SC/F14/J09

Yearly trend in the proportion of pregnant animals
among mature female Antarctic minke whales in the
JARPA and JARPAII period
Takeharu Bando and Takashi Hakamada
Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
Contact e-mail: bando@cetacean.jp
ABSTRACTThe proportion of pregnant animals among mature females (PPF) in Antarctic minke whales
was examined from samples collected during 1987/88-2004/05 JARPA and 2005/06-2010/11
JARPAII surveys. Analysis was conducted for two biological stocks which were separated at
165ºE. The PPF of both stocks appeared stable at around 0.9 and when data from all years werecombined, the PPF was 0.932 for I-stock and 0.904 for P-stock. Linear regression analysis
showed no significant yearly trend in either stock during the JARPAII period. From logistic
regression analysis, no significant trend was detected during the JARPAII period but asignificant increasing trend was detected for P-stock during the JARPA and all period. Thislatter result was influenced by just two lower PPF values in the 1990/91 and 1994/95 seasons.
Although a high PPF was observed throughout the survey period, monitoring of PPF isimportant to detect possible change in nutritional condition and reproduction which affectsustainable management of these whale stocks.
KEYWORDS: ANTARCTIC MINKE WHALE, PREGNANCY RATE, TRENDS
INTRODUCTION
Pregnancy rate is necessary to estimate composition of young whales which recruit into the stock, which
is an important parameter to evaluate reproductive condition of stock. Pregnancy rates of large baleen
whales were known to change in response to changes in abundance, food availability or competition withother species (Gambell, 1973; Kato, 1986a; 1986b; Masaki, 1979; Lockyer, 1984). Therefore, monitoringof pregnancy rate is important for sustainable management of baleen whale stocks.
Pregnancy rate in the Antarctic minke whale had been studied mainly from commercial whaling data by
several authors (Best, 1982; Kato, 1982; Masaki, 1979; Ohsumi et al., 1970; Ohsumi and Masaki, 1975).
Ohsumi et al. (1970) reported the proportion of pregnant animals among mature females (PPF) from
1967/68 commercial whaling data as 0.789 (n=114), and this value becomes 0.895 if individuals that had
corpus luteum but no fetus were considered as pregnant. Kato (1986a) examined apparent pregnancy ratefrom 1971/72 to 1982/83 commercial whaling data and reported no significant yearly trend during thisperiod and a mean value of 0.895. The first phase of JARPA was conducted from 1987/88 to 2004/05 anddata on pregnancy status were continuously collected during the survey period. Results were reported to
the JARPA review meeting held at Tokyo in 2006, which showed no yearly trend for two newly
identified stocks. The PPF was estimated as 0.929 and 0.854 for I-stock and P-stock, respectively (Bandoet al., 2006).
JARPA II started in 2005/06 and new data from six years surveys were accumulated. In this study,
yearly trend of PPF in Antarctic minke whale was examined for the hypothesized two stocks (I-stock andP-stock) for JARPA period, JARPA II period and both periods combined.
MATERIALS AND METHODS
Biological samples and data used
All of the mature female Antarctic minke whale samples collected during JARPA (1987/88 to 2004/05,

