Fish Sci (2012) 78:761–773
DOI 10.1007/s12562-012-0508-2

ORIGINAL ARTICLE

Fisheries

Basin-scale distribution pattern and biomass estimationof Japanese anchovy Engraulis japonicus in the western North
Pacific
Hiroto Murase • Atsushi Kawabata • Hiroshi Kubota •Masayasu Nakagami • Kazuo Amakasu • Koki Abe •Kazushi Miyashita • Yoshioki Oozeki

Received: 5 September 2011 / Accepted: 24 April 2012 / Published online: 1 June 2012Ó The Japanese Society of Fisheries Science 2012

Abstract The distribution pattern and biomass of theJapanese anchovy Engraulis japonicus in the offshore
region of the western North Pacific (north of 35°N and westof 170°E) were studied using a quantitative echosounder.
This is the first attempt at such a study in this region. Data
were collected in summer from 2004 to 2007. The biomass
was estimated using data collected at 38 kHz. Species
compositions in the backscatterings from pelagic fish wereassigned based on the results of trawl hauls taking account

H. Murase (&)
The Institute of Cetacean Research, 4-5 Toyomi-cho,
Chuo-ku, Tokyo 104-0055, Japane-mail: muraseh@affrc.go.jp
Present Address:
H. Murase
National Research Institute of Far Seas Fisheries, 2-12-4
Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan
A. Kawabata  H. Kubota  Y. Oozeki
National Research Institute of Fisheries Science, 2-12-4
Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan
M. Nakagami
Hachinohe Station, Tohoku National Fisheries ResearchInstitute, 25-259 Shimo-mekurakubo, Hachinohe,
Aomori 031-0841, Japan
K. Amakasu
Tokyo University of Marine Science and Technology,
4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan
K. AbeNational Research Institute of Fisheries Engineering,
7620-7 Hasaki, Kamisu, Ibaraki 314-0408, Japan
K. Miyashita
Field Science Center for the Northern Biosphere, Hokkaido
University, 3-1-1 Minato, Hakodate, Hokkaido 041-8611, Japanof sea surface temperature (SST). Japanese anchovy tended
to be high density to the west of 153°E and were distributedin an SST range of 9–24 °C. Although the temporal andspatial coverage of the survey differed each year, at least
1.5–3.4 million tons of Japanese anchovy were present in
the survey area between 2004 and 2007. To take account of
the spatial coverage of the survey each year, the mostreliable biomass estimate for this region in the time period
was 3.4 million tons (coefficient of variation 0.22).
Keywords Abundance  Acoustic  Distribution 
Hydroacoustic  Kuroshio  Oyashio  Pelagic fish 
Stock assessment

Introduction
Old World anchovies, Engraulis spp., are distributed
worldwide, mainly in coastal waters [1]. Among these, theJapanese anchovy E. japonicus is distributed around Japanand adjacent waters. The Pacific stock of the Japanese
anchovy is distributed on the Pacific coast side of Japan [2].
Although this stock tends to inhabit the coastal region, likeother Engraulis spp., it is well documented that the distribution range expands offshore to as far as 180° longitude
when the stock is abundant [3–9].
Japanese anchovy in the offshore region is considered to
be transported from the spawning ground on the coastalside of the Kuroshio by the Kuroshio Extension [10, 11].
However, it has been reported that Japanese anchovy also
spawn offshore [12–14]. Therefore, offshore Japanese
anchovy are considered to be a mixture of the offspring of
inshore and offshore spawners [9, 15]. The results of several studies indicate that the biological characteristics ofJapanese anchovy distributed in inshore and offshore

123

762

waters are different. Funamoto and Aoki [12] demonstrated
that Japanese anchovy located offshore spawn at a colder
water temperature than inshore anchovy. Takasuka et al.
[16] further demonstrated that batch fecundities andspawning frequencies in relation to sea surface temperature(SST) are different for inshore and offshore. Yukami et al.
[17] suggested that Japanese anchovy grow faster offshore
than individuals located inshore based on otolith microstructure analysis. Tanaka et al. [15] found higher stablecarbon and nitrogen isotope ratios in Japanese anchovy
inshore than offshore. The difference in the isotope ratio
may reflect differences in feeding habits as well as differences in the baseline levels of food webs inshore andoffshore.
The Pacific stock of Japanese anchovy is an important
target species for coastal commercial fisheries. The abundance (number of individuals) and the biomass (total
weight) have been estimated by the Fisheries Agency ofJapan for the purpose of stock management based on acohort analysis using commercial catch data and an egg
production method using a systematic net sampling data
set. Since the cohort analysis and egg production data are
restricted to those obtained near coastal waters, the estimates of abundance and biomass are only available in the
coastal waters. Although it is generally believed that millions of tons of the Pacific stock of Japanese anchovy are
distributed offshore during periods of high abundance, theexact biomass has not been reported.
Echosounders have been commonly used to obtainbiomass estimations as well as in distributional studies of,for example, the European anchovy E. encrasicolus in the
Bay of Biscay [18–21] and Mediterranean Sea [22–24], theSouthern African anchovy E. capensis on the South African continental shelf [25–28], the northern anchovy E.
mordax off the west coast of Baja California [29], thePeruvian anchovy E. ringens in Peruvian waters [30, 31],
and the Argentine anchovy E. anchoita in Argentine waters[32]. Large-scale acoustic surveys have been conducted in
the East China Sea and the Yellow Sea to estimate thebiomass of the Japanese anchovy [33–35]. Regional studiesof the Japanese anchovy using echosounders have been
conducted in Japanese coastal waters [36–39]. However, todate, no attempt has been made to estimate the biomass of
the Pacific stock of Japanese anchovy in the offshore
region of the western North Pacific using a quantitativeechosounder.
Although there is little commercial fishing of the Japanese anchovy in the offshore region of the western North
Pacific, it plays a key role in the marine ecosystem as aprey of predators, including commercially important species such as neon flying squid Ommastrephes bartramii[40], salmonids [6], skipjack tuna Katsuwonus pelamis[41], and baleen whales [42, 43].

