FISHERIES OCEANOGRAPHY
Fish. Oceanogr. 16:5, 409–421, 2007
Horizontal and vertical movements of juvenile bluefin tuna
(Thunnus orientalis) in relation to seasons and oceanographic
conditions in the eastern Pacific Ocean
TAKASHI KITAGAWA,1,* ANDRE M.
BOUSTANY,2 CHARLES J. FARWELL,3
THOMAS D. WILLIAMS,3 MICHAEL R.
CASTLETON2 AND BARBARA A. BLOCK2
1
Ocean Research Institute, University of Tokyo, Nakano, Tokyo
164-8639, Japan
2
Stanford University, Tuna Research and Conservation Center,
Hopkins Marine Station, Oceanview Blvd., Pacific Grove, CA
93950-3094, USA
3
Tuna Research and Conservation Center, Monterey Bay
Aquarium, 886 Cannery Row, Monterey, CA 93940, USA
ABSTRACT
Electronically tagged juvenile Pacific bluefin, Thunnus
orientalis, were released off Baja California in the
summer of 2002. Time-series data were analyzed for 18
fish that provided a record of 380 ± 120 days
(mean ± SD) of ambient water and peritoneal cavity
temperatures at 120 s intervals. Geolocations of tagged fish were estimated based on light-based longitude
and sea surface temperature-based latitude algorithms.
The horizontal and vertical movement patterns of
Pacific bluefin were examined in relation to oceanographic conditions and the occurrence of feeding
events inferred from thermal fluctuations in the peritoneal cavity. In summer, fish were located primarily
in the Southern California Bight and over the continental shelf of Baja California, where juvenile Pacific
bluefin use the top of the water column, undertaking
occasional, brief forays to depths below the thermocline. In autumn, bluefin migrated north to the waters
off the Central California coast when thermal fronts
form as the result of weakened equatorward wind
stress. An examination of ambient and peritoneal
temperatures revealed that bluefin tuna fed during this
period along the frontal boundaries. In mid-winter, the
bluefin returned to the Southern California Bight
possibly because of strong downwelling and depletion
*Correspondence. e-mail: [email protected]
Received 10 April 2006
Revised version accepted 30 October 2006
Ó 2007 The Authors.
of prey species off the Central California waters. The
elevation of the mean peritoneal cavity temperature
above the mean ambient water temperature increased
as ambient water temperature decreased. The ability of
juvenile bluefin tuna to maintain a thermal excess of
10°C occurred at ambient temperatures of 11–14°C
when the fish were off the Central California coast.
This suggests that the bluefin maintain peritoneal
temperature by increasing heat conservation and
possibly by increasing internal heat production when
in cooler waters. For all of the Pacific bluefin tuna,
there was a significant correlation between their mean
nighttime depth and the visible disk area of the moon.
Key words: archival tag, bluefin tuna, eastern Pacific
Ocean, feeding event, horizontal and vertical
movements, upwelling indices
INTRODUCTION
Pacific bluefin tuna, Thunnus orientalis, is a highly
migratory species distributed mainly in the North
Pacific. The commercial value of this species exceeds
that of most other species of tunas. Characteristics that
contribute to its high market price include its size,
reaching at the maximum about 3 m in total length
(Collette and Nauen, 1983; Foreman and Ishizuka,
1990; Bayliff, 1994), and its unique taste. It is,
however, universally agreed that southern bluefin,
T. maccoyii, and Atlantic bluefin, T. thynnus, are
overfished, and that Pacific bluefin may become
overfished in the near future. Bluefin tunas are at the
center of global international management concerns
because of depleted populations.
The North Pacific Ocean contains one population
of bluefin tuna. Pacific bluefin tuna spawn to the south
of Japan and in the Sea of Japan (Sund et al., 1981).
Juvenile bluefin remain in the western Pacific during
the first year of life, and late in the year or early in the
second year, some bluefin migrate from the KuroshioOyashio transition region in the western Pacific to the
eastern Pacific (Orange and Fink, 1963; Clemens and
Flittner, 1969; Bayliff et al., 1991; Bayliff, 1994; Itoh
doi:10.1111/j.1365-2419.2007.00441.x
409
410
T. Kitagawa et al.
et al., 2003a). After a sojourn in the eastern Pacific,
bluefin tuna are thought to return into the western
Pacific (Bayliff, 1994). However, the mechanism for
this east-to-west migration has not been clarified.
Polovina (1996) put forward the hypothesis that
migration of juvenile bluefin into the eastern Pacific
increased in years when the abundance of sardines off
Japan was declining.
