JA/J05/JR17

What has happened to the Antarctic minke whale stocks?
- A interprepation of results from JARPA YOSHIHIRO FUJISE, HIROSHI HATANAKA AND SEIJI OHSUMIThe Institute of Cetacean Research,4-5, Toyomi-cho, Chuo-ku, Tokyo 104-0055, JAPAN.
Contact e-mail: fujise@cetacean.jp

ABSTRACT
Historical changes in the Antarctic minke whale stocks were examined based on various results from
JARPA including age at sexual maturity, growth curve, blubber thickness, prey consumption, and
ADPT-VPA analysis of the stocks as well as results from research on mercury accumulation etc. It
has been assumed that feeding conditions of the minke whale improved with the removal of large
baleen whales such as the blue whale by commercial whaling, which promoted rapid growth andyounger age at sexual maturity; however, around 1970, conditions gradually shifted unfavourably,
resulting in slower rates of change in the foregoing parameters. These changes were then arrested by
the 1980s to the 1990s. Reflecting these unfavourable changes, it was observed that blubber
thickness and stomach content weight were reduced, which indicated less prey consumption. There
was also a decrease in the accumulation of mercury resulting from less prey consumption. Also, thedistribution area of humpback and fin whales in the feeding season expanded southward in the
Antarctic from around 1990, suggesting further deterioration of feeding conditions for the Antarctic
minke whales.
KEYWORDS: ANTARCTIC MINKE WHALE; BLUE WHALE; FIN WHALE; HUMPBACK WHALE,
COMPETITION; LONG-TERM CHANGE; HEAVY METALS; AGE AT SEXUAL MATURITY;
PREGNANCY RATEINTRODUCTION
Abundance and biological parameters of Antarctic minke whales have greatly changed over the years, andsince the late 1970s the IWC/SC has spent considerable time discussing possible reasons and trying tounderstand this phenomenon (see IWC, 19xx).
Due to uncertainties related to the estimation of biological parameters, there was no agreement at that time
concerning the data necessary for stock management, including natural mortality rate. This resulted indifficulties in calculating the catch quota and was the primary reason for the moratorium on commercial
whaling. The Government of Japan, therefore, launched a whale research program under the Article VIII of
the International Convention for the Regulation of Whaling (ICRR), in the Antarctic in 1987/88, with theestimation of biological parameters necessary for the management of Antarctic minke whales as the firstobjective. The elucidation of the role of cetaceans in the Antarctic ecosystem was the secondary objective.
The program was called JARPA, and in 1994 and 1995, the elucidation of the effects of environmental
changes on cetaceans and, the Antarctic minke whale stock structure were added to the research objectives.
The JARPA program spanning over eighteen years will end in 2004/05. Research in various fields has been
conducted together with surveys and analyses related to the program objectives. Phenomena suggesting
qualitative and quantitative changes on the Antarctic minke whale stocks have been observed, some of whichhave been reported at the IWC/SC ( e.g. IWC, 1988).
This paper rearranges the results reflecting changes in the Antarctic minke whale stocks, examines what has
happened in these stocks, and predicts possible future changes.
MATERIALS AND METHOD
Examination of the changes in the Antarctic minke whale stocks since whaling began, is based on analysesof sexual maturity age and growth curve (Kato, 1987; Zenitani et al., 2005), blubber thickness (Ohsumi et al.,
1997; Konishi and Tamura, 2005), and estimated prey consumption (Tamura and Konishi, 2005) using
samples from commercial whaling and JARPA, ADPT-VPA analysis of the stocks (Butterworth et al.,1999),37

