Technical Reports of the Ins�tute of Cetacean Research (2022)

pp. 19–24

Technical Report (not peer reviewed)

What do we know about whales and ecosystem in the western North
Paciﬁc Ocean? Part 3: summary of results on biological parameters inbaleen whale species
Takeharu Bando1, *, Hikari Maeda2, Satoko Inoue1 and Tomoe Kuno1
1

Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan
Fisheries Resources Institute, 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

ABSTRACT

Three baleen whale species, sei, Brydeʼs and common minke whales were surveyed during the former Japanese scien�ﬁc whaling programs in the western North Paciﬁc Ocean conducted from 1994 to 2019. This paper
summarizes some life history-related biological parameters es�mated for the three species, based on dataand samples collected during the research programs. These biological parameters are important as input data
for models aimed at studying the popula�on dynamics of the species. Results therefore will contribute to the
management of these whale species in the western North Paciﬁc.

INTRODUCTION
Three baleen whale species, sei (Balaenoptera borealis),
Brydeʼs (B. edeni brydei) and common minke (B. acutorostrata scammoni) whales were surveyed by former
Japanese scien�ﬁc whaling program in the western
North Paciﬁc conducted by the Ins�tute of Cetacean Research (ICR). The target species for the Japanese Whale
Research Program under Special Permit in the North Paciﬁc (JARPN) (1994–1999) was the common minke whales(GOJ, 1994). The second phase of this program (JARPNII)(2000–2016) targeted Brydeʼs, common minke and sei
whales. The la�er was added as a target species in 2002
(GOJ, 2000; 2002).
A�er the comple�on of the JARPNII, the New Scien�ﬁcWhale Research Program in the western North Paciﬁc(NEWREP-NP) commenced in 2017 and lasted un�l 2019
(GOJ, 2017). The target species were sei and common
minke whales. Figure 1 shows the baleen whale species
targeted by the former Japanese scien�ﬁc whaling programs in the western North Paciﬁc.
During these research programs, systema�c biologicalsampling was conducted on the whales taken. These included earplugs for age determina�on, testes and ovaries
for determina�on of sexual maturity and es�ma�on of
reproduc�ve history, fetus for reproduc�ve studies andbody length for growth studies. Some life history-related
biological parameters, which are important as input datafor popula�on dynamics models, were es�mated for thethree whale species based on data and samples collectedduring the research programs.
This paper summarizes the results of biologicalparameter es�ma�ons in sei, Brydeʼs and common
minke whales in the western North Paciﬁc. It follows
more detailed analyses presented previously in Bando(2017; 2018), Bando and Maeda (2020) and Maeda et al.
(2017).

SUMMARY OF BIOLOGICAL PARAMETERS IN
THREE BALEEN WHALE SPECIESLaboratory and analy�cal procedures to es�mate biological parameters in baleen whales were explained in previous ar�cles (Bando, 2017; 2018; Bando and Maeda, 2020;
Maeda et al., 2017). Details of these approaches are notrepeated here for brevity. The results of biological parameter es�ma�ons for the three species in the western
North Paciﬁc are presented in this sec�on.
Earplug age readability
In many baleen whale species, ages are es�mated bycoun�ng the number of growth layers accumulated in theearplugs which are formed in the external auditory meatus (Lockyer, 1984a). During the scien�ﬁc whaling programs, earplug age readability had improved comparedto the reading based on earplug collected from pastcommercial whaling. This was due to careful sampling
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Technical Reports of the Ins�tute of Cetacean Research (2022)

Figure 1.

