Technical Reports of the Institute of Cetacean Research (2018)

pp. 32–37

Technical Report (not peer reviewed)

Earplug-based age determination and estimation of biologicalparameters in North Paciﬁc Brydeʼs whales based on samples
collected by JARPNII
Takeharu Bando*
Institute of Cetacean Research, Toyomi-cho 4–5, Chuo-ku, Tokyo 104–0055, Japan*Contact e-mail: bando@cetacean.jp

ABSTRACTThis study summarizes the results of earplug-based age determination and estimation of biological parameters in the western North Paciﬁc Brydeʼs whales sampled by the JARPNII surveys between 2000 and 2016. A
total of 730 whales (314 males and 416 females) was sampled in the western North Paciﬁc north of 35°N andbetween the Paciﬁc coast of Japan and 170°E. Age information was obtained from earplugs of 476 (65.2%)
whales. Earplug readability was higher for mature (76.4%) than immature (43.8%) whales and no diﬀerence
between sexes was observed. The growth curves for males and females were Lt =12.63(1−e−0.208(t+4.266)), andLt =13.28(1−e−0.172(t+4.834)), respectively. The age at sexual maturity for males and females were 10.24 and 8.56years, respectively. The annual ovulation rate was estimated as 0.526/year. It is concluded that substantial
biological information was obtained for the North Paciﬁc Brydeʼs whales during the 17 years of JARPNII. In
particular the readability of earplugs for age determination increased notably compared to that in the periodof commercial whaling. The analyses of biological data will contribute to the management of this whale species in the North Paciﬁc, for example by incorporating such data into the conditioning process of the RMP
Implementation Simulation Trials.

INTRODUCTION
Brydeʼs whales are widely distributed throughout theworld, especially in waters with temperature of 20°Cor more (Kato and Perrin, 2017). Two types of Brydeʼswhales are distributed around Japan, one in the southwestern part of Japan (coastal type) and the other inoﬀshore waters in the Paciﬁc side of Japan (oﬀshore type)(Kishiro, 1996; Yoshida and Kato, 1999).
Some authors (e.g., Wada et al., 2003) recognize these
Brydeʼs whale types as two separate species, the smaller
coastal one Balaenoptera edeni Anderson, 1879 (Edenʼswhale) and the larger oﬀshore one B. brydei Olsen, 1913
(Brydeʼs whale). Other authors (e.g., Kershaw et al. 2013)
assign these species a sub-speciﬁc status: B. edeni edeni
and B. edeni brydei, respectively. This study focused on
the larger, oﬀshore type Brydeʼs whale (Figure 1).
JARPNII started in 2000 with three research objectives,i) feeding ecology and ecosystem studies, ii) monitoringof environmental pollutants in cetaceans and marine
ecosystems and iii) elucidation of stock structure (GOJ,2000; 2002). Brydeʼs whale (oﬀshore type) was selected
as one of the target species for sampling. The estima32

Figure 1.

North Paciﬁc Brydeʼs whale (oﬀshore type). Notethe three distinct ridges in the head, which are characteristic of this species.

tion of biological parameters of whales was not included
among the main objectives of JARPNII, however biological samples for such estimates were collected systematically during the surveys. The JARPNII was completed in2016, and a total of 730 Brydeʼs whales were sampledduring the 17 years survey period.
Age data is one of the most important information forEarplug-based age determination and estimation of biological parameters in North Pacific Brydeʼs whale

stock assessment and management of large whales. Ageestimation based on earplugs is considered the most reliable tool for age determination in whales (Lockyer, 1984;
Maeda et al., 2016). Earplug readability of North PaciﬁcBrydeʼs whales was reported as 17.4% from commercial
whaling samples collected in the pelagic whaling groundof the North Paciﬁc from 1971 to 1974 (Ohsumi, 1977).
During the JARPNII, earplugs were collected carefully
from each whale sampled, and eﬀorts were made to increase the readability.
This study summarized the results of age determination in North Paciﬁc Brydeʼs whales based on earplugs
and some biological parameters based on age data. Theanalyses were based on Brydeʼs whale samples collectedby JARPNII in the period 2000–2016 in the western North
Paciﬁc.

