decadal variability in the abundance of pacific saury and its response to climatic/oceanic regime...

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Journal of Marine Systems 52 (2004) 235–257

Decadal variability in the abundance of Pacific saury and its

response to climatic/oceanic regime shifts in the northwestern

subtropical Pacific during the last half century

Yongjun Tiana,*, Yasuhiro Uenob, Maki Sudaa, Taturo Akaminea

aNational Research Institute of Fisheries Science, Fisheries Research Agency (FRA), Fukuura 2-12-4, Kanazawa, Yokohama 236-8648, JapanbTohoku National Fisheries Research Institute Hachinohe Branch, Fisheries Research Agency (FRA),

Same, Hachinohe, Aomori 031-0841, Japan

Available online 7 June 2004

Abstract

Pacific saury (Cololabis saira) is one of the most important, small-sized, pelagic fishes in the North Pacific. Using

correlation analysis and principal component analysis (PCA), we examined the relationships between climatic/oceanographic

indices (Asian monsoon index (MOI), Southern Oscillation Index (SOI), North Pacific Index (NPI), Arctic Oscillation Index

(AOI), Pacific Decadal Oscillation (PDO) index, air temperature, wind velocity, sea surface temperature (SST), and surface

current velocity (SCV) in the Kuroshio axis), and abundance/biological indices of Pacific saury (adult catch, catch per unit

effort, i.e., CPUE, condition factor, and body length and larval density) in order to detect the response of Pacific saury

abundance to the recent climatic/oceanic regime shifts (1976/1977, 1987/1988, and 1997/1998). Our oceanographic analyses

show that notable regime shifts occurred in 1987/1988 and possibly 1997/1998 in the Kuroshio region, while the same kind of

regime shift was not readily apparent there in 1976/1977. Results of our oceanographic/biological analyses show that the

decadal-scale variation pattern in Pacific saury abundance responded well to the regime shifts of 1987/1988 and 1997/1998.

These results indicate that only the regime shifts which occurred in the Kuroshio region can affect Pacific saury abundance. Our

results also showed that the abundance and biological indices of saury significantly correlated with both the SSTs in the

northwestern Kuroshio waters and the SCV in the Kuroshio axis in winter. These correlations suggest that winter oceanographic

conditions in the Kuroshio region strongly affect the early survival process and determine the recruitment success of Pacific

saury. The abundance of other major small pelagic species also changed greatly around 1989, suggesting that the regime shift in

the late 1980s occurred in the pelagic ecosystem basin. We concluded that Pacific saury could be used as a bio-indicator of

regime shifts in the northwestern subtropical Pacific.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Pacific saury; Abundance; Decadal variability; Regime shift; Bio-indicator; Kuroshio; Northwestern Pacific

0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmarsys.2004.04.004

* Corresponding author. Present address: Japan Sea National

Fisheries Research Institute, Fisheries Research Agency (FRA),

Suido-cho, Niigata 951-8121, Japan. Tel.: +81-25-228-0536; fax:

+81-25-224-0950.

E-mail address: [email protected] (Y. Tian).

1. Introduction

Pacific saury (Cololabis saira) is one of the most

commercially important, pelagic species in the north-

western Pacific, along with Japanese sardine and

Y. Tian et al. / Journal of Marine Systems 52 (2004) 235–257236

Japanese anchovy. However, Pacific saury exhibits

immense interannual variations both in abundance and

size composition (Fukushima, 1979; Kosaka, 2000;

Tian et al., 2002, 2003). Annual catches of saury in

Japan have fluctuated by an order of magnitude from

572,000 t in 1958 to 63,000 t in 1969, with an annual

average of about 257,800 t over the last half century

(Tian et al., 2003). In fact, despite a descending trend

in fishing efforts in the 1990s, both the catch and

catch per unit effort (CPUE) of saury experienced an

abrupt decline in 1998 indicating a strong influence of

environmental factors (Tian et al., 2003); however,

causes and mechanisms of the large variations in

abundance are not well understood.

Understanding fluctuations in marine fish stocks is

important for fisheries management; therefore, in-

creasing attempts have been made to demonstrate

links with oceanographic and climatic variability

(e.g., Francis et al., 1998; Beamish et al., 1999; Zhang

et al., 2000; Attrill and Power, 2002; Finney et al.,

2002). Recent studies have revealed that regime shifts

and/or decadal-scale variability in the atmospheric and

oceanic environments are dominant characteristics of

the mid-latitude North Pacific (Trenberth and Hurrel,

1994; Nakamura et al., 1997; Nakamura and Yama-

gata, 1999; Minobe and Mantua, 1999) and strongly

influence the dynamics of fish stocks and ecosystems

(e.g., McGowan et al., 1998; Sugimoto and Tadokoro,

1998; Zhang et al., 2000; Hollowed et al., 2001;

Sugimoto et al., 2001). Minobe (1999) identified three

main climatic regime shifts during the 1920s, 1940s

and 1970s which occurred in the atmospheric fields of

the North Pacific, and which were strongly associated

with the interannual–decadal variations in the atmo-

spheric and oceanic environment. Yasunaka and

Hanawa (2002) detected six climatic regime shifts

that occurred in the northern hemisphere sea surface

temperature (SST) fields during the 1910s to 1990s:

1925/1926, 1945/1946, 1957/1958, 1970/1971, 1976/

1977, and 1988/1989. Using 100 historical physical

and biological time series events, Hare and Mantua

(2000) confirmed that climatic regime shifts took

place in 1977 and 1989 in the northeast Pacific.

Recent evidence also indicates that a regime shift

possibly took place around 1998 in the North Pacific

(Minobe, 2002). From considering the above inves-

tigations, it seems that whether or not regime shifts are

detected depends on the kind of data or climatic

indices which are used. The climatic regime shift

which occurred in the mid-1970s is the most famous

one, and it has been well documented (Nitta and

Yamada, 1989; Graham, 1994; Parrish et al., 2000;

Hare and Mantua, 2000). It is generally accepted that

the 1976/1977 regime shift had a marked effect not

only on global climate, but also on the dynamics of

North Pacific ecosystems, from low trophic plankton

production to fisheries resources (Hare and Francis,

1995; Hollowed and Wooster, 1995; Francis et al.,

1998; Sugimoto and Tadokoro, 1998; McFarlane et

al., 2000), while agreement on the possibility of

further climatic regime shifts in 1988/1989 and in

the late 1990s is yet to be achieved (McFarlane et al.,

2000).