With regard to the vertical distribution and movements of the Pacific bluefin, recent archival tagging
studies in the western Pacific demonstrated that
juvenile bluefin remain predominately in the surface
mixed layer, with occasional dives to depths of 120 m
during the period of occupation in the East China Sea
(Kitagawa et al., 2000). Average swimming depths
were much shallower during summer months than in
winter, when the water column experiences significant
mixing. In addition, Kitagawa et al. (2001, 2002) noted that juvenile bluefin (50–70 cm in fork length)
used behavioral thermoregulation to thermally buffer
the rapid ambient temperature drops that occur as the
bluefin swims repeatedly below the thermocline for
short periods. However, these studies described the
bluefin behavior only in a localized area and they did
not clarify the advantage of diving below the thermocline. Kitagawa et al. (2004) examined the relationship between the tuna’s vertical movement pattern
for feeding and the thermocline depth during summer
in the Kuroshio-Oyashio transition region, comparing
it to that in the East China Sea. They conclude (as did
Marcinek et al., 2001) that, in addition to the ambient
temperatures, the vertical and horizontal distributions
of prey species play an important role in their feeding
behavior and vertical distribution, and that bluefin
tuna in the Oyashio frontal area, where both the
horizontal and the vertical thermal gradients are much
steeper, spent most of the time on the warmer side of
the front and often traveled horizontally to the colder
side during the day, perhaps to feed.
Comparatively little is known about the horizontal
and vertical movements of bluefin tuna in the eastern
Pacific. Hanan (1983), Bayliff et al. (1991) and Bayliff
(1994) have collated information about horizontal
distribution off Baja California using fish catch data
during the fishing season. Marcinek et al. (2001) used
ultrasonic telemetry to track pressure and temperature
of six bluefins of 11.8–57.6 kg mass, revealing that
bluefin in the Southern California Bight have a preference for the upper mixed layer but occasionally dive
through the thermocline.
This study showed that bluefin in the Southern
California Bight had a preference for the upper mixed
layer, but occasionally dove through the thermocline.
Marcinek et al. (2001) and Farwell (2001) reported on
the application of pop-up satellite archival tags to
juvenile bluefin in the eastern Pacific, demonstrating
movements from the Central California to the Baja
region. Both authors’ results indicated a predominant
use of the continental shelf waters and vertical
movements in the mixed layer in tracks that ranged
from 5 to 90 days. Domeier et al. (2005) analyzed the
data recovered from 11 pop-up satellite archival tags
and three surgically implanted archival tags to juvenile
bluefin. They reported that the fish spend winter and
spring off central Baja California, and summer through
fall was spent moving as far northward as Oregon with
a return to Baja California in the winter months.
In this paper, we report on the initial experiments
involving bluefin tuna archival tagging in the Tagging
of Pacific Pelagic (TOPP) research project, a pilot
program of the Census of Marine Life (COML) (Block
et al., 2002; Block, 2005). As a part of this project,
bluefin tuna were electronically tagged off the west
coast of North America beginning in August of 2002.
In the present paper, we analyzed the data on vertical
distribution and movement of juvenile Pacific bluefin
tuna obtained from the archival tags released in 2002.
Our objectives were to examine the differences in
horizontal and vertical movement patterns among
seasons for the years 2002 and 2003 in relation to the
oceanographic conditions in the eastern Pacific. We
also examined their vertical movements through the
thermocline in relation to the occurrence of feeding
events. The influence of temporal and spatial movement patterns on the thermal excess observed between
peritoneal cavity temperatures and ambient temperatures are examined and discussed.
MATERIALS AND METHODS
Tags and data
The archival tag used for this study was the LTD2310
(Lotek Wireless Inc., St Johns, NF, Canada). The
cylindrical tag was 76 mm in length and 16 mm in
diameter, with a weight (in air) of 45 g. A thin, Teflon-coated flexible stalk (270 mm long and 2.0 mm in
diameter) extends from the main body of the tag. The
light sensor, a 478-nm dye responsive to ambient light
over nine decades, and the sensors for external temperature measurement are embedded in the end of the
stalk, whereas those for pressure and internal temperature measurement are installed in the main body
of the tag. The tags are designed for implantation into
the peritoneal cavity with the stalk protruded outside
the tag. The archival tag logs external and internal
temperatures (with a resolution of 0.05°C), swimming
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Horizontal and vertical movements of bluefin tuna in the eastern Pacific
depth (resolution of 0.05%; this changes with depth),
and ambient light levels at an interval of 60–120 s. In
addition, geolocations were estimated daily using
threshold-based methods based on the detection of
the time of sunrise and sunset (Musyl et al., 2001;
Ekstrom, 2004).
Juvenile Pacific bluefin tuna were captured by pole
and line fishing off Baja California aboard the fishing
vessel F/V Shogun. One hundred and sixty juvenile
bluefin tuna with fork lengths of 87–125 cm, which
were 2–3 yr old (Bayliff, 1993; Itoh, 2001), were tagged and released with archival tags inserted into the
peritoneal cavity on 5–6 August 2002 (Fig. 1). The
tagging procedure was described in detail by Teo et al.
(2004) and Boustany (2006). Twenty individuals were
recovered by commercial or recreational fishers and
data were obtained from 18 of the archival tags placed
in these fish. Fish used in this study were recaptured in
the eastern Pacific after more than 3 months at liberty
for analysis of diving behavior and movement patterns
between seasons. The other two fish (bluefin 405 and
437) were recaptured in the same area after a very
Figure 1. Estimated track (colored circles with line) from
daily geolocation positions of bluefin 315 and sites (squares)
of observation of synoptic sea-level pressure for upwelling
indices by the Pacific Fisheries Environmental Laboratory.
411
short period at liberty and were not included in the
analyses.