Baleen whale species targeted by the former Japanese scien�ﬁc whaling programs in the western North
Paciﬁc: sei whale (upper le�); Brydeʼs whale (upper right); and common minke whale (lower).

of earplugs by experienced researchers and introduc�onof a new gela�nized extrac�on method during the scien�ﬁc whaling programs (Bando, 2021; Bando and Maeda,
2020; Maeda, et al., 2013).
Sei whales
Bando and Maeda (2020) reported earplug age readability of immature sei whales as 55.9% and 48.9% for males
and females, respec�vely. Readability in sexually mature
animals was higher. The values were 72.5% and 67.4% formales and females, respec�vely.
Brydeʼs whales
Similar readability values were reported for Brydeʼswhales. They were 43.8% and 40.8% for immature males
and females, respec�vely, and 74.0% and 76.4% for mature males and females, respec�vely (Bando, 2021).
Common minke whales
In the case of common minke whale, the readability
was somewhat lower. They were 35.4% and 34.7% for
immature males and females, respec�vely, and 49.2%
and 59.0% for mature males and females, respec�vely(Maeda et al., 2016).
Growth curve
The growth curve of each whale species was es�mated by
20

ﬁ�ng the von Bertalanﬀy growth model to body lengthand age.
Sei whales
The growth curve of sei whales was es�mated as Lt =14.14(1−e−0.174(t+6.650)) for males, and Lt =15.17(1−e−0.150(t+7.407))
for females (Bando and Maeda, 2020).
Brydeʼs whales
The asympto�c length of Brydeʼs whales was slightly
smaller than that of sei whale. The growth curve was es�mated as Lt =12.65(1−e−0.189(t+5.250)) for males, and Lt =13.30(1−e−0.170(t+4.929)) for females (Bando, 2021).
Common minke whales
The asympto�c length of common minke whales wasmuch smaller than the other two species. The growth
curve was es�mated as Lt =7.49(1−e−0.41(t+0.90)) for males,and Lt =8.66(1−e−0.11(t+7.60)) for females (Maeda, 2012).
In the three whale species, the asympto�c length was
greater in females than in males.
Maximum life span
Sei whales
Bando and Maeda (2020) reported the maximum age ofsei whales (es�mated from earplug reading) as 54 years
for males and 43 years for females.

What do we know about whales and ecosystem in the western North Pacific Ocean? Part 3

Brydeʼs whales
Bando (2021) reported the maximum age of Brydeʼswhales as 51 and 56 years for males and females, respec�vely.
Common minke whales
Maeda et al. (2016) reported maximum age of maleand female common minke whales as 49 and 41 years,
respec�vely.
Age at sexual maturity
There are several methods to es�mate age at sexual
maturity of baleen whales (Lockyer, 1984b). Age at 50%
sexual maturity (tm50%) is es�mated by calcula�ng thera�o of immature and mature whales in each age. Age at
ﬁrst ovula�on (tmov) is es�mated by mean age of whales
with one corpus luteum and no corpus albicans in bothovaries. It is generally known that the transi�on phase inearplugs of large baleen whales indicates sexual maturity(Lockyer, 1972; Masaki, 1979; Kato, 1983; Ohsumi, 1986)
and age at sexual maturity (tmp) is es�mated by mean
age of whales when the transi�on phase is formed.
Sei whales
Bando and Maeda (2020) es�mated tm50% of sei whales
as 6.7 (SE= 0.29) years for males and 6.9 (SE= 0.27) years
for females, respec�vely. Tmov was es�mated as 8.6 (SD=3.14) years (Bando, 2017).
Brydeʼs whales
For Brydeʼs whales, tm50% were es�mated as 7.72 (SE=0.49) and 8.56 (SE= 0.39) years for males and females,
respec�vely (Bando, 2021). Tmov was es�mated as 10.1
(SD= 1.13) years (Bando, 2017).
Common minke whales
Tmp was es�mated as 7.68 (SD= 1.81) years for males and7.82 (SD= 1.58) years for females (Maeda et al, 2017).
Tmov was es�mated as 8.75 (SD= 1.5) years (Maeda etal, 2017).
Body length at sexual maturity
Body length at 50% sexual maturity (Lm50%) and ﬁrst
ovula�on (Lmov) are es�mated in the same manner as
the age at 50% sexual maturity (tm50%) and age at ﬁrst
ovula�on (tmov). Lm50% is es�mated by calcula�ng thera�o of immature and mature whales in each body lengthclass. Lmov is es�mated by mean body length of whales
with one corpus luteum and no corpus albicans in bothovaries.

