Technical Reports of the Institute of Cetacean Research (2017)

pp. 58–67

Technical Report (not peer reviewed)

Temporal trend of total mercury levels in common minke, sei and
Brydeʼs whales in the western North Paciﬁc in the period 1994–2014
Genta Yasunaga* and Yoshihiro Fujise
Institute of Cetacean Research, Toyomi-cho 4–5, Chuo-ku, Tokyo 104–0055, Japan*Contact e-mail: yasunaga@cetacean.jp

ABSTRACTTo examine yearly changes of total mercury (Hg) concentrations in common minke (Balaenoptera
acutorostrata), sei (B. borealis) and Brydeʼs (B. edeni) whales in the western North Paciﬁc, total Hg concentrations were measured in muscle samples from whales collected between 1994 and 2014 by JARPN/JARPNII.
Averages and standard deviations of total Hg concentrations in the samples of mature male common minke
whales from coastal area oﬀ Kushiro and Sanriku were 0.22±0.05, 0.22±0.06, ppm wet wt, and from oﬀshore sub-areas 7, 8 and 9 were 0.22±0.05, 0.22±0.06 and 0.24±0.10 ppm, respectively. The concentrationin sei whales from sub-area 9 was 0.044±0.013 ppm, and that in Brydeʼs whales from sub-areas 8 and 9 was
0.044±0.013 ppm. Multiple robust linear regression analysis was carried out considering several variables:
sampling year, sampling location (longitude and latitude), sampling date, body length, blubber thickness andmain prey item observed in their stomach. Results of the analyses can be summarized as follow: no signiﬁcant
yearly changes of total Hg concentrations in muscle of common minke whales oﬀ Kushiro, Sanriku and subarea 8; no signiﬁcant yearly changes of total Hg concentrations in muscle of Brydeʼs whales in sub-areas 8 and
9; signiﬁcant yearly changes of total Hg concentrations in muscle of minke whales from sub-areas 7 and 9, andsigniﬁcant yearly changes of total Hg concentrations in muscle of sei whales from sub-area 9. Temporal trendsof total Hg concentrations have not been observed in environmental samples, lower trophic organisms andbaleen whales, except for those cases mentioned above. Temporal changes in total Hg concentrations in common minke whales (sub-areas 7 and 9) and sei whales (sub-area 9) may reﬂect changes in their food habitsrather than changes of background levels of total Hg in the marine environment. Consequently, it is concluded
that the temporal trend of total Hg concentrations in the marine habitat of baleen whales in the western North
Paciﬁc remained stable in the research period.

INTRODUCTION
Mercury (Hg) is one of the most neurotoxic chemicals
to human and wildlife in the environment. It is released
into the ocean from a variety of sources such as volcanic
emission, degassing from soil and combustion of coalfrom power plants. It is released in the form of elemental
mercury, and is accumulated into top predators throughthe food chain in the marine ecosystem. There is a needto monitor Hg in the oceans.
Cetaceans have been used to monitor spatial and temporal trends of Hg in the oceans (Sanpera et al., 1993;
Borrell and Reijnders, 1999). Pollutants can be monitoredin wide areas and integrated in some way over time byusing cetacean as indicators because they are located at
the top of the food chain in the marine ecosystem, andare mobile and long-lived animals.
58

The present study examines temporal changes of total
Hg concentrations in common minke, sei and Brydeʼswhales in the western North Paciﬁc. To understand thepattern of accumulation of total Hg in whales it is important to consider some biological information such assex and sexual maturity of the animals, and ecologicalinformation such as feeding habits of whales and indicators of body condition. The present study, which is basedon samples collected by the Japanese Whale Research
Program under Special Permit in the western North
Paciﬁc (JARPN) and its Second Phase, JARPNII in the period 1994–2014, considers such biological and ecologicalinformation to understand and interpret the pattern ofaccumulation of Hg in whales.

Temporal trend of total mercury levels in common minke, sei and Brydeʼs whales in the western North Pacific in the period 1994–2014

MATERIALS AND METHODS
Whale sampling
Common minke (1994–2014), sei (2002–2014) and
Brydeʼs (2002–2013) whales were sampled in sub-areas
7, 8 and 9 under the JARPN and JARPNII. In addition common minke whales were sampled in coastal areas oﬀ Sanriku (2003–2014) and oﬀ Kushiro (2002–2014) (Figure 1).
Details of the sampling procedures are given in Tamura
et al. (this TEREP-ICR issue).
For all sampled whales, biological sampling and measurements were conducted on the research base vessel, Nisshin-maru in the oﬀshore component (JARPN/
JARPNII), and at the land research stations in the case of
the coastal component (JARPNII).

