SC/D06/J15
Temporal trend of age at sexual maturity of Antarctic minke whales based on transition
phase in earplugs obtained under JARPA surveys from 1987/88-2004/05
Ryoko Zenitani.*and Hidehiro Kato***: The Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan.
**: Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-Ku, Tokyo 108-8477,
JapanABSTRACT
Using transition phase (TP) and age readings from a total of 2,803 individuals collected in
1987/88 to 2004/05 special permit cruise under JARPA research program, the present study
examined temporal trend of age at sexual maturity of the Antarctic minke whales following by thenew stock boundary for I (Eastern Indian Ocean Stock) and P (Western South Pacific Ocean stock)stocks. Through analyses the present study confirmed again the similarly decline of age at sexual
maturity of Antarctic minke whale from 1940s cohorts to late of 1960s cohorts which wasidentified by previous studies; the mean age at sexual maturity of both stocks declined from themid 1940s cohort or the mid 1950s cohorts at around 10-12 years to the late 1960s cohorts or theearly 1970s cohorts at around 7 years with statistical significance at 5% level. However, it was
newly identified that mean age at sexual maturity for both stocks and sexes until the early 1970scohorts no longer declined and remained constant at 7-8 years or slightly increased in the early
1970s cohorts onward to the early 1990s cohorts. These temporal trends represent changes innutritional condition for Antarctic minke whales.
KEYWORDS: ANTARCTIC MINKE WHALE, AGE AT SEXUAL MATURITY, TRENDINTRODUCTION
One of the objectives of the Japanese Whale Research Program under Special Permit in theAntarctic (JARPA) is ‘estimation of the biological parameters to improve the stock managementof Antarctic minke whales’. After the moratorium on commercial whaling, the JARPA has beencollecting data in order to achieve the stated objective. There are some methods of estimation ofage at sexual maturity. Among which age at sexual maturity estimated from transition phase inearplug data (tmp) and mean age at first ovulation (tmov) are considered free from biases due tothe reproductive segregation and the catch selectivity (Kato, 1987).
It is generally known that transition phase in earplug indicated the age at sexual maturity (Lockyer,1972; Masaki, 1979; Kato, 1983a). The indirect indicator of the age at sexual maturity of baleen whales
was discovered by Lockyer (1972). Several authors have reported a decline of Antarctic minke whales age
at sexual maturity from 14 years in 1940s cohort to 6 years in late 1960s cohort from the analysis oftransition phase in earplugs collected commercial whaling (Masaki, 1979; Best 1982; Kato 1983b, 1985). Itwas considered that the changes of biological parameters estimated such as decline of age at sexual
maturity and increase of the growth rate using commercial data in the Antarctic minke whales, were causedby reduction of blue and fin whales populations which are in strong competition with minke whales, which
led to greater availability of food for minke whales (Masaki, 1979; Best, 1982; Kato, 1983b, 1985, 1987,
Kato and Sakuramoto, 1991).
However, Cooke and de la Mare (1983) argued that the observed decline trend of the age at sexual
maturity was equally explainable as a result of aging error and truncation sampling problem in the data set
used. Kato (1987) and Kato and Sakuramoto (1991) examined the possible biases affecting the estimationof age at sexual maturity and growth curve. They concluded and argued that the observed decline in age atsexual maturity in the mid 1940s (12-13 years) to the early 1970s (7-8 years) cohorts is real because it isfree from possible truncation biases (Kato, 1987; Kato and Sakuramoto, 1991). Although many studies
1have been carried out in this field, nevertheless, IWC/SC is still not able to reach a general consensus on
the decline of the age at sexual maturity of the Antarctic minke whales (IWC, 1985). Only agreement isreached at the mid-term JARPA review meeting in 1997 in which the Working Group agreed that there had
been a real decline in age at sexual maturity of minke whales (Thomson et al., 1997; Cooke et al., 1997).
In January 2005, JARPA review meeting was hosted by the government of JAPAN, weshowed long-term changes in age at sexual maturity of Antarctic minke whales from 1940s cohort to1980s cohort in Area IV by using transition phase data (Zenitani and Kato, 2005). The present study
examines a temporal trend in age at sexual maturity of Antarctic minke whales by the new stockboundary for I (Eastern Indian Ocean Stock) and P (Western South Pacific Ocean stock) stocks
which are revealed from several analyses using JARPA data (Pastene, 2006).
MATERIALS AND METHODS
Biological data used
The present study used a total of 2,803 age and transition phase (TP: age at transition phase layer)
data collected in 1987/88-2004/05 JARPA in Area IIIE, IV, V and VIW. Furthermore, data of
the age at first ovulation in 1987/88-2004/05 JARPA in Area IIIE, IV, V and VIW were also used.
Data in Areas IIIE, IV and VW for I-stock and data in Areas VE and VIW for P-stock which are
newly identified biological stocks were used (Pastene, 2006).
Age determination
Age of whales was determined by reading growth layers appearing on the bisected surface of theearplug, assuming an annual deposition of growth layers (i.e. one pair of dark and pale laminae
accumulated per year) in accordance with Best (1982) and Lockyer (1984). Individual ages from1987/88-1989/90 and 1992/93 JARPA data were determined using growth layers in earplug thatwere mainly counted by reader-K. Individual ages of 1990/91 and 1993/94-2004/05 JARPA weredetermined using growth layers in earplug that were counted by reader-Z. The age at transition
layer was recorded as if it was present.
Cohort (= year of birth)
Cohorts (Year classes) were defined by; Cohort = (starting year of season) – (age at capture).
Age at sexual maturity“Tmp” (Kato, 1987): Mean age at sexual maturity estimated for both sexes using transition phase in earplug
as an indicator of sexual maturity (Lockyer, 1972; Kato, 1983a; Ohsumi, 1986).
“Tmov” (Kato, 1987): Mean age at the first ovulation is defined as mean age of females identified
by the presence of a corpus luteum and no corpus albicansin both ovaries.
Statistical analyses
When declining, constant or increasing trend was observed in the range of cohorts, we fitted a regression
line and examined whether it was significantly different from zero or not at 5 % level.
Corrected truncation bias
We corrected truncation bias using the same method of Kato (1985). To estimate tmp we only usedTP data for animals older than following ages in cohort groups; 36 years (1951-55), 29 years(1956-60), 25 years (1961-65), 19 years (1966-70), 15 years (1971-75), 12 years (1976-80), 13years (1981-85) and 13 years (1986-95).
RESULTS
Evaluation of two readers
Individual ages were determined using growth layers in earplug that were counted by reader-K for
samples collected 1987/88-1989/90 and 1992/93 and by reader-Z for others. We examine
difference of tmp between two readers. Fig. 1 shows tmp against cohort based on TP data for each sexand stock. The temporal trends of two readers both sexes were similar and tmp values of two readers almost
coincide with each other. No differences of temporal trend of tmp were found between two readers bycohort.
2

