963; Maeda et al., 2016). The le� and right earplugs
were collected and immediately ﬁxed in 10% formalinun�l age determina�on. In the laboratory, the surface
along the central axis of the earplug was cut using a sharp
blade, then it was placed on a wet stone to expose theneonatal line and growth layers (Figure 3). Growth layers
were counted under water using stereoscopic microscope. A growth layer group (GLG) was deﬁned as one
pair of light and dark laminae in the core and considered
as one year of age.
Es�ma�on of biological parameters
Growth curve
To es�mate growth curve, the von Bertalanﬀy growthmodel was ﬁ�ed to the body length and age as:

Lt = L∞ (1 − e− K (t −t0 ) )where
Lt is the body length at age t,
L ∞ is the asympto�c length,
K is the growth rate coeﬃcient, andt0 is the theore�cal �me at zero length.
Age at sexual maturity
Age at sexual maturity (tm) was es�mated by the following equa�on (Cooke, 1984).

50

 Ia a=ga

  N M a Ia

 N ( N − 1)a=g

Figure 3.

h2
aa

where
Ma is the number of mature animals in age a,
Ia is the number of immature animals in age a,
Na is the total number of animals in age a,
g is the age of the youngest mature animal in the sample,
andh is the age of the oldest immature animal in the sample.
Body length at sexual maturity
Body length at sexual maturity (Lm) was es�mated by
the following equa�on (Cooke, 1984; Kato, 1992). Whencalcula�ng, body length data was rounded to the nearest
0.1 m.
Lm = j − 0.05 + 0.1
var(lm ) = qkk

 Ib b= jb

  N M b Ib

 N ( N − 1)b= j2bb

whereMb is the number of mature animals in body length b,
Ib is the number of immature animals in body length b,
Nb is the total number of animals in body length b,j is the body length of the smallest mature animal in thesample, andk is the body length of the largest immature animal in thesample.
Annual ovulation rate
The annual ovula�on rate was es�mated by applying
linear regression analysis between age and the total number of corpora (corpora lutea and albican�a). The regression line was ﬁ�ed to ages 14 years and older because
all females were sexually mature at the age of 14 years
in this study.

RESULTS AND DISCUSSIONSAge readability
Earplugs were read for whales collected up to 2016
(n=1,354). Age informa�on was obtained for 887 whales(65.5%). The readability of earplugs varied dependingon the maturity status. Readability of sexually immatureindividuals was 55.9% for males and 48.9% for females(Table 2). Readability of earplugs in sexually mature animal was higher, 72.5% and 67.4% for males and females,

Age determina�on and biological parameters of North Paciﬁc sei whales at the Ins�tute of Cetacean Research
Table 2
Earplug-age readability of sei whales sampled during 2002–2016 surveys, by sex and sexual maturity status.
MaturitystatusMale

FemaleNumber ofwhalesReadable
earplugs

Immature
Mature

152
466

85
338

55.9
72.5

174
562

85
379

48.9
67.4

Total

618

423

68.4

736

464

63.0

Figure 4.

Readability Number of(%)
whalesReadable
earplugs

Total
Readability Number of(%)
whalesReadable
earplugs

Readability
(%)

326
1028

170
717

52.1
69.7

1354

887

65.5

Rela�onship between body length and age in sei whales. The solid lines show the von Bertalanﬀy growth
curves.

respec�vely (Table 2). Readability of all samples was
65.5%, which was higher than for western North Paciﬁc
common minke whales (44.1%) (Maeda et al., 2016), andcomparable to Brydeʼs whales (65.2%) (Bando, 2018),
which were based on samples collected by JARPN andJARPNII.
Growth curve
For both sexes, the growth rate was high at younger ages
and stabilized a�er 15 years (Figure 4). The following vonBertalanﬀy growth curves were es�mated:

Lt = 14.14 (1 − e −0.174(t+6.650) )Male :
Female : Lt = 15.17 (1 − e −0.150(t+7.407) )Masaki (1976) es�mated growth curves of sei whales
based on samples collected from the commercial whaling
in the North Paciﬁc in the 1970s. However, the es�matedcurve was incomplete due to the lack of young individuals. During JARPNII and NEWREP-NP, samples from a wide
range of ages, including young individuals, were collected, and for the ﬁrst �me a reasonable growth curve
could be obtained for western North Paciﬁc sei whales.
Age at sexual maturity
Sexually mature males ﬁrst appeared at the age of 4 years,

and from 13 years old onwards all animals were sexually mature (Figure 5). Tm for males was es�mated as 6.7years (SE=0.29). For females, sexually mature whales ﬁrst
appeared at the age of 3 years, and from 14 years old onwards all animals were sexually mature (Figure 5). Tm wases�mated as 6.9 years (SE=0.27). Masaki (1976) reported
the Tm of sei whales from the commercial whaling in theNorth Paciﬁc in the 1970s as 2.5 and 6.5 years for males
and females, respec�vely. However, these values wereunderes�mated as the ages of younger animals are o�enes�mated as less than their actual ages. This is because
the so�er and smaller earplug of young animals, together
with a lack of neonatal layers, in some cases, make it difﬁcult to recognize growth layers (Masaki, 1976).
Body length at sexual maturity
The rela�onship between sexual maturity and bodylength is shown in Figure 6. Sexually mature male ﬁrst
appeared at body length of 12.2 m, and from 13.6 m onwards all animals were sexually mature. Lm was es�matedas 12.72 m (SE=0.12). For females, sexually mature
whales ﬁrst appeared at the body length of 12.6 m, andfrom 14.0 m all animals were sexually mature (Figure 6).
Lm was es�mated as 13.31 m (SE=0.12). Masaki (1976)es�mated Lm of male sei whales as 12.9 m based on the51

Technical Reports of the Ins�tute of Cetacean Research (2020)

Figure 5.

Sexual maturity status by age and sex in sei whales. Numbers in the bars show the numbers of samples
examined.

Figure 6.

Sexual maturity status by body length and sex for sei whales. Numbers in the bars show the numbers ofsamples examined.

commercial whaling samples from the North Paciﬁc,which was slightly higher than the value es�mated in thisstudy. However, Masaki (1976) determined the sexual
maturity of males by tes�s weight rather than histologicalobserva�on, which might have inﬂuenced the results. For
females, a similar value was es�mated from commercial
whaling samples (13.4 m) (Masaki, 1976).
Annual ovula�on rate
The corpus luteum and albicans ﬁrst appeared at the ageof 3 years and the number of corpora increased linearlya�er the age of 14 years (Figure 7). Annual ovula�on rate
was es�mated as 0.744. The es�mated value means thatthe majority of sei whales give birth (or ovula�on occurs)
three �mes in four years. This value was higher than
0.604 which was es�mated from the 1970s commercial
whaling samples (Masaki, 1976).

CONCLUDING REMARKS
Substan�al biological informa�on was obtained for theNorth Paciﬁc sei whale during the 17 years of JARPNII andNEWREP-NP. In par�cular, more age data from young in52

Figure 7.

Ji�er plot showing the rela�onship between age
and number of corpora in sei whales. The linear
regression line was ﬁ�ed to ages 14 years and over.

dividuals were obtained, resul�ng in improved es�matesof biological parameters than those obtained in the period of commercial whaling. The analyses of biological data

Age determina�on and biological parameters of North Paciﬁc sei whales at the Ins�tute of Cetacean Research

will contribute to the management of this whale species
in the North Paciﬁc.

ACKNOWLEDGEMENTS
Our sincere thanks to all researchers and crews who collected data and samples during the JARPNII and NEWREPNP surveys. We thank the members of the Ins�tute ofCetacean Research (ICR) and Japan Fisheries Researchand Educa�on Agency for useful sugges�ons as well as assistance in the laboratory work. H. Kato (Tokyo Universityof Marine Science and Technology and ICR) and Yoshihiro
Fujise (ICR) encouraged us to conduct this study and provided valuable ideas for the analyses.