Figure 1.

Sub-areas surveyed by the JARPN and the JARPNII
surveys, excluding the EEZ of foreign countries.

Figure 2.

Tissue sampling for Hg analysis
Figure 2 shows the overall process for Hg analysis fromthe tissue sampling at the ﬁeld to the laboratory work.
Muscle tissues were collected for total Hg analysis.
Biological information
The kind of biological information obtained from eachsampled whale was described by Bando (this TEREP-ICR
issue). In this study information on body length, blubber
thickness and reproductive status of whales was used.
Only mature male animals were used. Males of common
minke, Brydeʼs and sei whales were deﬁned as sexuallymature by testis weight of more than 290 g, 560 g and
1,090 g (heavier side), respectively. Only common minke
whales from the Okhotsk Sea-West Paciﬁc stock (O-stock)
as identiﬁed by the microsatellite DNA analysis (Gotoet al., this TEREP-ICR issue), were used.
The forestomach contents have proved suﬃcient for
determination of the common minke whale diet (seeKonishi et al., this TEREP-ICR issue). Data on prey composition and preyʼs Hg concentrations used in the present
study were based on contents from forestomach andanalyses conducted by Yasunaga and Fujise (2009) and
MHLW (2005).
Tables 1–3 show the number of samples by sub-areaand year, and the biological information used for common minke, sei and Brydeʼs whales in this study.
Measurement of total mercury concentration in muscle
of whales
Total Hg analysis was conducted at the laboratory of the Institute of Cetacean Research (ICR) in Tokyo and also at the

Schematic diagram of total Hg analysis in muscle tissue of whales.
59Technical Reports of the Institute of Cetacean Research (2017)
Table 1
Biological data and total mercury concentrations in muscle of common minke whales (mature males) from the western North Paciﬁcduring 1994 and 2014.
Sub areaYearnBody length(m)

Blubber thickness(cm)

Muscular Hg(ppm wet wt.)

7

1996
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2009
2011
2012
Total

18
35
31
6
27
33
11
6
17
16
41
11
23
19
294

(7.36±0.28)(7.50±0.27)(7.41±0.29)(7.30±0.25)(7.45±0.31)(7.52±0.27)(7.42±0.40)(7.56±0.21)(7.51±0.23)(7.39±0.39)(7.47±0.26)(7.71±0.26)(7.47±0.34)(7.37±0.34)(7.46±0.30)(3.1±0.4)(2.6±0.4)(3.6±0.8)(3.6±1.0)(2.4±0.5)(3.7±0.8)(2.4±0.5)(4.1±0.6)(3.3±0.9)(2.4±0.7)(3.0±0.7)(3.2±0.7)(3.5±0.6)(2.6±0.4)(3.1±0.8)(0.23±0.05)(0.20±0.05)(0.20±0.04)(0.24±0.04)(0.21±0.04)(0.22±0.03)(0.20±0.07)(0.22±0.02)(0.27±0.04)(0.22±0.05)(0.23±0.04)(0.21±0.06)(0.25±0.05)(0.24±0.08)(0.22±0.05)

Kushiro

2002
2004
2005
2006
2007
2008
2009
2010
2011 spring2011 autumn
2012
2013
2014
Total

14
23
25
10
14
5
6
3
4
10
5
23
10
152

(7.22±0.34)(7.43±0.24)(7.44±0.32)(7.52±0.29)(7.33±0.59)(7.26±0.21)(7.54±0.37)(7.49±0.21)(7.40±0.41)(7.26±0.37)(7.48±0.21)(7.33±0.35)(7.33±0.28)(7.38±0.35)(4.3±1.0)(4.2±0.6)(3.5±0.6)(4.4±0.8)(4.1±0.8)(4.3±1.0)(4.3±0.4)(3.8±0.4)(3.2±0.6)(4.4±1.0)(5.0±0.7)(5.0±0.9)(5.0±0.8)(4.3±0.9)(0.21±0.06)(0.20±0.03)(0.25±0.05)(0.22±0.03)(0.23±0.06)(0.25±0.01)(0.21±0.07)(0.23±0.02)(0.24±0.03)(0.23±0.02)(0.21±0.06)(0.19±0.04)(0.21±0.09)(0.22±0.05)

