Technical Reports of the Institute of Cetacean Research (2018)

pp. 1–13

Technical Report (not peer reviewed)

Cetaceans as indicators of historical and current changes in the
Antarctic ecosystem
Yoshihiro Fujise* and Luis A. Pastene
Institute of Cetacean Research, Toyomi-cho 4–5, Chuo-ku, Tokyo 104–0055, Japan*Contact e-mail: fujise@cetacean.jp

ABSTRACT
Changes in the Antarctic ecosystem are triggered by anthropogenic and natural factors. This paper reviews thescientiﬁc information on whales produced mainly by the Institute of Cetacean Research (ICR), that could be indicative of changes in the Antarctic ecosystem in the context of two hypotheses, the ʻkrill surplusʼ hypothesis in
the middle of the past century, and the hypothesis of recovery of krill-eater large whales since the 1980ʼs. The
information in the Indo-Paciﬁc region of the Antarctic (70°E–170°W) showed that the increased krill availability
in the middle of the past century could have been translated into better nutritional conditions for some krill
predators like the Antarctic minke whale, resulting in a decreasing trend in the age at sexual maturity and anincreasing trend in the recruitment rate and total population size of this species between approximately 1940
and 1970. The evidence available since the 1980ʼs showed a sharp increase in the abundance of some species
such as the humpback and ﬁn whales, which had been heavily exploited in the ﬁrst half of the past century.
The evidence also showed that, in parallel to this increase, the nutritional condition of Antarctic minke whales
has deteriorated as revealed by a decrease in energy storage and stomach content weight since the 1980ʼs.
This observation is consistent with the stable trend of age at sexual maturity, recruitment and abundance of
this species after the 1980ʼs. Therefore the historical demographic changes observed in the Antarctic minke
whale are consistent with the pattern expected under the ʻkrill surplusʼ hypothesis, while the demographic
and ecological changes observed in recent years are consistent with the changes expected under the reverseof the ʻkrill surplusʼ hypothesis.

INTRODUCTION
The Antarctic ecosystem is a very dynamic one withchanges in species composition and habitat occurring
through time. Within this ecosystem the Antarctic krill(Euphausia superba) is a key prey species supporting different species of baleen whales, pinnipeds, birds and ﬁsh.
Changes in the ecosystem can be derived from humaninterventions or from natural causes. One example ofthe former is the large-scale exploitation of whales in theﬁrst half of the 20th Century, which has been discussed
by several authors, notably by Laws (1977; 1985). Exploitation of large whales in the Antarctic Ocean started in
1904. Several species of krill-eater large whales such as
Antarctic blue (Balaenoptera musculus intermedia) andhumpback (Megaptera novaeangliae) whales were heavily depleted by commercial whaling by the ﬁrst half of thepast century. Other species such as the ﬁn (B. physalus)
whales were depleted during the second half of the 20th
Century. Large changes in the biomass of some whale

species had a strong eﬀect on the demography of otherkrill-eater predators in the Antarctic ecosystem (Laws
1977; 1985).
One example of the latter are the environmental
changes reported in recent years for some parts of the
Antarctic. For example Fraser et al. (1992) suggested
that the increase in chinstrap penguin populations in thelast four decades (1950ʼs–1990ʼs) were due to a gradualdecrease in the frequency of cold years with extensive
winter sea ice cover resulting from environmental warming. Their observation was supported by a multidisciplinary winter expedition to the Scotia and Weddell seas,satellite images of ocean ice cover, and the analysis of
long-term surface temperature records and penguin
demography. Less ice cover means wider open waters
available for penguinʼs feeding on krill. This observation
contradicted the hypothesis that the increases in the
penguin population resulted from an increase in preyavailability brought on by the decrease in baleen whale
stocks. They cited modeling work that predicted that the1Technical Reports of the Institute of Cetacean Research (2018)

Figure 1.

Schematic diagram of ecosystem perturbation in the Scotia Sea (after Trivelpiece et al., 2011).

Table 1
Biological and ecological parameters monitored from whales and their environment to investigate change in the ecosystem.
Research parameter2

How the information is obtained

Relevance of monitoring

Krill biomass

Echo-sounder and net surveys

Krill is a key species in the Antarctic ecosystem.
Changes in its abundance have eﬀects on predatorsand the whole ecosystem.

Whale abundance

Systematic sighting surveys

Fluctuation of abundance of whales through time isimportant for their management. Diﬀerent levels of
whale abundance have diﬀerent impacts on krill.

