SC/F16/JR53

Basic information of earplugs as age character of common
minke whales in western North Pacific
HIKARI MAEDA 1, TAKEHARU BANDO2, TOSHIYA KISHIRO1, TOSHIHIDE KITAKADO3 and
HIDEHIRO KATO3
1

National Research Institute of Far Seas Fisheries 2-12-4, Fukuura, Kanazawa, Yokohama, Kanagawa 2368648, Japan
2
Institute of Cetacean Research, Toyomi-cho 4-5, Chuo-ku, Tokyo 104-0055, Japan
3
Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan
Contact e-mail: hikarim@affrc.go.jp

ABSTRACTAge reading from the earplugs of the common minke whale (Balaenoptera acutorostrata) are generally
believed to be difficult and impractical because of their softness and poor formation of growth layers.
However, under JARPN and JARPNII surveys, all earplugs of common minke whales were carefullycollected and tried to read growth layers. We provide basic information of earplug readability and age
composition of North Pacific common minke whales collected from 1994 to 2013. Age readability was
45.2% for male, 41.2% for female. Readability of mature animals (49.2% for male and 59.0% for female)
was higher than immature animals (35.4% for male and 34.7% for female) in both sexes. From inter-reader
calibration experiment, the age reading outcomes of two readers appeared similar. Distinct difference wasobserved in body length distribution and also age distribution of common minke whales between coastal andoffshore areas. Whales of less than five years old were dominant in coastal areas. On the other hand, males ofmore than six years old were dominant in offshore areas. Result of age distribution supported segregation ofcommon minke whales by sex and maturity status in western North Pacific. It had been believed that
earplugs of common minke whales were not available for age characteristics. However, in conclusion,
earplugs of common minke whales in western North Pacific were considered useful as a valid age tool.

KEYWORDS: EARPLUG, AGE DATA, COMMON MINKE WHALE, WESTERN NORTH PACIFICINTRODUCTION
Information on whale age is of key importance for estimating life-history parameters that can be used for stock
management. Earplugs were examined as an age character for the first time in baleen whales by Purves (1955).
Even today, using earplugs as an age character, age estimations have been conducted in several species ofbaleen whales (Lockyer, 1984a; Gabriele et al., 2010; Nielsen et al., 2012; Kitakado et al., 2013). As shown in
Figure 1, the earplug accumulates in the external auditory meatus (Lillie, 1910) and consists of a core and outer
covering (Purves, 1955; Ichihara, 1959). The outer covering is secreted by epithelial cells in the external
auditory meatus, whereas the core, comprising concentric light and dark laminae, is secreted by papillae on thesurface of the glove finger (Purves, 1955). Fat content tends to be lower in the dark layer and higher in the lightlayer from histological observations of fin whale (Balaenoptera physalus) earplugs (Roe, 1967). The light anddark layers are formed during the feeding and breeding periods, respectively (Roe, 1967). Because many baleen
whales migrate between a breeding place in low-latitude waters (winter) and a feeding place in high-latitude
waters (summer), which is approximated by an annual cycle, such a living cycle is reflected as a growth layer;
therefore, one growth layer represents 1 year (Roe, 1967; Lockyer, 1972; 1984b). In addition, the neonatal line
(NL) which is created at birth (Ichihara, 1964), is formed at the apical portion of the core (Figure 1).
Age reading from earplugs of the common minke whale (B. acutorostrata) are generally believed to be difficultand impractical because of their softness and poor formation of growth layers (Sergeant, 1963; Mitchell andKozicki, 1975; Christensen, 1981; Larsen and Kapel, 1983; Christensen et al., 1990; Kato, 1992; Auðunsson etal., 2013). However, under JARPN and JARPNII survey, all earplugs of common minke whales were carefully

1	
  
	
  