Sanriku

2003
2005
2006
2007
2008
2009
2010
2012
2013
2014
Total

8
3
6
10
3
1
5
2
2
2
42

(7.10±0.35)(7.27±0.13)(7.24±0.32)(7.50±0.30)(7.49±0.32)(6.85±)(7.32±0.24)(7.31±)(7.52±)(7.38±0.39)(7.32±0.32)(2.9±0.5)(2.5±0.3)(2.8±0.2)(2.5±0.4)(3.2±0.3)(2.8± )
(3.1±0.6)(3.4± )
(3.5± )
(2.7±0.3)(2.8±0.5)(0.18±0.04)(0.29±0.02)(0.19±0.05)(0.25±0.08)(0.19±0.03)(0.21±)(0.20±0.05)(0.18±)(0.30±)(0.31±0.04)(0.22±0.06)

ﬁeld research stations of the coastal and oﬀshore components. The muscle sample was stripped externally to avoid
contamination before analysis, approximately 100 mg oftissue was weighed, and set on the total Hg analyser (Nippon Instruments Co., MA-3000) which applies a thermal
60

decomposition system (Figure 2). To increase the accuracyof the determination, triplicate analyses were performedon each sample. Accuracy and precision of the present
system were conﬁrmed using standard reference materials, DORM-3 (NRCC, muscle of dogﬁsh).

Temporal trend of total mercury levels in common minke, sei and Brydeʼs whales in the western North Pacific in the period 1994–2014
Table 1
Table 1 minke whales (mature males) from the western North Paciﬁc
Biological data and total mercury concentrations in muscle of common
Continued.
during 1994 and 2014.
Sub areaYearnBody length(m)

Blubber thickness(cm)

Muscular Hg(ppm wet wt.)

8

1996
1997
1998
2001
2002
2003
2005
2006
2007
2008
2009
Total

11
26
28
13
5
27
2
24
10
3
6
155

(7.44±0.33)(7.44±0.36)(7.54±0.25)(7.78±0.34)(7.71±0.21)(7.50±0.24)(7.54±)(7.51±0.26)(7.55±0.26)(7.52±0.05)(7.23±0.34)(7.52±0.30)(2.6±0.5)(3.5±0.6)(2.6±0.4)(2.3±0.4)(3.2±0.6)(2.4±0.4)(2.2± )
(2.9±0.6)(2.3±0.4)(2.7±0.4)(3.2±0.6)(2.8±0.6)(0.25±0.04)(0.27±0.06)(0.23±0.05)(0.21±0.05)(0.20±0.11)(0.18±0.06)(0.16±)(0.21±0.05)(0.22±0.06)(0.23±0.02)(0.20±0.06)(0.22±0.06)

9

1994
1995
1997
2000
2001
2002
2003
2004
2005
2006

16
68
39
12
19
21
28
50
25
16

(7.42±0.27)(7.45±0.31)(7.41±0.32)(7.51±0.23)(7.69±0.29)(7.55±0.21)(7.50±0.32)(7.47±0.21)(7.49±0.29)(7.56±0.30)(3.5±0.7)(3.1±0.5)(3.1±0.5)(2.6±0.5)(2.5±0.4)(2.9±0.6)(2.9±0.6)(3.9±0.7)(3.5±0.6)(3.4±0.6)(0.36±0.16)(0.27±0.06)(0.25±0.11)(0.16±0.03)(0.19±0.05)(0.20±0.05)(0.18±0.05)(0.20±0.07)(0.17±0.05)(0.28±0.08)

2007
2008
2009
2010
2013
Total

4
36
5
9
3
351

(7.56±0.20)(7.45±0.26)(7.41±0.41)(7.66±0.23)(7.59±0.22)(7.49±0.28)(2.2±0.3)(3.1±0.7)(3.0±0.8)(3.2±0.5)(4.0±1.3)(3.2±0.7)(0.18±0.09)(0.30±0.15)(0.19±0.06)(0.22±0.03)(0.19±0.06)(0.24±0.10)

Table 2Biological data and mercury concentrations in muscle of sei whales (mature males) from the western North Paciﬁc during 2002 and 2014.
Sub area
9

Total

Year

n

Body length(m)

Blubber thickness(cm)

Muscular Hg(ppm wet wt.)