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n=2,045) and JARPAII (2005/06 to 2010/11, n=1,198) surveys were used for the analysis (Table 1).
Proportion of pregnant animals among the mature females (PPF)
The proportion of pregnant animals among the mature females is defined as the proportion of pregnant
females within the sample of sexually mature females. Sexual maturity for females was determined by thepresence of corpora luteum or albicans in both ovaries.
Stock identification
Whales collected in Areas IIIE, IV and VW were treated as the ‘Eastern Indian Ocean Stock’ (I-stock)and those collected in Areas VE and VIW were treated as the ‘Western South Pacific Stock’ (P-stock),following Pastene (2006). It should be noted here that the stock structure hypothesis has been refinedrecently (e.g. Kitakado et al., 2014) and that new grouping for estimating PPF would be necessary infuture.
Statistical method
Two statistical methods were applied to examine yearly trend of PPF. Firstly, linear regression analysis
was applied to catch year of JARPAII (2005/06 to 2010/11) and PPF following Government of Japan(2005). The null hypothesis was set that the slope = 0 (H0) to examine whether the null hypothesis can berejected at 5% level. Secondly, logistic regression analysis was conducted to examine yearly trend of PPF
during JARPA, JARPAII and during all period, respectively.
RESULTS
The PPF of I-stock whales was stable at around 0.9 throughout survey period and no significant yearly
trend was detected during JARPAII by linear regression analysis (Figure 1, Tables 1, 2). No significant
yearly trend was detected by logistic regression analysis during JARPA, JARPAII and all period (Table 3).
When data from all years was combined, the PPF was calculated as 0.932.
For P-stock whales, the PPF was stable at around 0.9 but a slightly lower value was observed in 1990/91(PPF= 0.770) and 1994/95 (PPF= 0.740) (Figure 1, Table 1). No significant yearly trend was detectedduring JARPAII by either linear or logistic regression analysis but a significant increasing trend was
detected during the JARPA and all period by logistic regression analysis (Table 2, 3). When data from all
years was combined, the PPF was calculated as 0.904.
DISCUSSION
Full scale exploitation of Antarctic minke whale started at 1971/72 and the reported apparent pregnancy
rate was stable at a high value throughout the commercial whaling period (Kato, 1982; 1986a; Masaki,
1979; Ohsumi et al., 1970; Ohsumi and Masaki, 1975; Zenitani, et al., 2001). A high PPF was also
observed during the JARPA and JARPAII periods in this study. A significant increasing trend was
detected in P-stock but this is attributed to lower PPF observed in just two year in the early period of
JARPA (1990/91 and 1994/95). Therefore this ‘significant’ increasing trend result should be seen withcaution.
The observed high PPF during the commercial whaling period and the JARPA and JARPAII periods
means that Antarctic minke whale stocks have maintained a good reproductive condition from the 1970s.
Age at sexual maturity of minke whale was reported to have decreased from 10-12 in the 1940s cohorts to
7-8 in the 1970s cohorts and stabilized until the 1990s cohorts (Kato, 1983; Zenitani and Kato, 2006;
Bando et al., 2014). The decreasing trend from the 1940’s seems to be associated with improved
nutritional conditions from the mid of the past century.
On the other hand, the increasing trend of abundance in large baleen whale species such as blue,
humpback and fin whales have been reported in recent years (Branch, 2011; Hakamada and Matsuoka,,
2014; Matsuoka et al., 2011; Matsuoka and Hakamada, 2014). Antarctic minke whales utilize krill as theirfood species and are thought to compete with other baleen whales. Therefore, an increase in theabundance of other whale species might have an adverse effect on the nutritional condition of Antarctic
minke whales. Blubber thickness and stomach content weight of Antarctic minke whales have been

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reported to be decreasing during the JARPA/JARPAII period (Konishi et al., 2008; Konishi and Walløe,
2014a; 2014b), which might indicate possible changes in the feeding environment of minke whales.
It is known that the PPF of Antarctic minke whale might be biased by segregation or date of sampling(Kato, 1986b; Ohsumi and Masaki, 1975). Whales that conceive earlier in the breeding season tend to
migrate to Antarctic feeding areas earlier (Kato and Miyashita, 1991; Kato, 1995). Therefore, theobserved PPF in this study might be biased upward from the true pregnancy rate, which was estimated as
0.78 from the commercial whaling data in the breeding area (Best, 1982). However, the PPF would reflectmigration strategy, distribution pattern and feeding environment of Antarctic minke whales and could be
an important parameter for monitoring of this stock.
Although continuation of the high PPF was observed during the JARPA II period, monitoring of the PPF
is important to detect possible change in nutritional condition and reproduction which affect sustainable
management of these whale stocks.
ACKNOWLEDGEMENTS
Our sincere thanks are due to all researchers and crews who participated in the JARPA and JARPA II
surveys. We thank Hiroshi Hatanaka and Luis A. Pastene, of the Institute of Cetacean Research, for usefulcomment on this manuscript.
REFERENCES
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presented to the JARPA Review workshop, Tokyo, 4-8 December 2006. 28pp.
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whales examined by transition phase in earplugs collected during JARPA and JARPA II surveys.
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Branch, T. A. 2011. Humpback whale abundance south of 60ﾟS from three complete circumpolar sets ofsurveys. J. Cetacean Res. Manage. (Special Issue 3): 53-69.
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special permit in the Antarctic (JARPA II) -Monitoring of the Antarctic ecosystem anddevelopment of new management objectives for whale resources. Paper SC/57/O1 presented to the
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whale in Areas IIIE-VIW, south of 60 º S, based on JARPA and JARPAII sighting data
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935-945.
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segregation by genetics and morphometrics in Antarctic minke whales. Paper SC/F14/J29
presented to this meeting.
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in the Antarctic minke whale Balaenoptera bonaerensis in Southern Ocean. Polar Biol. 31:
1509-1520.
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during the JARPA and JARPAII research periods. Paper SC/F14/J13 presented to this meeting.
Konishi, K. and Walløe, L. 2014b. Time trends in the stomach contents weight in the Antarctic minke
whales during the JARPA and JARPAII research periods. Paper SC/F14/J14 presented to thismeeting.
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Whal. Commn 29: 375-395.
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data (1989/90-2008/09). Paper SC/F14/J5 presented to this meeting.
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Inst. 22: 75-125.
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population level. J. Fish. Res. Board Can. 32: 995-1004.
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 the JARPA Review workshop,
Tokyo, 4-8 December 2006. 24pp.
Zenitani, R., Kato, H. and Fujise, Y. 2001. Year to year trends of some biological parameters of Antarctic
minke whales from the view point of population monitoring. Paper SC/53/IA13 presented to the
IWC Scientific Committee, 2001(unpublished). 16pp.
Zenitani, R. and Kato, H. 2006. 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.
Paper SC/D06/J15 presented to the JARPA Review workshop, Tokyo, 4-8 December 2006. 9pp.