Examination of among sampling years
In order to examine that there is no truncation of sampling problem in sampling years, tmp plots against
cohort based on crude data: three groups of sampling years (1987/88-1992/93, 1993/94-1998/99, and1999/00-2004/05) were examined. Fig. 2 shows tmp against cohort based on TP data by sampling year
groups, sex and stocks. The temporal trends of four sampling groups of both sexes and stocks were similar.
The tmp values by cohort almost coincide to different sampling years. No differences of temporal trend of
tmp were found among three sampling years.
Temporal trend of mean age at sexual maturity by transition phase
As described above, no differences of temporal trend of tmp were found between two readers and amongfour sampling years. We estimated tmp using all TP data combined two readers and four samplinggroups. Fig.3 shows tmp against cohort based on TP data by sex and stocks.
In I-stock, tmp of male declined from the mid 1940s cohorts at around 12 years to the late1960s cohort at around 7 years. The decline of tmp had been stopped in the late 1960s cohorts tothe early 1970s cohorts, it was found that the mean age remained constant at 7-8 years or slightly
increased in the early 1970s cohorts to the early 1990s cohorts. The tmp of female declined fromthe early 1950s cohorts at around 12 years to the late 1960s at around 7 years. From the early
1970s cohorts to the early 1990s cohorts the temporal trend of tmp similar to the male was found.
In P-stock, tmp of male declined from half of 1950s cohorts at around 10 years to the late1960s at around 7 years. The decline of tmp had been stopped in the late 1960s cohorts to the early
1970s cohorts, it was found that the mean age remained constant at 7-8 years or slightly increasedin the early 1970s to the early 1990s cohorts. The tmp of female declined from the early 1955
cohorts at around 11 years to the late 1960s at around 7 years. From the early 1970s to 1990s
cohorts, the temporal trend of tmp similar to the male was found.
We fitted regression line in cohort range and the regressions of tmp weighted by sample size for both
sexes and stocks are expressed by the following formulae;
I-stock