and study of mercury accumulation in the whale body (Honda, 1985; Honda et al., 1987; Fujise et al., 1997;
Yasunaga and Fujise, 2005) and other reports.
Age at sexual maturity estimated by the transition phase of the growth layer in cetacean earplugs has beenused here.
Data for age at sexual maturity for fin and Antarctic minke whales used in this study are from Lockyer(1972) and Kato (1987), respectively. Apparent pregnancy rates for blue and fin whales are from Gambell(1972). Apparent pregnancy rate for humpback whales are from Bando et al. (2005) which were calculated
from the International Whaling Statistics.
RESULTS AND DISCUSSIONSCatch History and Biomass of Large Cetaceans in the Antarctic Ocean
Commercial whaling began in the Antarctic in 1904. Initially, whaling mainly targeted the blue whales that
had high commercial value and humpback whales that are slow swimmers. Later, the object of commercial
whaling gradually shifted to the fin, sei and Antarctic minke whales with reduction of the former target
whale stocks (Fig. 1).
Blue whales
In the 1911/12 season, more than 1,000 blue whales were taken. In 1928/29, the yearly catch exceeded
10,000, and in the 1930/31 season, 29,410 whales were taken as the largest catch on record. Annual catches
continued in the tens of thousands until the 1939/40 season. It temporarily decreased during World War II,
but recorded 9,192 in the 1946/47 hunting season, then declined rapidly (Fig. 1). The take of blue whales
was banned in 1964. Total catch amounted to 331,644 whales. In terms of biomass, this means that amaximum of 2,941,000 tons in a year, and an average of 526,408 tons per year were removed from thestocks, calculated with an average body weight of 100 tons (Fig. 2).
Fin whales
Fin whales were also hunted from the initial period. More than 10,000 were taken in the 1929/30 season,and in 1937/38, more than 20,000, exceeding the catch of blue whales, when the fin became the major targetof commercial whaling. As with blue whales, catches temporarily decreased during World War II, but
recovered when whaling was resumed after the end of the War, and in the 1951/52 season it again exceeded
20,000. Annual catches continued in the tens of thousands until the 1963/64 season (Fig. 1). The largest
catch on record was made in the 1960/61 season, when 28,761 whales were taken. Total catches came to691,890 until the take of fin whales was banned in 1976. In terms of biomass, this means that a maximum of
1,581,855 tons in a year, and an average of 528,527 tons per year were removed from the stocks, calculatedwith an average body weight of 55 tons (Fig. 2).
Sei whales
There is a record of the take of sei whales in 1905/06, but full-scale hunting began in the 1957/58 season.
Since then annual catches increased to a peak of 20, 380 in the 1964/65 season and decreased rapidly afterthat (Fig. 1). Total catch till 1977/78 was 149,594 or, an average of 2,301 whales taken annually. In termsof biomass, this means that a total of 2,917,083 tons, a maximum of 397,410 tons in a year, and an averageof 44,878 tons per year were removed from the stocks, calculated with an average body weight of 19.5 tons
(Fig. 2).
Antarctic minke whales
The take of Antarctic minke whales has been recorded in 1951/52, but it was in the 1971/72 season that fullscale whaling for them began. About 6,000 whales were taken annually from the 1972/73 to the 1986/87
season, when commercial whaling was suspended. Since then, 330 or less have been taken under the special
permit of the Japanese Government up till 1994/95, and 440 or less till 2003/04 (Fig. 1). The largest catchon record was made in the 1976/77 season, when 7,900 were taken. Total catch came to 97,810 whales until
the 1986/87 season, when commercial whaling was suspended or, an average of 2,877 whales taken annually(for reference, total catch is 104,165 and yearly average 2,042 whales if those taken under special permit areincluded). In terms of biomass, this means that a total of 723,794 tons, a maximum of 58,460 tons in a year,
and an average of 21,288 tons per year were removed from the stocks, calculated with an average body38