Sei whales
Lm50% of sei whales was es�mated as 12.72 m (SE= 0.12)
for males and 13.31 m (SE= 0.12) for females (Bando and
Maeda, 2020). Lmov was es�mated as 13.70 m (SD= 0.47)(Bando, 2017).
Brydeʼs whales
Lm50% of Brydeʼs whales was slightly smaller than that ofsei whale, 11.41 m (SE= 0.25) for males and 11.75 m (SE=0.23) for females (Bando, 2021). Lmov was es�mated as
12.13 m (SD= 0.62) (Bando, 2017).
Common minke whales
There are no available es�mates of Lm50% and Lmov forcommon minke whales. Common minke whales are known
to segregate in the western North Paciﬁc by sex and sexual
maturity status (Zenitani et al., 2000a; 2000b). This fact will
bias the es�mate of length at sexual maturity.
Pregnancy rate
Pregnancy rates are usually calculated as the propor�onof pregnant females among captured mature females.
However, in some cases, this value may diﬀer from thetrue pregnancy rate due to biases such as segrega�on andthe eﬀect of mother and calf pairʼs occurrence.
Sei and Brydeʼs whales
Most sei and Brydeʼs whales are thought to give birth
every two or more years. Females accompanied by calf
are considered to have a low pregnancy rate because
they were pregnant in the previous year. If females withcalf were excluded from samples, pregnancy rate may beoveres�mated. Bando (2017) es�mated pregnancy rateof sei and Brydeʼs whales with correc�on of the eﬀect ofmother and calf pair (which were not targeted for sampling in most years) as 0.650 and 0.491, respec�vely.
Common minke whales
Pregnancy rates of common minke whale have not beenes�mated due to the small number of mature female
samples. Most of the mature females distribute outsideof the research area (Hatanaka and Miyashita, 1997;
Wada, 1989).
Concep�on Date
In baleen whales, growth of fetus a�er the early stage ofdevelopment is considered to be linear with �me, andmethods for es�ma�ng concep�on date from foetal lengthand date of capture has been developed (Hugge� andWiddas, 1951; Kato and Miyashita, 1991; Lockyer, 1984b).
21

22
72.5% (n=466)
68.4% (n=618)

MatureTotal

tmp
13.70 m0.650

13.31 mReferences

Bando (2017)
Bando and Maeda (2020)

1.007

0.481

Li�er size

0.744/year
0.519

Annual ovula�on rate

Peak from December to February

12.72 m—Foetal sex ra�oConcep�on datePregnancy rate

Reproductive characteristics

Lm50%

—

8.6 years
6.9 years

tm50%

43 years

Lt =15.17(1−e−0.150(t+7.407))

63.0% (n=736)

67.4% (n=562)

48.9% (n=174)

Femaletmov
6.7 years

54 years

Lt =14.14(1−e−0.174(t+6.650))

55.9% (n=152)

Immature

Body length at sexual maturity Lmov

Age at sexual maturity

Sexual maturity

Maximum life span

Growth curve

GrowthEarplug readability

Age reading

Male

Sei whale

0.491

11.75 m12.13 m—8.56 years

10.1 years

56 years

Lt =13.30(1−e−0.170(t+4.929))

66.1% (n=416)

76.4% (n=296)

40.8% (n=120)

Female

1.011

0.467

0.526/year

Bando (2017); (2018); (2021)

0.533

Year round with peak from October to May

11.41 m—7.72 years

51 years

Lt =12.65(1−e−0.189(t+5.250))

63.9% (n=313)

74.0% (n=208)

43.8% (n=105)

Male

Brydeʼs whale

Table 1
Summary of biological parameters of three baleen whale species in the western North Paciﬁc.