It is widely accepted that regime shifts can occur

not only in the SST and the atmospheric circulation

fields, but also in fish productivity and pelagic eco-

systems in the North Pacific (e.g., Mantua et al., 1997;

Zhang et al., 2000). In the northeast Pacific, regime

shifts in marine ecosystems took place in 1977, 1989,

and possibly around 1998 (Hare and Mantua, 2000;

McFarlane et al., 2000). Beamish et al. (2000) con-

firmed that fisheries, including salmon, herring, and

groundfish in British Columbia, showed three regimes

during the last half century: 1950–1976, 1977–1988,

and 1989–1997. In the northwest Pacific, regime

shifts in 1976 and 1988 were detected in Korean

waters, which largely affected fisheries resources

(Kang et al., 2000; Zhang et al., 2000). The response

of oceanic ecosystems to oceanographic regime shifts

and long-term climate changes has caused great con-

cern (e.g., Mantua et al., 1997; Francis et al., 1998;

Sugimoto and Tadokoro, 1998; Beamish et al., 1999;

Zhang et al., 2000). Although many studies have

qualitatively demonstrated relationships between the

fluctuations in fisheries catch and climatic variability;

the processes by which climatic changes are linked to

the dynamics of fish stocks are not well understood

(Hare and Francis, 1995; Hollowed and Wooster,

1995).

Variations of oceanic physical variables such as

water temperatures are clearly involved in many

aspects of the variability of biological populations

on seasonal and interannual time scales; however,

there is great uncertainty about the mechanisms by

which decadal variations of physical variables in-

fluence biological populations (Miller and Scheider,

Y. Tian et al. / Journal of Marine Systems 52 (2004) 235–257 237

2000). In addition to the different variation patterns

in the large-scale SST fields observed within the

northeastern and northwestern Pacific Ocean,

responses of ecosystem to oceanic and atmospheric

conditions can vary regionally, even within large

scale regime shifts such as 1976/1977 (Hanawa,

1995; Nakamura et al., 1997; Yasunaka and

Hanawa, 2002). In general, the temporal–spatial

response scale of a specific regional oceanic eco-

system is smaller than that of a climatic regime

shift, hence it is important to identify the regional

variation pattern in the specific oceanic environment

which affects the dynamics of the fish stocks and

ecosystem (Sugimoto and Tadokoro, 1998). Further-

more, even in a specific region such as the north-

western subtropical Pacific, the responses of

individual species will vary with their life history

Fig. 1. Schematic diagram showing the life history and migration pattern

coast of Japan. Solid and broken arrows indicate main current direction an

larval survey. S1–S11 indicate the stations for meteorological and oc

Toyimisaki; S4: Shiomisaki; S5: Yirousaki; S6: Nojimasaki; S7: Choshi;

strategies and trophic levels. Therefore, it is rea-

sonable to assess the regional physical processes

determining the recruitment of fishes from the

viewpoints of life history and migration patterns.

Pacific saury make extensive migrations from

subtropical to subarctic regions throughout the Kur-

oshio–Oyashio Transition Zone (TZ) (Fukushima,

1979; Gong, 1984). The main spawning of saury

occurs in the Kuroshio region in winter (Kurita,

2001). Their larvae and juveniles grow up in subtrop-

ical waters while overwintering, start their northward

migration in the spring, feeding on the plentiful food

in the Oyashio region during the summer, and then are

fished during their southward migration off the north-

east coast of Japan (Fig. 1). It is interesting that

Pacific saury inhabit different oceanic systems from

the subtropics to the subarctic throughout the TZ

of Pacific saury with the oceanographic structures along the Pacific

d migration pattern, respectively. TL indicates transect lines for the

eanographic observations. S1: Tanegashima; S2: Kagoshima; S3:

S8: Onahama: S9: Sendai; S10: Kushiro; S11: Nemuro.


during different life stages. Pacific saury have a short

life-span of 1–2 years, therefore, impacts of climate

change on saury abundance should appear after a

short time lag.

The area of the northwestern Pacific adjacent to

Japan is characterized by dynamic oceanographic

conditions and high productivity, with more that 600

fish species. The area supports the largest fisheries

production in the world, including Japanese sardine

(Sardinops melanostictus), Japanese anchovy

(Engraulis japonicus), jack mackerel (Trachurus

japonicus), chub mackerel (Scomber japonicus), Jap-

anese common squid (Todarodes pacificus), and saury

(Chikuni, 1985; Belyaev and Shatilina, 1995). How-

ever, it is also a unique region in that biological

responses have not been adequately studied in com-

parison with those in the northeast Pacific (Limsakul

et al., 2001). Pacific saury exhibits decadal-scale

variation patterns, together with large interannual

fluctuations in abundance (Tian et al., 2002, 2003).

Our previous studies involving observational data

analysis and modeling revealed that the population

dynamics of saury is largely affected by oceanic and

climatic factors rather than by fishing, and that inter-

annual variations in the abundance of Pacific saury are

largely associated with El Nino-Southern Oscillation

(ENSO) events (Tian et al., 2002, 2003, 2004). In the

present study, we focus on the decadal-scale variabil-

ity in Pacific saury abundance, and its linkage with the

oceanic regime shifts in the northwestern subtropical

Pacific. In particular, we concentrate on the Kuroshio

region, which is an important spawning and nursery

ground for a number of small pelagic species such as

sardine, anchovy, mackerel, and common squid. The

objectives of this study are: (1) to identify the physical

characteristics of oceanic variability and regime shifts

in the Kuroshio water in winter; (2) to clarify the

decadal variability in Pacific saury abundance; and (3)

to examine the effects of the oceanographic and

climatic regime shifts on Pacific saury abundance

from the viewpoint of their life history strategy. We

demonstrate that a notable oceanic regime shift oc-

curred in 1987/1988, and possibly around 1997/1998,

in the Kuroshio region during winter, and that these

were largely associated with the decadal variation

patterns in saury abundance. We also conclude that

Pacific saury could be used as a bio-indicator of

regime shifts in Kuroshio waters.

2. Data and methods

2.1. Climatic indices of the North Pacific

The Southern Oscillation Index (SOI), Monsoon

Index (MOI), North Pacific Index (NPI), Arctic

Oscillation Index (AOI), and Pacific Decadal Oscil-

lation (PDO) index were chosen as climatic indices

of the North Pacific. These indices are well docu-

mented and largely associated with the interannual–

interdecadal variability in atmospheric and oceanic

conditions in the North Pacific (Beamish et al.,

2000).