As was pointed out by Welch and Eveson
(1999), errors in geolocations estimated from
archival tags can be considerable. As latitude estimation from day length was unreliable, we adjusted
the latitude by using the sea surface temperature
(SST), as recorded in the summary file for each day,
according to the methods of Teo et al. (2004) and
Block (2005). Boustany (2006) and Boustany and
Block (unpublished data) provide more details on
the horizontal movements of the fish tagged in the
eastern Pacific.
Data analysis, and definition of feeding and diving behavior
Data analysis began with creating daily averages of the
external and internal temperature data, as well as the
swimming depth data, logged by the tags. The daily
average data were further averaged over the period of a
month for all individuals. Frequency distributions of
swimming depth in relation to ambient water temperature were calculated for the month.
Figure 2 shows part of the time-series data of
ambient water and peritoneal cavity temperatures as
well as swimming depths for bluefin 315. In this figure,
an arrow indicates a drop of peritoneal cavity temperature. Ingestion of food, which results in a decline
in temperature either from the prey or from water
entering the peritoneal cavity during a feeding event,
provides an event marker for foraging (Carey et al.,
1984; Gunn et al., 1994; Itoh et al., 2003b; Kitagawa
et al., 2004). We considered the peritoneal cavity
temperature drops to be an indicator of a feeding event
Temperature (°C)
0
50
100
150
200
250
30
20
Peritoneal cavity
temperature
0:00
6:00
15-Feb-03
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Ambient
temperature
10
12:00
18:00
0:00
Depth (m)
Figure 2. An example of temperature changes in peritoneal
cavity recorded by an archival tag (bluefin 315). Upper,
middle and lower lines indicate depth, peritoneal cavity
temperature, and ambient temperature, respectively. An
arrow indicates temperature drop in peritoneal cavity due to
feeding. Black bars on the bottom indicate nighttime.
Aug. 6, 2002– Aug. 6, 2002– Aug. 6, 2002– Aug. 6, 2002–
Jul. 22, 2003 Jul. 15, 2004 Jun. 18, 2003 Jul. 23, 2003
109
86
91
97
120
120
120
120
–
0.19
0.23
0.22
Duration of the data Aug. 6, 2002– Aug. 6, 2002– Aug. 6, 2002– Aug. 6, 2002– Aug. 6, 2002–
analyzed
Jul. 3, 2003
Aug. 3, 2003 Jun. 9, 2003
Aug. 3, 2003 Jul. 23, 2003
Fork length (cm)
88
94
91
91
90
Sampling interval (s) 60
60
60
60
120
Threshold (°C/120 s) 0.28
0.22
0.34
0.2
0.39
488
485
475
471
446
423
415
414
412
Duration of the data Aug. 7, 2002– Aug. 7, 2002– Aug. 6, 2002– Aug. 7, 2002– Aug. 7, 2002– Aug. 7, 2002– Aug. 7, 2002– Aug. 7, 2002– Aug. 6, 2002–
analyzed
Jul. 24, 2003 Jun. 20, 2003 Jul. 17, 2003 Jul. 23, 2003 Jun. 6, 2003
Jul. 11, 2004 Aug. 5, 2003 Jun. 18, 2003 Jul. 22, 2003
Fork length (cm)
91
102
88
104
92
91
91
100
92
Sampling interval (s) 60
60
60
60
60
60
120
120
60
Threshold (°C/120 s) 0.17
0.2
0.32
0.32
0.29
0.2
0.26
0.25
0.18
403
364
345
342
340
334
333
315
Oceanographic and fish landing data
In order to investigate the vertical movement of the
bluefin tuna in relation to vertical thermal structure,
daily or monthly vertical ambient temperature profiles
were computed by estimating mean ambient temperature at 10-m depth intervals.
In the present paper, the timing of horizontal
movements in relationship to oceanographic condition was examined. For these analyses fish geolocations
were superimposed on the coastal upwelling indices
calculated by Pacific Fisheries Environmental Laboratory, U.S. National Marine Fisheries Serves (NMFS).
The data were downloaded from a web site (http://
www.pfeg.noaa.gov/javamenu.html) and a daily index
was calculated at nine positions from 21° to 45°N
along the west coast of North America (Fig. 1), using
synoptic sea level pressure gridded fields. Detailed
information concerning this data set is available on
the web site.
Some authors reported that juvenile bluefin tuna
consumed mainly anchovies (Blunt, 1958; Bell,
1963a; Pinkas, 1971). However, recent studies have
shown that climate-ocean and biological systems are
not stable, but shift from one regime to another in
the Pacific Ocean (Ebbesmeyer et al., 1991; McFarlane et al., 2002). This ‘regime shift’ results in
oscillations of anchovy and sardine abundance
cycles. An anchovy regime was in place from 1950s
Bluefin no.
only when the temperature change could not be
explained by changes in the ambient water temperature, by using the method reported in Kitagawa et al.
(2004). The thresholds for feeding events were
established as the maximum absolute value of the rate
of increase of peritoneal temperature (0.20–0.39°C per
120 s, Table 1). The threshold in only one individual
(bluefin 471) could not be established because the fish
dove to colder depths for longer durations compared
with the other fish. In this individual the drop in
temperatures caused by feeding could not be detected.