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1.0

PPF

0.8
0.6
0.4
0.2

2010/11

2009/10

2008/09

2007/08

2006/07

2005/06

2004/05

2003/04

2002/03

2001/02

2000/01

1999/00

1998/99

1997/98

1996/97

1995/96

1994/95

1993/94

1992/93

1991/92

1990/91

1989/90

1988/89

I-stock
1987/88

0.0

1.0

PPF

0.8
0.6
0.4
0.2

2010/11

2009/10

2008/09

2007/08

2006/07

2005/06

2004/05

2003/04

2002/03

2001/02

2000/01

1999/00

1998/99

1997/98

1996/97

1995/96

1994/95

1993/94

1992/93

1991/92

1990/91

1989/90

1988/89

P-stock
1987/88

0.0

Figure 1. Yearly trend of PPF in I-stock and P-stock Antarctic minke whales collected during JARPA and
JARPA II surveys. Linear regression line was applied for both stocks during JARPAII (2005/06 to
2010/11) periods.
Table 1. Number of pregnant, not pregnant matured females and PPF in I-stock and P-stock Antarctic
minke whales.
I-stock
Number of matured
Not
Pregnant
pregnant
female1987/88
57
1
58
1988/89
1989/9080
585
1990/9151
5
56
1991/92
72
10
82
1992/9364
1
65
1993/9462
6
68
1994/95
14
0
14
1995/96
96
8
104
JARPA
1996/9754
3
57
1997/9843
3
46
1998/9956
662
1999/00
106
6
112
2000/01
23
3
26
2001/02141
9150
2002/0335
1
36
2003/04154
15
169
2004/0539
4
43
2005/0622615
241
2006/07
JARPA 2007/08
168
13
181
II
2008/0930
131
2009/10184
13
197
2010/11
Total
1755
128
1883
PeriodSeasonPPF

P-stock
Not
Number of matured
Pregnant
pregnant
female

PPF

0.983
0.941
0.911
0.878
0.985
0.912
1.000
0.923
0.947
0.935
0.903
0.946
0.885
0.940
0.972
0.911
0.907
0.938
0.928
0.968
0.934
0.932

5

96

9

105

0.914

57

17

74

0.770

54

2

56

0.964

54

19

73

0.740

112

19

131

0.855

16

2

18

0.889

94

6

100

0.940

92

3

95

0.968

144

16

160

0.900

262

24

286

0.916

162

7

169

0.959

87
1230

6
130

93
1360

0.935
0.904

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Table 2. Result of linear regression analysis for both stocks of Antarctic minke whales during JARPA II
period.

I-stock
value
Intercept
0.916
Coefficient 0.00115

P-stock
SE
0.162
0.008

t
5.647
0.151

p-value
0.030
0.894

value
Intercept
0.830
Coefficient 0.00485

SE
0.209
0.009

t
3.974
0.512

p-value
0.157
0.699

Table 3. Result of logistic regression analysis for both stocks of Antarctic minke whales during JARPA,
JARPA II and all period.
I-stock: JARPA
Estimate Std. Error
Intercept
35.48644.153Year-0.0160.022

z value
0.804-0.745

Pr(>|z|)
0.422
0.456

P-stock: JARPA
Estimate Std. Error
Intercept -89.238
39.380
Year
0.046
0.020

z value-2.2662.317

Pr(>|z|)0.023 *
0.021 *

I-stock: JARPA II
Estimate Std. Error
Intercept
15.427193.561Year-0.006
0.096

z value
0.080-0.066

Pr(>|z|)
0.936
0.947

P-stock: JARPA II
Estimate Std. Error
Intercept -264.655 243.264
Year
0.133
0.121

z value-1.0881.099

Pr(>|z|)
0.277
0.272

I-stock: JARPA+JARPA II
Estimate Std. Error
Intercept
7.396
28.183Year-0.0020.014

z value
0.262-0.170

Pr(>|z|)
0.793
0.865

P-stock: JARPA+JARPA II
Estimate Std. Error
Intercept -98.810
25.960
Year
0.051
0.013

z value-3.806
3.891

Pr(>|z|)0.000 *
0.000 *

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