SC/65b/IA02

Yearly trend in the proportion of pregnant animals
among mature female Antarctic minke whales in the
JARPA and JARPAII period
SC/65b/J9Rev
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
ABSTRACTThis paper is a revised version of SC/F14/J9 presented to the JARPAII Review Workshop. The
discussion of the paper was modified to take into consideration some recommendations from the
Review Workshop. The 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 whichwere separated at 165ºE. The PPF of both stocks appeared stable at around 0.9 and when datafrom all years were combined, 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 a significant 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 is important
to detect possible change in nutritional condition and reproduction which affect sustainable
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 isan 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 with otherspecies (Gambell, 1973; Kato, 1986a; 1986b; Masaki, 1979; Lockyer, 1984). Therefore, monitoring ofpregnancy 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 (Bando et 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 and P-stock)
for JARPA period, JARPA II period and both periods combined.
MATERIALS AND METHODS
Biological samples and data used1

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All of the mature female Antarctic minke whale samples collected during JARPA (1987/88 to 2004/05,
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) andthose 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 refined recently (e.g.
Kitakado et al., 2014) and that new grouping for estimating PPF would be necessary in future.
Statistical method
Two statistical methods were applied to examine yearly trend of PPF. Firstly, linear regression analysis wasapplied 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 be rejected
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 detected during
JARPAII by either linear or logistic regression analysis but a significant increasing trend was detectedduring 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 and1994/95). Therefore this ‘significant’ increasing trend result should be seen with caution.
The observed high PPF during the commercial whaling period and the JARPA and JARPAII periods meansthat Antarctic minke whale stocks have maintained a good reproductive condition from the 1970s. Age atsexual 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 etal., 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, humpbackand 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 their foodspecies and are thought to compete with other baleen whales. Therefore, an increase in the abundance ofother 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 reported to be
decreasing during the JARPA/JARPAII period (Konishi et al., 2008; Konishi and Walløe, 2014a; 2014b),2

SC/65b/J9Rev

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, the observed
PPF in this study might be biased upward from the true pregnancy rate, which was estimated as 0.78 fromthe commercial whaling data in the breeding area (Best, 1982). However, the PPF would reflect migration
strategy, distribution pattern and feeding environment of Antarctic minke whales and could be an importantparameter 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.
The JARPAII Review Workshop suggested the investigation of the limitations of the criteria used to
identify a pregnant female:(1) Further discussion of the limitations of the criteria used to identify a pregnant female – inparticular, better account should be taken of the fact that small foetuses may be missed by scientists
and that foetuses may be lost due to abortion during the hunt or loss during towing leading to anunderestimate of true pregnancy rate;(2) To assist with (1), the authors should consider examination of other criteria for pregnancy (insome cases this may be possible from existing data or tissue samples) including diameter of the largest
uterine horn, histology of the uterine mucosa, examination of the mammary glands and examinationof hormone levels in blubber.
JARPAII researchers on board the research base ship examine ovaries and uterus of all females to identify
sexual maturity and pregnancy status. Table 4 shows the number of females in each reproductive categorycollected during JARPAII surveys. The possibility that abortion could occur during hunt or towing cannot
be denied. However, only six individuals among 1,701 females (0.35%) were classified as “Unknown
mature” which means corpora lutea or albicantia exist in the ovary (only a part of ovary was collected insome case) but presence of fetus could not be confirmed due to damage by harpoon. There was no case inunbroken uterus that a part of umbilical code or placenta existed but no fetus was found, which could
indicate abortion. These results suggests that the case of abortion or loss during towing is rare. ‘Unknown
mature’ were excluded from the calculation of PPF.
Among 1,205 matured females collected during JARPAII, 19 individuals (1.6%) were classified as“ovulating”, which means corpora lutea exists in ovary but no fetus was found in uterus (Table 4). Although
the proportion is small, among the ‘ovulating’ females there is the possibility of occurrence of fetus thatwas not detected, which leads to an underestimation of true pregnancy rate. The Review Workshop
recommended the examination of four other criteria for identification of pregnancy. Examination of uterus
would be useful to identify pregnancy status (Lockyer and Smellie, 1985). Diameter of both uterine horn
were measured for all females during 1987/88 to 2004/05 JARPA survey and collection of uterine mucosa
was conducted during the same periods. Therefore, a feasibility study to identify the usefulness of thesedata and samples for identification of pregnancy can start within this year. Examination of the mammary
glands will be only applicable to late pregnancy because development of mammary gland will start at latestage of pregnancy (Laws, 1961; Lockyer, 1984). There is a possibility that early pregnant and ovulation
without fertilization could not be differentiated from the examination of hormone in blubber (Mansour etal., 2002; Kellar et al., 2013).
Although recommendations (1) and (2) will increase the accuracy of PPF, the authors note that the degreeof changes will be small (may be less than 1.6%) and therefore the conclusion of this paper (high PPF
throughout JARPA and JARPA II periods) will not change.
The Review Workshop noted that ‘in terms of effects at the population level the ultimate parameters ofinterest are successful births and calf survival rather than pregnancy rates per se’. In this context the3