123

Fish Sci (2012) 78:761–773

In this paper, the results of the study on the distribution
pattern and biomass of Japanese anchovy in the offshore
region of the Kuroshio–Oyashio transition zone of thewestern North Pacific are presented. Data obtained by trawl
and echosounder surveys conducted from 2004 to 2007 areused in this study. This is the first attempt at a basin scale
study designed to determine the distribution and to estimate
the biomass of the stock of Japanese anchovy in the western North Pacific.

Materials and methods
Survey area, period and vessels
The survey area was in the western North Pacific (Fig. 1).
The southern, northern, eastern, and western boundaries ofthe survey area were 35°N, the economic exclusive zone
(EEZ) boundaries claimed by foreign countries (except to
the west of 147°E), 170°E, and the eastern coastline ofJapan, respectively. The continental shelf region of Japan(shallower than 200 m) was not included except in the2006 survey when a small survey effort was undertaken on
the shelf. Eight small blocks were established to takeaccount of the general oceanographic conditions, as suggested in Yasuda [44], in the survey area and to estimate
the biomass using a stratified random sampling method[45]. The echosounder survey was conducted in summer(May–August) (Table 1). A cetacean sighting survey vessel, Kyoshin-Maru No. 2 (KS2: 368GT), engaged in an
acoustic survey as a part of the Japanese Whale Research
Program under Special Permit in the western North PacificPhase II (JARPN II). Two trawler fisheries research vessels, Ohmi-Maru (OHM; 403 GT) and Hokuho-Maru(HOK: 664 GT), towed midwater trawls as part of theannual survey of Pacific saury Cololabis saira abundance
conducted by Tohoku National Fisheries Research Institute. The trawl data were collected in June and July.
Because the echosounder and trawl surveys were undertaken independently, the tracklines and timing of thesesurveys were not exactly the same.
Acoustic data collection
Acoustic data were collected by KS2 while she conductedcetacean sighting surveys on the survey tracklines. Zigzag
tracklines were set systematically within the survey area. A
calibrated quantitative echosounder (Simrad EK500, Norway) was operated during the daytime on KS2 to acquire
acoustic data with the operating frequency at 38 kHz. The
transducer was hull-mounted at a depth of 4.3 m below thesea surface. The data were recorded with the aid of
Echoview software (Myriax, Australia).

Fish Sci (2012) 78:761–773

763

Trawling
OHM and HOK performed midwater trawls at predetermined stations to collect biological samples, with the primary purpose of estimating the abundance of Pacific saury.
Meridional tracklines were set approximately 4° apart in thelongitudinal direction, and sampling was conducted at threestations per day along the tracklines as well as while moving
between the tracklines. The midwater trawls (NST-99,
Nichiomo, Japan) were 86.3 m long with a mouth openingof 900 m2 and a 6.0 m cod end with a 17.5 9 17.5 mm
mesh inner. The specifications of the midwater trawl are
given in detail in Ueno et al. [46]. The towing speed of the
trawl net was 4–5 knots, and the water depth layer sampled
by the midwater trawl was 0–20 m. Floats were attached tothe bridle of the trawl so that the trawl could be towed at thesurface where the Pacific saury is mainly distributed [46].
The towing continued for 1 h at each station.
Sea surface temperature

Fig. 1 Survey area (a) and locations of midwater trawl samplingstations (b). Eight survey blocks were established in the area

The SST was recorded every hour by using a temperature
sensor positioned at the bottom of KS2 ,while acoustic data
were collected along the tracklines. The SST at eachtrawling station was also recorded by using sampled waterwith a thermometer. Level 3 seasonal composite SST datain the summer (21 June to 20 September of each year)
derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the satellite Aqua were used
to observe the qualitative relationship between the SST and
the distribution patterns of Japanese anchovy. The Aqua
MODIS data were downloaded from Ocean Color Web(http://oceancolor.gsfc.nasa.gov/).