Whale distribution

Systematic sighting surveys

Distribution of whale species can change with time inresponse to changes in abundance and/or changes inoceanographic conditions/krill availability.

Whale recruitment

Population dynamic models
that use age and abundance
information from whales

Same as above. Index of young whale abundance.

Blubber thickness,
fat weight and girth

Direct measurements fromsampled whales

Index of body condition. Better nutritional condition(e.g. better availability of krill) will be reﬂected inthicker blubber, heavier fat and larger girth.

Stomach content weight

Direct measurements fromsampled whales

Index of body condition. Better nutritional condition
will be reﬂected in heavier stomach contents.

Age at sexual maturity (ASM)

Examination of transition
phase in earplugs; examination of ovaries and testis.

Better nutritional conditions will be reﬂected in a shiftof the ASM to younger ages e.g. whales will be able toreproduce at younger ages.

Pregnancy rate

Examination of ovaries anduterus

Better nutritional conditions will be reﬂected in higher
pregnancy rates.

Oceanographic conditions

Systematic oceanographic surveys based on CTD and XCTD

Changes in oceanographic conditions will aﬀect distribution and krill biomass and in turn the abundanceand distribution of whales. Changes in oceanographic
conditions might indicate an eﬀect of climate changes.

Cetaceans as indicators of historical and current changes in the Antarctic ecosystem

beneﬁts of a reduction in baleen whales would have been
most directly conferred to predators of similar size andbehavior, namely, other whales (Fraser et al., 1992). However more recent analyses in the Antarctic Peninsula andScotia Sea concluded that the chinstrap penguin instead
may be among the most vulnerable species aﬀected by awarming climate (Trivelpiece et al., 2011).
Trivelpiece et al. (2011) presented a very informative
diagram of ecosystem perturbation in West Antarctic(Scotia Sea) due to both, anthropogenic reasons suchas the commercial over-exploitation of marine resources (sealing from 1820 to the 1960ʼs, whaling from1900 to 1970; ﬁshing for ice ﬁshes and Notothenids
from 1960–1980), and natural reasons (climate-change
eﬀects from the 1970ʼs). They noted that a pelagic trawl
ﬁshery for krill also developed from the 1970ʼs, and thatin the 2000ʼs, the once-depleted marine mammal populations had recovered or were recovering, that the krill ﬁshery was expanding, that climate change was progressing
and that Adelie and chinstrap penguin populations weredecreasing (Figure 1). The causes of ecosystem changes
in the Antarctic are therefore complex, and to understandthose causes, long-term monitoring research programs
focused on collecting biological data of krill predators aswell as data on sea ice cover and environmental variables,
are important.
Previous studies have documented ecosystem changes
in West Antarctic, and have considered environmental
variables and demographic information of land-based
krill predators. The objective of this study was to review
the scientiﬁc evidence for expected changes in the IndoPaciﬁc region of the Antarctic, in the context of someestablished hypotheses. The evidence is based mainly
on biological and ecological data of baleen whales, seabased predators, collected by ICR. Table 1 shows the data
monitored in whales and their environment, which are
informative of changes in the ecosystem.

KEY BALEEN WHALES IN THE ANTARCTIC ECOSYSTEM
Baleen whale species, except the Brydeʼs whale, migrateseasonally between low latitude breeding areas in winterto high latitude feeding areas in the Antarctic in summer. The main prey of baleen whales such as Antarctic
blue, ﬁn, humpback and Antarctic minke (B. bonaerensis)
whales is the Antarctic krill therefore their summer migrations to the Antarctic are related to areas of krill concentrations as those shown in Figure 2. Breeding areas of
the humpback whales occurring in the Antarctic researcharea are located in both Western and Eastern Australia.

Figure 2.

Areas of krill distribution in the Antarctic associatedwith gyres (after Nicol, 2006).