SC/65b/J9Rev

Workshop provided some useful comments. For example it recommended that:(3) The Panel recommends that the feasibility of using histological examination of mammary glands
to assess recent lactation is examined. If this is possible, the proportions of recently lactated to[apparently] pregnant females and to mature females would be a more appropriate index of
reproductive success that would likely be more sensitive to environmental fluctuations than crude
pregnancy rates because perinatal mortality or abandonments of newborns by females in low nutritive
condition is known to occur in many mammal species.
The authors agree that the proportion of recently lactated to (apparently) pregnant females and to mature
females would be more appropriate index of reproduction. Although Antarctic is feeding grounds for
Antarctic minke whales, a trace of the secretion of milk might remain in the mammary gland(Chittleborough, 1958; van Utrecht, 1968). Measurement and collection of mammary gland was conducted
for all females during 1987/88 to 2010/11 JARPA/JARPAII surveys. The authors will start a feasibility
study to examine the possibility of recent lactation by observation of histological sample of mammary glandwithin 2014.
(4) Existing sampling protocols be examined to ensure that uterine horn and mammary glandmeasurements are recorded and mammary gland tissue and uterine mucosa are collected forhistological analysis
Data and samples of uterine and mammary gland collected during JARPA/JARPA II were as follows:
Measurement of uterine horn:
1987/88-2004/05
n=3,151
Collection of uterine mucosa:
1987/88-2004/05
n=3,151
Measurement of mammary gland:
1987/88-2010/11
n=4,852
Collection of mammary gland:
1987/88-2010/11
n=4,852
Data and samples were collected from all females irrespective of sexual maturity status. Measurement ofuterine horn and collection of uterine mucosa was not conducted after 2005/06 but these research items will
be resumed if the feasibility study showed effectiveness of these sample for identification of pregnancy.
(5) The programme takes the opportunity to examine the efficacy of approaches to examine pregnancy
using blubber analyses and faecal samples found for some other mysticetes (see Mansour et al., 2002;
and Kellar et al., 2013)
The authors agree that these approaches could be useful in other species, although PPF could not be
estimated by the blubber or faecal analysis because immature and resting (mature but not pregnant) could
not be differentiated by the hormone level analysis (Mansour et al., 2002; Kjeld, et al., 2004; Rolland, etal., 2005; Kellar et al., 2013). In addition, the authors have the opinion that this approach is of limited
practical utility for the case of the Antarctic minke whales because the collection of biopsies (for blubbersamples) (particularly in offshore areas where JARPAII operates) and faeces in the field is not feasible(Ohsumi et al., 2007).
(6) Given the proponents’ primary objective of studying biological parameters in order to monitorchanges in the ecosystem, the Panel recommends that the relationship between reproductive
parameters and the information available on body condition, krill consumption and environmental
covariates such sea surface temperature (SST), location, and other factors be examined on an annual
basis. This may help to explain anomalous years like the 1990-91 or the 1994-95 seasons, when low
apparent pregnancy rate values were observed.
The authors note that this recommendation is in the context of the integration which JARPAII researchers
have identified as a priority for the second part of JARPAII, and the authors will consider implementing
this recommendation with high priority.
ACKNOWLEDGEMENTS
Our sincere thanks are due to all researchers and crews who participated in the JARPA and JARPA II surveys.

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We thank Hiroshi Hatanaka and Luis A. Pastene, of the Institute of Cetacean Research, for useful commenton this manuscript.
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Inst. 22: 75-125.
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population level. J. Fish. Res. Board Can. 32: 995-1004.
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population research and their application in JARPA. Paper SC/59/O2 presented to the IWC
Scientific Committee, 2007 (unpublished). 12pp.
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Rolland, R. M., Hunt, K. E., Kraus, S. D. and Wasser, S. K. 2005. Assessing reproductive status of rightwhales (Eubalaena glacialis) using fecal hormone metabolites. Gen. Comp. Endocrin. 142: 308-317.
Van Utrecht, W. L. 1968. Notes on some aspects of the mammary glands in the fin whale, Balaenoptera
physalus (L), with regard to the criterion “lactating”. Norsk Hvalfangst-Tedende 57: 1-13.
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.
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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

I-stock
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

1987/88

0.0

1.0

PPF

0.8
0.6
0.4
0.2

P-stock
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

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
femalepregnant
1987/88
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
femalepregnant

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

7

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

P-stock

value
Intercept
0.916
Coefficient 0.00115

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 *

Table 4. Number of female Antarctic minke whales classified by sexual maturity and pregnancy status. PPF
was calculated as {(C)+(E)}/{(A)+(B)+(C)+(D)+(E)}FemaleYear
2005/06
2006/07
2007/08
2008/09
2009/102010/11
Total

Immature Ovulating Resting Pregnant Lactating
149
63
96
104
70
14
496

(A)
4
4
3
7
1
19

(B)
11
20
12
5
6
5
59

(C)
224
262
164
188
174
87
1099

8

(D)

1

1

Pregnant Unknown
Total& Lactating mature
(E)
(F)
3
391
2
351
4
1
278
4
304
10
2
269
1
108
21
6
1701