2002
2003
2004
2005
2006
2007
2011
2012
2013
2014
2002–2014

5
5
5
5
5
5
15
21
26
21
113

(13.60±0.16)(13.80±0.43)(13.61±0.04)(13.54±0.38)(13.84±0.51)(13.92±0.08)(13.82±0.42)(13.74±0.45)(13.76±0.50)(13.80±0.36)(13.76±0.41)(4.7±0.5)(4.7±0.7)(4.1±0.6)(4.8±0.6)(4.9±0.6)(5.8±0.7)(4.8±0.8)(5.0±0.5)(5.6±0.8)(4.8±0.5)(5.0±0.8)(0.050±0.012)(0.057±0.007)(0.050±0.009)(0.049±0.011)(0.052±0.006)(0.058±0.010)(0.039±0.009)(0.030±0.011)(0.040±0.009)(0.053±0.009)(0.044±0.013)

61

Technical Reports of the Institute of Cetacean Research (2017)
Table 3
Biological data and mercury concentrations in muscle of Brydeʼs whales (mature males) from the western North Paciﬁc during 2002 and 2013.
Sub area
8, 9

Total

Year

n

Body length(m)

Blubber thickness(cm)

Muscular Hg(ppm wet wt.)

2002
2004
2006
2007
2011
2012
2013
2002–2013

5
5
5
5
3
6
9
38

(12.67±0.43)(12.59±0.36)(12.36±0.47)(12.63±0.35)(12.31±0.27)(12.85±0.48)(12.79±0.38)(12.64±0.41)(4.5±0.6)(4.4±0.7)(4.5±0.5)(3.9±0.7)(3.6±0.5)(4.8±1.0)(4.7±0.8)(4.4±0.8)(0.051±0.008)(0.045±0.008)(0.040±0.011)(0.049±0.008)(0.022±0.005)(0.046±0.019)(0.047±0.010)(0.044±0.013)

Statistical analysis
The yearly changes of total Hg concentrations in muscle
of whales were assessed by multiple robust linear regression in the context of several variables (R Development
Core Team, 2006). The following independent variables
were considered: ʻYear,ʼ ʻDate,ʼ ʻLatitude,ʼ ʻLongitude,ʼʻBody length,ʼ ʻBlubber thicknessʼ and ʻMain prey itemʼ.
All variables except the main prey item, were logarithmically transformed.
Categorical parameters of main prey items used in theanalyses were the following: minke whale and sub-area
7 (Japanese anchovy: Engraulis japonicus, Euphausiids, Japanese ﬂying squid: Todarodes paciﬁcus, mackerel: Scomber japonicus, Japanese sardine: Sardinops
melanostictus, Paciﬁc saury: Cololabis saira and walleye
pollock: Theragra chalcogramma); minke whale andsub-area 8 (anchovy, copepods, Euphausiids, Japanese
ﬂying squid, mackerel and Paciﬁc saury); minke whaleand sub-area 9 (anchovy, Atka mackerel: Pleurogrammus monopterygius, Copepods, Euphausiids, mackerel,minimal armhook squid: Berryteuthis anonychus, oceanic lightfish: Vinciguerria nimbaria, Paciﬁc pomfret:Brama japonica, salmonids, Paciﬁc saury); sei whale andsub-area 9 (Japanese anchovy, Copepods, Euphausiids,
mackerel, minimal armhook squid, Japanese sardine
and Paciﬁc saury); Brydeʼs whale and sub-areas 8 and 9(Japanese anchovy, Euphausiids and mackerel). Anchovyis the baseline prey item in sub-areas 7–9 so the eﬀects
of anchovy being the main prey item are included in themodel intercepts. Categorical parameters of main prey
items were not used in the case of Sanriku and Kushiro. In
Sanriku the prey items of common minke whales were almost all sand lance. In Kushiro the diversity of prey items
was too high in relation to the number of whale samples.
The cases of whales with empty stomach and damageby harpoon were excluded from the analysis. Furthermore, a generalized additive model (GAM) was used to62

examine the ﬂexion point in the yearly changes of total Hgconcentrations in muscle of whales. A p value of less than
0.05 was considered to indicate statistical signiﬁcance inall tests. These statistical analyses were performed using
the free software R, version 3.3.0.

RESULTS
Tables 1–3 show the total Hg concentrations in muscle ofcommon minke, sei and Brydeʼs whales, respectively.
The results of multiple robust linear regression to examined yearly changes in the context of several variables
are given in Tables 4–10. F statistics showed that theoverall regression was statistically signiﬁcant in all cases(p<0.05). Only in the cases of minke whales from subareas 7 and 9 (Tables 4, 8), and sei whales from sub-area
9 (Table 9), the total Hg concentrations were signiﬁcantly
associated with sampling year. In these cases the simple
and GAM plots against research year are shown in Figures
3 and 4 for minke whales in sub-areas 7 and 9, respectively, and in Figure 5 for sei whale in sub-area 9.
Slight ﬂexion points of yearly trends of muscular total
Hg were observed in minke whales from sub-area 9 in2008 and in sei whales from sub-area 9 in 2012.