collected and tried to read growth layers. Furthermore, we tried to improve age readability especially young
animals to prevent breakage and loss of neonatal line using new collection technique “gelatinized extraction”(Maeda et al., 2013). Here we provide basic information of earplug readability and age distribution of North
Pacific common minke whales collected 1994 to 2013.
MATERIALS AND METHOD
Samples and data used
All earplugs of common minke whales were collected during the 1994 to 2013 JARPN and JARPANII surveys.
Earplugs were collected from 2,572 whales (1,888 males and 684 females). Some biological datasets, including
body length, sex, weight of testis, number of ovulations and genetic information were used. Each whale was
assigned to the O and J stock based on the microsatellite DNA analysis presented in Pastene et al. (2016a:
SC/F16/JR38). Only animals with membership probability of over 90% were included in the analysis. Males
with a single testis weight of 290g or more, and females that had at least one corpus luteum or albicans in theirovaries were regarded as sexually mature.
Age estimation using earplugs
The age of the whales was estimated by the counting of growth layers of earplug (Purves, 1955; Ohsumi, 1964;
Lockyer, 1972; Maeda and Kato, 2012). The left and right earplugs with glove-finger were collected carefully,
and immediately fixed in 10% formalin solution until age estimation. Traditionally, earplugs have beencollected by exposing and cutting the external auditory meatus and then directly obtaining the internal earplugs
using tweezers. According to Maeda et al. (2013), earplugs of small individuals were collected using the methodof gelatinized extraction which was introduced in 2007.
In the laboratory, after cutting flat along the central axis of the earplug using a sharp blade, it was ground on awet stone to expose the neonatal line and growth layers. Growth layers were counted under water using
stereoscopic microscope under low magnification (3.15x – 31.5x). A year of age was defined as one pair of thelight and dark laminae in the core.
Age reading was conducted in the following manner: i) earplug of the left side was read. If the earplug growthlayers were ambiguous, earplug from the right side was also read. Reading from the less ambiguous side wasadopted; ii) age reading was conducted only once without any knowledge of biological information such as
body length or sex; iii) when reading of all sample was completed, age data was compared with biological datasuch as body length or sexual maturity, and some samples were re-read to check outlier, incomplete sample orinvalid reading. All earplugs collected during the 1994 to 2013 were read by a single reader (Maeda).
Inter-reader calibration
The inter-reader calibration experiment and analysis were conducted following the method of Kitakado et al.
(2013) which was applied for Antarctic minke whales. A total of 150 earplugs were chosen from three periodsof year (1994 to 2003, 2004 to 2008, 2009 to 2013) from male common minke whales of O-stock sampledduring 1994 to 2013. Fifty earplugs were randomly selected from each period and read by control reader(Bando) and Maeda.
RESULTS AND DISCUSSION
Age readability
Earplugs of immature animals had soft and light-colored core, and its outer covering was undeveloped (Figure 2,
A). On the other hand, mature animals had toughened and dark-colored core with developed outer covering(Figure 2, B and C). Readability of mature animals (49.2% for male and 59.0% for female) was higher than
immature animals (35.4% for male and 34.7% for female) in both sexes. Figure 3 shows readability by bodylength class and stocks. Readability increased with body length class in both stocks. Age readability of all
animals was 45.2% for male, 41.2% for female, respectively.
Most of unreadable earplugs of mature animals had growth layers with unclear formation such as irregularlamination and partially-formed growth layers. About lower readability in immature animals, it was difficult to
identify growth layers because of its solid and light-colored core and easily damaged because of its softness andsmall size. In the past, Kato (1992) reported that readability of common minke whales off Northern Japan
collected by commercial whaling was only 8.7% (321 animals, 183 males and 133 females). However, under

2	
  
	
  