Breeding areas for the other species are assumed to be
in low latitudes of the eastern Indian and western South
Paciﬁc Oceans.
Antarctic blue whale
This is the largest baleen whale species. The record for awhale killed in the Southern Hemisphere in the ﬁrst halfof the past century was a body length of more that 30 m
and weight of nearly 180 tons. During the austral summer
season blue whales are distributed mainly above the Antarctic Convergence to the ice edge. Current abundance inall of the Antarctic is approximately 3,000 animals.
Fin whale
This is the second largest baleen whale species witha maximum length of 27.1 m and weight of 120 tons.
During the austral summer season ﬁn whales are foundextensively south of 50°S, most commonly north of 60°S
(Branch and Butterworth, 2001). In the Antarctic ﬁnwhales feed on krill. The existing estimates from limitedparts of the range covered are of the order of several
thousand animals.
Humpback whale
This is a highly migratory species with a maximum bodylength of 17 m and weight of 40 tons. During the australsummer season it is distributed south of the Antarctic
Convergence to the ice edge but just north of the main3

Technical Reports of the Institute of Cetacean Research (2018)

distribution area of Antarctic minke whale. In the Antarctic, humpback whales feed on krill. The total current
Southern Hemisphere abundance is probably at least
60,000.
Antarctic minke whale
This is one of the smallest baleen whale species with amaximum body length of 10.7 m and weight of 10 tons.
During the austral summer season it is distributed mainly
around the pack-ice and feeds on krill. Abundance ofAntarctic minke whale in the Antarctic was estimated at
515,000 animals (IWC, 2013).

RESEARCH AREA
The review was focused on the Indo-Paciﬁc region of
the Antarctic, in the longitudinal sector between 70°E
and 170°W, south of 60°S, including both Indian (Area
IV according to International Whaling Commission (IWC)
terminology, 70°–130°E) and Paciﬁc (Area V according to
IWC terminology, 130°E–170°W) sectors. The researcharea includes Prydz Bay as its western boundary and theRoss Sea as the eastern boundary.

CHANGES IN THE ANTARCTIC ECOSYSTEM
The review of evidence of changes in the Indo-Paciﬁcregion of the Antarctic is treated in two periods, historicaland current. The former corresponds to the period approximately from the start of commercial whaling in the
Antarctica in 1904 until approximately 1970ʼs, which
Trivelpiece et al. (2011) characterized as ʻfavourable
climate conditions and reduced competition for krillʼ(Figure 1). The latter corresponds to the period from the1970ʼs–1980ʼs to recent years, which those authors char-

Figure 3.
4acterized as ʻunfavorable climate conditions and increasing competition for krillʼ (Figure 1).
Historical changes
The ecological eﬀect of the large-scale exploitation ofwhales in the ﬁrst half of the 20th Century has been discussed by several authors, notably by Laws (1977; 1985).
Exploitation of large whales in the Antarctic Ocean started in 1904. Several species of large whales such as blueand humpback whales were heavily depleted by the ﬁrst
half of the past century (Figure 3). Other species such as
the ﬁn and sei whales were depleted during the second
half of the 20th Century. Commercial exploitation of the
Antarctic minke whale started in the early 70ʼs when allother baleen whale species were already depleted. Table
2 shows the original and current (by 1980) abundanceof large whales exploited in the Southern Hemisphere(mainly Antarctic). In the Southern Hemisphere the abundance of large whale species such as the blue and humpback whales decreased to a minimal percentage of theoriginal abundance before exploitation. In the case of the
Antarctic minke whales, which were not exploited during
the ﬁrst half of the past century, the current abundance
has not changed with regard the original abundance.
As noted earlier, the Antarctic krill is a key prey species
in the Antarctic ecosystem supporting diﬀerent species ofbaleen whales, pinnipeds, birds and ﬁsh. Some researchers have suggested that following the main exploitation
period of large baleen whales in the Antarctic (approximately at the middle of the past century) some 150 million tons of ʻsurplusʼ annual production of Antarctic krill(Figure 4) became available for other krill predators, suchas Antarctic minke whales, crabeater seals, fur seals, pen-

Catches of baleen whales in the southern Hemisphere, 1910–1977 (after Allen, 1980).

Cetaceans as indicators of historical and current changes in the Antarctic ecosystem

Figure 4.

Initial and present (1970ʼs) biomass and annual krill consumption of exploited large baleen whale species
and other predators in the Antarctic (after Laws, 1977; Sahrhage, 1984).
Table 2
Original and current (by 1980) abundance of exploited large whales in the Southern
Hemisphere (modiﬁed of Allen, 1980).