DISCUSSION
Results of the analyses can be summarized as follows:
no signiﬁcant yearly changes of total Hg concentrations
in muscle of common minke whales oﬀ Kushiro, Sanriku
and sub-area 8; no signiﬁcant yearly changes of total Hgconcentrations in muscle of Brydeʼs whales in sub-areas
8 and 9; signiﬁcant yearly changes of total Hg concentrations in muscle of minke whales from sub-areas 7 and 9,
and signiﬁcant yearly changes of total Hg concentrations
in muscle of sei whales from sub-area 9.
It should be noted here that no signiﬁcant trends oftotal Hg concentrations were found in prey species suchas the krill, Japanese anchovy and Paciﬁc saury from 1994

Temporal trend of total mercury levels in common minke, sei and Brydeʼs whales in the western North Pacific in the period 1994–2014
Table 4
Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of common minke whales from subarea 7ʼ as
the dependent variable.

Table 6
Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of common minke whales from oﬀ Sanrikuʼ
as the dependent variable.

a) Analysis of Variancea) Analysis of Variance

SourceSourceRobust residual SE

0.20

Robust residual SE

0.2543

R2

0.135

R2

0.2374

Adjusted R2

0.0969Adjusted R2

0.1067b) Variablesb) Variables

Model
Intercept

p value

Model

B

SE

−198.59771 53.16048 −3.736 p<0.05

Intercept

−50.2

286.8

−0.175 0.862

Year

31.50

44.65

0.71

0.485

B

SE

T

Year

26.054

6.788

3.84

p<0.05T

p value

Body length−0.138

0.237

−0.58

0.559

Body length−0.029

0.025

−1.15

0.257

Blubber thickness

0.059

0.060

0.98

0.328

Blubber thickness−0.400

0.274

−1.46

0.153

Latitude−0.156

0.541

−0.29

0.773

Latitude23.79217.674

1.35

0.187

Longitude−0.064

0.568

−0.11

0.911

Longitude−56.288

43.990

−1.28

0.209

Date

0.038

0.017

2.21

p<0.05

Date

1.079

0.569

1.90

0.066

MainPrey_Euphausiids

0.076

0.045

1.70

0.089

MainPrey_JFSquid

0.062

0.054

1.15

0.250

MainPrey_Mackerel

0.372

0.026

14.53

p<0.05

MainPrey_Sardine−0.294

0.036

−8.25

p<0.05

MainPrey_Saury

0.097

0.047

2.07

p<0.05

MainPrey_WalleyePollock

0.138

0.049

2.83

p<0.05

Table 7
Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of common minke whales from subarea 8ʼ as
the dependent variable.
a) Analysis of Variance

Table 5Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of common minke whales from oﬀ Kushiroʼ
as the dependent variable.
a) Analysis of Variance

SourceRobust residual SE

0.2439R20.3021

Adjusted R2

0.2473b) Variables

Source

Model

B

SE

T

p value

Robust residual SE

0.2834R2Intercept

163.35009

95.2101.716 0.088

0.3472Adjusted R2

Year−17.288

13.636

−1.27

0.207

0.3176

Body length

1.219

0.534

2.29

p<0.05

Blubber thickness

0.288

0.108

2.67

p<0.05Latitude−0.281

1.628

−0.17

0.863

Longitude−7.181

2.778

−2.59

p<0.050.481b) Variables
SET

p value

Model

B

Intercept

63.6

98.2

0.648

0.518

Date

0.495

0.699

0.71

Year

−2.14

10.61

−0.20

0.840

MainPrey_Copepods−0.020

0.080

−0.25

0.805

Body length

0.904

0.843

1.07

0.285

MainPrey_Euphausiids−0.104

0.144

−0.72

0.471

Blubber thickness−0.322

0.077

−4.15

p<0.05

MainPrey_JFSquid−0.137

0.101

−1.36

0.175

Latitude

5.384

6.941

0.78

0.439

MainPrey_Mackerel

0.309

0.170

1.82

0.072

Longitude−14.215

13.618

−1.04

0.298

MainPrey_Saury

0.180

0.063

2.84

p<0.05

Date

0.119

0.149

0.80

0.424

to 2008 (Yasunaga and Fujise, 2009), and that total Hgconcentrations in surface water of the North Paciﬁc did
not change from the 1980ʼs to 2000ʼs (Laurier et al., 2004;
Sunderland et al., 2009).
Firstly those cases where signiﬁcant yearly trend wasobserved, are discussed.