weight of 7.4 tons (for reference, total biomass is 770,821 tons, and annual average 15,114 if those taken
under special permit are included) (Fig. 2).
Humpback whales
Humpback whales were taken as the major target species from the 1904/05 season, when commercial
whaling began in the Antarctic Ocean, since they were easy to hunt. Their status as the major target
continued until the 1913/14 season. In the 1910/11 season, more than 8,000 whales were taken, but after the1913/14 season, the target shifted to blue and fin whales, and catch gradually decreased until the 1916/17
season, after which it declined to 1,000 or less. From the 1934/35 to 1940/41, and from the 1949/50 to
1959/60 seasons, 1,000 to 2,000 whales were taken, but the take of humpback whales was banned in 1963
(Fig. 1). The largest catch on record was made in the 1936/37 season, when 4,477 whales were taken. Total
catch came to 68,294 or, an annual average of 1,102. In terms of biomass, this means that a total of
2,117,114 tons and an average of 33,605 tons per year were removed from the stock, calculated with anaverage body weight of 31 tons (Fig. 2).
Until around 1970 while fin whales were being hunted, about 2 million tons of whales had been removed
from the stocks. Krill consumed by the whales would have amounted to 60,000 tons per day, assuming that
consumption is 3% of body weight of whales. This all became a surplus since whales were removed by thehunts. If we assume that the blue, fin, humpback and other large whales stayed in the Antarctic Ocean for
one hundred days, as much as 3 million tons of krill would have been left over as surplus every year.
Age at sexual maturity
Age at sexual maturity of the fin and Antarctic minke whales is discussed here since biological data for these
species are comparatively abundant.
Fin whales
Fig. 3 shows the changes in the age at sexual maturity of fin whales in the Antarctic (Lockyer, 1972). Theircatch continued from 1904, when whaling began in the Antarctic, to 1976. Age at sexual maturity declined
markedly from year classes in 1920 to 1930 and this trend is observed until the 1957 year class for which
there are data.
Antarctic minke whales
Fig. 4 shows the changes in age at sexual maturity deduced from the transition phase in earplugs of female
Antarctic minke whales (Kato, 1987). Full-scale whaling for the Antarctic minke whales began in the
1971/72 season, but the age at sexual maturity already began to decline before the season which full-scale
whaling was started. It seems the decline began from around the 1932 year class. Significant decline wasobserved from the 1950 to 1977 year classes (Kato, 1987).
After the moratorium on commercial whaling was implemented in 1987, samples were collected by the
JARPA, which are shown in Fig. 5 (Zenitani and Kato, 2005). The decline tendency in the age at sexual
maturity gradually slowed down around the 1960s, and almost stopped around 1965 to 1980. In case of
females, increasing trends was observed after the 1990 year class.
Pregnancy Rate
Fig. 6 shows the changes in the pregnancy rates of the blue, fin, and humpback whales over the years. The
rates of the three whale species show the tendency to increase from the 1930s, although there are yearlyfluctuations. Pregnancy rates of blue whales increased from about 1930 to 1960, fin whales from around1930 to 1970, and humpback whales from about 1930 to 1960, although the yearly fluctuation is great.
It is considered that Antarctic minke whales are capable of reproducing every year, and the apparent
pregnancy rate estimated from those migrating to the Antarctic is high and constantly in the ninetiethpercentile since 1970s. However, they are segregated by sex and reproductive status, and the 78% (Best,1982) estimated in their breeding ground is considered to be appropriate. No detailed reports are available
on the changes over the years.

39

Growth Curve
The growth curve of Antarctic minke whales using samples from commercial whaling is shown in Fig. 6(Kato, 1987). Kato (1987) reported that growth rate increased from year classes in 1940 to 1949 and for
those in 1970 to 1979, and that they matured at a younger age and became larger in size.
We compared the growth curve after the above period, using JARPA samples, and found that the growth rate
has slowed down for year classes in the 1990s, compared with those in the 1980s. The growth curve showsthat the 1990s year group tend to be below those for 1980s (Fig. 8).
VPA Analysis of Antarctic Minke Whale Stocks
Catch-at-age analysis using VPA and others have shown that recruitment of Antarctic minke whales
increased from 1944 to 1968 (Sakuramoto and Tanaka, 1985; 1986; Butterworth et al., 1999). It thendecreased until about 1980, after which the declining trend has halted. The causes for these changes havelong been the subject of discussion at the IWC (see IWC, 1989; 1990; 1991; 1992a; 1992b; 1995;
Sakuramoto and Tanaka, 1986; Butterworth et al., 1999; Butterworth and Punt, 1999).
Accumulation of Pollutants
Honda et al. (1987) analyzed heavy metals in the livers of Antarctic minke whales commercially taken in the
1980/81 season. He found that Hg concentration did not increase with age; rather it decreased after age ten.
It is well known that Hg concentration in the liver increases with age in marine mammals including
cetaceans. However, he reported that such age-related accumulation was not detected in the minke whale
samples. Changes in Hg concentration in the prey was not a plausible explanation, since Hg in theenvironment has not greatly changed, and he considered that it was a result of an increase in prey
consumption, hence the increase of Hg intake. As a factor, he pointed out that feeding conditions had
improved for the Antarctic minke, with the decrease in the abundance of larger whales (Honda et al., 1987).
Further, in order to understand the apparent decreasing pattern of Hg concentration, Honda (1985) estimatedthe total mercury loads (burdens) accumulated in the whale, and examined conditions that would best fit the
mercury accumulation age curve using an accumulation model. In this simulation, he assumed that Hgconcentration in the environment (prey) has not greatly changed and used 4.6 years for the biological halflife of mercury as determined in the striped dolphin. The assumed feeding conditions that fit the curve wasthat in 1980/81, whales up to ten years old consumed prey amounting to 15% of body weight, while
consumption decreased from 15% to 5% for those from ten to thirty; and for those over thirty, it was 5% (Fig.
11). The trend was observed by analyzing samples taken during the 1980/81 to 1981/82 whaling seasons,from which can be deduced that feeding conditions became favourable around 1950, which is thirty years
prior to these sampling dates.
Fujise et al. (1997) continued to monitor Hg accumulation in Antarctic minke whales using JARPA samples.
They clarified the process that reflects an increase in mercury intake for all ages. Examination of theaccumulation changes up to recent years, shows a rise in mercury intake due to increased consumption of
prey. This was found in all age groups. It was confirmed that Hg concentration also increased in Antarctic
minke whales with age as with other cetaceans (Fig. 12).
Furthermore, Hg concentration tends to be lower in the younger age group (one to five year olds), when thedetails of these accumulation curves for the most recent (2003/04) JARPA samples are examined (Fig. 13).
Blubber Thickness
Ohsumi et al. (1997) examined blubber thickness using data from commercial whaling which began in
1971/72 and the JARPA data from 1987/88 to 1995/96. They reported that a reduction in blubber thickness
was observed after 1978 (Fig. 14).
Konishi and Tamura (2005) analyzed blubber thickness using the JARPA data from 1987/88 to 2003/04, andreported that the decrease in thickness found by Ohsumi et al. (1997) was still in evidence (Fig. 15).
Changes in Stomach Content Weight of Antarctic Minke Whales
Tamura and Konishi (2005) examined the stomach contents of Antarctic minke whales using data from the
JARPA 1987/88 to 2003/04 seasons. They reported that the decreasing pattern in stomach content weights ofmature minke whales was observed since the 1987/88 season in which JARPA was started (Fig. 16).
40