The SOI is defined as the difference in sea level

pressures (SLP) between Darwin (Australia) and

Tahiti. It is an index of ENSO; the extreme ne-

gative (positive) values represent El Nino (La Nina)

episodes (Trenberth, 1990). The MOI is defined as

the difference in the SLP between Irkutsk (Russia)

and Nemuro (Japan), representing conditions of the

winter Asian monsoon (Tian et al., 2002). The NPI

is defined as the area-averaged SLP anomaly in

the region 160jE–140jW, 30–60jN; large (small)

NPI represents the strong (weak) Aleutian Low

(Trenberth and Hurrel, 1994). The AOI is defined

as the leading empirical orthogonal function (EOF)

of wintertime SLP poleward of 20jN, and is

largely associated with the variability in the mid-

latitude North Pacific (Thomson and Wallace,

1998). The PDO index is defined as the time

coefficient of the leading EOF created from month-

ly SST anomalies poleward of 20jN in the Pacific

basin; positive (negative) PDO indices represent

the cooling (warming) phase in the central North

Pacific (Mantua and Hare, 2002). Monthly values

for the above four climatic indices except MOI

were provided by the Climate Predictive Center of

NOAA for the period 1950–2000, and the MOI

data for the same period were obtained from Tian

et al. (2002).

2.2. Oceanographic and meteorological data from

Japan

In addition to the above five global climatic

indices, we also used various oceanographic and

meteorological indices representing the regional cli-

mate and oceanic conditions adjacent to Japan which

Fig. 2. Anomaly values (vertical bars) and their cumulative sums (CuSum, solid circles) for the five climatic indices from 1950 to 2000. (a)

Annual Southern Oscillation Index (SOI) averaged during May through April. (b) Winter North Pacific Index (NPI). (c) Winter Pacific Decadal

Oscillation (PDO) index. (d) Winter Arctic Oscillation Index (AOI). (e) Winter Asian Monsoon Index (MOI).


Fig. 4. The first principal component (PC) scores for wind velocity

at four Japanese stations (Tanegashima, Shiomisaki, Onahama, and

Kushiro). The PC scores are normalized time series and the solid

(open) bars indicate the positive (negative) values.


affect the habitats of Pacific saury directly. These

long-term time series included wind velocity (WV),

air temperature (AT), SST, and Kuroshio surface

current velocity (SCV), obtained from the archived

data set of Japan Meteorological Agency (JMA) and

Japan Oceanographic Data Centre (JODC). Monthly

WV at four stations (Tanegashima, Shiomisaki, Ona-

hama, and Kushiro) and AT at eight stations (Choshi,

Kagoshima, Sendai, and Nemuro in addition to the

above four stations for wind) around the Pacific

coast of Japan are calculated from the original daily

observational data set of JMA for the period 1951–

2000 (Fig. 1). Monthly SCV and SST in the path of

the Kuroshio Current measured from four capes in

the southeast region of Japan (Toyimisaki, Shiomi-

saki, Yirousaki, and Nojimasaki, located from 31j to

35jN) during the period 1968–2000 were calculated

from a JODC monitoring data set (measured twice

each month).

Fig. 3. The first two principal component (PC) scores for the five clim

normalized time series and the solid (open) bars indicate the positive (neg

In this study, we also used three, area-averaged

time series of monthly mean SSTs extracted from the

original 1j grid (longitude� latitude) JMA data set to

atic indices (SOI, NPI, PDO, AOI, and MOI). The PC scores are

ative) values.

Y. Tian et al. / Journal of Marine

represent the oceanic conditions of Japan (Tian et al.,

2003). These three SST series are for the Kuroshio

region (28–35jN, 128–145jE), the TZ (35–40jN,140–160jE), and the Oyashio region (40–45jN,140–160jE) for the period 1950–2000, which ap-

proximately represent the subtropical frontal zone, the

TZ, and the subarctic frontal zone, respectively. These

three zones also generally cover the distribution and

migration areas of Pacific saury (Fig. 1) and almost

correspond to the spawning grounds in winter and

spring, and the feeding ground in summer, respective-

ly (Tian et al., 2003).

Fig. 5. Anomaly values (vertical bars) and their cumulative sums (CuSum,

Japan. (a). North Japan (averaged for Nemuro and Kushiro). (b). East Jap

(averaged for Shiomisaki, Kagoshima, and Tanegashima).

2.3. Saury abundance and biological indices

The population of Pacific saury is dominated by

two size groups, large- and medium-sized (Tian et al.,

2003). The raw catch in number of fish by size group

estimated from length frequency analysis (LFA) for

the period 1951–2000 was used as the abundance

index (Tian et al., 2003). Saury achieve 30 cm in body

length within 10 months (Watanabe et al., 1988),

therefore, since their life-span is 1–2 years, most of

the fisheries catch consists of the recruitment of the

current year class. In addition to the abundance index,

Systems 52 (2004) 235–257 241

solid circles) of air temperature in winter from the south to north of

an (averaged for Sendai, Onahama, and Choshi). (c). South Japan


annual CPUE (in tons per haul) for the period 1971–

2000 provided by the Tohoku National Fisheries

Research Institute (TNFRI) of Japan, was also used

as a biomass index of saury.

In addition to the above abundance indices for

saury adults, a time series data set for larval and

juvenile saury density was used as an index of the

early survival process (TNFRI, 2000). Saury larvae

and juveniles were collected in the waters off Iwate,

northeast Japan (around 39.5jN, also see Fig. 1)

during the summer–autumn season using a cylindri-

cal–conical net with a 1.3 m diameter mouth and 0.45

mm mesh aperture towed for 10 min at 2 km (1.03 m/

s) (Watanabe et al., 1997). Annual mean larval and

juvenile density was calculated from the summer and

autumn monthly survey data.

Long-term time series data of adult mean body

length and condition factor were used as biological

indices representing the growth and nutritional state of

saury (Wootton, 1990). The mean body length by size

group for the period 1951–2000 was estimated from

LFA and obtained from Tian et al. (2003). The

Fig. 6. The first two principal component (PC) scores for winter air temper

the solid (open) bars indicate the positive (negative) values.

condition factor (K) is defined by the following

equation:

K ¼ W=L3

where W and L are body weight (g) and body

length (mm), respectively. These data were gathered

from data sets of condition factors broken up into

size groups measured during the fishing season

(from late August to early September), and were

provided from TNFRI for the period 1972–2000

and from Kosaka (2000) for the period 1952–1971.