In sub-area 7, total Hg concentrations in muscle ofcommon minke whales (period 1996–2012) were signiﬁcantly associated with Intercept (−), Year (+), Date (+),
Main prey items (mackerel, sardine, saury and Walleye
pollock) (Table 4). A comparison of total Hg concentrations in muscle of common minke whale and in the whole63

Technical Reports of the Institute of Cetacean Research (2017)
Table 8Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of common minke whales from subarea 9ʼ as
the dependent variable.
a) Analysis of Variance

Table 9Results of multiple linear regression analysis with ʻtotal Hg levels
in muscle of sei whales from subarea 9ʼ as the dependent variable.
a) Analysis of Variance
SourceSourceRobust residual SE

0.2526

Robust residual SE

0.2834R2

0.3344R2

0.3472Adjusted R2

0.2293

2

Adjusted R

0.3176b) Variablesb) Variables
B

SE

Intercept

330.3

55.7

5.932 p<0.05

Year−43.74

7.31

−5.99

Body length

0.150

0.441

T

p value

Model

0.34

T

p value

Model

B

SE

Intercept

405.9

137.4

2.954 p<0.05

Year−54.46

18.35

−2.97

p<0.05p<0.05

Body length

3.579

1.029

3.48

p<0.05

0.735

Blubber thickness

0.024

0.262

0.09

0.927

Blubber thickness

0.164

0.092

1.78

0.076

Latitude

0.910

1.188

0.77

0.446

Latitude

1.809

0.672

2.69

p<0.05Longitude−1.233

1.431

−0.86

0.392
0.215

Longitude−0.784

0.899

−0.87

0.384

Date

−0.602

0.481

−1.25

Date

−1.424

0.301

−4.73

p<0.05

MainPrey_Copepods−0.182

0.096

−1.90

0.061

MainPrey_AtkaMackerel−0.368

0.076

−4.86

p<0.05

MainPrey_Euphausiids−0.259

0.159

−1.63

0.107

MainPrey_Copepods

0.044

0.224

0.20

0.843

MainPrey_Mackerel−0.064

0.097

−0.66

0.510

−0.012

0.105

−0.12

0.906

MainPrey_Euphausiids−0.177

0.159

−1.12

0.266

MainPrey_Min.arm squid

MainPrey_Mackerel

0.050

0.139

0.36

0.722

MainPrey_Sardine

0.193

0.104

1.85

0.069

MainPrey_MAFSquid−0.386

0.209

−1.84

0.066

MainPrey_Saury−0.136

0.096

−1.41

0.164

MainPrey_OceanicLightfish−0.439

0.060

−7.30

p<0.05

MainPrey_PaciﬁcPomfret

0.930

0.335

2.78

p<0.05

MainPrey_Salmonids

0.213

0.132

1.61

0.108

MainPrey_Saury

0.188

0.050

3.73

p<0.05body of the main prey species in the stomachs is shownin Table 11. Total Hg concentrations in minke whales having mackerel in the stomach were the highest while thosehaving sardine in stomach was the lowest. The numberof minke whales having mackerel and sardine in theirstomach were only one and two whales, respectively.
These observations indicate that total Hg concentrationsof common minke whales from sub-area 7 may be lessaﬀected by total Hg in the prey species.
In sub-area 9, total Hg concentrations in muscle ofcommon minke whales (period 1996–2012) were signiﬁcantly associated with Intercept (+), Year (−), Latitude (+),
Date (−) and Main prey items (Atka mackerel, oceanic
lightfish, Paciﬁc pomfret and saury) (Table 8). A comparison of total Hg concentrations in muscle of minke whale
and in the whole body of the main prey species in thestomachs is shown in Table 12. Total Hg concentrations
in common minke whales having Paciﬁc pomfret in theirstomach were the highest, and total Hg concentrationsin Paciﬁc pomfret were one or two orders of magnitude
higher than those in the other prey items. Also total
Hg concentrations in common minke whales having
zooplankton such as copepods and euphausiids were
64

Table 10
Results of multiple robust linear regression analysis with ʻtotal
Hg levels in muscle of Bryde's whales from subareas 8 and 9ʼ as
the dependent variable.
a) Analysis of Variance
SourceRobust residual SE