Competition with Larger Baleen Whales (Humpback and Fin Whales)
In recent years, it has become known that larger baleen whale stocks, including humpback whales, arerecovering. Matsuoka et al., (2005) estimated abundance of humpback and fin whales in the Antarctic Areas
III (East), IV, V and VI (West), using the whale sighting data from JARPA. They reported that the number of
humpback and fin whales migrating to Antarctic Areas IV and V have been increasing (Fig. 17) and that thebiomass of these species has become larger than that of Antarctic minke whale in recent years (Fig. 18;
Matsuoka et al., 2005). In Area IV, especially, the humpback whale has been seen to be encroaching on the
distribution range of Antarctic minke whales. Many minke whales were sighted in the research area south oflat. 60°S in recent years, but it was observed in the 2003/04 surveys that they tended to be pushed back into
the pack ice (Ishikawa et al., 2004).
Humpback and fin whales used to be only sighted offshore until the 1990s, but in recent years, theirdistribution range has become overlapped with that of the Antarctic minke whale. The overlapping of two or
more whale species in the feeding grounds of the Antarctic suggests competition among these whale species
occurs over krill, which is the key species, especially in a simple marine ecosystem structure as the Antarctic
Ocean. The reduced range of Antarctic minke whales in their feeding ground suggests that those niches havebecome sub-optimal.
What Has Happened to the Antarctic Minke Whale Stocks?
The above considerations can be summarized as follows:
Changes found from 1940 to 1970:* Increase in recruitment (VPA)* Acceleration in growth rate (growth curve)* Decrease in the age at sexual maturity (earplug transition phase)* Increase in mercury intake
Changes found from 1970 to 1980:* Decrease in recruitment (VPA)* Halt in the decreasing trend in the age at sexual maturity (earplug transition phase)* Mercury intake stabilizes at fixed level
Changes from 1980 onwards:* Decreasing growth rate (growth curve)* Decrease in blubber thickness (from 1980 to present)* Decrease in stomach contents weight (from latter half of 1980 to present)* Decrease in mercury intake
Examining the above phenomenon comprehensively, the following possible changes are considered to haveoccurred in the Antarctic minke whale stocks.
Feeding conditions became favourable for the Antarctic minke whales around 1940, at the latest, with thedepletion in large baleen whales such as blue and other whales due to overhunting, and the nutritional statusof each minke whale individual improved. Thus, growth rate increased for the minke, and they grew tomature body length earlier, reaching sexual maturity younger. The increased breeding capability resulted inincreased abundance. Declining age at sexual maturity and increasing pregnancy rates were also observed infin, humpback and other large baleen whales, suggesting improved feeding conditions for them as well.
Overall improvement in feeding conditions is indicated, arising from lower population density due to thedecrease in the number of large baleen whales which consumed huge amounts of krill in the Antarctic Ocean(Kato, 1987).
41