guins and some albatrosses which took advantage of thisfood surplus to increase their abundance. This is the socalled ʻkrill surplusʼ hypothesis (Laws, 1977; 1985).
Inter-speciﬁc eﬀects of the surplus hypothesis
Antarctic minke whale
As shown in Table 2, Antarctic minke whales were notexploited in the ﬁrst half of the past century and thecommercial exploitation of this species only started in
the 1970ʼs at a moderate scale under management of
the IWC. The expectation here was that the krill surplus
by the middle of the past century translated to better
nutritional conditions, which should have been reﬂectedin changes in demographic parameters of this species.
In particular we focus here on the information on age atsexual maturity and total abundance.
Changes in the age at sexual maturity (ASM) indicate
change in nutritional conditions of the whales, which inturn could indicate less or more food availability in theenvironment. Better nutritional conditions will be reﬂected in a shift of the ASM to younger ages e.g. whales
will be able to reproduce at younger ages and as a consequence the populations will growth faster (Table 1). Oneof the methods for determining age at sexual maturity
in whales is examination of the ʻtransition phaseʼ in theearplugs. The earplugs of several baleen whale stocks exhibit seasonal growth layers which have been shown forsome species to indicate the age of the animals. A transition from early, irregular layers, to later, more regularlayers can be seen in these earplugs, and this is thought
to indicate the age at maturity of the whale (Thomson
et al., 1999). Historical changes in the age at sexual maturity can be investigated when the analyses are carriedout on cohorts (groups of whales born in the same year).
Earplugs of Antarctic minke whales were collected during the period of commercial whaling in the early 1970ʼs,
and during Japanese whale research in the Indo-Paciﬁcregion of the Antarctic (JARPA and JARPAII), for morethan 25 years. Thomson et al. (1999) showed a decline
in the average age at transition in Antarctic minke whalesin the Indian sector from roughly 11 for the cohorts of
the 1950ʼs to roughly 7 for those of the 1970ʼs, and thetrend was similar for females and males. This work was
updated by Bando et al. (2014) by using a large numberof samples, and analyses conducted on two biologicalstocks, one mainly distributed in the Indian Ocean sector
and the other in the Paciﬁc Ocean sector.
These results suggested that the nutritional conditions
were optimal for Antarctic minke whales after a period inwhich other krill-predators species such as the Antarctic
5Technical Reports of the Institute of Cetacean Research (2018)

blue and humpback whales were already depleted (Figure 3).
The Scientiﬁc Committee (SC) of the IWC has been
applying statistical catch-at-age (SCAA) analyses on Antarctic minke whales since 2005. A summary history of theapplication of SCAA to this species was presented by Punt(2014), and an assessment of Antarctic minke whales
using SCAA was reported by Punt et al. (2014).
The data used when conducting assessment by SCAA
on Antarctic minke whales consisted of catches, abundance estimates, length frequency data, and conditional
age-at-length data. Diﬀerent series of abundance estimates were used e.g. those from the IWC SCʼs International Decade of Cetacean Research/Southern Ocean
Whale and Ecosystem Research (IDCR/SOWER) as wellas those from Japanese dedicated sighting surveys in the
Indo-Paciﬁc region of the Antarctic. Biological data wereavailable from the period of commercial whaling (Japanand the former Soviet Union), and from Japanese whale
research in the Indo-Paciﬁc region of the Antarctic (JARPA
and JARPAII), for more than 25 years.
The SCAA assessment on Antarctic minke whale involved a ʻreferenceʼ case and several sensitivity tests.
These sensitivity tests explored sensitivity to the weightassigned to the various data sources and penalties, theassumptions related to vulnerability, natural mortality
and catchability, and to the use or otherwise of the Japanese researchʼs index data (Punt et al., 2014).
Results presented here refer to the ʻreferenceʼ casein Punt et al. (2014), which was robust to the sensitivity
tests conducted (Figure 6 for the Indian sector). As shownin Figure 6, the stock was estimated to have increased
from 1930 until the early 1970ʼs, with the stock having
declined subsequently thereafter. The increase in abundance was due primarily to an increase in recruitment
owing in turn to an increase in carrying capacity (Puntet al., 2014).

Figure 6.

6

For historical changes examined in this section, it isinteresting to note that the increase from 1930 to 1970ʼsroughly coincided with the period in which the age atsexual maturity decreased. Again these results suggested
that the nutritional conditions were optimal for Antarctic
minke whales after a period in which other krill-predator
species such as the Antarctic blue and humpback whales
were already depleted (Figure 3).
Summary
According to the ʻkrill surplusʼ hypothesis, the increased
krill availability was translated into better nutritional conditions for some krill predators like the Antarctic minke
whale. In the case of this species this resulted in some

Figure 5.

Changes in the age at sexual maturity of Antarctic
minke whale, by cohort (Indian Ocean sector). Ageat sexual maturity changed from around 11 years in
1950s cohorts to around 7 years in 1970s cohorts(middle curve) (after Thomson et al., 1999).

