J-STAGE Advance published date: January 31, 2021

IMPROVED ESTIMATES OF LIFE-HISTORY PARAMETERS FOR BRYDE’S WHALESIMPROVED ESTIMATES OF SOME LIFE-HISTORYPARAMETERS OF THE PELAGIC SUBSPECIES OF
BRYDE’S WHALE IN THE
WESTERN NORTH PACIFIC
Takeharu BANDO
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan
Corresponding author: bando@cetacean.jp

Abstract
Estimates of several life-history parameters of the pelagic subspecies of Bryde’s whale(Balaenoptera edeni brydei) in the western North Paciﬁc were made based on samples collected from the second phase of the Japanese Whale Research Program under Special Permit
in the Western North Paciﬁc (JARPNII) from 2000 to 2016. A total of 730 individuals weresampled in the western North Paciﬁc, between the Japanese coast and 170°E and between
35°N and approximately 42°N. Age information was obtained from earplugs of 475 individuals (65.2%). The growth curves were estimated as Lt=12.65 (1−e−0.189 (t+5.250)) and Lt=13.30(1−e−0.170 (t+4.929)) for males and females, respectively. The mean age at sexual maturity wasestimated as 7.72 (SE=0.49) and 8.56 (SE=0.39) years, and the mean body length at sexualmaturity was estimated as 11.41 m (SE=0.25) and 11.75 m (SE=0 .23) for males and females,respectively. The annual ovulation rate was estimated as 0.526/year. Increased readability ofearplug age in the JARPNII resulted in improved estimates of age-related life-history parameters for the western North Paciﬁc Bryde’s whale compared with those based on samples collected during the past commercial whaling period.
Key words:

life-history parameter, Bryde’s whale, earplugs, western North Paciﬁc.

Bryde’s whale is widely distributed throughout the world, especially in waters with temperaturesof 20°C or more (Kato and Perrin, 2017). Two types of Bryde’s whale are found around Japan, one
along the coast of the southwestern part of Japan and the other on pelagic waters in the Paciﬁc side ofJapan (Kishiro, 1996; Yoshida and Kato, 1999). Some authors (e.g., Wada et al., 2003) recognize these
two types as separate species, a smaller coastal species Balaenoptera edeni Anderson, 1879 (Eden’swhale) and a larger pelagic species B. brydei Olsen, 1913 (Bryde’s whale). Other authors (e.g., Kershaw et al. 2013; Kato and Perrin, 2017) assign these species a subspeciﬁc status: B. edeni edeni andB. edeni brydei. This study focused on the larger, pelagic subspecies of Bryde’s whale (Fig. 1).
The second phase of the Japanese Whale Research Program under Special Permit in the Western
North Paciﬁc (JARPNII) started in 2000, under a permit issued by the Government of Japan. The pelagic subspecies of Bryde’s whale was selected as one of the target species for sampling. Biologicalinvestigations, such as body length measurements and sampling of reproductive organs and earplugs,
were conducted on whole animals in order to estimate life-history parameters. The JARPNII was completed in 2016, and a total of 730 Bryde’s whales (314 males and 416 females) were sampled during
the 17 year survey period.
Age data are important for assessment of stock and management of large whales. Age estimation
based on the number of growth layers accumulated in the earplugs is considered the most reliable toolfor age determination in baleen whales (Lockyer, 1984a; Maeda et al., 2016). During the JARPNII,Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

239

BANDO

Fig. 1. Pelagic subspecies of Bryde’s whale in the western North Paciﬁc. Note the three distinct ridges in thehead, which are characteristic of this species.

Fig. 2. Research area of the second phase of the Japanese Whale Research Program under Special Permit inthe Western North Paciﬁc (JARPNII) and location of sightings of Bryde’s whales sampled during 2000–2016
surveys.

earplugs were collected carefully from each whale by experienced biologists.
Age-related life-history parameters are important for monitoring the status of stocks and can beused in various analyses, including population dynamics models (e.g., the statistical catch-at age model; Punt et al., 2014). Previous estimates of life-history parameters of Bryde’s whale were based on
samples collected during the commercial whaling period (Kato and Yoshioka, 1995; Ohsumi, 1977).
However, samples from commercial whaling are not representative of the stock, as whaling operations
targeted large individuals and a legal size limit was imposed in some operations. Therefore, somelife-history parameters estimated from these samples, such as age at sexual maturity are thought to bebiased (Kato, 1982; Kato and Yoshioka, 1995; Ohsumi, 1977). JARPNII collected samples randomly,
regardless of body length or age, resulting in less biased estimates (Bando et al., 2016).
This article summarizes the results of estimation of several life-history parameters in the pelagic
subspecies of Bryde’s whale in the western North Paciﬁc based on randomly collected samples, which