A feeding depth was defined as the depth in which a
feeding event occurs.
A dive was defined as starting when a descending
tuna passed below 10 m and ended when it ascended
to 10 m. Dive maximum depths were defined as the
greatest depth reached during a dive. Dive duration
was the time elapsed during one dive. In this study,
data from dive depths of more than 50 m were used for
dive analysis. Daily averages were calculated for dive
depth, dive frequency, dive duration, feeding events
and feeding frequency. The daily average data were
further averaged over the period of a month for all
individuals.
411
T. Kitagawa et al.
Table 1. Fork length at release, sampling interval, the duration of the data analyzed, and threshold values for feeding events analysis for each individual.
412
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Horizontal and vertical movements of bluefin tuna in the eastern Pacific
through the early 1970s followed by a sardine cycle
(Chavez et al., 2003). High sardine stocks off Japan
and Chile declined in the early 1990s, but those off
California have not declined yet (McFarlane et al.,
2002). The tagging cruise in 2002 revealed that
most of the bluefin tuna had Pacific sardines in their
stomachs (B.A. Block, personal observation). In
recent years, therefore, the Pacific bluefin tuna in
the eastern Pacific would have primarily consumed
Pacific sardines (Sardinops sagax). To look at the
monthly changes in prey availability to bluefin, we
downloaded Pacific sardine commercial landing data
at Los Angeles and Monterey (Fig. 1) from the web
site of Southwest Fisheries Science Center, NMFS
(http://las.pfeg.noaa.gov:8080/las_fish1/servlets/dataset).
The sum of the Pacific sardine in all ports of California in 2002 was about 58 348 tonnes, which is
more than 10-fold higher than that for Northern
anchovy, Engraulis mordax (4648 tonnes, California
Department of Fish and Game, 2002).
(a)
(b)
(c)
Figure 3. Time series in (a) August
2002, (b) November, (c) February 2003,
and (d) May for swimming depth
together with vertical thermal structure
(upper), peritoneal cavity temperature
(middle), and ambient temperature
(lower) obtained from bluefin 315.
Arrows indicate temporary drops of
ambient temperature irrelevant to dives.
(d)
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
413
RESULTS
Seasonal change in swimming depth in relation to
vertical thermal structure
The time-series data for a representative individual
bluefin (bluefin 315), which was released in August
2002 and tracked for 351 days, are shown in Fig. 3.
Isotherms depicted in Fig. 3 were estimated from daily
ambient water temperature and depth profiles acquired
from the diving fish carrying the archival tag. Vertical
movements for the other tagged bluefin tuna were
similar to those of this fish (Table 2). The frequency
distributions of the swimming depths recorded for
bluefin 315 are shown in Fig. 4 together with vertical
profiles of the monthly mean ambient temperature in
the daytime. The average depth for all individuals in
the daytime was significantly greater than that in the
nighttime (Table 2), and power spectra of the swimming depth calculated by Fast Fourier Transform
(FFT) had a marked peak at about 24 h for most of the
414
T. Kitagawa et al.
Table 2. Monthly averages of daytime depth, nighttime depth, and daytime dive duration for all individuals from 2002 through
2004.
August
Depth ± SD (daytime, m)
Depth ± SD (nighttime, m)
Dive duration (min)
September
17.9 ± 3.9
13.0 ± 2.1
17.8 ± 5.6
22.6 ± 6.0
8.6 ± 2.1
18.8 ± 7.9
February
Depth ± SD (daytime, m)
Depth ± SD (nighttime)
Dive duration (min)
March
28.3 ± 6.4
15.4 ± 2.5
19.5 ± 8.5
October
November
December
January
21.1 ± 9.0
8.8 ± 2.8
19.1 ± 8.6
14.8 ± 4.4
10.9 ± 3.2
21.3 ± 6.6
23.4 ± 6.6
16.5 ± 4.1
27.0 ± 9.9
34.1 ± 8.6
21.7 ± 3.4
25.9 ± 8.1
April
26.5 ± 6.6
15.6 ± 3.2
17.1 ± 5.9
29.8 ± 6.4
14.0 ± 2.3
16.2 ± 4.5
May
26.3 ± 6.3
13.6 ± 2.7
14.9 ± 5.3
June
22.1 ± 2.9
14.0 ± 2.5
11.6 ± 3.4
July
17.9 ± 3.4
10.1 ± 1.1
9.1 ± 2.4
seasonal thermocline possibly regulates vertical distribution of the juvenile bluefin tuna or its prey species.
In October to November, the fish moved to the
Central California coast off Monterey Bay (Fig. 1),
where colder waters occurred in comparison with
those in August (Fig. 3b). The temperature of surface
water was 15.6 ± 0.9°C (mean ± SD, Fig. 4b). A
seasonal thermocline gradient was apparent, however
its depth often varied (Fig. 3b). For example before 6
November 2002, the thermocline formed at depths
shallower than 50 m, while after 9 November 2002, it
formed below 50 m (Fig. 3b). In addition, cold water
began upwelling around 23 November 2002. These
events indicate that this region dramatically changes
temporally and/or spatially. In relationship to the
individuals. These data indicate that bluefin make diel
vertical migrations. Dive duration in the daytime
ranged from about 9.1 to 27.0 min (Table 2).