Time trajectories of total (1+) population size of Antarctic minke whales in the Indian sector (base-case).
The dotted lines indicate 95% asymptotic conﬁdence intervals (after Punt et al., 2014).

Cetaceans as indicators of historical and current changes in the Antarctic ecosystem

biological changes such as the observed decreasing trend
in the age at sexual maturity between approximately
1940 and 1970 (Figure 5), which coincide with the period
of depletion of some key krill-eater large whale species(Figure 3). A low age at sexual maturity favored an increase in the recruitment rate and total population size ina similar period (Figure 6).
The Antarctic minke whale perhaps reached the increased maximum carrying capacity by the late 1960s,
and then the stock responded by stabilizing the age atsexual maturity at 7–8 years old (Figure 5). This biologicalprocess should have resulted in a stable continuous highrate of newborns after 1970 since the recruitment was
rather stable after 1970 (Figure 6). The trend in recruitment is consistent with the trend in total abundance ofAntarctic minke whale estimated by sighting data which
has been broadly stable since the 1980ʼs.
Finally it should be noted that the period of demographic changes observed in Antarctic minke whales
coincides with the period 1900–70ʼs (ʻwhaling eraʼ) in
Trivelpiece et al. (2011), which they characterized as ʻfavorable climate conditions and reduced competition for
krillʼ (Figure 1). Such conditions allowed for rapid growthand increase in populations of the Antarctic minke whaleand other krill-eater predators in that period.

IWC SC (IWC, 2012). The IWC SC carried out three circumpolar sighting surveys under the IDCR/SOWER programs.
Japanese researchers also carried out dedicated sighting
surveys in the Indo-Paciﬁc region using the same methodology and guidelines.
Matsuoka et al. (2011) reported abundance estimatesand abundance trends for two populations of the humpback whales, stock D in the Indian sector (Area IV) andstock E in the Paciﬁc sector (Area V). Results showed thatthe abundance of both stocks has been increasing in thelast decades. Abundance and population growth rates of
humpback whale in the Indian sector and in the Paciﬁc
sector were 31,000 (16.4% increase by year with 95% CI:
9.5–23.3%) and 9,000 (12.1% increase by year with 95%
CI: 1.7–22.6%), respectively. The current abundance ofthe stock in the Indian sector is close to that at its preexploitation level. Figure 7 shows the abundance trend

Current changes
Commercial whaling of humpback, Antarctic blue and ﬁnwhales in the Antarctic was banned in 1963, 1964 and1976, respectively. As an eﬀect of these conservation
measures, the abundance of these species increased inrecent decades, which points toward a recovery frompast commercial exploitation. The speed of recovery varies among these species and among stocks within these
species. The increase in the abundance of large whale
species that feed on krill in the Antarctic means less foodavailable for species like the Antarctic minke whale thatonce beneﬁted from the surplus of krill. It was expectedthat these conditions, which correspond to the ʻreverseʼ
of the krill surplus hypothesis, should have some eﬀect
on the nutritional conditions and on demographic parameters of Antarctic minke whale and other species thatonce beneﬁted from the surplus of krill.
Evidence for the increase in abundance of large whales in
the Indo-Paciﬁc region of the Antarctic
To estimate abundance of large whales in the Antarctic,
systematic sighting surveys have been carried out under
the DISTANCE sampling method (Buckland et al., 2015),
and the guidelines for surveys and analyses agreed by the

Figure 7.

Abundance trend of humpback whale in the Indian
sector (Area IV) and Paciﬁc sector (Area V) based onJapanese dedicated sighting surveys. Black dots areabundance estimates based on the IDCR/SOWER
surveys, which are consistent with those from theJapanese surveys (after Matsuoka et al, 2011).
7

Technical Reports of the Institute of Cetacean Research (2018)of this stock in the Indian and Paciﬁc sectors for the period 1989/90–2003/04, and compares it with the trend
estimated using IDCR/SOWER sighting data (Matsuoka
et al., 2011). There is a clear increasing trend that is consistent between both Japanese and IDCR/SOWER surveys(see Figures 7).
The abundance and annual population growth rates ofﬁn whales in the Indian sector and Paciﬁc sector based onJapanese dedicated sighting surveys were estimated at
6,514 (16.0% CV: 0.78 increase rate) and 5,241 (12.8% CV:
0.60 increase rate) for the period 1989/90–2004/05, respectively (Matsuoka et al., 2005). It should be noted thatabundance estimates for ﬁn whales were made based ondata obtained south of 60°S because the Japanese dedicated sighting surveys did not cover the main distribution areaof ﬁn whales in the austral summer, which is north of 60°S.
The circumpolar abundance of the Antarctic blue
whales based on IDCR/SOWER surveys was estimatedat 1,700 (95% Bayesian interval 860–2,900) in 1996.
Although this ﬁgure corresponds to less than 1% of theoriginal levels, blue whales are increasing at 7.3% perannum (95% Bayesian interval 1.4–11.6%) (Figure 8)(Branch et al., 2004).