Table 1 Timing of theechosounder survey for eachblock in each year

2004

2005

2006

14–29 May–

29 June–6 July

Block

1

Year
2007
22–24 June
28–30 July2
330 May–8 June–

17–19 May
25–29 June

24–27 June
25–28 July

10–16 June

17–26 May

18–24 June

10–16 June

22–29 June

12–17 July

11–17 July

27 June–2 July

4–

27 June–5 July

19–25 May

20–24 July5–

22 August

5–26 August

15–28 May

2–18 August

18 July–21 August

17 July–4 August

29 May–10 June

27 May–11 June

7–18 June

2–10 July

12–26 June

25 May–5 June

10–20 July6

22–23 August
7

30 June–2 July
7–8 July
19–31 July

8

–

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Fish Sci (2012) 78:761–773

Fig. 2 Typical echosigns ofpelagic fish schools recordedwith the echosounder. The data
were recorded at 38°130 N and
144°520 E on 23 May 2004. The
grid size is 0.1 n.mile(horizontal direction) 9 10 m(vertical direction)

Data analysis
Japanese anchovy, Japanese sardine Sardinops melanostictus, and mackerel (including both chub Scomber japonicus
and spotted S. australasicus mackerel) are the only small
pelagic fish species to form dense schools in the survey area.
Although Pacific saury also form dense schools, they are
distributed near the sea surface [46]. Therefore, they are
rarely detected with an echosounder. For this reason, Pacificsaury were not considered in this analysis. Species seen inechosigns could not be identified by trawling in the field
because the echosounder and trawl survey vessels wereoperated independently. Accordingly, pelagic fish weredistinguished from the other backscatterings based on theirshapes and auxiliary acoustic characteristics (e.g., backscattering intensity) [47, 48], although the species compositions of pelagic fish schools could not be identified. Typicalechosigns of pelagic fish observed during the survey are
shown in Fig. 2. To add the species composition information
to the acoustic data, the proportion of the species composition by number of individuals was obtained using trawl data.
The proportion was calculated for each 1 °C SST increment,
as were the arithmetic mean lengths. Because anchovy larvae
have a different acoustic property from that of juveniles andadults [49], these were not considered in the analysis. The
target strengths (TS) of anchovy and mackerel corresponding to their mean lengths were calculated using formulas (1)[50] and (2) [51], respectively:
20 z
TS ¼ 20 log Lt  log 1 þ 67:6
ð1Þ
3
10
TS ¼ 20 log Lt  70:9

ð2Þ

where TS is in decibels (dB), Lt is the total length (cm), andz is the water depth (m) corresponding to the Japanese123

anchovy distribution. Formula (1) was used as a
substitution for sardine. The mean distribution depths (z)of Japanese anchovy were used in the formula as suggested
by Murase et al. [52] and were calculated as:P
zi sAið3Þz ¼ P
sAi
where zi is the ith 1 m depth bin and sAi is the nautical area
scattering coefficient (m2/n.mile2) of pelagic fish in the ith
depth bin. For this purpose, the sA of pelagic fish was
calculated every 1 n.mile of the survey transect in each 1 m
depth bin for a water depth of 10–150 m. Because the scale
length [Ls; from the most anterior part of the fish to themost posterior part covered by scales (cm)] of Japanese
anchovy was measured in the field, it was converted to Lt(cm) using the following formula:
Lt ¼ 1:092Ls þ 0:848:

ð4Þ

This formula was estimated by using data collected by theauthors in other locations (n = 591, R2 = 0.98). The Ls of
the Japanese sardine and the fork length of the mackerel
were used as substitutes for Lt because no conversion formula was available. The arithmetic mean lengths of theJapanese sardine and the mackerel were calculated for each
1 °C SST increment.
Acoustic data were analyzed with the aid of Echoview,
with the noise threshold set at -60 dB. The sA was calculated every 1 n.mile of the survey tracklines over adefined depth interval (10–150 m) for the purpose of biomass estimation. SST data recorded along the tracklines
every hour, corresponding to the midpoints of the 1 n.mile
intervals, were assigned to acoustic data. Time at the
midpoints of the intervals was rounded to the nearest hour.
A sA was allocated to Japanese anchovy, Japanese sardine,Fish Sci (2012) 78:761–773

765

and mackerel, respectively, based on their proportions in
the 1 °C SST increments. These proportions were applied
to the 1 n.mile intervals corresponding to the SST.
Although most Japanese anchovy are distributed in shallow
water (e.g., \50 m), they are also distributed in deeper
water ([50 m) according to our unpublished data as well as
results from other studies [53, 54], with the maximum
recorded water depth reported being 150 m [53]. Based on
information regarding the vertical distribution pattern ofJapanese anchovy, acoustic data obtained at depths from 10
to 150 m were used in the analysis. The biomass density(q; t/n.mile2) was calculated as:
 
sA
q¼
w
ð5Þ
rsp

The coefficient of variation (CV) of Bk was calculated as:pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃvarðBk ÞCVðBk Þ ¼
ð11Þ
Bk

where rsp is a spherical scattering cross-section and w is
the wet weight of anchovy corresponding to the mean
length calculated by using the SST in 1 °C increments. A
weight–length relationship formula reported by Kubota
et al. [2] was used:

ð14Þ

w ¼ 0:010L3:00
swhere w is the wet weight (g) of an individual. The
parameters in the formula are mean values between 1998
and 2007, with mean of n = 13,470 and mean of
R2 = 0.944. The rsp is derived as:
rsp ¼ 4p100:1TS

where, qk = mean q in the kth block, Nk = number oftransects in the kth block, qki = mean q on the Ith transect
in the kth block, and nki = number of 1 n.mile intervals on
the ith transect in the kth block. In this formula, eachtransect is regarded as a single biomass density sample.
The variance of qk was then calculated as:
P Nk
2 2
Nk
i¼1 ðqki  qk Þ nkivarðqk Þ ¼
:
ð8Þ


P Nk
2
Nk  1i¼1 nki
The biomass (Bk) in the kth block was estimated as:ð9Þ

where, Ak is the area of the kth block (n.mile2). The
variance of Bk was calculated with the following formula:varðBk Þ ¼ A2k varðqk Þ

B0 ¼

N
X

ð12Þ

Bk

k¼1

The variance of B0 was:varðB0 Þ ¼

NXvarðBk Þ

ð13Þ

k¼1

The CV of B0 was calculated as:pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃvarðB0 Þ
CVðB0 Þ ¼
B0

The degree of coverage (d) defined by Aglen [55] was used
to measure the sampling intensity of the echosoundersurvey. The degree of coverage is written as:
D
d ¼ pﬃﬃﬃ
A

ð15Þ

where D is the total length of the trackline (n.mile) and A isthe size of the survey area (n.mile2).

ð6Þ

The biomass and its variance in the survey area were
estimated based on the stratified random sampling method[45] as follows. The weighted mean of q for each block
was:
P Nk
i¼1 qki nki
qk ¼ Pð7Þ
Nki¼1 nkiBk ¼ Ak qk

The biomass (B0) in the survey area was calculated as:

ð10Þ

Results
Biological samples of pelagic fish were obtained at a totalof 347 trawl stations from 2004 to 2007 (Fig. 1). Amongthese, both Japanese anchovy and mackerel were sampled
simultaneously at 68 stations, with Japanese anchovy being
dominant ([90 % of the total catch in numbers) at 38
stations and mackerel being dominant at seven stations.
Japanese anchovy and mackerel were moderately mixed(up to 50%) at the remaining 23 stations. The species
composition of the pelagic fish in 1 °C increments is given
in Table 2. Japanese anchovy was the dominant catch inmost cases, but the proportion of mackerel increased when
the SST exceeded 14 °C, especially in 2004 and 2007. The
relatively large CV suggested that the number of sampledindividuals in each haul varied greatly.
The length frequency of Japanese anchovy in each 1 °C
SST increment indicated that small individuals tended tooccur when the SST was high, while large individuals
tended to occur when the SST was low, although no consistent change in length frequency was observed along the
SST gradient. The mean Lt (range 4.8–15.3 cm) and corresponding w (range 0.5–23.2 g) for anchovy are summarized in Table 3.

123

123
%

0

CV

%

CV

Mackerels
%

CV

9

13

6

7

4

7

7

1

3

1

Year

12

13

14

15

16

17

18

19

20

21

SST (°C)