0.2025R2

0.3592Adjusted R2

0.1029b) Variables
Model

B

SE

InterceptTp value−101.1

162.8

Year

9.93

22.02

−0.621 0.541
0.45

0.657

Body length

2.431

1.453

1.67

0.110

Blubber thickness−0.320

0.317

−1.01

0.325

Latitude

1.693

1.534

1.10

0.283

Longitude

1.933

2.810

0.69

0.499

Date

0.503

0.376

1.34

0.196

MainPrey_Euphausiids−0.106

0.071

−1.51

0.148

MainPrey_Mackerel−0.240

0.274

−0.88

0.391

lower than the others. Total Hg concentrations in thezooplankton were one or two orders of magnitude lower
than those in the other prey items. Furthermore, yearlychanges were observed in food items of the common
minke whales in the same period (Konishi et al., 2016).

Temporal trend of total mercury levels in common minke, sei and Brydeʼs whales in the western North Pacific in the period 1994–2014

Figure 3.

Simple plots and smoothing plots using the generalized additive model of total Hg concentrations inmuscle of common minke whales (mature males, O-stock) in sub-area 7 against research years during theperiod 1996–2012.

Figure 4.

Simple plots and smoothing plots using the generalized additive model of total Hg concentrations inmuscle of common minke whales (mature males, O-stock) in sub-area 9 against research years during theperiod 1994–2013.

Figure 5.

Simple plots and smoothing plots using the generalized additive model of total Hg concentrations inmuscle of sei whales (mature males) in sub-area 9 against research years during the period 2002–2014.

A peak of total Hg concentration was observed in 2008
(Figure 4), and Paciﬁc pomfret was observed in the stomach contents of common minke whales from sub-area 9in that year (Tamura et al., 2009). These results indicatethat changes of total Hg concentrations in common minke
whales in sub-area 9 reﬂect changes in food habits of thiswhale species rather than changes of background levelsof total Hg in the marine environment.
In sub-area 9, total Hg concentrations in sei whales (period 2002–2014) were signiﬁcantly associated with Intercept (+), Year (−) and Body length (+) (Table 9), whereas

those in 2012 were lower than the others in the GAM
plots (Figure 5). A comparison of total Hg concentrations
in muscle of sei whale and in the whole body of the main
prey species in the stomachs are shown in Table 13. Total
Hg concentrations in sei whales having anchovy and sardine in the stomach were slightly higher than the othertwo whale species from sub-area 9. These results indicatethat total Hg concentrations of sei whales from sub-area
9 may be less aﬀected by total Hg in the food items.
Temporal trends of total Hg concentrations have notbeen observed in environmental samples, lower trophic
65

66

Ave.±SDn
Ave.±SDn( 0.21±0.051)
238(0.037±0.025)
20*( 0.22±0.049)
33(0.005±0.003)
19*

Euphausiids

Copepods

Euphausiids( 0.17±2(0.018±66**

Sardine

)

)

Saury

Mackerel(0.19±0.12)
5MAFSquid( 0.11±1

Oceanic
Lightfish

Ave.±SDn
Ave.±SDn(0.049±0.013)
11(0.037±0.025)
20*

Anchovy(0.043±0.014)
48(0.005±0.003)
5*

Copepods(0.045±0.016)
9(0.005±0.003)
19*

Euphausiids(0.045±0.016)
9(0.020±0.002)
5*

MackerelSaury( 0.24±0.056)
28(0.045±)2*

Walleye Pollock(0.040±0.005)
3

Min.arm squid(0.052±)2(0.018±)66**

Sardine(0.041±0.015)
7(0.038±0.015)
41*

Saury

) ( 0.44±0.26 ) ( 0.32±0.08 ) ( 0.24±0.09 )65257( 0.23±0.03 ) (0.027±) (0.038±0.015)
3*
41**
41*

Paciﬁc PomfretSalmonids( 0.23±0.043)
26(0.038±0.015)
41*

Table 13Comparison between Hg concentrations in muscle of sei whales and in the whole body of prey species in sub-area 9.

*: Yasunaga and Fujise (2009); **: Ministry of Health, Labour and Welfare (2005)

Whole of prey spp.

Muscle of whales

Mackerel( 0.33±)
1(0.020±0.002)
5*) ( 0.17±0.07 ) ( 0.18±0.09 ) ( 0.19±0.04 )615
3) (0.005±0.003) (0.005±0.003) (0.020±0.002)
5*
19*
5*

Atka Mackerel( 0.22±0.10 ) ( 0.13±
83
1(0.037±0.025) (0.086±
20*
61**

Anchovy*: Yasunaga and Fujise (2009); **: Ministry of Health, Labour and Welfare (2005)

Ave.±SDnWhole of prey spp. Ave.±SDnMuscle of whales( 0.22±0.03)
8(0.058±)57**

JFSquid

Table 12Comparison between Hg concentrations in muscle of common minke whales and in the whole body of prey species in sub-area 9.