JARPN and JARPANII surveys, readability of common minke whales was improved dramatically, because of
careful collection and efforts in technical development of earplugs sampling.
Age reading error
Figure 4 showed the scatter plots of age estimates of common minke whales from two independent readers(Bando and Maeda) on 1st trial (read 150 samples). Figure 5 and 6 showed scatter plots of age estimates fromreading data for three trials. The age reading outcomes of each reader appeared similar. Table 2 and Figure 7
showed the result of age reading error with Bando assuming control reader (unbiased). Compared to Bando,
Maeda tended to estimate on the small side, however, the extent of the difference is not so large. Thus, it wasconsidered that age data read by a single reader (Maeda) from earplugs collected during 1994 to 2013 had aconstant reliability.
Age and body composition
Male and female were distributed over a wide range of age, however sample size of female was smaller thanmale and there were biased under ten years old. From age composition by stock (Figure 8), sample size of Jstock was smaller than O-stock and there were few old individuals. Distinct difference was observed in age
distribution between coastal and offshore areas (Figure 9). Whales of less than five years old were dominant incoastal areas. On the other hand, males of more than six years old were dominant in offshore areas. Hatanaka
and Miyashita (1997) reported on segregation of common minke whales in feeding season from body lengthcomposition. From body length distribution and maturity composition of JARPN/JARPANII data (1994 to2001), Zenitani et al. (2002) concluded that common minke whales were distributed widely from coastal tooffshore with segregation depending on sex and reproductive status. Furthermore, Kanda et al. (2010) analyzed
length composition and sex ratio data from bycaught minke whales around Japan from 2001 to 2007 and
JARPN/JARPANII data (1994 to 2007). They reported immature animals were distributed in mainly coastal
areas whereas mature animals were distributed mainly in offshore areas. Results of age distribution from thisstudy supported segregation of common minke whales by sex and maturity status in western North Pacific.
From body length composition (Figure 10 and 11), it was shown that body length compositions of readable
earplugs in each stock and survey are not always exactly reflected in all whales. Especially under six meteranimals which considered immature animals had a large proportion of unreadable earplugs in both stock and sex.
O stock male or offshore male had also a large proportion of unreadable earplugs in seven to eight meter bodylength ranges. The age structure could be reliably estimated by summarizing the relationship between age andlength (Age length key). It was assumed that Age length key works to some extend to fill in gaps of outcomes
between readable and unreadable earplug for under six meter animals.
It had been believed that earplugs of common minke whales were not available for age characteristics. However,in conclusion, earplugs of common minke whales in western North Pacific were considered useful as a valid age
tool.
ACKNOWLEDGEMENTS
We would like to express our gratitude to all survey leaders, researchers, crews and staff of the research stationthat participated in JARPN and JARPNII surveys.
REFERENCES
Auðunsson, G.A., Nielsen, N.H., Elvarsson, B.T., Víkingsson, G.A., Kato, H., Halldórsson, S.D., Gunnlaugsson,
T. and Hansen, S.H. 2013. Age estimation of common minke whales (Balaenoptera acutorostrata) in
Icelandic waters by aspartic acid racemization (AAR),-AAR and earplug readings of Antarctic minke
whales (B. bonaerensis) used as a reference. Paper SC/65/SP15 presented to the IWC/SC, 2012
(unpublished). 17pp.
Christensen, I. 1981. Age determination of minke whales, Balaenoptera acutorostrata, from laminatedstructures in the tympanic bullae. Rep. Int. Whal. Commn. 31: 245–253.
Christensen, I., Haug, T., and Wiig, Ø. 1990. Morphometric comparison of minke whales Balaenoptera
acutorostrata from different areas of the North Atlantic. Mar. Mamm. Sci. 6: 327-338.
Gabriele, C. Lockyer, C. Straley, M.J. Jurasz, M.C. and Kato, H. 2010. Sighting history of a naturally marked
humpback whale (Megaptera novaeangliae) suggests ear plug growth layer groups are depositedannually. Mar. Mamm. Sci. 26: 443-450.

3	
  
	
  

Hatanaka, H. and Miyashita, T. 1997. On the feeding migration of Okhotsk sea -West Pacific stock of minke
whales, estimates based on length composition data. Rep. Int. Whal. Commn. 47: 557-564
Ichihara, T. 1959. Formation mechanism of ear plug in baleen whales in relation to glove-finger. Sci. Rep.
Whales Res. Inst. 14: 107-135.
Ichihara, T. 1964. Prenatal development of ear plug in baleen whales. Sci. Rep. Whales Res. Inst. 18:29-48.
Kato, H. 1992. Body length, reproduction and stock separation of minke whales off northern Japan. Rep. Int.
Whal. Commn. 42: 443-453.
Kanda, N., Pastene, L.A. and Hatanaka, H. 2010. Length composition and sex ratio of western North Pacificminke whales and their consistencies with stock structure hypotheses. Paper SC/D10/NPM7 presented
to the First Intersessional Workshop of western North Pacific minke whale Implementation, December
2010 (unpublished). 12pp.
Kitakado, T., Lockyer, C. and Punt, E. A. 2013. A statistical model for quantifying age reading errors and its
application to the Antarctic Minke whales. J. Cetacean Res. Manage. 13(3): 181-190.
Larsen, F. and Kapel, F. O. 1983. Further biological studies of the West Greenland minke whale. Rep. Int. Whal.
Commn. 33: 329-338.
Lillie, D. G. 1910. Observation on the anatomy and general biology of some of members of the lager cetacea.
Proc. Zool. Soc. Lond. 769-792.
Lockyer, C. 1972. The age at sexual maturity of the southern fin whale (Balaenoptera physalus) using annual
layer counts in the ear plug. J. Cons. Int. Explor. Mer. 34: 276-294.
Lockyer, C. 1984a. Age determination by means of the ear plug 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. and Kato, H. 2012. Technical improvements of the age character (earplug) collections in common
minke whales. Bull. Jpn. Soc. Fish. Oceanogr. 76(2):59-65. (Japanese)
Maeda, H., Kawamoto, T. and Kato H. 2013. A study on the improvement of age estimation in common minke
whales using the method of gelatinized extraction of earplug. NAMMCO Scientific Publications 10:
17pp.
Mitchell, E. and Kozicki, V. M. 1975. Supplementary information on minke whale (Balaenoptera
acutorostrata) from Newfoundland fishery. J. Fish. Res. Bd. Can. 32 (7): 985-94
Nielsen, N.H., Garde, E., Heide-Jørgensen, M.P., Lockyer, C.H., Ditlevsen, S., Òlafsdóttir, D., and Hansen, S.H.
2012. Application of a novel method for age estimation of a baleen whale and a porpoise. Mar. Mamm.
Sci. 29: E1-E23.
Ohsumi, S. 1964. Examination on age determination of the fin whales. Sci. Rep. Whales Res. Inst. 18:49-88.
Pastene, L.A., Goto, M., Taguchi, M. and Kitakado, T. 2016. Temporal and spatial distribution of the ‘J’ and ‘O’stocks of common minke whale in waters around Japan based on microsatellite DNA. Paper
SC/F16/JR38 presented to the JARPNII special permit expert panel review workshop, Tokyo, February
2016 (unpublished). 14pp.
Purves, P.E. 1955. The wax plug in the external auditory meatus of the Mysticeti. Discovery Rep. 27:293-302.
Roe, H.S.J. 1967. Seasonal formation of laminae in the ear plug of the fin whale. Discovery Rep. 35:1-30.
Sergeant, D. 1963. Minke whale, Balaenoptera acutorostrata lacepede, of the Western North Atlantic. J. Fish.
Res. Board Can. 20: 1489-1504.
Zenitani, R., Fujise, Y., Kawahara, S. and Kato, H. 2002. Examination of the distribution and reproductive
status of western North Pacific minke whales collected in sub-areas 7, 8 and 9 during JARPN and
JARPN II from 1994 to 2001. Paper SC/J02/NP12 presented to the IWC/SC North Pacific minke whale
RMP/IST meeting, January 2002 (unpublished). 9pp.