18,96486

16,96136,547

4,989

12,041

58,261

3,485

20,82012,954

673

140

97

100

71

85

99

28

100

59

80

99

100

100

0.00

CV

0.00

1.41

0.00

1.43

2.26

1.70

2.44

2.20

3.46

2.81

2.63

2.13

2.00

0

0

0

13

2

0

2

64

0

3

5

0

0

0

0

%

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0.00

CV

0.00

0.00

0.00

2.45

0.00

1.73

2.41

0.00

3.46

2.83

0.00

0.00

0.00

0

Mackerels

42

35

3,086

532

12,80837

39,959

878

135

31

0

4

0

0

%

0

29

15

1

72

0

41

20

1

0

0

3

0

0.00

CV

0.00

1.41

0.00

2.00

2.39

1.55

1.63

2.13

3.44

2.83

0.00

2.67

0.00

7

–

–

–

–

9

5

8

2

8

9

7

10

4

9

6

8

9

1

15

16

17

18

19

12

8

6

11

10

10

10

14

4

9

13

6

11

8

0

1,211

1,005

343

11,360

44

5,632

5

1,817

0

0

23

0

100

100

90

100

100

100

100

100

0

0

100

0.00

1.41

1.75

1.49

2.81

1.05

1.79

2.24

2.48

0.00

0.00

2.24

0

2

0

0

0

0

1

0

0

1

0

0

Numberof individuals

Numberof individuals%

Japanese sardine

Japanese anchovy

0

0

0

0

0

0

0

0

0

100

0

0.00

2.83

0.00

0.00

0.00

0.00

2.65

0.00

0.00

1.73

0.00

0.00

0

4

0

40

11

0

0

0

0

0

0

0

Numberof individuals

0

0

0

10

0

0

0

0

0

0

0

0

0.00

1.54

0.00

2.24

2.83

0.00

0.00

0.00

0.00

0.00

0.00

0.00

3

5

8

12

8

13

13

8

11

5

4

1

Numberof hauls

11

11

100

Numberof hauls

13

10

1

0

2007

5

Number ofindividuals

2006

6

9

Number ofindividuals

Number ofindividuals

Numberof hauls

Japanese sardine

Numberof hauls

Japanese anchovy

2005

2004

Year

8

SST (°C)

–

–

–

–

–

–

–

–

99

99

96

99

52

6,437

847

90,160

6,826

3,332

14,762

5,217

81

60

3

0

0

Numberof individuals

0

89

38

85

20

31

100

97

99

100

100

%

0

100

100

0

0

0

%

Japanese anchovy

50,645

8,385

13,243

192

12

7,825

3

0

0

0

Number ofindividuals

Japanese anchovy

Table 2 Species composition in 1 °C increments for each year. Number of hauls and coefficient of variation are also shown

0.95

1.68

1.54

2.93

1.95

1.82

2.37

1.23

1.18

1.33

0.00

0.00

CV

–

–

–

–

2.35

2.00

1.94

0.99

1.33

2.81

2.45

0.00

0.00

0.00

CV

11

34

50

2

20

4

19

0

0

0

0

0

Numberof individuals

Japanese sardine

–

–

–

–

64

31

13

0

0

0

0

0

0

0

Number ofindividuals

Japanese sardine

0

2

0

0

0

0

0

0

0

0

0

0

%

–

–

–

–

0

0

0

0

0

0

0

0

0

0

%

0.90

2.00

2.35

2.24

2.65

2.67

2.64

0.00

0.00

0.00

0.00

0.00

CV

–

–

–

–

2.26

2.00

2.65

0.00

0.00

0.00

0.00

0.00

0.00

0.00

CV

766

1,347

15,69626,486

7,327

2

146

1

0

0

0

0

Numberof individuals

Mackerels

–

–

–

–

204

19

580

1

11

0

0

0

0

0

Number ofindividuals

Mackerels

11

60

15

80

69

0

3

1

0

0

0

0

%

–

–

–

–

0

0

4

1

48

0

0

0

0

0

%

1.25

1.23

2.01

3.31

1.46

2.35

3.41

2.65

0.00

0.00

0.00

0.00

CV

–

–

–

–

1.73

2.00

2.59

1.00

2.15

0.00

0.00

0.00

0.00

0.00

CV

766
Fish Sci (2012) 78:761–773

SST Surface sea temperature, CV coefficient of variation

1.51

0.00
31

40
18,786

4
0.00

0.98
0

0
0

114
1.43

0.00
69

60
27,931

9
1

4
–

0.00
0

–
–

0
0.00

–

0
0.00
0

0

–
–
–

1
21

0

–
20

–

Numberof individuals
CV–

Numberof individuals
Numberof individuals%
Numberof individuals
Numberof individuals

767%
Numberof individuals

Numberof hauls

2006

Year
SST (°C)

Table 2 continued

Japanese anchovy

Japanese sardine%

CV

Mackerels%CV

Numberof hauls

2007Japanese anchovy

CV

Japanese sardine%

CV

Mackerels%

CV

Fish Sci (2012) 78:761–773

The mean distribution depth of anchovy in each blockfor each year is summarized in Table 4. The mean distribution depth from 2004 to 2007 was 24 m.
The estimated biomass of anchovy in each block foreach year and in the survey area for each year is shown in
Tables 5 and 6, respectively. Not all of the blocks weresurveyed in 2004 and 2005, and although all the blocks
were surveyed in 2006 and 2007, the degree of coverage
indicated that the coverage of the 2006 survey (d = 5.3)
was greater than that of the 2007 survey (d = 3.2). The
estimated biomass in 2004, 2005, 2006 and 2007 was 2.84
(CV 0.22), 1.52 (CV 0.43), 3.36 (CV 0.22), and 2.76
(CV 0.31) million tons, respectively. The contribution ofblocks 1, 2, and 3, which were located relatively closer toJapan, to the estimated biomass over the entire survey area
was high in 2004 (93 %), 2006 (86 %), and 2007 (82 %).
In general, the biomass in blocks further offshore thanblock 3 were relatively low, except in block 6 in 2005.
Maps of the Japanese anchovy distribution from 2004 to2007 are shown in Fig. 3. No anchovy was observed to thenorth of 47°N in 2006 and 2007 while the water to thenorth of 47°N was not surveyed in 2004 and 2005. There
were no apparent latitudinal density trends in the region to
the south of this latitude. Japanese anchovy tended to beconcentrated to the west of 153°E in 2004, 2006, and 2007,
although the annual distribution patterns were not consistent. No survey was undertaken in the region to the west of
150°E in 2005. The mean density from 2004 to 2007 bylongitude in 1° increments indicated that anchovy weremainly distributed to the west of 153°E (Fig. 4). The
highest density (1,477 t/n.mile2) in 1 n.mile interval wasobserved at 44°330 N and 166°480 E in 2005. The meandensities from 2004 to 2007 by SST in 1 °C increments
indicated that the anchovy densities were comparable from11 to 23 °C except at 21 °C (Fig. 5), with the density
declining beyond this temperature range. The horizontal
distribution patterns of Japanese anchovy in relation to SST
depicted in Fig. 3 also indicate that Japanese anchovy were
distributed in an intermediate SST range in the survey area.