Long-term series of catch data (tons) for four

major small pelagic species (Japanese anchovy, jack

mackerel, chub mackerel, and common squid) for the

period of 1951–2000 were also examined together

with the catch data of saury; these weight data were

obtained from the statistics of the Fisheries Agency

of Japan. These small pelagic species have similar

life histories, share almost the same nursery ground

in the Kuroshio region in winter, and have the same

atures in eight stations. The PC scores are normalized time series and


trophic level as saury, i.e., they prey on small

zooplankton.

In the present study, seasonal means were calcu-

lated from monthly data. Winter was defined as the

period from January through March for biological

and oceanographic indices such as SST and SCV,

and from December through February for atmo-

spheric variables such as MOI and NPI.

2.4. Correlation analysis and PCA

Correlation analysis and principal component

analysis (PCA) were used to examine the relation-

ships between climatic/oceanographic indices and

Pacific saury abundance/biological indices. The goal

of the PCA was to concentrate most of the variance

of a large data set into a small number of physically

interpretable patterns of variability (Hare and Man-

tua, 2000). We used PCA to objectively isolate the

most important patterns of common variability in the

various physical and biological time series. The

Fig. 7. Annual changes in (a) winter sea surface temperature (SST) and (b

four stations) from 1968 to 2000. Bold lines indicate running means.

cumulative sum (CuSum) of the anomalies from

the time series data was also used to examine the

trend in the index. The method of CuSum is a simple

addition of a datum to the sum of all previous data

points, and the CuSum graph provides a visual

picture to study the trend in a time series (Beamish

et al., 1999).

3. Results

3.1. Atmospheric and climatic conditions

Fig. 2 shows the time series of anomalies for the

five climatic indices and their cumulative sums

(CuSum). The annual mean (averaged during May

through April) SOI showed large interannual varia-

tions. A distinct change occurred in 1976, from this

year on, the pattern of SOI changed from positive to

negative anomalies (except 1988), indicating that El

Nino trends tended to be intense after 1977, partic-

) surface current velocity (SCV) in the Kuroshio axis (averaged for


ularly during 1990 to 1995, while La Nina trends

were evident until 1976 and after 1998. The CuSum

curve showed abrupt changes in 1976 and 1998.

Winter NPI showed a distinct change in 1977 from

positive to negative values indicating weakening of

the Aleutian Low after 1977. Both PDO and AOI

showed decadal-scale variation patterns; there were

distinct changes in 1977 and 1989 both for PDO and

AOI (also a changes in 1971 for AOI). Winter MOI

showed abrupt changes around 1987 from positive to

negative anomalies indicating the weakening Asian

Monsoon, while no distinct change occurred in the

mid-1970s. It seems that the abrupt changes which

occurred in 1976/1977 are common for SOI, NPI,

PDO, and AOI, while the changes which occurred in

the end of the 1980s are founded in MOI, PDO, and

AOI. These five indices also point to a possible

change around the late 1990s.

Fig. 3 shows the first two principal components

(PC) scores for the above five climatic indices. The

two PCs explained about 67% of the total variance.

The first principal component (PC1) has a short

Fig. 8. The first two principal component (PC) scores for winter oceanogra

four stations) in the Kuroshio axis. The PC scores are normalized time ser

time scale of a few years, and shows shifts around

1960, 1971, 1977, and 1989. The second principal

component (PC2) showed decadal scale patterns

with distinct changes in 1977, and possibly in

1999. These results strongly suggest that regime

shifts in the atmospheric and climatic conditions

occurred in 1976/1977, 1988/1989, and possibly in

1998/1999.

There were no differences in WVs between the

four stations from the south to the north of Japan. The

WVs show a slow oscillation or long-term scale

variation, with a shift occurring during the mid-

1970s from positive to negative anomalies indicating

weakening wind after the mid-1970s (Fig. 4). On the

other hand, ATs were higher during the 1950s and

1990s, and were lower during the 1960s to 1980s,

with large interannual and regional variations (Fig. 5).

A distinct shift took place in the late 1980s (1986/

1987 in the east and south of Japan, 1988/1989 in the

north of Japan). The PC1 for winter ATs at eight

stations showed an abrupt shift in 1986/1987, while

the PC2 showed four regime shifts which occurred in

phic indices (sea surface temperature and surface current velocity at

ies and the solid (open) bars indicate the positive (negative) values.


1961/1962, 1976/1977, 1988/1989, and 1997/1998

(Fig. 6).

3.2. Oceanographic conditions in the Kuroshio region

The Kuroshio Current normally flows northeast-

ward south of Japan, leaves the coast at Cape Yinu-

bozaki (36jN) and turns to flow eastward (see Fig. 1),

hence oceanographic conditions in the Kuroshio re-

gion south of Japan are largely affected by the

Kuroshio Current. Fig. 7 shows annual SST and

SCV changes in the Kuroshio axis in winter averaged

from measurements off four capes (Toyimisaki, Shio-

misaki, Yirousaki, and Nojimasaki) during 1968–

2000. Both SST and SCV primarily increased around

1987/1988. Fig. 8 shows the first two PCs for winter

SST and SCV in the Kuroshio axis off four capes,

which accounted for about 71% of the total variance.

Fig. 9. Anomaly values (vertical bars) and their cumulative sums (CuSum, s

for three oceanic regions. (a). The Kuroshio region—winter. (b). The K

region—summer.

The PC1 shows slow oscillations and an abrupt shift

in 1987/1988, while PC2 shows large interannual

variations with abrupt changes in 1978/1979, 1988/

1989, and 1996/1997.

Sea surface temperatures in the northwestern

Pacific off Japan show large differences between

oceanic regions. Over the last half century, averages

of annual mean SST were 23.0jC (with a variance of

7.0%) in the Kuroshio region, 17.8jC (with a

variance of 23.6%) in the TZ, and 9.9jC (with a

variance of 21.0%) in the Oyashio region. Fig. 9

shows the annual variation in SSTs for the appro-

priate season associated with saury habitat utilization

of the three geographic areas. The SSTs showed

decadal variation patterns together with large inter-

annual variations. There was a warm period during

the 1950s and after the late 1980s in the Kuroshio

region in winter, while in the TZ and Oyashio region

olid circles) of shifting seasonal mean sea surface temperature (SST)

uroshio–Oyashio Transition Zone (TZ)—spring. (c). The Oyashio


the SSTs were low during mid-1960s and then from

the early 1980s to late 1990s. The SSTs were high

until the early 1960s in the three areas, but the

patterns are different after late 1980 in the Kuroshio

region. The PCA of winter SSTs for the three areas

showed distinct changes in 1987/1988 for PC1 and

in 1977/1978 for PC2 (Fig. 10).