Figure 8.

Posterior abundance trajectory (and 95% interval)
from 1965, and the IDCR/SOWER abundance estimates for blue whales in the Antarctic (d) (after
Branch et al., 2004).

Figure 9.

8Possible inter-speciﬁc eﬀects of the recovery of large
whalesAntarctic minke whale
Trend in abundance
Hakamada et al. (2013) estimated abundance and abundance trends of Antarctic minke whale in the Indian sector (Area IV) and Paciﬁc sector (Area V) based on Japanese
dedicated sighting surveys (JARPA and JARPAII), under
the assumption of g(0)=1. Abundance estimates for the
Indian sector range from 16,562 (CV=0.542) in 1997/98
to 44,945 (CV=0.338) in 1999/00, while those for the Paciﬁc sector range from 74,144 (CV=0.329) in 2004/05 to
151,828 (CV=0.322) in 2002/03. Estimates of the annual
rates of increase in abundance are 1.8% with a 95% CI of[−2.5%, 6.0%] for the Indian sector and 1.9% with a 95%
CI of [−3.0%, 6.9%] for the Paciﬁc sector. Adjustments toallow for the g(0) being less than 1 were made by theapplication of a regression model, developed from theresults of the Okamura-Kitakado (OK) method estimateof minke whale abundance from the IDCR-SOWER surveys, which provides estimates of g(0) from the statisticsof the minke whale school size distribution in a stratum.
With this adjustment, abundance estimates increased byan average of 32,333 (106%) for the Indian sector and89,245 (86%) for the Paciﬁc sector, while the estimatesof annual rates of increase and their 95% CIs changedslightly to 2.6% [−1.5%, 6.9%] for the Indian sector and1.6% [−3.4%, 6.7%] for the Paciﬁc sector. See Figure 9 forthe abundance trends in the two sectors.
In 2012 the IWC SC agreed to a new best abundance
estimate for Antarctic minke whale in Antarctic open waters south of 60°S, based on IDCR/SOWER sighting data.
The estimates were 720,000 based on the sighting datacollected during the CPII (1985/86–1990/91) with 95% CI(512,000, 1,012,000), and 515,000 based on the sighting
data collected during the CPIII (1992/93–2003/04) with

The best case estimates of annual abundance of Antarctic minke whale in the Indian sector (Area IV) andPaciﬁc sector (Area V) together with their 95% CIs. The IDCR/SOWER estimates for a common northernboundary for CPII and CPIII are shown by the open triangles. Conﬁdence intervals include allowance for
additional variance. The dashed curves indicate the 95% Cis for the exponential model (after Hakamada
et al., 2013).

Cetaceans as indicators of historical and current changes in the Antarctic ecosystem