The amount of available prey (krill) per minke whale as a consumer became restricted about 1970. Aroundthat time, growth rate and the declining the age at sexual maturity slowed down. By 1980 a halt in the latter
trend was observed. It seems that the growth rate of whales in year classes of the 1990s was lower than thatof those in year classes of the 1980s. The change in feeding conditions was reflected in blubber thickness.
Blubber thickness of Antarctic minke whale has shown a constant decline since 1978/88. Also, the numberof humpback and fin whales which are higher in their niches than the number of Antarctic minke whales,
migrating to the Antarctic increased from 1990. This suggests the recovery of these stocks, while indicating
a further deterioration in feeding conditions for Antarctic minke whales.
Other factors that may have contributed to the changes in feeding conditions include environmental changes
such as global warming. Melting of fast ice in the seas around the Antarctic Peninsula has been reported,and a decrease in the number of penguins which incubate on ice has been observed (Croxall et al., 2002).
However, Areas IV and V of the Antarctic Ocean, which are the concern of this paper,, are on the opposite
side of the Antarctic Peninsula and no major melting of fast ice has been reported to date.. Analysis ofsatellite data and oceanographic observations carried out in the JARPA programs have not shown any
constant change in the marine environment, although annual fluctuations due to El Nino and La Nina havebeen observed (Watanabe et al., 2005).
It is highly possible that nutritional conditions for the Antarctic minke whales have changed due tocompetition with other whale species such as the humpback whale, and possibly the change in carrying
capacity, in the Antarctic marine ecosystem.
For the appropriate management and sustainable use of Antarctic minke whale stocks, we would need tocollect not only data on abundance and biological parameters of major whale species, but also data on theirhabitat environment and their responses to environmental changes.
ACKNOWLEDGMENTS
We thank Dr. Hidehiro Kato, National Research Institute of Far Seas Fisheries (NRIFSF), Ms. Ryoko
Zenitani, the Institute of Cetacean Research (ICR), for preparing the information on sexual maturity of whale
samples. We also thank Mr. Koji Matsuoka, ICR, Dr. Tsutomu Tamura, ICR, Mr. Takeharu Bando, ICR,
and Dr. Kenji Konishi, ICR, for providing the information in the referenced research documents. Our sincere
thanks are also due to all researchers and crews who were participated in the JARPA surveys from 1987/88
to 2003/04 seasons.
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JARPA Review meeting, January 2005.
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Manage., 1(1): 11-32
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mercury accumulations of Southern minke whales in past fifteen years. Paper SC/M97/20 presented to the
IWC Intersessional Working Group to Review Data and Results from Special Permit Research on Minke
whales in the Antarctic, May 1997 (unpublished). 16pp.
Gambell, R. 1973. Some effects of exploitation on reproduction in whales. J. Reprod. Fert., Suppl. 19: 533553.
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bioaccumulations of heavy met al.s in marine mammals). Ph. D. thesis, Univ. Tokyo, 101p.
Honda, K., Yamamoto, Y., Kato, H. and Tatsukawa, R. 1987. Heavy met al. accumulations and their recentchanges in southern minke whales Balaenoptera acutorostrata. Arch. Environ. Contam. Toxicol. 16:209216.
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Ishikawa, H., Otani, S., Kiwada, H., Isoda, T., Tohyama, D., Honjo, K., Hasegawa, A., Terao, T., Kushimoto,T., Ohshima, K., Sugiyama, K., Sasaki, T., Itoh, S., Takamatsu, T. and Yoshida, T. 2004. Cruise Report of
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Balaenoptera bonaerensis in Areas IV and V. Paper JA/J05/PJR9 presented to the JARPA Review meeting,January 2005.
Lockyer, C. 1982. Investigation of the ear plug of the southern sei whale, Balaenoptera borealis, as a validmeans of determining age. J. Cons. Int. Explor. Mer., 36(1): 71-81.
Matsuoka, K., Hakamada, T. and Nishiwaki, S. 2005. Distribution and abundance of humpback, fin and blue
whales occurring in the Antarctic Areas IIIE, IV, V and VIW (35°E-145°W). Paper JA/J05/PJR10
presented to the JARPA Review meeting, January 2005.
Ohsumi, S., Fujise, Y., Ishikawa, H., Hakamada, T, Zenitani, R. and Matsuoka, K. 1997. The fattyness of the
Antarctic minke whale and its yearly change. Paper SC/M97/18 presented to the IWC Intersessional
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Antarctic, May 1997 (unpublished). 21pp.
Sakuramoto, K. and Tanaka, S. 1986. A simulation study on management of whale stocks considering feedback systems. Paper SC/38/O10 presented to the IWC Scientific Committee, May 1986.
Tamura, T. and Konishi, K. 2005. Feeding habits and prey consumption of Antarctic minke whales,
Balaenoptera bonaerensis in Areas IV and V of the Antarctic Paper JA/J05/PJR8 presented to the JARPA
Review meeting, January 2005.
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Comparison of mercury accumulation in Antarctic minke whale collected in 1980-82 and 1984-86. Nippon
Suisan Gakkaishi, 64:105-109.
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JA/J05/PJR15 presented to the JARPA Review meeting, January 2005.
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to the JARPA Review meeting, January 2005.
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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/PJR7 presented to the JARPA Review meeting, January
2005.