These results suggest that a distinct regime shift

in the oceanographic conditions in the Kuroshio

region occurred in 1987/1988, while the shift which

occurred around 1976/1977 is in comparison weaker.

Fig. 11 shows the spatial difference in the winter

SST fields in the northwestern Pacific between

1977–1987 and 1988–1997 (1987/1988 regime

shift) as well as the difference between 1988–1997

and 1998–2000 (1997/1998 regime shift). During

the 1987/1988 regime shift, the northwestern Pacific

exhibited a warming pattern; a strong warming

particularly occurred in the Kuroshio region. A

similar warming is also found in the Kuroshio region

during the possibly 1997/1998 regime shift accom-

panied by a cooling in the Oyashio region (north-

Fig. 10. The first two principal component (PC) scores for winter SSTs in th

the Oyashio region. The PC scores are normalized time series and the so

ward of 40jN). The strong warming region

corresponds well to the spawning and nursery

ground of saury (see Fig. 1).

3.3. Variation patterns in the abundance of saury

Both the large and medium size groups of saury

have shown large interannual variations in their abun-

dances during the past 50 years (Fig. 12). The

abundance index for large saury increased from

1954 to 1962, decreased sharply from 1963 to 1977

(with a substantial recovery in 1973), experienced a

relatively stable low abundance period from 1978 to

1987, and increased rapidly from 1988 to 1997.

Abrupt changes occurred around 1953/1954, 1962/

1963, 1977/1978, 1987/1988, and 1997/1998. This

pattern strongly indicates decadal-scale variations

together with large interannual fluctuations in abun-

dance. The variation patterns for the medium size

group of saury are very similar to those of the large

size saury until 1976. After 1977, however, and

particularly after 1987, the medium size patterns are

e Kuroshio region, the Kuroshio–Oyashio Transition Zone (TZ) and

lid (open) bars indicate the positive (negative) values.

Fig. 11. Spatial difference in winter SST in the northwestern Pacific between 1988–1997 and 1998–2000 (a: top panel) and between 1977–1987

and 1988–1997 (b: bottom panel). Contour interval is 0.5jC.


completely the opposite of the large size group saury,

indicating that variations in abundance for the two

size groups are different and may respond to different

oceanic systems.

Fig. 13 shows annual changes in CPUE and larval

density during 1971–2000. Despite large interannual

variations, both indices increased markedly after 1987

indicating a shift occurred in the late 1980s. There is

no clear trend in mean body length for the two size

groups, although the mean length seems somewhat

small during the 1970s–1980s for the medium size

group (Fig. 14a). On the other hand, condition factors

for both large- and medium-sized saury are low until

the early 1970s and much higher after the mid-1980s

indicating a shift occurred in the early 1970s and

possibly again in the late 1980s (Fig. 14b). Fig. 15

shows the first two PC scores for the above eight

indices (abundance index, mean body length and

condition factor for the large and medium size groups,

and larval density and CPUE) of saury during 1971–

2000, which accounted for about 64% of the total

variance. The PC1 shows slow oscillations with a

distinct shift in 1986/1987, while PC2 shows large

interannual variations with changes in 1976/1977,

1982/1983, 1987/1988, and 1996/1997. These results

strongly suggest that a distinct regime shift occurred

Fig. 12. Anomaly values (broken line with open circles) and cumulative sums (CuSum: solid line with solid circles) of the abundance index by

size group for Pacific saury from 1951 to 2000.


in 1986/1987 for saury. The 1986/1987 shift was

found in all indices of saury, and represents a long-

term change in abundance, while shifts in 1963/1964,

1976/1977, and 1996/1997 can be interpreted as

short-term scale variations.

The community in the northwestern Pacific adja-

cent to Japan is characterized by abundant small

pelagic species which mainly include Japanese sar-

dine, Japanese anchovy, jack mackerel, chub macker-

el, saury, and common squid. Fig. 16 shows the PCAs

for annual catches (tons) of the above five short-lived

zooplanktivorous species (except the long-lived her-

bivorous sardine). The first two PCs accounted for

about 66% of the total variance. Both PC1 and PC2

show decadal-scale variation patterns with abrupt

changes occurring in 1969/1970 and 1989/1990 for

PC1 and in 1964/1965, 1977/1978, and 1995/1996 for

PC2. This result indicates that the late 1980s regime

shift occurred in the pelagic community basin, while

the 1976/1977 regime shift does not seem evident in

the northwestern Pacific.

3.4. Relationships between saury abundance and

oceanographic conditions

Table 1 shows the correlation coefficient matrix

between abundance/biological indices of saury and

oceanographic/climatic indices. The abundance and

condition factor for large-sized saury is significantly

correlated with MOI, SOI, and oceanographic indi-

ces in the Kuroshio region (SCV and SST), while

the abundance and condition factors of the medium-

Fig. 13. Annual changes in CPUE and larval density for Pacific saury from 1971 to 2000.


sized saury showed high correlations with AOI and

AT in the north of Japan. CPUE and larval density

also showed significant correlations with oceano-

graphic and meteorological indices in the south and

Fig. 14. Annual changes in mean body length (a) and condition fa

east of Japan indicating that the environments in

the Kuroshio region are strongly associated with

survival of saury. In particular, there are high

correlations between the CPUE and annual mean

ctor for Pacific saury by size group from 1951 to 2000 (b).

Fig. 15. The first two principal component (PC) scores for eight indices (abundance index, mean body length and condition factor for the large

and medium size groups, and larval density and CPUE) for Pacific saury. The PC scores are normalized time series and the solid (open) bars

indicate the positive (negative) values.


SCV and between larval density and SSTs in the

Kuroshio region, indicating that the Kuroshio Cur-

rent may play an important role in the survival

process of saury larvae (Table 1). Condition factors

showed positive correlations with the abundance

indices, CPUE and oceanographic/meteorological

indices in the Kuroshio region indicating that den-

sity-dependent effects on growth are not evident,

and that growth may be largely affected by physical

factors rather than by prey density. It also suggests

that recruitment success and growth of saury is

mainly associated with oceanographic conditions

in the Kuroshio region.

Catches of small pelagic species showed significant

correlations with meteorological and oceanographic

indices, particularly with oceanographic conditions

(SCV and SST) in the Kuroshio region (Table 1).

These results strongly suggest that the oceanographic

conditions in the Kuroshio region in winter not only

affect the abundance of saury, but are also largely

associated with the dynamics of the pelagic ecosys-

tems in the subtropical northwestern Pacific.