In August of 2002, bluefin 315 occupied the region
near the release location off the northern coast of Baja
California. The horizontal movements indicate this
fish was swimming in the Southern California Bight
(Fig. 1). The fish remained above the thermocline
most of the time while in this area, undertaking
occasional, brief vertical movements below the thermocline to depths greater than 250 m (Fig. 3a). In
Fig. 4a, a seasonal thermocline developed at the depth
from 20 to 50 m in August, which was very stratified
and stable. The swimming depths of the fish tended to
be confined to the shallow mixed layer and thermocline depths (0–50 m, Fig. 4a). This suggests that a
Frequency (%)
(a) 0
20
0
40
60
80
100
(c) 0
20
0
50
50
100
100
150
150
200
200
250
40
250
Depth (m)
80
100
Febuary 2003
August 2002
300
60
300
8
(b) 0
10
20
12
40
14
16
60
18
80
20
100
0
8
(d) 0
10
20
12
40
14
16
60
18
80
20
100
0
50
50
100
100
150
150
200
200
250
250
November 2002
300
8
10
12
14
16
18
May 2003
300
20
8
10
12
Mean ambient temperature (oC)
14
16
18
20
Figure 4. Frequency distributions of the
swimming depth of bluefin 315 and vertical profiles of mean ambient temperature in the daytime in (a) August 2002,
(b) November, (c) February 2003, and
(d) May. Bars indicate standard deviation.
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Horizontal and vertical movements of bluefin tuna in the eastern Pacific
415
vertical ambient temperatures were comparatively
higher than those at other times of the year. When the
water column was stratified (Figs 3d and 4d), the
swimming depths tended to be confined to the shallow
depths with the maximum frequency at the 0–10 m
depth (Fig. 3d). As shown in the other panels, the fish
often repeated brief vertical movements below the
thermocline in the daytime, although the fish spent
most of the time swimming within the surface mixed
layer. Mean depth of all bluefin was 26.5 ± 6.6 m
(Table 2).
The vertical distributions of all the bluefin were
influenced by the lunar phases. A significant correlation exists between the visible disk area of the moon
phase and their average nighttime depth distributions
(r ¼ 0.123–0.536, Table 3). Fish occupied significantly greater depths for periods around the full moon,
in contrast to the other days of the lunar cycle; this
tendency was more prominent from November
through March (Fig. 5).
changes in the depth of the thermocline, the bluefin
tuna changes its diving pattern. The other fish also
took a similar tendency. That is, all bluefin (including
this bluefin) spent most of their time within the surface mixed layer with a mean swimming depth of
14.8 ± 4.4 m in November during the daytime
(Table 2), and they ceased undertaking occasional
brief movements below the thermocline.
Importantly, there were temporary drops in ambient temperatures (to as cool as 13.0°C minimum at the
surface, indicated by arrows in Fig. 3b), although the
monthly surface water temperatures were relatively
constant. These temporary drops in temperature
indicate daytime horizontal excursions into cooler
waters occur from warmer areas and are potentially
indicative of the bluefin tuna moving across frontal
areas in these regions. These temporary drops were
seen in most of the bluefin tunas examined.
From December to January, bluefin 315 together
with the other bluefin returned back to the waters of
the Southern California Bight and remained off Baja
California (Fig. 1). The water column had a mixed
layer above depths of around 75–100 m where the fish
spend most of the time (Figs 3c and 4c). The fish made
occasional dives to depths through the thermocline,
which was not as steep as the thermocline in Fig. 3a,b.
These horizontal and vertical migration patterns were
seen in most of the bluefin tunas examined.
In May, the fish migrated further to the south to an
area off southern Baja California, to the south of Point
Eugenia and to the north of Magdelana Bay, where
Temporal and spatial variation in feeding
In order to detect temporal and spatial changes in the
bluefin tunas feeding activity, we calculated average
values for latitude, surface temperature, dive frequency
per day, feeding frequency per day, and feeding depth
in each month for all fish except bluefin 471 (Fig. 6).
During the months of August and September, we
observed high feeding frequency when the fish were
located 30°N in the Southern California Bight
(Fig. 6d,e). In these periods the bluefin dove to feeding
Table 3. Results of correlation analysis for relationships between lunar illumination and average nighttime depth, between
ambient water temperature and peritoneal temperature, and between ambient temperature and the thermal excess between
peritoneal cavity and ambient temperatures.
Bluefin no.
315
333
334
340
342
345
Correlation between lunar illumination
0.448 0.126*
0.406 0.459 0.495 0.449
and average nighttime depth
Correlation between ambient and
0.303 0.176** 0.285 0.322 0.199 0.367
peritoneal temperatures
Correlation between ambient temperature )0.675 )0.732
)0.536 )0.472 )0.729 )0.637
and thermal excess
412
414
Correlation between lunar illumination
0.217 0.413
and average nighttime depth
0.399 0.272
Correlation between ambient and
peritoneal temperatures
Correlation between ambient temperature )0.680 )0.716
and thermal excess.