—

—

—

7.82 years—

8.75 years

41 years

Lt =8.66(1−e−0.11(t+7.60))

41.2% (n=684)

59.0% (n=183)

34.7% (n=501)

Female———Bando et al. (2010)
Goto et al. (2021)
Maeda (2012)
Maeda et al. (2016); (2017)—

Peak from autum to winter (J-stock)
One peak in winter (O-stock)——

7.68 years—

49 years

Lt =7.49(1−e−0.41(t+0.90))

45.2% (n=1,888)

49.2% (n=1,340)

35.4% (n=548)

MaleCommon minke whale

Technical Reports of the Ins�tute of Cetacean Research (2022)

What do we know about whales and ecosystem in the western North Pacific Ocean? Part 3

Sei and Brydeʼs whales
Bando (2017) calculated concep�on dates based on theanalysis of fetuses from 414 sei and 177 Brydeʼs whales.
He reported that the concep�on season of sei whales
lasts from November to March, with a peak from December to February, and that of Brydeʼs whales is year-round,with a moderate peak from October to May.

Common minke whales
There are no es�mates available for common minke
whales.
Table 1 summarizes the es�mates of life history-related
biological parameters in sei, Brydeʼs and common minke
whales in the western North Paciﬁc.

Common minke whales
As for minke whales, two biological stocks are known
to exist in the western North Paciﬁc, the J and O stocks(see reviews in Goto et al., 2017; 2021). Both stocks arereported to have diﬀerent concep�on seasons, the Ostock from November to June with a single winter peak,
and the J-stock from August to March with a peak fromautumn to winter (Bando et al., 2010, Goto et al., 2021).

Several biological parameters of three baleen whale
species were es�mated from the samples collected byscien�ﬁc whaling programs conducted by the ICR. During these surveys, a wide range of whale samples, fromyoung to old, were collected by random sampling method
which led to a more accurate es�ma�on of biologicalparameters than that es�mated from past commercial
whaling data. Some biological parameters respond tochanges in their environment such as food availabilityor fecundity, which will aﬀect the status of whale stocks.
Biological parameters are also important as input data
for models aimed at studying the popula�on dynamics
of the species. The es�mated biological parameters willcontribute to the management of these whale species in
the western North Paciﬁc Ocean.

Annual ovula�on rate
Annual ovula�on rate was es�mated from regression ofage and number of corpora in the ovaries.
Sei and Brydeʼs whales
Annual ovula�on rate was es�mated as 0.744/year for sei
whales and 0.526/year for Brydeʼs whales, respec�vely(Bando and Maeda, 2020; Bando, 2021).
Common minke whales
There is no es�mate available for common minke whales.
Foetal sex ra�o
Sei and Brydeʼs whales
Bando (2017) reported foetal sex ra�o (male ra�o) of sei
whales as 51.9% (n=418) and that of Brydesʼs whales as
53.3% (n=169). No signiﬁcant diﬀerence from parity was
detected for sei and Brydeʼs whales.
Common minke whales
There are no es�mates available for common minke
whales.
Li�er size
In baleen whale species, the number of fetuses is usually
one. However, mul�ple fetuses (twins in most cases) are
observed occasionally (Kimura, 1957; Lockyer, 1984b).
Sei and Brydeʼs whalesLi�er sizes were es�mated as 1.007 for sei whales (n=420),
and 1.011 for Brydeʼs whale (n=177) (Bando, 2017).

CONCLUDING REMARKSACKNOWLEDGEMENTS
Our sincere thanks to all researchers and crews who collected data and samples during the JARPN, JARPNII and
NEWREP-NP surveys. We thank the members of the Ins�tute of Cetacean Research (ICR) and Japan Fisheries Research and Educa�on Agency for their useful sugges�ons
and assistance in the laboratory work. We also thank Luis
A. Pastene (ICR) for his assistance in preparing this report.
We also thank the Editorial Team of TEREP-ICR for theeditorial work on this manuscript.

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