4. Discussion

Table 2 summarizes marked shifts found in various

indices from PCAs. It seems that regime shifts took

place in the mid-to-late 1970s, late 1980s, and mid-to-

late 1990s in the northwestern Pacific. With regard to

the climatic indices in the North Pacific, the well-

known 1976/1977 regime shift is confirmed both in

PC1 and PC2, and showed an interdecadal scale;

however, the 1988/1989 shift was only observed in

PC1 and had a short time scale of several years (Fig.

3). On the other hand, regarding the oceanographic

and meteorological conditions in the northwestern

Pacific, and the biological indices of saury, the

1987/1988 regime shift is confirmed in both PC1

and PC2 and appeared to have a long time scale,

Fig. 16. The first two principal component (PC) scores for catches of five major small pelagic species. The PC scores are normalized time series

and the solid (open) bars indicate the positive (negative) values.


while the mid-1970s shift is only found in PC2, and

showed a short time scale (also see Figs. 6, 8, 10, and

15). It is strongly suggested that the 1987/1988 regime

shift is the most evident event in the northwestern

Pacific, particularly in the Kuroshio region during the

last half century, whereas the mid-1970s regime shift

is not clear in the Kuroshio region compared with

more distinct patterns found in global climatic indices

and the SST fields in the central and eastern North

Pacific (Yasunaka and Hanawa, 2002). That changes

occurred around 1996/1997 is also confirmed by

saury abundance and by the Kuroshio Current indices

(Table 2; also see Figs. 8 and 15).

For the five climatic indices, it is of interest to note

that the 1987/1988 regime shift is evident in MOI and

AO, but not in SOI, NPI, and PDO (Fig. 2). MOI is

significantly correlated with SSTs and ATs in the

Kuroshio region (Table 3), suggesting that the oceanic

variability in the Kuroshio region is linked with the

winter Asian monsoon (Limsakul et al., 2001; Tian et

al., 2002). The warm period after 1987 in the Kur-

oshio region corresponded well with the weakening of

Asian monsoon. AOI and SOI also showed significant

correlations with SSTs in the Kuroshio region and

with ATs in the south and east of Japan, while PDO

and NPI significantly correlated with SSTs in the TZ

and Oyashio region and with ATs in the north of Japan

(Table 3). It is suggested that the subtropical Kuroshio

water are mainly associated with SOI, AOI, and MOI,

while the subarctic TZ and Oyashio waters are mostly

affected by PDO and NPI.

Pacific saury showed interannual–decadal varia-

tion patterns in their abundance (Fig. 12). This is

confirmed by power spectral analysis in which saury

abundance showed cycles of around 4–7 and 16–17

years, corresponding to interannual and decadal

scales, respectively (Tian et al., 2002). It is important

to understand the effects of environmental factors with

different time scales. Interannual variations are largely

associated with ENSO, in which El Nino (La Nina)

Table 1

Cross-correlation coefficients between abundance/biological indices of saury and winter oceanic/climatic indices

AI-L AI-M BL-L BL-M CF-L CF-M CPUE LARVA SAURY ANCHO SQUID JACK CHUB

NPI 0.04 � 0.07 0.30 � 0.16 � 0.09 � 0.03 0.04 0.38 * 0.05 0.19 0.29 0.13 � 0.03

MOI � 0.44 ** 0.10 � 0.22 0.02 � 0.37 * � 0.25 � 0.33 * � 0.46 ** � 0.42 ** � 0.18 � 0.02 � 0.27 0.26

SOI � 0.48 ** 0.05 0.20 � 0.11 � 0.42 ** � 0.27 � 0.28 � 0.07 � 0.43 ** 0.13 0.22 � 0.09 0.15

PDO 0.10 0.18 � 0.33 � 0.03 0.15 0.15 0.04 � 0.19 0.07 � 0.41 * � 0.52 ** � 0.05 � 0.03

AOI 0.34 * � 0.60 *** 0.47** 0.40 * 0.40 * 0.31 * 0.34 * 0.33 * 0.09 0.13 0.26 0.27 � 0.49 **

WIND � 0.11 � 0.05 � 0.09 0.15 � 0.35 * � 0.26 � 0.31 * � 0.49 ** � 0.09 0.02 0.12 � 0.23 0.39 *

AT-NORTH 0.29 � 0.51 ** 0.31 * 0.11 0.42 * 0.32 * 0.52 ** 0.40 * 0.03 0.19 0.34 * 0.59*** � 0.50 **

AT-EAST 0.28 � 0.18 0.26 � 0.03 0.37 * 0.29 0.44 ** 0.60 *** 0.22 0.24 0.23 0.40 * � 0.43 **

AT-SOUTH 0.30 � 0.22 0.35 * 0.14 0.44 ** 0.37 * 0.37 * 0.55 *** 0.19 0.32 * 0.08 0.40* � 0.52 **

SCV-KC 0.18 � 0.30 0.30 0.42 * 0.42 ** 0.32 * 0.64 *** 0.25 0.15 0.33 * 0.35 * 0.72 *** � 0.47 **

SST-KC 0.38 * � 0.31 * 0.27 0.26 0.41* 0.36 * 0.45 ** 0.55 *** 0.21 0.37 * � 0.08 0.51 * � 0.50 **

SST-KR 0.49 ** � 0.26 0.20 0.10 0.49 ** 0.39 * 0.39 * 0.53 *** 0.32 * 0.21 � 0.11 0.28 � 0.44 **

SST-TZ 0.16 0.001 0.003 � 0.25 � 0.13 � 0.15 � 0.14 0.18 0.21 0.18 0.22 � 0.25 0.19

SST-OR � 0.0.5 0.18 0.27 0.14 � 0.14 � 0.04 � 0.14 0.12 0.10 0.33 * � 0.01 � 0.11 � 0.01

AI-L – � 0.32 * � 0.10 � 0.03 0.67 *** 0.63 *** 0.58 *** 0.20 0.79 *** � 0.21 � 0.10 0.23 � 0.27

AI-M – � 0.10 � 0.37 * � 0.25 � 0.21 � 0.18 � 0.26 0.24 � 0.11 � 0.05 � 0.30 0.47 **

BL-L – 0.53 *** 0.35 * 0.30 0.27 0.27 � 0.01 0.40 * 0.19 0.29 � 0.38 *

BL-M – 0.22 0.23 0.19 0.08 � 0.12 0.38 * 0.20 0.32 * � 0.36 *

CF-L – 0.89 *** 0.75 *** 0.38 * 0.52 ** � 0.11 � 0.13 0.50 ** � 0.62 ***

CF-M – 0.63 *** 0.31 * 0.46 ** � 0.12 � 0.31* 0.49 ** � 0.65 ***

CPUE – 0.40 * 0.56 *** � 0.09 0.23 0.70 *** � 0.48 **

LARVA – 0.09 0.17 0.10 0.33 * � 0.51 **

SAURY – � 0.18 0.03 0.13 0.003

ANCHO – 0.35 * 0.32 * � 0.28

SQUID – 0.24 0.20

JACK – � 0.59 ***

CHUB –

NPI: North Pacific Index. MOI: Winter Asian Monsoon Index. SOI: Southern Oscillation Index. PDO: Pacific Decadal Oscillation Index. AOI: Arctic Oscillation Index. WIND: mean