240

Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

IMPROVED ESTIMATES OF LIFE-HISTORY PARAMETERS FOR BRYDE’S WHALES

should be more representative than those obtained in past commercial whaling.
Whales were sampled between the Japanese coast and 170°E, and between 35°N and approximately
42°N (Fig. 2). Two biological stocks of the pelagic subspecies of Bryde’s whale have been suggested
for the western North Paciﬁc, one between the Japanese coast and approximately 165°E and the othereast of 180°, with a transition area between 165°E and 180° (IWC, 2018). Analyses were conducted on
the assumption that all samples were from the western stock because it is diﬃcult to identify stocks
for each individual and the number of samples from east of 165°E was not large (Fig. 2) (IWC, 2018).
Body length was measured to the nearest 1 cm in a straight line from the tip of the snout to the notchof the ﬂukes using stainless steel measuring tapes. Sexual maturity in males was determined by examination of a histological sample from the testis. Males with seminiferous tubules over 100 µm diameterand spermatid or open lumen in the tubules were considered to be sexually mature (Nishiwaki et al.,1954; Lockyer, 1984b). Sexual maturity in females was determined by the presence or absence of corpora lutea and albicantia in the ovaries. If there was at least one corpus luteum or corpus albicans in
the ovaries, the female was considered to be sexually mature (Nishiwaki et al., 1954; Lockyer, 1984b).
Earplugs were collected from all sampled animals, following the method developed for baleen
whales (Omura, 1963; Maeda et al., 2016). The left and right earplugs were collected carefully andimmediately ﬁxed in 10% formalin. In the laboratory, the surface along the central axis of the earplug
was cut using a sharp blade, and was placed on a wet stone to expose the growth layers. The growthlayers were counted under water using a stereoscopic microscope. A growth layer group was deﬁned
as one pair of light and dark laminae in the core and was considered to indicate 1 year of age. All earplugs were read by the author.
To estimate the growth curve, the von Bertalanﬀy growth model was ﬁtted to body length and age
asLt = L∞ (1 − e− K (t − t0 ) )(1)

where Lt is the body length at age t, L∞ is the asymptotic length, K is the growth rate coeﬃcient and t0
is the theoretical time at zero length.
Age at sexual maturity (tm) was estimated by the following equation (Cooke, 1984):

Tm = g − 0.5 +var (tm ) =hh

 Ia a= ga

  N M a Ia

 N ( N − 1)
a=g2
aa(2)
(3)

whereMa is the number of mature animals of age a,
Ia is the number of immature animals of age a,
Na is the total number of animals of 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 (lm) was estimated by the following equation (Cooke, 1984;Kato, 1992) (body length was rounded to 0.1 m):

Lm = j − 0.05 + 0.1
kk Ib b= jb

  N M b Ib

 N (N − 1)var (lm ) = 0.1

b= j

Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

2bb(4)(5)241

BANDO

whereMb is the number of mature animals of body length b,Ib is the number of immature animals of body length b,
Nb is the total number of animals of body length b,j is the body length of the smallest mature animal in the sample, andk is the body length of the largest immature animal in the sample.
The annual ovulation rate was estimated by applying linear regression analysis between age and thetotal number of corpora (corpora lutea and corpora albicantia). The regression line was ﬁtted to ages
10 years and older because almost all animals were mature at the age of 10 years.
The readability of earplugs varied depending on the maturity status of the individual. The readability of earplugs of sexually immature individuals was 43.8% for males and 40.8% for females. The
readability of earplugs of sexually mature individuals was higher than that of immature individuals:
74.0% for males and 76.4% for females. The readability of all samples was 65.2% (Table 1). Although
the proportion of immature individuals with lower earplug readability was higher in JARPNII samples
than in commercial whaling samples from the 1970s (30.9% vs. 14.3%) (Ohsumi, 1977), the proportion of samples from which age could be determined increased drastically from commercial whaling
samples (17.4%; Ohsumi, 1977) to JARPNII samples.
The relationship between age and body length is shown in Fig. 3. For both sexes, the growth rate
was high at younger ages and stabilized after the age of 20 years. The following von Bertalanﬀygrowth curves were estimated:
Male :
Female :Lt = 12.65 (1 – e – 0.189 ( t+5.250 ) )Lt = 13.30 (1 – e –0.170 (t + 4.929 ) ) .

Table 1. Readability of earplugs of Bryde’s whales for determination of age according to sex and sexualmaturity status.

Fig. 3. Relationship between body length and age in Bryde’s whale. The solid lines show the von Bertalanﬀygrowth curves.

242

Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

IMPROVED ESTIMATES OF LIFE-HISTORY PARAMETERS FOR BRYDE’S WHALES

Fig. 4. Sexual maturity status by age and sex in Bryde’s whale. Numbers in the bars are the numbers of samples examined.