Discussion
The results of this study confirm the general belief thatmillions of tons of the Pacific stock of Japanese anchovy
are distributed in the offshore area in the western North
Pacific. At least 1.7–3.4 million tons of Japanese anchovy
were present in the survey area between 2004 and 2007.
Because the temporal and spatial coverage of the survey
differed each year, the trend in terms of the biomass during
the period could not be estimated. Surveys were undertaken
in all survey blocks in 2006 and 2007. The degree ofcoverage in 2006 and 2007 was 5.3 and 3.2, respectively,

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Fish Sci (2012) 78:761–773

Table 3 Total length (Lt) and wet weight (w) of Japanese anchovy in 1 °C increments for each year
SST (°C)

Year
2004

2005

2006

2007

Lt (cm)CV of Ltw (g)

Lt (cm)CV of Ltw (g)

Lt (cm)CV of Ltw (g)

Lt (cm)CV of Lt

w (g)

8

–

–

–

–

–

–

14.9

0.07

21.3

–

–

–

9

9.6

0.00

5.1

–

–

–

–

–

–

–

–

–

10
11

11.5
8.6

0.13
0.15

9.2
3.6

–
10.5

–
0.15

–
6.9

–
13.9

–
0.05

–
17.2

8.9
7.8

0.12
0.18

3.9
2.6

12

13.2

0.07

14.6

15.3

0.05

23.2

10.3

0.21

6.5

7.6

0.20

2.3

13

13.3

0.05

14.8

7.1

0.32

1.9

13.7

0.07

16.4

10.1

0.07

6.0

14

14.0

0.10

17.6

4.8

0.18

0.5

10.9

0.29

7.8

13.5

0.12

15.6

15

13.6

0.06

16.0

13.2

0.16

14.4

9.7

0.09

5.4

8.9

0.25

4.0

16

13.4

0.07

15.3

10.7

0.07

7.4

8.0

0.26

2.8

14.3

0.05

18.5

17

13.7

0.09

16.2

7.5

0.23

2.3

6.3

0.15

1.2

10.0

0.08

5.8

18

12.8

0.18

13.2

–

–

–

8.8

0.47

3.8

9.7

0.12

5.3

19

14.5

0.05

19.5

–

–

–

–

–

–

7.2

0.28

1.9

20

9.8

0.10

5.5

–

–

–

–

–

–

9.5

0.09

4.9

21

9.7

0.13

5.4

–

–

–

–

–

–

9.9

0.03

5.6

Table 4 Mean distribution depth of Japanese anchovy in each block for each year
Block

Year
2004

2005

2006

2007

All years

Mean distribution
depth (m)CV

Mean distribution
depth (m)CV

Mean distribution
depth (m)CV

Mean distribution
depth (m)CV

Mean distribution
depth (m)

CV

1

26

0.40

–

–

33

0.59

22

0.42

28

0.51

2

19

0.26

–

–

20

0.37

38

0.89

28

0.91

3

19

0.58

13

0.33

20

0.39

30

0.79

22

0.64

4

–

–

15

0.19

–

–

84

0.45

16

0.65

5

–

–

14

0.26

18

0.36

–

–

14

0.31

6

17

0.19

11

0.27

18

0.24

–

–

12

0.30

7

17

0.19

16

0.42

22

0.22

21

0.99

18

0.49

8

–

–

13

0.69

95

0.44

30

0.59

23

1.11

All

22

0.41

12

0.40

25

0.66

35

0.89

24

0.82

with an d value of [6 indicating sufficient coverage [55].
Therefore, the 2006 estimate of 3.4 million tons (CV 0.22)
can be considered to be the most reliable biomass estimate
obtained in the survey area between 2004 and 2007. The
contributions of blocks 1, 2, and 3 to the estimated biomassin the entire survey area were high in 2004, 2006, and2007. Kubota et al. [2] carried out an assessment of the
Pacific stock of Japanese anchovy based on a cohort
analysis using commercial catch data for the coastal watersof Japan [2]. According to these authors, the biomass levelfrom 2004 to 2007 was high compared with that of theprevious 30 years. However, the trend declined from 2004