wind velocity in Japan. AT-NORTH, AT-EAST, and AT-SOUTH represent air temperature in the north, east, and south of Japan, respectively. SST-KR, SST-TZ, and SST-OR represent

SSTs in the Kuroshio region, in the Kuroshio–Oyashio Transition Zone, and in the Oyashio region, respectively. SCV-KC and SST-KC represent surface current velocity and SST in

the Kuroshio axis. AI-L, AI-M, BL-L, BL-M, CF-L, and CF-M represent abundance index, body length, and condition factor of saury by size group (large and medium), respectively.

LARVA represents saury larval density. SAURY, ANCHO, SQUID, JACK, and CHUB represent the catch (tons/year) of saury, anchovy, common squid, jack mackerel, and chub

mackerel, respectively. Single, double, and triple asterisks represent significance at P< 0.05, P < 0.01, and P < 0.001, respectively.

Y.Tianet

al./JournalofMarin

eSystem

s52(2004)235–257

252

Table 2

Catalogue of regime shifts found in various indices from principal

component analysis (PCA) during 1950–2000

Indices Shifts in

1950s–1960s 1970s 1980s 1990s

Climate PC1 1959/1960 1976/1977 1988/1989 1998/1999

PC2 – 1976/1977 – 1998/1999

AT PC1 1960/1961 – 1986/1987 –

PC2 1961/1962 1976/1977 1988/1989 1997/1998

Kuroshio PC1 – – 1987/1988 –

PC2 – 1978/1979 1988/1989 1996/1997

SST PC1 1958/59 1977/1978? 1987/1988 –

PC2 – 1977/1978 – –

Abundance PC1 – – 1986/1987 –

PC2 – 1976/1977 1986/1987 1996/1997

Catches PC1 1969/1970 1989/1990 –

PC2 1964/1965 1977/1978 1986/1987 1995/1996

All PC1 – – 1987/1988 –

PC2 – 1976/1977 – 1997/1998

Climate: five climatic indices (NPI, SOI, PDO, AOI, and MOI). AT:

winter air temperature for 8 stations from the south to the north of

Japan. Kuroshio Current: winter surface current velocity and SST in

the Kuroshio axis off of four capes. SST: winter SSTs for three areas

(Kuroshio region, TZ, and Oyashio region). Abundance: abundance

and biological indices of saury. Catches: catches of five small

pelagic species. All: all of above indices except the climatic indices.

PC1 and PC2 represent the first and second components of PCAs.

Table 3

Cross-correlation coefficients between winter oceanographic/met-

rological indices in the northwestern Pacific and five global climatic

indices

SOI NPI PDO AOI MOI

WIND 0.15 � 0.02 � 0.10 � 0.24 0.41*

AT-NORTH 0.27 0.51*** � 0.50*** 0.67*** � 0.38*

AT-EAST 0.04 0.43** � 0.42** 0.47** � 0.75***

AT-SOUTH � 0.09 0.36* � 0.25 0.49** � 0.82***

SCV-KC � 0.07 � 0.06 � 0.08 0.43** � 0.19

SST-KC � 0.22 0.12 0.04 0.36* � 0.63***

SST-KR � 0.43** 0.07 0.06 0.36* � 0.74***

SST-TZ 0.00 0.36* � 0.60*** 0.09 � 0.30

SST-OR 0.05 0.35* � 0.62*** 0.11 0.00

SOI: Southern Oscillation Index. NPI: North Pacific Index. MOI:

Winter Asian Monsoon Index. PDO: Pacific Decadal Oscillation

index. AOI: Arctic Oscillation Index. WIND: mean wind velocity in

Japan. AT-NORTH, AT-EAST, and AT-SOUTH represent air

temperature in the north, east, and south of Japan, respectively.

SST-KR, SST-TZ, and SST-OR represent SSTs in the Kuroshio

region, in the Kuroshio–Oyashio Transition Zone, and in the

Oyashio region, respectively. SCV-KC and SST-KC represent

surface current velocity and SST in the Kuroshio axis. Single,

double, and triple asterisks represent significance at P< 0.05,

P< 0.01, and P < 0.001, respectively.


events have positive (negative) effects on saury abun-

dance (Tian et al., 2003). On the other hand, this study

revealed that decadal variability in saury abundance is

largely associated with oceanic regime shifts in the

Kuroshio region. In particular, the abrupt change in

the abundance of saury which occurred in 1987/1988

appears to be a consequence of the oceanic regime

shift in the Kuroshio region, while the shifts in 1976/

1977 and 1997/1998 also seemed associated with

ENSO events (Figs. 8 and 15). This is supported by

a modeling study for Pacific saury which demonstrat-

ed that winter SSTs in the Kuroshio region approxi-

mately reproduced the regime shift that occurred at

the end of the 1980s, while the large interannual

variation was well explained by ENSO (Tian et al.,

2004).

Significant and positive correlations between the

abundance/biological indices of saury and winter

oceanographic conditions (SST, SCV) in the Kuroshio

region strongly suggest that oceanic environments in

the Kuroshio region during winter determine the

recruitment success of saury (Table 1). Fig. 17 sum-

marizes the process by which the 1988/1989 climatic

regime shifts link to increasing the abundance of

Pacific saury. It is notable that abrupt changes in

oceanographic conditions that occurred in the Kur-

oshio region are correspondent with the saury winter

spawning ground (also see Figs. 1, 7, and 11). Pacific

saury have a long spawning period from autumn

through spring (Watanabe et al., 1997). Recent studies

revealed that the main spawning of saury occurs in the

Kuroshio region in winter, and that the winter spawn-

ing cohort plays an important role in recruitment

(Kurita, 2001; Tian et al., 2003). Consequently, oce-

anic conditions such as SST in the Kuroshio region

could affect early survival and determine the recruit-

ment success of saury accordingly. This is consistent

with results of laboratory experiments which demon-

strated that saury larvae have high survival rates and

show positive growth with high sea temperature

(Oozeki and Watanabe, 2000).