415
446
471
403
411
0.470
0.380
0.469
0.452
0.224
0.338
)0.475 )0.768
)0.516
475
488
485
0.403
0.507
0.184
0.123*
0.536
0.453
0.340
0.603
0.395
0.472
0.387
0.337
0.125*
0.229*
)0.565 )0.729 )0.511 )0.690
*P < 0.05, **P < 0.01, and P < 0.0001 unless otherwise stated.
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
423
364
)0.606 )0.752
)0.580
416
T. Kitagawa et al.
New
30
Moon Phase
Mean depth in the night (m)
0
60
Full
Sep-02
Nov-02
Jan-03
Mar-03
May-03
Figure 6. (a) Average latitudinal position, (b) surface
temperature, (c) diving frequency, (d) feeding frequency,
(e) feeding depth averaged over month for all individuals
except bluefin 471 from 2002 through 2004. Bars indicate
standard deviation.
(a)
35
30
20
(b)
19
18
17
16
20
14
15
10
5
0
4
(d)
3
2
1
Feeding depth (m)
100
0
(e)
80
60
40
20
0
35°N (Fig. 6a), the surface sea temperatures were
lower (15.5°C in September and 15.3°C in November
on average, Fig. 6b). Bluefin tunas dove less frequently
and correspondingly they fed at shallower depths
(Fig. 6c,e). Feeding frequencies were significantly
lower than in August (P < 0.001, Sheffe test), and
were also lower than in September but not significantly so (Fig. 6d). Feeding frequencies decreased
during the period from December through April,
although vertical movement frequency gradually
increased. Mean feeding depths ranged from 38.0 to
46.4 m (Fig. 6d,e). In the months of May and June
when the bluefin tuna are located around 25°N, in the
south of Baja California, dive frequency was the
highest and feeding events also increased (Fig. 6a,d).
In July when they initiated northward migrations, the
feeding frequencies increased, although the feeding
depths did not vary so much (Fig. 6a,d,e).
(c)
–1
Diving frequency (day–1)
15
25
o
20
Ambient temperature ( C)
25
Feeding frequency (day )
o
Latitude ( N)
40
Jul-03
Figure 5. Lunar illumination (dotted
curve) and daily average nighttime depth
of bluefin 315.
A
S O N D
J F M A M J
Month
J
depths that ranged from 36.9 to 55.4 m (Fig. 6e). The
results indicate that bluefin are potentially feeding on
prey items at a variety of depths and access or search
for food by using repetitive vertical movements. In
October to November when they migrated to around
Timing of northward migration in relation to changes in
oceanography and food
In order to examine the northward migration of
bluefin tuna from October to November in relationship to the potential environmental cues, we examined the relationship between daily latitude positions
and coastal upwelling indices (Fig. 7). It is evident
from Fig. 7 that the bluefin tuna, located around 30°N
in August of 2002, initiated northward migrations just
after strong upwelling events from September to
October (blue) ceased in the region of 30°–45°N, and
that they remained in this northern region for a while.
When strong advection of surface waters (yellow to
red) accompanied by a downwelling came to be
dominant in the central coast of California region
from November to December, almost all of the fish
returned to the region around 28°–30°N. During this
period a few individuals moved out into the offshore
open ocean regions. During January through March,
most of the bluefin were distributed in the region
between 25° and 32°N once again where the upwelling
indices were almost zero (green), although some
individuals went on an excursion to the open ocean.
From April through June, the tunas were concentrated
in the vicinity of 25°N over the continental shelf, and
they initiated northward migrations in July. When the
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Horizontal and vertical movements of bluefin tuna in the eastern Pacific
417
Latitude (oN)
45
Figure 7. Time series for daily latitudinal positions for all individuals with
upwelling indices.
40
35
30
25
Jan-03
second year was represented in the track, a similar
cycle of movements was revealed in 2003–04.
We analyzed time series of the monthly landing
data of Pacific sardines in the ports of Monterey and
Los Angeles from the months of August 1993 through
July 2003. In both sets of landing data, a 1-year periodicity was detected in the fluctuations by auto-correlation analysis. This indicates that the landings
fluctuated periodically within the year for these 10 yr.
Figure 8 shows the average monthly landing data of
Pacific sardines in the ports of Monterey and Los
Angeles from August through July. In Monterey, the
landings of sardines increased from August and
reached the highest levels in October. Then the
landings from November to April gradually decreased
to almost zero. Strong downwelling along the Central
California coast is clearly observed in December
(Fig. 7). The landings remained low or go to zero from
April through July. In Los Angeles, on the other hand,
the monthly landings for all the months were significantly higher than those in Monterey (Fig. 8, Wilcoxon test P < 0.0001). As in Monterey, the landings
in Los Angeles increased from August through October. There was a decline in November and December
before reaching the highest levels from January to
Figure 8. Monthly mean Pacific sardine landing data plus
standard deviation of Monterey and Los Angeles for 10 yr
from August 1993 through July 2003.
16
Monterey
10
0
16
Los Angeles
Jul-03
Jan-04
Jul-04
March. Landings of sardines decreased to the lowest
levels from April to July.