REFERENCES
Bando, T. 2018. Earplug-based age determina�on and es�ma�on of biological parameters in North Paciﬁc Brydeʼs whales
based on samples collected by JARPNII. Technical Reports ofthe Institute of Cetacean Research (TEREP-ICR) No. 2: 32–37.
Cooke, J.C. 1984. The es�ma�on of mean ages at sexual maturity from age sample. Paper SC/36/O22 presented toIWC Scien�ﬁc Commi�ee, May 1984 (unpublished). 8 pp.
[Available from the IWC Secretariat].
Gambell, R. 1973. Some eﬀects of exploita�on on reproduc�onin whales. J. Reproduct. Fert. (Suppl.) 19: 533–553.
Government of Japan. 2000. Research plan for cetacean studies
in the western North Paciﬁc under special permit (JARPNII)(Feasibility study plan for 2000 and 2001). Paper SC/52/O1
presented to the IWC Scien�ﬁc Commi�ee, June 2000 (unpublished). 68 pp. [Available from the IWC Secretariat].
Government of Japan. 2002. Research plan for cetacean studies
in the western North Paciﬁc under special permit (JARPNII).
Paper SC/54/O2 presented to the IWC Scien�ﬁc Commi�ee,April 2002 (unpublished). 115 pp. [Available from the IWCSecretariat].

Government of Japan. 2017. Revised research plan for New
Scien�ﬁc whale Research Program in the western North
Paciﬁc NEWREP-NP). Paper SC/67a/SCSP10 presented to theIWC Scien�ﬁc Commi�ee, May 2017 (unpublished). 147 pp.
[Available from the IWC Secretariat].
Horwood, J. 2017. Sei whale Balaenoptera borealis. pp. 845–847. In: B. Würsig, J.G.M. Thewissen and K. Kovacs (eds.)
Encyclopedia of Marine Mammals, Third Edition. AcademicPress, San Diego. 1190 pp.
Kato, H. 1992. Body length, reproduc�on and stock separa�onof minke whales oﬀ northern Japan. Rep. int. Whal. Commn
42: 443–453.
Kato, H. and Sakuramoto, K. 1991. Age at sexual maturity ofsouthern minke whales: a review and some addi�onal analyses. Rep. int. Whal. Commn 41: 331–337.
Lockyer, C. 1984a. Age determina�on by means of the ear plugin baleen whales. Rep. int. Whal. Commn 34: 692–696.
Lockyer, C. 1984b. Review of baleen whale (Mys�ce�) reproduc�on and implica�ons for management. Rep. int. Whal.
Commn (special issue) 6: 27–50.
Maeda, H., Bando, T., Kishiro, T., Kitakado, T. and Kato, H. 2016.
Basic informa�on of earplug as age character of common
minke whales in western North Paciﬁc. Paper SC/F16/JR53
presented to the JARPNII special permit expert panel review
workshop, February 2016 (unpublished). 10 pp. [Availablefrom the IWC Secretariat].
Masaki, Y. 1976. Biological studies on the North Paciﬁc seiwhale. Bull. Far Seas Fish. Res. Lab. 14: 1–114.
Murase, H., Hakamada, T., Matsuoka, K., Nishiwaki, S., Inagake,D., Okazaki, M., Toji, N. and Kitakado, T. 2014. Distribu�on ofsei whales (Balaenoptera borealis) in the subarc�c-subtropical transi�on area of the western North Paciﬁc in rela�on tooceanic fronts. Deep Sea Res. II 107: 22–28.
Omura, H. 1963. An improved method for collec�on of ear plugs
from baleen whales. Norsk Hvalfangst-Tidende 10: 279–283.

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