123

to 2007 after reaching its peak in 2003. The estimatedbiomass reported in this paper can be considered to be atlevel compared with that over the past 30 years, but to be
declining during the past 5 years on the assumption thatinshore and offshore biomass estimates are linked.
Some of the larval Japanese anchovy that hatch in the
coastal side of the Kuroshio are believed to be transported
offshore by the Kuroshio Extension and then to migrate
northward into the Kuroshio–Oyashio transition region[56]. Consequently, it can be assumed that small individuals prevail in areas with a high SST, while large individuals prevail in areas with a low SST. The mean lengths

Fish Sci (2012) 78:761–773

769

Table 5 Estimated biomass of Japanese anchovy in each block for each year
Year

Block
Block 1

Block 2Block 3Block 4Block 5

No survey

No survey

Block 6Block 7Block 87

7

No survey

822

1,201

2004
Nk

8

6

6

Rni

424

1,055

1,259

q

35.14

9.43

1.30

0.69

0.87

Ak

50

81

101

137

108

Bk

1.77

0.76

0.13

0.09

0.09

CV (Bk)

0.33
1.89

0.23
3.72

0.28
3.95

0.52
2.22

0.66
3.65

No survey

No survey

8

7

5

1,097

924

399

5.16

1.57

4.03

d
2005
Nk

6

2

878

142

q

0.17

2.75

Ak

101

61

137

108

113

Bk

0.02

0.17

0.70

0.17

0.46

CV (Bk)

0.95

0.58

0.86

0.95

0.25

2.76

0.58

2.97

2.81

1.19

Rni

d

Excludeddue to low
coverage

2006
Nk

6

5

5

3

6

5

6

2

Rni

538

634

637

251

836

774

598

411

q

16.86

8.30

13.85

0.00

0.42

0.73

1.51

1.06

Ak

50

81

101

61

141

137

108

113

Bk
CV (Bk)

0.85
0.44

0.67
0.60

1.40
0.35

0.00
0.00

0.06
0.89

0.10
0.19

0.16
0.79

0.12
0.63

d

2.39

2.23

2.00

1.02

2.23

2.09

1.82

1.22

Nk

4

5

3

3

4

5

3

5

Rni

425

732

344

81

232

501

307

208

q

2.13

19.50

5.69

0.16

0.00

0.00

0.67

3.69

Ak

50

81

101

61

141

137

108

113

2007

Bk

0.11

1.57

0.58

0.01

0.00

0.00

0.07

0.42

CV (Bk)

0.46

0.39

0.94

0.47

–

–

1.23

0.40

1.89

2.58

1.08

0.33

0.62

0.93

0.93

d

2

0.62

Nk Number of transects in the kth block, ni number of 1 n.mile intervals, q biomass density (t/n.mile ), Ak area of the kth block (10 n.mile2), Bk
biomass in the kth block (106 t), d degree of coverageof the Japanese anchovy were calculated in 1 °C SST
increments in our study based on this assumption. However, no consistent change in length frequency wasobserved along the SST gradient although small individuals did tend to occur when at high SST while large individuals tended to occur at low SST. Small individuals (e.g.,juveniles) may have been underrepresented in the trawl
samples because of gear selection. The use of sampling
gears, such as a multiple layer opening/closing MatsudaOozeki-Hu-Trawl (MOC-MOHT) [57], specificallydesigned to sample larval and juvenile pelagic fish, isnecessary to have more realistic length frequency of the3

Japanese anchovy. Although the sampling locations arealso an important factor when determining mean length,
they were not considered in this study because the echosounder and trawl survey vessels operated independently interms of time and location. It has been reported that offshore Japanese anchovy spawn at an SST as low as 5 °C
[12]. Individuals hatched offshore at a low SST might
therefore be reflected in the length frequency at a low SST,
as seen at 11 °C in this study. There were several differences between the biological characteristics of inshore andoffshore adult Japanese anchovy [12, 15–17]. The distribution patterns of eggs and larvae of Japanese anchovy

123

770

Fish Sci (2012) 78:761–773

Table 6 Estimated biomass ofJapanese anchovy in the survey
area for each year

ni number of 1 n.mile intervals,
A0 surveyed area (103 n.mile2),B0 biomass (106t), d degree ofcoverage

Year
2004

2005

2006

2007

Surveyed blocks

1, 2, 3, 6, 7

3, 4, 6, 7, 8

1–8

1–8

Rni

4,761

3,440

4,679

2,830

A0

477

520

792

792

B0

2.84

1.52

3.36

2.76

CV (B0)