Relationship between ambient and peritoneal cavity
temperatures
The peritoneal cavity temperature showed a diel pattern often increasing in the daytime, showing smallscale temperature fluctuations corresponding to the
occasional dives below the thermocline, as well as
potential feeding events. Notably the peritoneal cavity
temperature always decreased in the nighttime
(Fig. 3a–d). According to FFT analysis, additionally,
these daily temperature fluctuations for all fish showed
a 24-h periodicity. This diel periodicity in the peritoneal temperatures indicates that the fish produce heat
in the peritoneal cavity during the daytime potentially
in association with morning feeding events and has a
decay of thermal excess in the night. Mean values for
ambient water and peritoneal cavity temperatures in
the daytime were calculated. The relationship for
bluefin 315 is shown in Fig. 9a. Positive correlations
were found between the ambient water temperatures
and peritoneal temperatures. This implies that ambient
temperature affects peritoneal cavity thermal excesses.
The results of correlation analysis for the relationships
are summarized in Table 3.
Importantly, the difference between peritoneal
cavity and ambient temperature (thermal excess)
increased as ambient water temperature significantly
decreased (Fig. 9b). The thermal excess was less than
approximately 4°C when ambient temperatures were
more than 18°C, while the thermal excess increased to
approximately 10°C when the mean ambient temperatures were 11–14°C. This large thermal excess was
especially apparent during October to December when
the fish moved up to northern waters off the central
coast of Monterey Bay (Fig. 9b).
DISCUSSION
10
0
Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
Jul
Month
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
The juvenile Pacific bluefin tuna with archival tags in
the eastern Pacific show strong preferences for depths
shallower than 50 m and remain above the thermocline most of the time (Figs 3 and 4, Table 2). This
result agrees with previous electronic tagging studies
418
T. Kitagawa et al.
Figure 9. (a) Relationship between daily mean ambient
water temperature and peritoneal cavity temperature for
daytime data, and (b) relationship between mean ambient
water temperature and thermal excess (peritoneal cavity
temperature minus ambient water temperature) for daytime
data for bluefin 315.
(a)
(b)
for juvenile bluefin tuna that were similar in size or
smaller (Kitagawa et al., 2000, 2002), but those studies
were conducted in the western Pacific. During the
daytime bluefin tuna in the eastern Pacific occasionally make vertical movements for durations of
9.1–27.0 min (Table 2). These dives are most likely
foraging dives. The diving behaviors and durations at
depth show distinctions from those observed in
archival tagged Pacific bluefin of similar size in the
East China Sea (Kitagawa et al., 2004). For example,
despite the fact that the thermal structure was occasionally almost homogenous in the California Current
up to 50 m depth in Fig. 3b,c, dive durations were
much shorter compared with those observed in the
East China Sea, which ranged from 16.7 min to more
than 167 min (Kitagawa et al., 2004). Potentially, the
decreased time spent at depth is due in part to the
cooler temperatures of the California Current regime.
The bluefin tuna showed a cyclical pattern of
movement swimming from the south off the Southern
California Bight to the north and back to Baja California. During this time, the bluefin remained along
the North American continental shelf (Fig. 1). In July
to August, when the Ekman indices decreased in the
waters north of 25°N (Fig. 7) and the ambient temperature became warm, the landings of sardines
increased in both Los Angeles and Monterey (Fig. 8).
These increases in landing potentially indicate the
success of recruitment of sardines hatched in prior
winters off the Southern California Bight. In these
months, bluefin move up and into the Southern California Bight from waters off Baja California, and their
feeding frequency increased to the highest levels
(Fig. 6). This area is one of the regions in which
bluefin tuna aggregate and actively feed: a hot spot in
the eastern Pacific most likely driven by seasonal prey
abundance induced by favorable environmental conditions (warm seasonal temperatures and high productivity).
From September to November, the landings of
sardines increases to the highest level in Central
California coincident with rising SST (Goericke et al.,
2004), although the arrival of bluefin tuna in the
Central California waters occurs at a period when the
temperatures of surface water were <15°C (Figs 3b and
6). This period is the beginning of the warmest SSTs
in the annual cycle of the region. This suggests a likely
hypothesis for why the Pacific bluefin move up the
coast. The northward movement or recruitment of
prey due to high productivity and warmer temperatures is accompanied by the result of weakened equatorward wind stress in the region, leading to the
northward migration of bluefin (Fig. 6). A thermal
window has opened for the bluefin tuna at this time –
providing access to the rich prey in the region. Bell
(1963b) stated that bluefin are caught by purse-seine
vessels off California and Baja California waters with
SST 17–23°C. However, the results shown in Fig. 9
indicate that in the eastern Pacific the juveniles
experience ambient temperatures ranging from 11 to
21°C. When bluefin move to the northward point of
their migration and occupy waters off the Central
California coastal region, they must produce significant heat (2.5 times as much from when they are in
the southern waters) as well as increase heat conservation to maintain their body temperatures.