MATERIALS AND METHODS
Whale sampling
A total of 730 (314 males and 416 females) Brydeʼswhales were sampled by JARPNII during 2000–2016. The
geographical distribution of the sampled whales is shownin Figure 2. Whales were sampled between the Japanese
coast and 170°E, and between 35°N and approximately
42°N. Two biological stocks of Brydeʼs whales (oﬀshoretype) have been suggested for the western North Paciﬁc,one distributed between the Japanese coast and approximately 165°E and the other, east of 180°, with a transitionarea between 165°E and 180° (IWC, 2017). The analyses
in this study are mainly focused on the former stock.
Table 1 shows the number of sampled whales by year
and sexual maturity status.
Biological data
Body length was measured in a straight line from the tipof snout to the notch of ﬂukes using stainless steel measuring tapes. Sexual maturity of females was preliminarily
determined in the ﬁeld by the presence or absence ofcorpora lutea/albicantia in ovaries from both sides andconﬁrmed later by counting corpora number at the laboratory. If there was at least one corpora lutea or albicantia
in the ovaries, the female was determined as sexually mature. Maturity of males was determined by testis weight.
Testis weight (heavier side) of more than 560 g was regarded as sexually mature (Bando unpublished data).
Earplug sampling and age determination procedure
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

Figure 2.

Research area of JARPNII and sighting positions ofBrydeʼs whales sampled during the 2000–2016 surveys.

Table 1
Sex and sexual maturity status of Brydeʼs whales sampled during 2000 to 2016 JARPNII.
MaleFemale

Immature Mature Total Immature Mature Total

Total

2000

9122161622432001

1341712213350

2002

1872511142550

2003

109199223150

2004

51419724

3150

2005

10112113162950

2006

813218212950

2007

1310235222750

2008

62430

41620502009

991814183250

2010

52025

42125502011

10102011193050

2012

65114

192334

2013

31013

3121528

2014

2465141925

2015

31114

291125

2016

110112121425

Total

131

183

314

120

296

416

730

were collected carefully, and immediately ﬁxed in 10%
formalin.
In the laboratory, the ﬂat along the central axis of theearplug was cut using a sharp blade, then it was placed ona wet stone to expose the neonatal line and growth layers (Figure 3). Growth layers were counted under waterusing a stereoscopic microscope. A growth layer group(GLG) was deﬁned as one pair of light and dark laminae inthe core and considered as one year of age. Age reading
was conducted in the following manner: i) earplug of the33

Technical Reports of the Institute of Cetacean Research (2018)

left side was read. If the growth layers were ambiguous,
earplug from the right side was also read. Reading fromthe less ambiguous side was adopted; ii) age reading wasconducted only once without any knowledge of biological information such as body length or sex; iii) when thereading of all sample was completed, age data was compared with biological data such as body length or sexual
maturity status, and some samples were re-read to check
outlier, incomplete sample or invalid reading. All earplugs
were read by a single reader (TB).
Estimation of biological parameters
Growth curve
To estimate growth curve, the von Bertalanﬀy growthmodel was ﬁtted 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 asymptotic
length, K is the growth rate coeﬃcient and t0 is the theoretical time at zero length.
Age at sexual maturity
Age at sexual maturity (tm) was estimated by the following equation (Cooke, 1984):
tm = l − 0.5 +Figure 3.

k Ia la  N var(tm) =kl

whereMa is
Ia is
Na is
l isk isM a Ia N ( N − 1)
2aa

number of mature animals in age anumber of immature animals in age atotal number of animals in age aage of youngest mature animal in the sample
age of oldest immature animal in the sample.

Ovulation rate
Annual ovulation rate was estimated by applying linear
regression analysis between age and total number of corpora (corpora lutea and albicantia). The regression line
was ﬁtted to age 10 and older because almost all animals
mature at the age of 10.

RESULTS AND DISCUSSIONSAge readability
The readability of earplugs varied depending on the maturity status. Readability of sexually immature individuals
was 46.6% for males and 40.8% for females (Table 2).
Readability of earplugs in sexually mature animal washigher, 76.5% and 76.4% for males and females, respectively (Table 2). Readability of all samples was 65.2%,
which was higher than that of western North Paciﬁc
common minke whales (44.1%) (Maeda et al., 2016), andBisected earplug surfaces of Brydeʼs whales (left: 5 GLGs, right: 20 GLGs). The scale shows 1 mm interval.
Table 2
Earplug-age readability of Brydeʼs whales by sex and sexual maturity status.