95% CI (361,000, 733,000). No signiﬁcant statistical diﬀerences were found between the CPII and CPIII estimates(IWC, 2013).
The estimates in the Indian sector (Area IV) were55,237 (CV: 0.17) in CPII and 59,677 (CV: 0.34) in CPIII. In
the Paciﬁc sector (Area V) were 300,214 (CV: 0.13) in CPII
and 183,915 (CV: 0.11) in CPIII.
By considering the 95% CIs of the estimates it can besuggested that the stocks of Antarctic minke whales arebroadly stable with at most a slight decline.
Changes in spatial distribution
Figures 10 and 11 show the temporal changes in whale
species composition in the Indian sector (Area IV) andPaciﬁc sector (Area V) based on Japanese dedicated sighting surveys. In the Indian sector, humpback whales weremore frequently observed than Antarctic minke whales
in recent years (Figure 10). In the Paciﬁc sector Antarctic
minke whale is still the predominant species (Figure 11).
The increase in abundance of the large whales once
depleted by commercial whaling implies changes in thespatial distribution of whale species. Murase et al. (2014)
examined Japanese dedicated sighting survey data to
study the spatial distribution of Antarctic minke whales
and humpback whales in the Indian sector (Area IV)
during three periods: early (1989/1990, 1991/1992
and 1993/1994), middle (1995/1996, 1997/1998 and1999/2000) and late (2001/2002, 2003/2004 and2005/2006). Spatial distribution was estimated using generalized additive models (GAM). Presence or absence ofwhales was used as the response variable while seaﬂoor
depth, distance from shelf break and longitude were used
as explanatory variables.
Mean probabilities of occurrence of Antarctic minke
whales in the survey area in early, middle and late periods were 0.41, 0.46 and 0.41 while those of humpback
whales were 0.14, 0.35 and 0.46. Occupied area indices(probabilities of occurrence of Antarctic minke whales
minus probabilities of occurrence of humpback whales)
were also calculated. If the index is 1, only Antarctic
minke whales were present in a grid cell while only humpback whales were present if the index is −1. If the index is0, probabilities of presence of Antarctic minke whales andhumpback whales in a grid cell were identical. Mean occupied area indices in early, middle and late periods were0.28, 0.11 and −0.07, respectively. The authors concluded
that the spatial distribution of humpback whales expanded during the period investigated while that of Antarctic
minke whales remained stable. The results suggested thepossibility that competition between humpback and Ant-

Figure 10.

Sighting composition of Antarctic minke, humpback and ﬁn whales in the Indian sector (AreaIV) for two Japanese dedicated sighting surveys,
1995/96 and 2005/06. Note that humpback
whales are more frequently observed than
Antarctic minke whales in recent years (see explanation of data source in Pastene et al., 2014).

Figure 11.

Sighting composition of Antarctic minke, humpback and ﬁn whales in the Paciﬁc sector (AreaV) for two Japanese dedicated sighting surveys,
1996/97 and 2006/07. Note that Antarctic minke
whale is still the predominant species. The composition of humpback whales in the total sighting
have increased but at a slower pace compared to
the Indian sector (see explanation of data source
in Pastene et al., 2014).

arctic minke whales for habitat in the Indian sector (AreaIV) during the period studied could have been intensiﬁed
as abundance of humpback whales increased. A summary of the results are presented in Figure 12.
It should also be mentioned here that Antarctic minke
9Technical Reports of the Institute of Cetacean Research (2018)

Figure 12.

Probability of occurrence of humpback (left) and Antarctic minke (right) whales in the Indian sector (AreaIV) in three periods: early, middle and late (after Murase et al., 2014). Red indicate high probability ofoccurrence.

whales have been observed more frequently in polynias within the pack-ice in recent years (Matsuoka, pers.
comm. 2016), reﬂecting perhaps a response to the geographical expansion of humpback whales.
Nutritional conditions
Nutritional condition in Antarctic minke whales has been
investigated through diﬀerent indices, blubber thickness
under the assumption that the amount of lipids increase
with the thickness of the blubber, girth and total fat.
These data have been collected for more than 25 years by
Japanese research (JARPA and JARPA II) in the Indo-Paciﬁcregion of the Antarctic (Areas IV and V).
Konishi et al. (2008) showed the results of the annual trend in energy storage in sexually mature Antarctic
minke whales based on data collected for a period of
18 years in the Indo-Paciﬁc region. Regression analyses
clearly showed that blubber thickness, girth and fat
weight have been decreasing for nearly two decades.
The decrease per year was estimated at approximately
10

0.02 cm for mid-lateral blubber thickness and 17 kg for
fat weight, corresponding to 9% for both measurements
over the 18-year period (Figure 13).
Konishi et al. (2014) reported the results of an analysisof temporal trend in stomach content weight in the Antarctic minke whale in a 20-year period. A linear mixedeﬀects analysis showed a 31% (95% CI 12.6–45.3%)
decrease in the weight of stomach contents over the 20
years since 1990/91. A similar pattern of decrease wasfound in both males and females, except in the case of
females sampled at higher latitude in the Ross Sea. These
results were consistent with the decline in energy storage reported above. Humpback whales are not found in
the Ross Sea, where both Antarctic krill and ice krill (E.
crystallorophias) are available, and where the authors
found no change in prey availability for Antarctic minke
whales.
These studies suggested a decrease in the availabilityof krill for Antarctic minke whales.

Cetaceans as indicators of historical and current changes in the Antarctic ecosystem

Figure 13.

Yearly trends in blubber thickness (a), fat weight (b) and girth (c) in Antarctic minke whale in the Indo-Paciﬁc region of
the Antarctic from 1987/88 to 2004/05 (after Konishi et al., 2008).