44

35000

Blue
Fin
Sei
Humpback
Minke

30000

Catches (whales)

25000

20000

15000

10000

5000

0
1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2001

Calender year
Fig. 1. Catch history of large sized baleen whales in the Antarctic since 1904.
4,000,000

Minke

3,500,000

Right
Sperm

Catch (tons)

3,000,000

Sei

2,500,000

HumpbackFin

2,000,000

Blue
1,500,000
1,000,000
500,000

1904
1 9 /05
06
1 9 /0708
1 9 /0910
1 9 /1112
1 9 /1314
1 9 /1516
1 9 /1718
1 9 /1920
1 9 /2122
1 9 /2324
1 9 /25
26
1 9 /2728
1 9 /2930
1 9 /3132
1 9 /33
34
1 9 /3536
1 9 /3738
1 9 /3940
1 9 /4142
1 9 /43
44
1 9 /4546
1 9 /47
48
1 9 /4950
1 9 /51
52
1 9 /5354
1 9 /55
56
1 9 /57
58
1 9 /5960
1 9 /6162
1 9 /63
64
1 9 /65
66
1 9 /67
68
1 9 /6970
1 9 /71
72
1 9 /7374
1 9 /75
76
1 9 /7778
1 9 /7980
1 9 /8182
1 9 /83
84
1 9 /85
86
1 9 /87
88
1 9 /8990
1 9 /91
92
1 9 /93
94
1 9 /95
96
1 9 /9798
2 0 /9900
2 0 /01
02
2 0 /03
04
/05

0

Year

Fig. 2. Total biomass of harvested large sized baleen whales in the Antarctic since 1904.

Age at sexual maturity(year)

15

10

5

0
1900

1905

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

Year

Fig. 3. Trend of age at sexual maturity of southern fin whales by cohort (Lockyer, 1982)

45

1980

1985

1990

1995

2000

Age at sexual maturity(year)

25
20
15
10
5
0
1900

1905

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Year

Fig. 4. Trend of age at sexual maturity of Antarctic minke whales by cohort (Kato, 1987)
26
24

Male

22
Mean age at sexual maturity

20
18

Mean±S.D.

16
14
12
10
8
6
4
2
0
1930

1935

1940

1945

1950

1955

1960
1965
Cohort

1970

1975

1980

1985

1990

1995

26
24

FemaleMean age at sexual maturity

22
20
18

Mean±S.D.

16
14
12
10
8
6
4
2
0
1930

1935

1940

1945

1950

1955

1960
1965
Cohort

1970

1975

1980

1985

1990

1995

Fig. 5. Trend of age at sexual maturity of Antarctic minke whales by cohort (Zenitani and Kato, 2005)

Pregnancy rate (%)

100
90

Blue

80

Fin
Humpback

70
60
50
40
30
20
10
2000

1995

1990

1985

1980

1975

1970

1965

1960

1955

1950

1945

1940

1935

1930

1925

1920

1915

1910

1905

1900

0

Year

Fig. 6. Yearly changes of apparent pregnancy rate of blue, fin and humpback whales in the Antarctic (data fromLockyer (1982) and BIWS).

46

Fig. 7. Yearly changes of growth curve of Antarctic minke whales using samples from commercial whaling (Kato,
1987).