	
  

4	
  
	
  

TABLES
Table 1
Age readability of common minke whale collected by JARPN and JARPNII surveys from 1994 to 2013 by sex
and maturity status.
Sex

Number of whales

Number of whales withreadable earplugs

Readability (%)

Total

Male
Female

1888
684

853
282

45.2
41.2

Immature

Male
Female

548
501

194
174

35.4
34.7

MatureMale
Female

1340
183

659
108

49.2
59.0

2572

1135

44.1

Total

Table 2
Result of the age reading error analysis based on Kitakado et al. (2013). The expected age-reading outcomes of
the control and unbiased reader (Bando) was assumed to be bL1 = 0, bH 1 = 50 for minimum and maximum ages 0
and 50 respectively, and those for the tested reader (Maeda), bL 2 , bH 2 , were estimated as the table below. The
estimates and associated SEs for the standard deviation for the two readers were also given in the following
table.

5	  	
  FIGURES

Figure 1. Bisected surface of an earplug of a common minke whale. a: Outer covering, b: Core. Scale bar: 5mmFigure 2. Bisected surface of an earplug of a common minke whale. A: 4 years old, B:18 years old, C:28 years
old. A scale indicates 0.5 mm.

6	
  
	
  

Figure 3. Age readability of common minke whales collected by JARPN and JARPNII (1994 to 2013) surveys
by body length class (upper; all combined, middle; O stock, bottom; J stock).

Figure 4. Scatter plots of age estimates of common minke whales from Bando against Maeda on 1st trial (read150 samples). The dashed line shows a 45 degree line. Solid line shows regression line.

7	
  
	
  

Figure 5. Scatter plots of age estimates of common minke whales from Bando’s age-reading data for three
trials. The solid line shows a 45 degree line.

Figure 6. Scatter plots of age estimates of common minke whales from Maeda’s age-reading data for three
trials. The solid line shows a 45 degree line.

Figure 7. Expected age estimates (left) for the control reader (Bando: solid line) and Maeda (dotted line) if thecontrol reader is assumed to be unbiased. Right: standard deviations of both readers.

8	
  
	
  

Figure 8. Age distributions of common minke whales collected during JARPN and JARPNII (1994-2013)
surveys (upper; all combined, middle; O stock, bottom; J stock).

Figure 9. Age distributions of JARPN and JARPNII (1994-2013) samples (NP; offshore, NPCS; coastal area
off Sanriku, NPCK; coastal area off Kushiro, by sex).

9	
  
	
  

Figure 10. Body length distributions of common minke whales collected during JARPN and JARPNII (19942013) surveys (upper; all combined, middle; O stock, bottom; J stock).

Figure 11. Body length distributions of JARPN and JARPNII (1994-2013) surveys (NP; offshore, NPCS;
coastal area off Sanriku, NPCK; coastal area off Kushiro).

10