0.22

0.43

0.22

0.31

d

6.89

4.77

5.26

3.18

Fig. 3 Distribution patterns ofJapanese anchovy from 2004 to
2007. Bars Relative densities ofanchovy. The highest density(1,477 t/n.mile2) in 1 n.mile
intervals was observed at
44°330 N and 166°480 E in 2005.
Thin black lines Surveyed
tracklines. Seasonal composite
surface seawater temperture(SST) data in the summer,
derived from the Moderate
Resolution Imaging
Spectroradiometer (MODIS)
aboard the satellite, Aqua, arealso shown

were estimated by performing a numerical particle-tracking
experiment [11] because anchovy can be transported passively by currents. However, the factors that determine the
distribution patterns of juvenile and adult Japanese
anchovy, which can move actively, have yet to be examined. It is expected that their distribution patterns are also
related to their life history as well as environmental factors
other than the SST and currents. The spawning season of
the Japanese anchovy is from spring to autumn [2]. The
timing and location of the hatching could also affect the
distribution patterns.
The mean distribution depth of pelagic fish, including
the Japanese anchovy, was around 20 m in this study.
Although the Japanese anchovy has been observed at
depths of up to 150 m [53], it is mainly confined to shallow
water. The mean distribution depth in block 8 in 2006
(95 m) and in block 4 in 2007 (84 m) was deeper than that123

in other blocks. However, as only a few schools observedin these two blocks contributed to the mean depths, they
can be considered to be exceptions. We assumed that the
species composition in water deeper than 20 m is the sameas that in water less than 20 m deep. The assumption is not
far from the true species composition given the mean distribution depth of pelagic fish. However, the vertical distribution patterns of pelagic fish in the survey area should
be studied to validate this assumption.
Both Japanese anchovy and mackerel were sampled
simultaneously at 68 stations, with mackerel being thedominant pelagic fish in terms of number at seven of thesesampling stations and Japanese anchovy and mackerel
being moderately mixed at 23 stations. Although theechosigns of mackerel near the coast of Japan are distinctive, they have rarely been reported offshore [47]. Japanese
anchovy and mackerel found offshore would be in the same

Fish Sci (2012) 78:761–773

Fig. 4 Mean density of Japanese anchovy by longitude from 2004 to
2007

Fig. 5 Mean density of Japanese anchovy by SST from 2004 to 2007

schools in some cases, as indicated by the trawl sampling
results. It is difficult to distinguish between these two
species based solely on echosign characteristics. Chub andspotted mackerel hatched inshore are considered to betransported offshore by currents in a manner similar to theJapanese anchovy [58, 59]. Japanese anchovy and mackerel
hatched inshore at almost the same location and time would
end up in almost the same offshore environment. Based on
the above assumption, species composition was assigned
according to acoustic data in relation to the SST. As withmean length, the sampling locations were not considered inthis study because the echosounder and trawl survey vessels were operated independently with respect to time andlocation. However, the species composition in schools of
pelagic fish could be related to factors other than SST.
Such factors should be investigated in a future study.
Several factors could contribute to either over- or
underestimation. There are three potential factors that
could contribute to underestimation. First, the continental

771

shelf area of Japan where Japanese anchovy fishing is
mainly conducted has not been well surveyed. Any biomass estimation undertaken on the continental shelf region
will increase the biomass reported in this study. Second,
Japanese anchovy distributed at depths above 10 m were
not considered because the echosounder could not detectechoes in water shallower than 10 m. Third, the biomass ofJapanese anchovy larvae was not included in the data
because it has a different acoustic property from juvenile
and adult anchovy [49].
A change in the assumptions associated with TS will
either increase or decrease the estimated biomass reported
here. Murase et al. [52] pointed out that the effect of pitch
angle on biomass estimation is not negligible even if thedepth-dependent TS is taken into account. A change inpitch angle will result in either the over- or underestimationof the biomass depending on the angle. Uncertainties
related to the species identification methods, as describedabove, could affect the estimated biomass. Other sources of
uncertainties stemming from the calibration, length-toweight model, bubble attenuation, and signal thresholding
could also affect the accuracy of the biomass estimation[60].
It has been well documented that an abundance of small
shoaling pelagic fish (Japanese anchovy, Japanese sardine,
Pacific saury and mackerel) show decadal fluctuations in
response to climatic regime shifts [61–63]. However, theeffect of such shifts on the distribution pattern and biomassof the Japanese anchovy in the offshore region of thewestern North Pacific has not been well documented. The
continuation of a systematic acoustic survey independentof fishing activity is critically important for monitoring thechanges in the distribution pattern and biomass of theJapanese anchovy in the western North Pacific.
Acknowledgments The authors express their thanks to the crews
and the researchers who dedicated themselves to collecting data in thefield. We would like to express our special thanks to Ms. Tomoko
Hasegawa who supported the handling of huge numbers of data sets.
Comments from two anonymous reviewers and the editor greatly
improved the manuscript. The results in this paper constitute part of a
project, ‘‘Marine fisheries stock assessment and evaluation for Japanese waters (fiscal year from 2005 to 2009)’’, conducted by the
Fisheries Agency of Japan and the Fisheries Research Agency ofJapan. This survey was supported by the Fisheries Agency of Japan,
the Fisheries Research Agency of Japan, and the Institute of Cetacean
Research. We thank these institutions for their support.

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