34MaleFemaleTotal

Number of Readable
whalesearplugs

Readability Number of Readable(%)
whalesearplugs

Readability Number of Readable(%)
whalesearplugs

Readability(%)

Immature
Mature

131
183

61
140

46.6
76.5

120
296

49
226

40.8
76.4

251
479

110
366

43.8
76.4

Total

314

201

64.0

416

275

66.1

730

476

65.2

Earplug-based age determination and estimation of biological parameters in North Pacific Brydeʼs whale

Figure 4.

Figure 5.

Earplug age readability of Brydeʼs whales by body length class and sex.

Relationship between body length and age of Brydeʼs whales. The solid line shows the von Bertalanﬀygrowth curve.

was comparable to that of sei whales (63.0%) (Bandoet al., 2016), which were estimated based on samples
collected by JARPN/JARPNII.
Readability increased with body length for both sexes(Figure 4). An inter-reader calibration experiment, following the method of Kitakado et al. (2013), will be conducted in the near future.
Growth curve
For both sex, the growth rate was high at younger ages
and stabilized after 20 years old (Figure 5). The following
von Bertalanﬀy growth curves were estimated:
Male:
Female:(L = 13.28 (1 − eLt = 12.63 1 − e −0.208( t+4.266)
t− 0.172( t+4.834)))

Ohsumi (1977) estimated growth curves of Brydeʼswhales based on samples collected by the commercial
whaling in the North Paciﬁc, and estimated maximum
length as 12.8 m (42 ft) and 13.4 m (43.8 ft) for males andfemales, respectively. Similar values were obtained forBrydeʼs whales sampled by JARPNII.
Age at sexual maturity
Sexually mature males ﬁrst appeared at the age of 8,
and from 16 years old all animals were sexually mature(Figure 6). Age at sexual maturity (tm) for males was esti-mated as 10.24 years (SE=0.60).
Sexually mature females ﬁrst appeared at the age of 7,
and from 11 years old all animals were sexually mature(Figure 6). Tm was estimated as 8.56 years (SE=0.39).
Ohsumi (1977) reported age at 50% sexual maturityof Brydeʼs whales collected by the commercial whaling
in the North Paciﬁc in the 1970ʼs as 10 and 8 years formales and females, respectively. Consideration should begiven to the possibility of bias arising from legal size limit
and the selectivity of large animals during commercialwhaling. However similar estimates were obtained forBrydeʼs whales collected during past commercial whaling
and JARPNII.
Annual ovulation rate
The corpora lutea/albicantia ﬁrst appeared at the age of
7 and the number of corpora increased linearly after theage of 10 (Figure 7). Annual ovulation rate was calculated
as 0.526. The estimated annual ovulation rate means thatthe majority of Brydeʼs whale in the North Paciﬁc givebirth (or ovulation occurs) every two years. The estimatedvalue was higher than the 0.455 estimated from the1970ʼs commercial whaling (Ohsumi, 1977).

CONCLUDING REMARKS
It is concluded that substantial biological information was35

Technical Reports of the Institute of Cetacean Research (2018)

Figure 6.

Figure 7.

Sexual maturity status by age and sex in the Brydeʼs whales. Numbers in the bar shows the number ofsamples examined.

Relationship between age and number of corpora inBrydeʼs whales. Linear regression line was ﬁtted toages 10 and more.

obtained for the North Paciﬁc Brydeʼs whales during the17 years of JARPNII. In particular the readability of earplugs for age determination increased notably comparedto that in the period of commercial whaling. The analysesof biological data will contribute to the management of
this whale species in the North Paciﬁc for example byincorporating such data into the conditioning process of
the RMP Implementation Simulation Trials (Bando andKato, 2017).

ACKNOWLEDGEMENTS
Our sincere thanks are due to all researchers and crews
that collected data and samples during the JARPNII surveys. Kazuyoshi Nakai (ICR) helped in the preparation ofearplugs for age determination.

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