Understanding the thermal structure in the waters
offshore of regions such as Monterey Bay from September to November is important for understanding
the vertical distribution of bluefin tuna. As shown in
previous tuna studies (Laurs et al., 1977, 1984; Fiedler
and Bernard, 1987; Block et al., 1997; Inagake et al.,
2001; Kitagawa et al., 2004), bluefin aggregate in
frontal regions. They make daytime horizontal excursions to the cooler coastal waters to potentially feed on
schools of sardine, which might stray away from their
established zone of relative abundance near the coast,
and they discover the area of enhanced food concentration along the frontal region (Bakun, 2001). It is
likely that the resultant abundant schools of sardines
in these locations are restricted in their vertical distribution by temperature (Kitagawa et al., 2004),
making foraging easier. Bluefin tuna foraging depths
are very shallow in these waters potentially due to the
restriction in vertical habitat of the prey species. Thus,
this region appears to be another seasonal hot spot of
bluefin as the elevated thermocline becomes a trap for
the prey because of thermal limitations.
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
Horizontal and vertical movements of bluefin tuna in the eastern Pacific
Strong downwelling along the Central California
coast in December causes a local decrease in the
productivity of the region, reducing the concentration
of sardines in the zone. This change of oceanographic
conditions most likely results in the bluefin tunas
returning to the waters off Southern California and
moving further south to Baja California. In addition,
there may be an increased energy cost to feeding in
these waters, as the bluefin tuna not only have to
conserve heat but also potentially have to generate
increased amounts of body heat to maintain their body
temperatures when in waters off Central California. As
shown above, ambient temperatures of 11–14°C were
much lower than those around the Southern California Bight (14–18°) (Fig. 9), yet the fish maintain
significant gradients in thermal excess in the peritoneal cavity. Recent metabolic data on similar-sized
Pacific bluefin tuna also demonstrate that metabolic
rate goes up in cooler waters and tail beat frequency
increases in the cold, suggesting a possible mechanism
for increased heat production (Blank et al., 2005).
In this study, furthermore, feeding frequency was
lowest in November to December off Central California in the vicinity of Monterey Bay (Fig. 6). If
bluefin tuna stayed longer off Monterey Bay without
acquiring food, they would pay a high energetic cost
because of foraging in cooler waters. Overall, the
energetic constraints of maintaining body temperature
may generate a net caloric loss once the prey is less
dense, when occupying these cooler regimes. We
hypothesize that the motivation for the southward
migration may be a tradeoff between energy acquisition off the central coast waters and metabolic costs.
Migration to the south may be a mechanism for saving
metabolic energy expended while in cooler waters.
From January to April, the sardine landings in Los
Angeles were at a high level while they were low or
zero in Monterey (Fig. 8). The landings data indicate
that sardines are most likely more aggregated around
the Southern California Bight and recruit to the
fishery resource. The presence of the sardines is most
likely the reason the bluefin tuna remain in these
regions. Although the growth rate of juvenile bluefin
in winter is slower than in summer (Bayliff, 1994),
they are able to find sardines continuously available
there during the winter months.
From April to July when the landings of Pacific
sardine in Los Angeles decreased to their lowest level,
they remained low in Monterey (Fig. 9). Ekman
indices were high in the waters north of 25°N (Fig. 7).
These months are the coldest of the year in Monterey
because of strong winds (Goericke et al., 2004). There
was very strong offshore transport and high levels of
Ó 2007 The Authors, Fish. Oceanogr., 16:5, 409–421.
419
induced turbulence, and lower temperatures along the
Central California coastal waters. The wind events
and resultant upwelling do not appear to provide ideal
conditions for the bluefin tuna or their prey to feed or
to reproduce (Bakun, 1996). In this season, therefore,
sardine populations might be depleted or migrate
south because of poor environmental conditions for
sardines. We hypothesize that bluefin migrate to
waters around 25°N for feeding during these months,
where they potentially take advantage of locally
abundant species such as red crabs (Pleuroncodes
planipes) or Humboldt squid (Dosidicus gigas).
The significant correlation between average nighttime depth distributions and the visible disk area of
the moon all year round (Fig. 5) is consistent with
similar behavior found in southern bluefin and bigeye
tuna (Gunn and Block, 2001; Musyl et al., 2003).
Musyl et al. (2003) suggest that the fish were mirroring
the movements of nocturnally migrating organisms of
the sound scattering layer, which were, in turn,
attempting to occupy an isolume. However, bluefin
appear to be less active foragers during the night. From
peritoneal temperatures and depth data, it appears that
bluefin are not feeding as much (Kitagawa et al.,
2004). Thus the behavior of moving down in the water
column most likely is a behavior to hide their bulletshaped silhouette caused by the moon light. We
hypothesize that the movements with the lunar cycle
are primarily to avoid predation events.
ACKNOWLEDGEMENTS
This study is part of the tuna archival tagging program
in the TOPP research project, a pilot program of the
COML. We thank Captain Norm Kagawa and crews
of the F/V Shogun. We thank Ted Dunn for advice and
support. We also thank A. Seitz, R. Schallert, K.
Weng and the Stanford undergraduate assistants from
the Block laboratory for assisting in the archival tagging of bluefin tuna. We thank G. Strout, D. Kohrs, H.
Dewar, S. Teo, C. Perle, A. Walli and R. Matteson
for assistance in data analysis. Funding is from the
National Science Foundation, NOAA Sea Grant, the
Sloan, Monterey Bay Aquarium, the Dave and Lucile
Packard, and Gordon and Betty Moore Foundations.
T. Johnson, Ocean Research Institute, University of
Tokyo, provided comments that substantially improved the manuscript.
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