1980’s1980’s1990’sArea IV

1990’sArea IV

1980’s1980’s1990’sArea V

1990’sArea V

Fig. 8. Comparison of growth curve of body length in each cohort for Antarctic minke whales. Left column indicate
male, and right female. Upper and lower figures indicate Area IV and Area V, respectively. Age data in this figure wereused from Ms. Zenitani (unpublished data).

47

Hg ( μ g / g , dr y wt . )Fig. 9. Boot strap estimates of median (solid line), and 5%- and 95%-iles (dotted lines), for recruitment Ny,2 (relative to
its estimated 1968 level for the corresponding bootstrap replicate) for Area IV for the base-case estimator when M isfixed at its corresponding best estimate of o.o57yr-1. (Butterworth et al., 1999)

A g e (ye ar )Fig. 10. Age trend of hepatic mercury concentrations (μg/g) in Antarctic minke whales using samples fromcommercial whaling in 1980/81 season . (Honda et al., 1987)

Fig. 11. Age trend of mercury load (mg) in the body of Antarctic minke whales, and accumulation curve obtained fromthe simulation of accumulation model (Honda, 1984).

48

100
80
60
40
01/0202/03 97/9898/99 94/9595/96 88/8989/90

20

84/8585/86

Survey year group

1-5

80/81- 0
81/82

6-15

16-25

Hg concentration (ng/g wet wt)

120

0
26-

Age group

Fig. 12. Comparison of age trends of hepatic Hg concentrations (ng/g) of Antarctic minke whales during five survey
year groups (1980/81+1981/82, 1984/85+1985/86, 1988/89+1989/90, 1994/95+1995/96, 1997/98+1998/99,
2001/02+2002/03).
120

1-5
6-15
16-25
26-

H g c o n c e n tratio n s ( n g/ g)

100

80

60

40

20

0
80/8181/82

84/8585/86

88/8989/90

94/9595/96

97/9898/99

01/0202/03

S u rve y ye ar g ro u p

Fig. 13. Yearly changes of hepatic Hg concentrations (ng/g) in minke whales from four age groups (1-5, 6-15, 16-25, 26
or more).

6.0

BT/BL(*100)

5.5

5.0

4.5

4.0
1970

1975

1980

1985
Year

1990

1995

2000

Fig. 14. Yearly changes in average fattyness index of blubber thickness in February (closed circle indicates males,
open circle females. (Ohsumi et al., 1997)

49

6.0

Mean blubber thickness

5.5

5.0

y = -0.1117x + 4.5923
2
R = 0.6016
4.5

4.0

3.5

3.0
0

1
1987/88

2
3
4
5
6
7
8
9
1999/00
2003/04
1989/90
1993/94
1995/96
1997/98
2001/02
1991/92

10

Survey year

Fig. 15. Yearly changes in average blubber thickness of pregnant females in February (Konishi and Tamura, 2005).

Area VI

Area V
140.0

140.0

Male
120.0

120.0
y = -1.5017x + 3077.6
2
R = 0.2133

100.0

kg

kg

100.0

Male

y = -1.0507x + 2172.8
2
R = 0.0606

80.0

80.0

60.0

60.0

40.0
1988

1990

1992

1994

1996

1998

2000

2002

40.0
1988

2004

1990

1992

1994

1996

1998

2000

2002

2004

140.0

140.0

FemaleFemale

120.0

120.0
y = -3.3534x + 6782.2
2
R = 0.3655

100.0

kg

kg

100.0

y = -3.6764x + 7424.1
2
R = 0.6431

80.0

80.0

60.0

60.0

40.0
1988

1990

1992

1994

1996

1998

2000

2002

40.0
1988

2004

Year

1990

1992

1994

1996

1998

2000

2002

2004

Year

Fig. 16. Yearly changes of stomach contents (kg) of mature Antarctic minke whales in Area IV and V (Tamura andKonishi, 2005). Data used was from individuals collected from an area south of 63 degree S in January andFebruary. The content weight was combined the first and second stomachs.

50

Fig. 17. Yearly changes of abundance estimates for humpback and fin whales in Antarctic Area IV (70E-130E). Upper
figure shows humpback whales, lower figure fin whale (Matsuoka et al., 2005)

Fig. 18. Biomass of Humpback and Antarctic minke whales in Antarctic Area IV (70E-130E) using data from 1989/90
to 2003/04 JARPA (Matsuoka et al., 2005).

51