Fig. 5. Sexual maturity status by body length and sex in Bryde’s whale. Numbers in the bars are the numbersof samples examined. Body length values were rounded to 0.1 m.

Figs. 4 and 5 show sexual maturity status by age and body length. Males ﬁrst reached sexual maturity at the age of 6 years, and all males were sexually mature by the age of 11 years (Fig. 4). The mean
age at sexual maturity (tm) for males was estimated as 7.72 years (SE=0.49). Females ﬁrst reachedsexual maturity at the age of 7 years, and all females were sexually mature by the age of 11 years. Tmwas estimated as 8.56 years (SE=0.39).
Males ﬁrst reached sexual maturity at a body length of 10.4 m, and all males with a body length of
12.9 m or greater were sexually mature (Fig. 5). The mean body length at sexual maturity (lm) was estimated as 11.41 m (SE=0.25). Females ﬁrst reached sexual maturity at a body length of 11.2 m, and all femaleswith a body length of 12.5 m or greater were sexually mature. Lm was estimated as 11.75 m (SE=0.23).
The corpora lutea and corpora albicantia ﬁrst appeared at the age of 7 years and the number of corpora increased linearly after the age of 10 years (Fig. 6). The annual ovulation rate was estimated as 0.526.
The life-history parameters of the western North Paciﬁc Bryde’s whale have been estimated byseveral authors using commercial whaling samples. Ohsumi (1977) estimated the growth curves ofBryde’s whales based on commercial whaling samples from the western North Paciﬁc in the 1970s.
The results were preliminary because of the small numbers of younger individuals in the samples
due to the small numbers of young individuals in the catch and diﬃculty in collecting, preparing, andcounting the number of layers in the earplugs of young animals (Ohsumi, 1977).
Ohsumi (1977) reported the age at sexual maturity of Bryde’s whales collected by commercial whaling in the western North Paciﬁc as 10 years for males and 8 years for females. Although the estimatedCetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

243

BANDO

Fig. 6. Jitter plot showing the relationship between age and number of corpora lutea and corpora albicantia inBryde’s whale. The linear regression line was ﬁtted to ages 10 years and more.

values in the present study are similar to the estimates from commercial whaling, consideration should
be given to the possibility of bias arising from legal size limits and the selection of large animals duringcommercial whaling. Large whales were preferred and individuals of the same age but large bodylength, which has high probability of maturity, are caught among the younger whales during commercial
whaling, which leads to overestimation of the sexual maturity rate of younger individuals and underestimation of the age at sexual maturity. Further analysis including other parameters that are free from bias,
such as age at ﬁrst ovulation, is needed to determine whether any changes in the age at sexual maturity
have occurred between the period of commercial whaling and the period of the JARPNII program.
Kato and Yoshioka (1995) reported body length at sexual maturity of Bryde’s whales collected by
the commercial whaling in the coastal region of the western North Paciﬁc in the 1970s as 11.4 m and11.8 m for males and females. These results are comparable to the results of the present study. In Katoand Yoshioka’s study, testis weight was used as a criterion of male sexual maturity. The present study
used histological examination of the testis, which is more desirable than testis weight as a criterion ofmale sexual maturity (Lockyer, 1984b), and which would enhance the credibility of the estimated value.
The annual ovulation rate was estimated as 0.455 from the 1970s commercial whaling samples(Ohsumi, 1977), which was lower than the value estimated in this study (0.526). As in the case ofage at sexual maturity, there may be an underestimation bias in estimates from commercial whalingsamples. In commercial whaling, whales with the same age but large body length, which would growand reached sexual maturity faster and have more corpora lutea and albicantia, are caught among theyounger whales, which leads to underestimation of the annual ovulation rate.
In conclusion, in this study improved estimates of some life-history parameters are derived for thepelagic subspecies of Bryde’s whale in the western North Paciﬁc from randomly collected samples ofanimals with a wide range of ages and body lengths. Further analysis, including examination of yearly
trends will contribute to understanding and management of this whale stock.
Acknowledgements

My sincere thanks are due to all researchers and crews that collected data and samples during the JARPNII surveys. I also
thank Dr. Tsutomu Tamura and colleagues of the Institute of Cetacean Research for useful suggestions and help in conducting laboratory work. Dr. Yoshihiro Fujise, director-general of the Institute of Cetacean Research, provided the original idea
for this analysis. Dr. Hidehiro Kato, Professor Emeritus of Tokyo University of Marine Science and Technology gave me theopportunity to conduct this study. Finally, I greatly appreciate the anonymous reviewers for their valuable suggestions.