P-stock

male: tmp＝−0.207×cohort＋415.410（1946 cohorts-1970 cohorts）
tmp＝ 0.012×cohort−17.167 （1970 cohorts-1991 cohorts）
female: tmp＝−0.256×cohort＋509.558（1952 cohorts-1968 cohorts）
tmp＝ 0.043×cohort−78.068 （1968cohorts-1991cohorts）
male: tmp＝−0.177×cohort＋356.570（1955 cohorts-1971 cohorts）
tmp＝ 0.020×cohort−31.793 （1971 cohorts-1991 cohorts）
female : tmp＝−0.256×cohort＋510.519（1957 cohort-1969 cohort）
tmp＝ 0.008×cohort−8.956 （1969 cohorts-1991cohorts）

All slopes in cohort range at declining trend were significantly different from zero at 5 % level (both sexesand both stocks: p<0.001). In cohort range at constant or slightly increase trend, except for a slope offemale in P-stock other slops were significantly different from zero (I-stock male: p<0.001, I-stock female:p<0.001, P-stock male: p=0.028, P-stock female: p=0.064).
DISCUSSION
The present analysis revealed a temporal trend of age at sexual maturity in I and P stocks in theAntarctic minke whales from the mid 1940s cohorts to the early 1990s cohorts using TP data
accumulated by JARPA surveys. Several authors have reported a decline of age at sexual maturity fromthe 1940s cohorts to the late 1960s cohorts using transition phase earplugs in Antarctic minke whales
collected during commercial whaling (Masaki, 1979; Best, 1982; Kato, 1983b, 1985, 1987, Kato andSakuramoto, 1991, Thomson et al., 1997; Cooke et al., 1997). Through the present analysis
incorporating bias correction confirmed again similarly declined of tmp of Antarctic minke whalefrom 1940s cohorts to late of 1960s cohorts which was identified by previous studies. The tmp of I
and P stocks declined from the mid 1940s cohorts or the mid 1950s cohorts at around 10-12 years
to the late 1960s cohorts or the early 1970s cohorts at around 7 years with statistical significance
at 5% level. However, it was newly identified that tmp for both stocks and sexes until the early
1970s cohorts no longer declined and remained constant at 7-8 years or slightly increased in the3early 1970s cohorts onward to the early 1990s cohorts. Except for one trend, these slightly
increasing trends in recent cohorts are statistical significant at 5 % level.
The tmov and tmp are considered to be free from the segregation and catch selectivity (Kato, 1987).
Although the number of first ovulation data is very small, the values of tmov in both stocks are similar totmp and remain almost constant from the 1978 cohort to the 1995 cohort. The trend of remaining
constant of tmp is supported by direct estimates from tmov (shown in Fig. 4).
Kato (1983b, 1987) argued that one possible mechanism for decline of age at sexual maturity in theAntarctic minke whales is the result of an increased growth rate by improvements of nutrition in theadolescent growth spurt stage due to the increase of food availability per capita in the Antarctic
feeding ground. However, the present analysis found that the decline of tmp until the early 1970s
cohort no longer continued. The constant or slightly increased trend of tmp in the recent cohortsuggest the nutritional condition for Antarctic minke whales to support their decline of age at sexual
maturity have shifted to condition not to support decline trend of tmp. It is consider that these temporal
trends of tmp represent change in nutritional condition for Antarctic minke whales.
Thus, the age at sexual maturity can understand for stock condition therefore the age at sexual
maturity can be useful indicator in monitoring for population level. Continuous monitoring of the age atsexual maturity can contribute improvement the stock management of the Antarctic minke whales.
ACKNOWLEDGMENTS
We sincerely thank Dr. Hiroshi Hatanaka, Dr. Yoshihiro Fujise and Dr. Kayo Ohmagari of the Institute ofCetacean Research (ICR) for useful comments and suggestion in preparing this paper. We also thank all
research leaders, researchers, Captain and crewmembers contributed to JARPA surveys.
REFERENCES
Best, P.B. 1982. Seasonal abundance, feeding, reproduction, age and growth in minke whales off Durban(with incidental observation from the Antarctic). Rep. int. Whal. Commn 32: 759-86.
Cooke, J.G and de la Mare. 1983. The effects of variability in age data on the estimation of biologicalparameters of minke whales (Balaenoptera acutorostrata). Rep. int. Whal. Commn 33: 333-38.
Cooke, J.G., Fujise, Y. and Kato, H. 1997. Analysis of maturity stage and transition phase data from minke
whales collected during JARPA in Area IV, 1987/8/8 through 1995/96. Paper SC/M97/22
submitted ton the Intercessional working group to review data and results from special permit
research on minke whales in the Antarctic, 16pp, 1997.
International Whaling Commission, 1985. Report of the Scientific Committede. Rep. int. Whal. Commn 35:
31-94.
Kato, H. 1983a. Relation of earplug growth and skull growth in the Antarctic minke whale. Paper
SC/A83/AW5 presented to the IWC Minke Whale Ageing Workshop, April 1983.
Kato, H. 1983b. Some Consideration on the Decline in Age at Sexual Maturity of the Antarctic Minke
Whale. Rep. int. Whal. Commn 33: 393-99.
Kato, H. 1985. Further examination of the age sexual maturity of the Antarctic minke whale as determinedfrom earplug studies. Rep. int. Whal. Commn 35: 273-7.
Kato, H. 1987. Density dependent changes in growth parameters of the southern minke whale. Sci. Rep.
Whales Res. Inst., Tokyo 38: 47-73.
Kato, H. and Sakuramoto, K. 1991. Age at sexual maturity of southern minke whales: A review and some
additional analyses. Rep. int. Whal. Commn 41: 331-37.
Lockyer, C. 1972. The age at sexual maturity of southern fin whale (Balanenoptera physalus) using annual
layer counts in the earplug. J. CIEM, 34, 276-94.
Lockyer, C. 1984. Review of baleen whale (Mysticeti) reproduction and implications for management. Rep.
int. Whal. Commn (special issue 6): 27-50.
Masaki, Y. 1979. Yearly change of the biological parameters for the Antarctic minke whale. Rep. int. Whal.
Commn 29: 375-95.
Ohsumi, S., 1986. Earplug transition phase as an indicator of sexual maturity in female Antarctic minkewhales. Sci. Rep. Whales Res. Inst., 37: 17-30.
Pastene, L. A. 2006. What do we know about the stock structure of the Antarctic minke whale? Asummary of studies and hypotheses. Paper SC/D06/J12 presented to this meeting.
Thomson, R.B., Butterworth, D.S. and Kato, H. 1999. Has the age at trnsition of Southern Hemisphere4minke whales declined over recent decreases? Marine Mammal Science. 15(3): 661-82.
Zenitani, R. and Kato, H. 2005. Long-term trend of age at sexual maturity of Antarctic minke whales bycounting transition phase in earplugs. Paper JA/J05/JR7 presented to the JARPA Review meeting,January 2005.