PCAs for the five major small pelagic species in

the northwestern Pacific suggest that the regime shift

in the late 1980s occurred in the pelagic ecosystem

basin. Among the five small pelagic species, Japanese

common squid has very similar life history character-

istics and showed a synchronous variation pattern in

abundance with Pacific saury. It is of interest to note

Fig. 17. Schematic diagram to show the possible process of the

effect of the 1988/1989 regime shift on the variability of abundance

of Pacific saury.


that both the abundance and body size of common

squid showed an abrupt shift around 1988, and that

the body size correlated positively with abundance

(Kidokoro, 2001). It is believed that the stock fluctu-

ation of common squid is related to climatic regime

shifts, in particular to winter SST regimes in the

Kuroshio region (Sakurai, 2001). This is consistent

with changes both in the abundance and the condition

factors of saury which occurred in the late 1980s,

indicative of the responses to the 1987/1988 oceanic

regime shifts in the northwestern Pacific.

Our analyses revealed that a marked oceanic re-

gime shift occurred around 1987/1988 in the Kuroshio

region, and that it was strongly associated with the

dynamics of the pelagic community in the northwest-

ern Pacific, whereas the 1976/1977 regime shift was

not evident there. Noto and Yasuda (1999) showed

that SSTs in the Kuroshio Extension abruptly in-

creased during the winter of 1987–1988 and was

associated with the collapse of Japanese sardine.

Using long-term time series (1960–1997) in oceano-

graphic and meteorological data, Lin et al. (2001)

showed that only the 1988/1989 regime shift was

observed in the Bohai Sea. Regime shifts in 1976

and 1988 were detected in Korean waters and fisheries

resources were greatly affected by these regime shifts

(Kang et al., 2000; Zhang et al., 2000); however, it

appeared that the 1976 shift was not as strong or as

influential as the 1988 shift. Yasuda et al. (2000)

found that there was a regime shift in the winter

mixed layer depth (MLD) in the Kuroshio Extension

changing from a thick to a thin phase in 1985. The

MLD is affected by the winter Asian monsoon, and is

strongly associated with primary production in the

northwestern subtropical Pacific (Limsakul et al.,

2001; Sugimoto et al., 2001). Both the Chl-a concen-

tration and zooplankton biomass in the offshore

waters of the Kuroshio Current south of Japan have

increased since the end of the 1980s (Sugimoto et al.,

2001). Zooplankton biomass in Korean waters also

has showed an abrupt increase after 1990 (Zhang et

al., 2000; Kang, 2001). These studies strongly sug-

gested that the 1987/1988 regime shift occurred not

only in the oceanic environment, but also in the

ecosystems from low trophic levels to pelagic fish

species in the northwestern Pacific, and that the

regime shift was linked with large-scale climate

changes such as the Asian monsoon. Increasing evi-

dence supports the conclusions that a regime shift

took place in late 1980s, and possibly around 1998,

both in the pelagic ecosystem and the oceanic envi-

ronment in the northeastern Pacific (e.g., Beamish et

al., 1999, 2000; McFarlane et al., 2000; Hare and

Mantua, 2000; Minobe, 2002), suggesting that the

1987/1988 regime shift occurred on a scale of the

entire North Pacific basin. However, since the varia-

tion patterns in SST fields are different in the north-

western and northeastern Pacific (Yasunaka and

Hanawa, 2002), it is important to focus on regional

oceanic/climatic indices representing the environmen-

tal habitat of specific species. Our analysis led us to

conclude that oceanic conditions such as winter SSTs

in the Kuroshio region are useful indices for predict-

ing long-term abundance trends of Pacific saury, and


the apparently strong saury abundance response to

these physical conditions suggests that Pacific saury

could be used as a bio-indicator of regime shifts in the

Kuroshio waters.

5. Summary and conclusions

1. The PCAs on oceanographic, meteorological, and

climatic indices suggest that oceanic regime shifts

took place in the mid-1970s, late 1980s, and

possibly late 1990s in the northwestern Pacific. In

particular, the 1987/1988 regime shift is the most

evident event in the Kuroshio region during the last

half century, whereas the mid-1970s regime shift is

not clear in the Kuroshio region compared with its

distinct pattern found in global climatic indices and

SST fields in the central and eastern North Pacific.

2. The oceanic regime shifts in the Kuroshio region

are strongly associated with the dynamics of the

pelagic community.

3. Decadal variations in Pacific saury abundance

corresponded well with the regime shifts of 1976/

1977, 1987/1988, and possibly 1997/1998 in the

Kuroshio region. In particular, the abrupt change in

saury abundance which occurred around 1987/

1988 appears to be a consequence of the oceanic

regime shift in the Kuroshio region, while the shifts

in 1976/1977 and 1997/1998 also seemed associ-

ated with ENSO events.

4. Since the abundance/biological indices of saury are

significantly correlated with winter oceanographic

conditions (SST, SCV) in the Kuroshio region,

oceanic environmental factors, such as SST in the

Kuroshio region, appear to influence the early

survival and recruitment success of saury.

5. Positive correlations were found between condition

factors and saury abundance indices, CPUE and

oceanographic/meteorological indices in the Kur-

oshio region indicating that a density-dependent

effect on the growth is not evident, and that

recruitment success and growth of saury are

strongly affected by oceanographic conditions in

the Kuroshio region, rather than by biological

factors such as prey density.

6. The PCAs for the main, small pelagic species

investigated suggest that the regime shift in the late

1980s occurred basin-wide in the pelagic ecosys-

tem of the northwestern Pacific, and that Pacific

saury could be used as a bio-indicator of

oceanographic regime shifts in Kuroshio waters.

Acknowledgements

We are grateful to Dr. T. Watanabe of NRIFS for

providing MOI data and to the Japanese Fisheries

Agency for providing the condition factor data for

saury. We thank Dr. T. Watanabe of JSNFRI for his

kind help in drawing spatial maps, and also Drs. A.

Yatsu and T. Yirie of NRIFS for their discussions and

support. Comments from two reviewers helped to

improve this manuscript. This study was partially

supported by grants from the VENFISH project of the

Japanese Ministry of Agriculture, Forestry and

Fisheries.

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