244

Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

IMPROVED ESTIMATES OF LIFE-HISTORY PARAMETERS FOR BRYDE’S WHALES

References
Bando, T., Yasunaga, G., Tamura, T., Matsuoka, K., Murase, H., Kishiro, T. and Miyashita, T. 2016. Methodology and survey
procedures under the JARPNII—oﬀshore component—during 2008 to 2014 with special emphasis on whale sampling
procedures. Paper SC/F16/JR4 presented to the JARPNII special permit expert panel review workshop, Tokyo, February 2016 (unpublished). 14 pp. [Available from the secretariat of IWC].
Cooke, J.C. 1984. The estimation of mean ages at sexual maturity from age sample. Paper SC/36/O22 presented to IWC Scientiﬁc Committee, 1984 (unpublished). 8 pp. [Available from the IWC Secretariat].
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 Scientiﬁc Committee, 2000 (unpublished). 68 pp. [Available from the secretariat of IWC].
International Whaling Commission. 2018. Report of the Workshop on the Implementation Review of western North Paciﬁc
Bryde’s whales. J. Cetacean Res. Manage. (Suppl.) 19: 561–593.
Kato, H. 1982. Some biological parameters for the Antarctic minke whale. Rep. Int. Whal. Commn. 32: 935–945.
Kato, H. 1987. Density dependent changes in growth parameters of the southern minke whale. Sci. Rep. Whales Res. Inst. 38:
47–73.
Kato, H. 1992. Body length, reproduction and stock separation of minke whales oﬀ northern Japan. Rep. Int. Whal. Commn.
42: 443–453.
Kato, H. and Perrin, W.F. 2017. Bryde’s whales Balaenoptera edeni/brydei. pp. 158–163. In: B. Würsig, J.G.M. Thewissenand K. Kovacs (eds.) Encyclopedia of marine mammals, Third Edition. Academic Press, San Diego. 1190 pp.
Kato, H. and Yoshioka, M. 1995. Biological parameters and morphology of Bryde’s whales in the western North Paciﬁc, withreference to stock identiﬁcation. Paper SC/47/NP11 presented to the IWC Scientiﬁc Committee, 19 pp. (unpublished).
19 pp. [Available from the secretariat of IWC].
Kershaw, F., Leslie, M.S., Collins, T., Mansur, R.M., Smith, B.D., Minton, G., Baldwin, R., LeDuc, R.G., Anderson, R.C.,Brownell, R.L. and Rosenbaum, H.C. 2013. Population diﬀerentiation of 2 forms of Bryde’s whales in the Indian andPaciﬁc Oceans. J. Hered. 104 (6): 755–764.
Kishiro, T. 1996. Movements of marked Bryde’s whales in the western North Paciﬁc. Rep. Int. Whal. Commn. 46: 421–428.
Lockyer, C. 1984a. Age determination by means of the earplug in baleen whales. Rep. Int. Whal. Commn. 34: 692–696.
Lockyer, C. 1984b. Review of baleen whale (Mysticeti) reproduction and implications for management. Rep. Int. Whal.
Commn. (Special Issue 6): 27–50.
Maeda, H., Bando, T., Kishiro, T., Kitakado, T. and Kato, H. 2016. Basic information 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, Tokyo, February 2016 (unpublished). 10 pp. [Available from the secretariat of IWC].
Nishiwaki, M., Hibiya, T. and Kimura, S. 1954. On the sexual maturity of the sei whales of the Bonin waters. Sci. Rep.
Whales Res. Inst. 8: 165–177.
Ohsumi, S. 1977. Bryde’s whales in the pelagic whaling ground of the North Paciﬁc. Rep. Int. Whal. Commn. (SpecialIssue 1): 140–148.
Ohsumi, S. 1986. Yearly change in age and body length at sexual maturity of a ﬁn whale stock in the eastern North Paciﬁc.
Sci. Rep. Whales Res. Inst. 37: 1–16.
Omura, H. 1963. An improved method for collection of earplugs from baleen whales. Norsk Hvalfangst-Tidende 10: 279–283.
Punt, A.E., Hakamada, T., Bando, T. and Kitakado, T. 2014. Assessment of Antarctic minke whales using statistical catch-atage analysis. J. Cetacean Res. Manage. 14: 93–116.
Wada, S., Oishi, M. and Yamada, T. 2003. A newly discovered species of living baleen whale. Nature 426: 278–281.
Yoshida, H. and Kato, H. 1999. Phylogenetic relationships of Bryde’s whales in the western North Paciﬁc and adjacentwaters inferred from mitochondrial DNA sequences. Mar. Mamm. Sci. 15: 1269–1286.

Received: October 31, 2019
Accepted: September 16, 2020
Published online: January 31, 2021

Cetacean Popul. Stud. (CPOPS), Short noteVol. 3, 2021, 239–245

245