Figure 14.

Yearly age-related changes of hepatic Hg concentrations in the Antarctic minke whale (after Honda et al., 2006).

Pollutant loads
Honda et al. (2006) studied yearly age-related accumulations of Hg in the Antarctic minke whales collected in the
period 1980–1999 in the Indo-Paciﬁc region. One of therelevant results was that of Hg levels and growth rates
in young age-groups (1–5 years old): Hg levels and bodyweights for the 1997/99 season groups were signiﬁcantly
lower than those for the 1988/90 groups. On the other
hand they examined the Hg levels in krill from the whale
stomach, and no yearly signiﬁcant diﬀerences were
found. They concluded that the diﬀerences in Hg levels inyounger age groups was due to diﬀerences in the amountof food intake by the whale i.e. whale are consuming lessfood in recent years (Figure 14). These results are consistent with the decrease in stomach contents weight in
Antarctic minke whales in recent years.

arctic minke whales has stabilized or slightly increased
since the 1970ʼs and 1980ʼs. The recruitment has consistently been broadly stable since the 1970ʼs and the totalabundance estimated by SCAA has been stable or slightly
decreasing (Figure 6). These observations are consistent
with the observation of deteriorated nutritional conditions since the 1980ʼs.
Other species
A recent study showed that the regional population of
the krill-eater Adelie penguin have almost doubled inabundance since the 1980ʼs and have been increasing
since the earliest counts in the 1960ʼs. These results ofdecadal-scale change were based on a combination ofextensive new population survey data, new population
estimation methods, and re-interpretation of historicalsurvey data (Southwell et al., 2015).

Demographic parameters
As shown in Figure 5 the age at sexual maturity for Ant11

Technical Reports of the Institute of Cetacean Research (2018)

Summary
Evidence available since the 1980ʼs has shown that thenutritional conditions for Antarctic minke whales havenot been optimal as revealed by the decrease in energy
storage and stomach content weight since the 1980ʼs.
This observation is consistent with the observations thatage at sexual maturity has stabilized or slightly increasedin the 1970ʼs and that the recruitment was stable after
1970. The stable trend in recruitment is consistent withthe total abundance of Antarctic minke whale estimatedby sighting data which has been broadly stable since the1980ʼs.
The observations above suggest less availability of krill
for Antarctic minke whales. Less availability of krill for thisspecies could result from competition with other recovering krill-eater large whale species, e.g. the reversal ofLawsʼ ʻkrill surplusʼ hypothesis. The implication of theseʻdeterioratedʼ nutritional conditions is the possibility ofa decrease of Antarctic minke whale abundance in thenear future.
Southwell et al. (2015) provided two explanations forthe increase of the Adelie penguins in East Antarctica.
The ﬁrst was that harvesting of baleen whales, krill andﬁsh across East Antarctic waters through the 20th century
could have reduced competition between Adelie penguins and other predators for food, and improved preyavailability. The second was that a proposed reduction in
sea-ice extent in the mid-20 century may also have beneﬁted Adelie penguins by enabling better access to theocean for foraging. Since a strong recovery of krill-eater
large baleen whales has been reported since the 1980ʼs,
it can be suggested that environmental factors are moreplausible for explaining the increase of Adelie penguinssince the 1980ʼs. Probably environmental changes havestronger eﬀects on land-based predators such as the penguins than on sea-based predators such as whales.

CONCLUDING REMARKSThe review of scientiﬁc evidence for ecosystem changes
in the Indo-Paciﬁc region of the Antarctic has highlighted
the importance of long-time monitoring research programs focused on the collection of biological data of krill
predators (land-based and sea-based predators) as well
as data on sea ice cover and environmental variables.
Evidence of environmental changes are more marked inWest Antarctic than East Antarctic. An implication of thisis that in East Antarctic (Indo-Paciﬁc region), competition for space and food could better explain the patternof changes in biological and demographic parameters
observed among sea-based krill predators. However to12further investigate the plausibility of this hypothesis it
will be necessary to obtain information on krill biomass
trends in the research area. There is some partial information based on dedicated krill surveys in the past but
the information is scattered and needs to be combinedwith new surveys in a comprehensive and consistent way
so that time series data can be obtained.

ACKNOWLEDGEMENTS
We thank researchers and crew members of the JARPA/
JARPAII and IDCR/SOWER surveys that collected samples
and data used in many published studies, on which thisreview is based.

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