5I-stock16
Mean age at sexual maturity

14Male

12
10
8
64
2

Reader-K (n=171)
Reader-Z (n=795)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Cohort16
Mean age at sexual maturity

14Female

12108642Reader-K (n=89)
Reader-Z (n=503)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortP-stock
16
Mean age at sexual maturity

14Male1210864Reader-K (n=32)
Reader-Z (n=386)

20
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Cohort16
Mean age at sexual maturity

14Female

1210864Reader-K (n=72)
Reader-Z (n=366)

20
1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortFig.1. A plot of mean age at sexual maturity against cohorts based ontransition phase data of both stocks separated by two age readers.

6I-stock16
Mean age at sexual maturity

14Male12108642

1987-1992 (n=249)
1993-1998 (n=396)
1999-2004 (n=321)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Cohort16
Mean age at sexual maturity

14Female

12108642

1987-1992 (n=136)
1993-1998 (n=177)
1999-2004 (n=279)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortP-stock
16
Mean age at sexual maturity

14Male12108642

1987-1992 (n=51)
1993-1998 (n=173)
1999-2004 (n=194)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Cohort16
Mean age at sexual maturity

14Female

12108642

1987-1992 (n=106)
1993-1998 (n=137)
1999-2004 (n=195)

01930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortFig.2. A plot of mean age at sexual maturity against cohorts based on transition phase data
separated by 3 groups of sampling years each sex and stock.
7

I-stock16
Mean age of sexual maturity

14Male1210864201930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortMean age of sexual maturity1614Female

1210864201930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortP-stock
16Mean age of sexual maturity

14Male1210864201930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortMean age of sexual maturity

1614Female

1210864201930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
CohortFig.3. Temporal trend of mean age at sexual maturity derived from transition phasein earplug by cohort for each sex and stock.
Open circle is mean age and solid line is range of standard deviation.
8Mean age at sexual maturity

1614I-stock

1210864201965

Tm p
Tm ov

1970

1975

1980

Cohort

1985

1990

1995

2000

16
Mean age at sexual maturity

14P-stock121086420
1965

Tm p
Tm ov

1970

1975

1980

Cohort

1985

1990

1995

2000

Fig.4. Comparison of two kinds of estimates of age at sexual maturity for female in each stock.

9