*: Yasunaga and Fujise (2009); **: Ministry of Health, Labour and Welfare (2005)

Whole of prey spp.

Muscle of whales

Anchovy

Table 11Comparison between Hg concentrations in muscle of common minke whales and in the whole body of prey items in sub-area 7.

Technical Reports of the Institute of Cetacean Research (2017)

Temporal trend of total mercury levels in common minke, sei and Brydeʼs whales in the western North Pacific in the period 1994–2014

Figure 6.

Schematic diagram showing total Hg ﬂow in the western North Paciﬁc food web.

organisms and baleen whales, except for common minke
whales collected from sub-areas 7 and 9, and sei whales
from sub-area 9 from 1990ʼs to 2000ʼs. Total Hg concentrations in these exceptions may reﬂect changes in food habits rather than changes of background levels of total Hg in
the marine environment. This phenomenon is illustrated
in Figure 6, which shows that diﬀerences in food habitat
explain the pattern of Hg accumulation of baleen whales.
Consequently, it is concluded that temporal trend of
total Hg concentrations in the marine habitat of baleen
whales in the western North Paciﬁc remained stable in
the research period. In future, other variables such asage of the animals, should be included in the analyses toinvestigate yearly trend of Hg concentrations in baleen
whales.

ACKNOWLEDGEMENTS
We gratefully acknowledge the researchers and crew
members that participated in JARPN/JARPNII surveys forthe collection of data and samples.

REFERENCES
Borrell, A. and Reijnders, P.J.H. 1999. Summary of temporal
trends in pollutant levels observed in marine mammals. J.
Cetacean Res. Manage. (Special Issue 1): 149–155.
Konishi, K., Isoda, T. and Tamura, T. 2016. Decadal change of
feeding ecology in sei, Brydeʼs and common minke whales in
the oﬀshore of the Western North Paciﬁc. Paper SC/F16/JR23
presented to the JARPNII Review Workshop, Tokyo, February
2016 (unpublished). 19 pp.

Laurier, F.G.J., Mason, R.P., Gill, G.A. and Whalin, L. 2004.
Mercury distribution in the North Paciﬁc Ocean: 20 years ofobservations. Mar. Chem. 90: 3–19.
Ministry of Health, Labour and Welfare. 2005. Survey results ofmercury levels in ﬁsheries. http://www.mhlw.go.jp/topics/
bukyoku/iyaku/syoku-anzen/suigin/dl/050812-1-05.pdf
R Development Core Team 2006. R: A language and environment for statistical computing. R Foundation for StatisticalComputing, Vienna, Austria. ISBN 3-900051-07-0, URL
http://www.R-project.org.
Sanpera, C., Capelli, R., Minganti, V. and Jover, L. 1993. Totaland organic mercury in North Atlantic ﬁn whales: distribution
pattern and biological related changes. Mar. Pollut. Bull. 26:
135–139.
Sunderland, E.M., Krabbenhoft, D.P., Moreau, J.W., Strode, S.A.
and Landing, W.M. 2009. Mercury sources, distribution, andbioavailability in the North Paciﬁc Ocean: Insights from data
and models. Global Biogeochem. Cycles 23: GB2010.
Tamura, T., Otani, S., Isoda, T., Wada, A., Yonezaki, S., Mori, M.,
Tsunekawa, M., Fukutome, K., Nakai, K., Satoh, H., Nomura,I., Nagatsuka, S., Umatni, M., Koyanagi, T., Takamatsu, T.,
Kawabe, S., Kandabashi, S., Watanabe, H, Kumagai, S., Sato,H. and Ogawa, T. 2009. Cruise Report of the second phaseof the Japanese Whale Research Program under Special
Permit in the Western North Paciﬁc (JARPN II) in 2008 (partI)—Oﬀshore component—. Paper SC/61/O4 presented to the
IWC Scientiﬁc Committee, May 2009 (unpublished). 49 pp.
Yasunaga, G. and Fujise, Y. 2009. Temporal trends and factors aﬀecting mercury levels in common minke, Brydeʼs
and sei whales and their prey species in the western North
Paciﬁc. Paper SC/J09/JR23 presented to the JARPNII Review
Workshop, Tokyo, January 2009 (unpublished). 14 pp.

67